4. Electrochemistry + 5. Chemical energetics Flashcards
(329 cards)
1
Q
What happens to a molten ionic compound when a current is passed through it?
A
When an electric current is passed through a molten ionic compound the compound decomposes or breaks down
2
Q
In what forms can an ionic compound undergo electrolysis? Why?
A
in a molten state
in an aqueos state
Ionic compounds in the solid state cannot conduct electricity either since they have no free ions that can move and carry the charge
3
Q
Why can covalent compounds not undergo electrolysis?
A
Covalent compounds cannot conduct electricity hence they do not undergo electrolysis
4
Q
Electrode definition
A
Electrode is a rod of metal or graphite through which an electric current flows into or out of an electrolyte
5
Q
Electrolyte definition
A
Electrolyte is the ionic compound in molten or dissolved solution that conducts the electricity
6
Q
Anode definition
A
Anode is the positive electrode of an electrolysis cell
7
Q
Anion definition
A
Anion is a negatively charged ion which is attracted to the anode
8
Q
Cathode definition
A
Cathode is the negative electrode of an electrolysis cell
9
Q
Cation definition
A
Cation is a positively charged ion which is attracted to the cathode
10
Q
How would you set up the equipment for the electrolysis of lead (III) bromide)
A
Add lead (II) bromide into a beaker and heat so it will turn molten, allowing ions to be free to move and conduct an electric charge
Add two graphite rods as the electrodes and connect this to a power pack or battery
Turn on power pack or battery and allow electrolysis to take place
11
Q
What happens at the anode during the electrolysis of lead (III) bromide?
A
Negative bromide ions move to the positive electrode (anode) and lose two electrons to form bromine molecules. There is bubbling at the anode as brown bromine gas is given off
12
Q
What happens at the cathode during the electrolysis of lead (III) bromide?
A
Positive lead ions move to the negative electrode (cathode) and gain electrons to form a grey lead metal which deposits on the surface of the electrode
13
Q
Half equation for anode
electrolysis of lead (III) bromide
A
2Br- —> Br2 + 2e-
14
Q
Half equation for cathode
electrolysis of lead (III) bromide
A
Pb2+ + 2e- —> Pb
15
Q
When using an aqueos solution, what needs to be taken into account?
A
Aqueous solutions will always have water (H2O)
H+ and OH– ions from the water are involved as well
16
Q
To which electrode are OH- ions attracted?
A
OH– ions and non-metal ions (anions) are attracted to the positive electrode
17
Q
What will be formed at the anode? (electrolysis of an aqueos solution)
A
Either OH– or non-metal ions will lose electrons and oxygen gas or gas of non-metal in question is released, eg. chlorine, bromine, nitrogen
18
Q
What is the rule for deciding what non-metal will be formed at the anode? (electrolysis of an aqueos solution)
A
The product formed depends on which ion loses electrons more readily, with the more reactive ion remaining in solution
19
Q
What is the reactivity series for anions?
A
More reactive
→ SO42-
→ NO3-
→ OH-
→ Cl-
→ Br-
→ I-
Less reactive
20
Q
What will form at the cathode? (electrolysis of an aqueos solution)
A
H+ and metal ions attracted to the negative electrode but only one will gain electrons
Either hydrogen or metal will be produced
21
Q
What is the rule for deciding what will be formed at the cathode? (electrolysis of an aqueos solution)
A
If the metal is above hydrogen in reactivity series, then hydrogen will be produced and bubbling will be seen at the cathode
22
Q
What factor (apart from reactivity) also influences the products of the electrolysis of an aqueos solution?
A
Concentrated and dilute solutions of the same compound give different products
23
Q
What is the rule for which anion will be produced in a concentrated solution?
A
For anions, the more concentrated ion will tend to be produced over a more dilute ion
24
Q
In the electrolysis of a molten compound, what will be produced at the cathode and the anode?
A
cathode - metal
anode - non-metal
25
How can you determine that hydrogen was produced at the cathode?
If the gas produced at the cathode burns with a ‘pop’ when a sample is lit with a lighted splint then the gas is hydrogen
26
How can you determine that oxygen was produced at the anode?
If the gas produced at the anode relights a glowing splint dipped into a sample of the gas then the gas is oxygen
27
What colour is bromine gas?
red-brown
28
What colour is chlorine gas?
yellow-green
29
What colour is fluorine gas?
pale yellow
30
How would you set up the equipment for copper refining?
In the set-up, the impure metal is always the anode, in this case the impure copper
The cathode is a thin sheet of pure copper
The electrolyte used is an aqueous solution of a soluble salt of the pure metal at the anode, e.g: CuSO4
31
Why is electrolysis used to purify metals?
Electrolysis can be used to purify metals by separating them from their impurities
32
What happens to copper atoms during copper refining?
Copper atoms at the anode lose electrons, go into solution as ions and are attracted to the cathode where they gain electrons and form now purified copper atoms
33
What happens to the anode during copper refining?
The anode thus becomes thinner due to loss of atoms and the impurities fall to the bottom of the cell as sludge
34
What happens to the cathode during copper refining?
The cathode gradually becomes thicker
35
Where do electrons move during electrolysis?
During electrolysis the electrons move from the power supply towards the cathode
36
Where do positive ions move during electrolysis?
Positive ions within the electrolyte move towards the negatively charged electrode which is the cathode
37
Where do positive ions move during electrolysis?
Positive ions within the electrolyte move towards the negatively charged electrode which is the cathode
38
What happens to positive ions at the cathode?
Here they accept electrons from the cathode and either a metal or hydrogen gas is produced
39
Where do negative ions move during electrolysis?
Negative ions within the electrolyte move towards the positively charged electrode which is the anode
40
What happens to negative ions at the anode, if it is inert?
If the anode is inert (such as graphite or platinum), the ions lose electrons to the anode and form a nonmetal or oxygen gas
41
What happens to negative ions at the anode, if it is reactive?
If the anode is a reactive metal, then the metal atoms of the anode lose electrons and go into solution as ions, thinning the anode
42
What is an electrochemical cell a source of?
An electrochemical cell is a source of electrical energy
43
What is the simple design of an electrochemical cell?
The simplest design consists of two electrodes made from metals of different reactivity immersed in an electrolyte and connected to an external circuit
44
How does an electrochemical cell work, using zinc and copper as an example?
Zinc is the more reactive metal and forms ions more easily, releasing electrons as its atoms form ions
The electrons give the more reactive electrode a negative charge and they then flow around the circuit to the copper electrode
45
What causes a voltage to be produced by an electrochemical cell?
The difference in the ability of the electrodes to release electrons causes a voltage to be produced
46
What is the correlation between the reactivity of a metal and the voltage produced? (electrochemical cell)
The greater the difference in the metal’s reactivity, the greater the voltage
47
Electroplating definition
Electroplating is a process where the surface of one metal is coated with a layer of a different metal
48
What is the rule when deciding which metal is used to coat the other one? (electroplating)
The metal being used to coat is a less reactive metal than the one it is covering
49
What is the basic setup of equipment for electroplating?
The anode is made from the pure metal used to coat
The cathode is the object to be electroplated
The electrolyte is an aqueous solution of a soluble salt of the pure metal at the anode
50
What are 2 uses of electroplating?
Electroplating is done to make metals more resistant to corrosion or damage, e.g: chromium and nickel plating
It is also done to improve the appearance of metals, e.g: silver plating cutlery
51
Conductor definition
Conductors of electricity allow electrical charge to pass through them easily
52
3 examples of conductors
Conductors can be:
Solids such as metals or graphite
Liquids such as molten lead bromide or molten metals
Solutions such as sodium chloride solution
53
What metal is a good conductor and is used in electrical wiring? What other quality does it have which makes it suitable?
Copper is used extensively in electrical wiring as it is an excellent conductor and is malleable and easy to work with
54
What metal is used in overhead cables?
Aluminium is used in overhead cables which are reinforced with a steel core
55
Why do aluminium cables have a steel core?
The steel core provides extra strength and prevents the cable from breaking under its own weight
56
Why is aluminium used in overhead cables rather than copper?
Although not as good a conductor as copper, it is less dense and cheaper than copper
57
Insulator definition
Insulators resist the flow of electricity and do not conduct
58
What are 3 examples of insulators?
Most insulators are solids of plastic, rubber or ceramic
59
How are plastics used as insulators?
Plastics are used as insulators and are placed around electrical wiring and for some tool and machine handles
60
When are ceramics used as insulators?
Ceramics are used in very high voltage lines where contact between the power line and the metal of the pylon would be dangerous
61
What type of metals and metal compounds does the earth’s crust contain? What is the problem?
The Earth’s Crust contains metals and metal compounds such as Gold, Iron Oxide and Aluminium Oxide, but when found in the Earth, these are often mixed with other substances
62
What needs to be done with metals found in the earth’s crust to make them useful?
To be useful, the metals have to be extracted from their ore
63
What processes can be used to extract a metal from its compound?
electrolysis, using a blast furnace or by reacting with more reactive material
64
How are metals above carbon extracted?
Extracted by electrolysis of molten chloride or molten oxide.
65
How are metals below carbon extracted?
Extracted by heating with a reducing agent such as carbon or carbon monoxide in a blast furnace.
66
What is the difference in costs between electrolysis and reduction?
E - large amounts of electricity required, so an expensive process
R - cheap process as carbon is cheap and can be source of heat as well
67
1. What is bauxite purified to?
Extraction of aluminium
The Bauxite is first purified to produce Aluminium Oxide Al2O3
68
2. What is aluminium oxide dissolved in and why?(Extraction of aluminium)
Aluminium Oxide has a very high melting point so it is first dissolved in molten cryolite producing an electrolyte with a lower melting point, as well as a better conductor of electricity than molten aluminium oxide. This also reduces expense considerably
69
3. What is the electrolyte?
Extraction of aluminium
The electrolyte is a solution of aluminium oxide in molten cryolite at a temperature of about 1000 °C.
70
4. What keeps the electrolyte molten?
Extraction of aluminium
The cell operates at 5-6 volts and with a current of 100,000 amps. The heat generated by the huge current keeps the electrolyte molten
71
5. How is aluminium extracted from the cell?
Extraction of aluminium
The molten aluminium is siphoned off from time to time and fresh aluminium oxide is added to the cell.
72
What is the half-equation at the cathode?
Extraction of aluminium
The Aluminium melts and collects at the bottom of the cell and is then tapped off:
Al3+ + 3e- → Al
73
What is the half-equation at the anode?
Extraction of aluminium
2O2- – 4e– → O2
74
What happens at the anode which leads to it being reduced in size?
(Extraction of aluminium)
Some of the Oxygen Produced at the positive electrode then reacts with the Graphite (Carbon) electrode to produce Carbon Dioxide Gas:
C (s) + O2 (g) → CO2 (g)
*This causes the carbon anodes to burn away, so they must be replaced regularly.
75
What is brine?
Brine is a concentrated solution of aqueous sodium chloride
76
What are the products when brine is electrolysed?
When electrolysed it produces chlorine, hydrogen and sodium hydroxide
77
What ions does brine contain?
The electrolyte is concentrated sodium chloride which contains the following ions: Na+, H+, Cl– and OH–
78
What happens at the cathode during the electrolysis of brine?
The H+ ions are discharged at the cathode as hydrogen gas
79
What happens at the anode during the electrolysis of brine?
The Cl– ions are discharged at the anode as chlorine gas
80
What remains behind after the electrolysis of brine?
The Na+ and OH– ions remain behind and form the NaOH solution
81
Exothermic definition
A reaction in which energy is given out to surroundings (temperature of environment increases)
82
What are 3 examples of exothermic reactions?
Combustion of fuels
Reaction of acids and metals
Neutralisation reactions
83
Endothermic definition
A reaction in which energy is taken in from surroundings (temperature of environment decreases)
The energy change is positive
84
What are 3 examples of endothermic reactions?
Thermal decomposition of carbonates
Electrolysis
First stages of photosynthesis
85
What determines whether a reaction is endo/exothermic?
energy
Whether a reaction is endothermic or exothermic depends on the difference between the energy needed to break bonds and the energy released when the new bonds are formed
86
How can you determine if a reaction is endothermic by the energy?
If more energy is absorbed than is released, this reaction is endothermic
More energy is required to break the bonds than that gained from making the new bonds
87
Is the change in energy positive or negative if the reaction is endothermic? Why?
The change in energy is positive since the products have more energy than the reactants
- positive ΔH value
88
What symbol is used to show the change in heat energy?
The symbol ΔH (delta H)
89
What is H the symbol for, and what does this mean?
H is the symbol for enthalpy, which is a measure of the total heat of reaction of a chemical reaction
90
How can you determine if a reaction is exothermic by the energy?
If more energy is released than is absorbed, then the reaction is exothermic
More energy is released when new bonds are formed than energy required to break the bonds in the reactants
91
Is the change in energy positive or negative if the reaction is exothermic? Why?
The change in energy is negative since the products have less energy than the reactants
Therefore an exothermic reaction has a negative ΔH value
92
What are energy level diagrams?
These are graphical representations of the heat changes in chemical reactions
93
What is shown on the y-axis?
energy level diagram
The enthalpy of the reactants and products is displayed on the y-axis
94
What is shown on the x-axis?
energy level diagram
The reaction pathway is shown on the x-axis
95
What do the arrows show?
energy level diagram
Arrows on the diagrams indicate whether the reaction is exothermic (downwards pointing) or endothermic (upwards pointing)
96
In an exothermic reaction, do the reactants or products have more energy? Why?
During an exothermic reaction, energy is given out
This means that the energy of the products will be lower than the energy of the reactants, so the change in enthalpy (ΔH) is negative
97
How is an exothermic reaction represented on an energy level diagram?
This is represented on the energy-level diagram with a downwards arrow (from the reactants to the products) as the energy of the products is lower than the reactants
98
In an endothermic reaction, do the reactants or products have more energy? Why?
During an endothermic reaction, energy is absorbed
This means that the energy of the products will be higher than the energy of the reactants, so the change in enthalpy (ΔH) is positive
99
How is an endothermic reaction represented on an energy level diagram?
This is represented on the energy-level diagram with an upwards arrow (from the reactants to the products) as the energy of the products is higher than the reactants
100
What does each chemical bond have which is specific to it?
Each chemical bond has a specific bond energy associated with it
101
What is bond energy?
This is the amount of energy required to break the bond or the amount of energy given out when the bond is formed
102
What can bond energy be used to calculate?
This energy can be used to calculate how much heat would be released or absorbed in a reaction
103
What is necessary to calculate the heat released or absorbed during a reaction?
To do this it is necessary to know the bonds present in both the reactants and products
104
How can you calculate the heat released or absorbed during a reaction?
Add together all the bond energies for all the bonds in the reactants – this is the ‘energy in’
Add together the bond energies for all the bonds in the products – this is the ‘energy out’
Calculate the energy change: Energy change = energy in – energy out
105
Fuel definition
A fuel is a substance which releases energy when burned
106
What is produced upon combustion when a fuel is a hydrocarbon?
When the fuel is a hydrocarbon then water and carbon dioxide are produced in combustion reactions
107
What shows the efficiency of a fuel?
The efficiency of a fuel refers to how much energy is released per unit amount
108
How can we measure the efficiency of fuels?
We can measure the efficiency of fuels by calorimetry
109
How can we calculate the heat of combustion?
A known mass of the fuel is combusted and used to heat up a known mass of water to calculate its heat of combustion
110
What is the method for an experiment with a calorimeter?
Using a measuring cylinder, place 100 cm3 of water into a copper can
Measure and record the initial temperature of the water
Fill the spirit burner with test substance and measure and record its mass
Place the burner under the copper can and light the wick
Stir the water constantly with the thermometer and continue heating until the spirit burner burns out
Measure and record the highest temperature of the water
111
What calculations are needed? (experiment with a calorimeter)
Temperature change of water = final temperature – initial temperature
Number of moles burned = change in mass ÷ molecular mass
Amount of energy = change in temperature x mass of water x specific heat capacity
Amount of energy per mole (J mol-1) = total amount of energy ÷ moles burned
112
How is hydrogen used as a fuel?
Hydrogen is used in rocket engines and in fuel cells to power some cars
113
What are advantages of hydrogen as a fuel?
It releases more energy per kilogram than any other fuel (except for nuclear fuels)
It does not pollute as it only produces water on combustion, no other product is formed
114
What are disadvantages of hydrogen as a fuel?
Expensive to produce and requires energy for the production process
Difficult and dangerous to store and move around (usually stored as liquid hydrogen in highly pressurised containers)
115
How is Uranium-235 used in power stations?
Uranium-235 undergoes decay and gives off heat energy which nuclear power stations harness
The heat it produces is used to heat water to steam, which in turn is used to power turbines to generate electricity
116
What are advantages of nuclear fuel?
Nuclear fuel energy is clean as it does not produce pollutants such as CO2 or oxides of nitrogen or sulfur
117
What are disadvantages of nuclear fuel?
But nuclear power plants are expensive to build and maintain as well as being potentially dangerous in the event of an accident as radioactive materials may be released
118
What is a fuel cell?
A fuel cell is an electrochemical cell in which a fuel donates electrons at one electrode and oxygen gains electrons at the other electrode
119
In which industry are fuel cells becoming more common?
These cells are becoming more common in the automotive industry to replace petrol or diesel engines
120
What is pumped through the electrodes in a fuel cell?
H2 and O2 are pumped through two porous electrodes where the half-reactions occur
121
What is the reaction at the anode? (fuel cell)
2H2 → 4H+ + 4e-
122
What is the reaction at the cathode? (fuel cell)
4H+ + O2 + 4e- → 2H2O
123
What is the overall reaction in a fuel cell?
2H2 + O2 → 2H2O
124
How does a fuel cell work?
The electrons move around the external circuit from the cathode to the anode
This movement of electrons is used to drive an electric motor
125
What are advantages of fuel cell?
They do not produce any pollution
They produce more energy per kilogram than either petrol or diesel
No power is lost on transmission as there are far fewer moving parts than in an internal combustion engine
126
What are disadvantages of fuel cell?
Materials used in producing fuel cells are expensive
High-pressure tanks are needed to store the oxygen and hydrogen in sufficient amounts
Fuel cells are affected by low temperatures, becoming less efficient
Hydrogen is expensive to produce and store
127
Electrolyte
A liquid containing free moving ions that is broken down by electricity during the process of electrolysis.
128
Electrolysis
The breakdown of a substance containing ions by electricity.
129
The positive electrode
Anode
130
The negative electrode
Cathode
131
What sort of compounds are usually electrolysed?
Ionic compounds.
Covalent compounds can only do it if they react in the water to form ions.
132
Equation at the anode
n- -> n + e-
133
Equation at the cathode
n+ + e- -> n
134
Will the more or less reactive element be formed at the cathode?
Less reactive
135
What is the ‘order of discharge’ at the anode?
Halide ion > Hydroxide > All other negatively charged ions.
136
What gas is often given off at the anode?
Oxygen
137
Name of aluminium ore
Bauxite
138
What substance is mixed with aluminium during electrolysis and why?
Molten cryolite
It reduces the melting point
139
What happens at the cathode during the electrolysis of aluminium?
Al3+ ions are reduced to form Al atoms.
This forms aluminium at the bottom of the tank and is then siphoned off.
140
What happens at the anodes during the electrolysis of aluminium?
O2- ions turn into oxygen atoms.
These bond in pairs to form oxygen gas.
The oxygen reacts with the carbon anodes, forming Carbon Dioxide.
Due to the this the anodes must be replaced regularly.
141
What three products are formed in the electrolysis of brine and where?
> Chlorine gas- anode
Hydrogen gas- cathode
Sodium Hydroxide solution
142
How is brine electrolysed in industry?
Using a diaphragm cell, where the two electrodes are separated by a porous membrane.
143
Why is sodium hydroxide produced in the electrolysis of brine?
During the electrolysis of brine, Na+, Cl-, H+ and OH- are present. Chlorine and Hydrogen gas are emitted during the electrolysis process, which leaves only sodium and hydroxide.
144
2 uses of chlorine.
> When reacted with sodium hydroxide it can be used as bleach.
Plastics such as PVC
145
Use for hydrogen
> Makes margarine when combined with veritable oils.
146
Use for sodium hydroxide
> Important alkali in industry
Bleach
Soap
Paper
147
Electroplating
The process of depositing a thin layer of metal onto an object during electrolysis.
148
Why do we electroplate objects?
> To protect the metal beneath from corrosion.
To make the object look more attractive.
To increase the hardness or resistance to scratching.
To save money.
149
In electroplating, what substances make up the anode and cathode?
> Cathode- Object to be electroplated
> Anode - Metal to plate the Object.
150
Exothermic
A reaction that gives out energy to the surroundings.
151
Endothermic
A reaction that takes in energy from the surroundings.
152
Sign for enthalpy change
△H
153
Are the products of exothermic energy calculations positive or negative?
Negative
154
Uses of exothermic reactions
> Hand warmers
> Self-heating cans
155
Use of endothermic reactions
> Cold packs for sports injuries.
156
Test for water
Anhydrous copper sulfate (white) + Water = Hydrated copper sulfate (blue)
157
If a reaction releases energy when it goes in one direction…
… It will absorb exactly the same amount of energy when it goes in the opposite direction.
158
energy released =
mass of object heated x specific heat capacity x change in temperature
159
What is the unit of energy?
Joules (although calories are still used sometimes)
160
Calorimeter
A machine used to measure the amount of energy released from an object.
They can be made inaccurate by energy losses.
161
How do catalysts increase the rate of reaction?
By providing an alternative pathway for the products which has a lower activation energy.
This means that a greater proportion go reactant particles have enough energy to react.
162
Breaking bonds…
aBsorbs energy (endothermic)
163
foRming bonds…
Releases energy (exothermic)
164
How can you measure enthalpy change?
By using calorimetry
165
What does calorimetry allow you to measure?
The amount of energy transferred in a chemical reaction
166
What reactions does calorimetry measure?
Dissolving
Displacement
Neutralisation
167
How do you measure the energy transferred in these reactions?
Take the temperature of the reagents (making sure they are the same)
Mix them
Measure the temperature of the solution at the end of the reaction
168
What can you put the reagents into when carrying out the experiment?
A polystyrene cup (insulator)
169
Why should the polystyrene cup be put in a beaker of cotton?
Gives more insulation as lots of energy can be lost to the surrounds
Also a lid can be put on the cup to reduce energy lost by evaporation
170
How can you measure combustion using calorimetry?
Burn a fuel and use the flame to heat up some water - this method uses a metal container, usually copper, as it conducts heat well.
171
What is the method for working out the enthalpy change in combustion using calorimetry
Put 50g of water in the copper container and record the temperature
Weigh the spirit burner and lid
Put the spirit burner underneath the container and light the wick
Heat the water stirring constantly until the temperature reaches about 50 degrees
Put out the flame and measure the final temperature
Calculate the enthalpy change from the measurements
172
What does the combustion experiment involve?
Heating water by burning a liquid fuel
173
What do you need to measure to get how much energy is supplied by each gram of fuel?
How much fuel you’ve burned
- Temperature change of the water
174
What is specific heat capacity?
The amount of energy needed to raise the temperature of 1 gram of water by 1 degrees in water
175
What is the specific heat capacity for water?
4.2 J/g/degrees - so it takes 4.2 joules to raise the temperature of 1g of water by 1 degrees
176
Work out the mass of methylated spirit burnt using the following figures:
Mass of spirit burner + lid before heating = 68.75g
Mass of spirit burner + lid after heating = 67.85g
68.75 - 67.85 = 0.90g
177
Work out the temperature rise of 50g of water due to heating with the following figures:
Temp of water in copper before heating = 21.5 degrees
Temp of water in copper after heating = 52.5 degrees
52.5 - 21.5 = 31.0 degrees
178
Work out the total energy produced in this experiment
(knowing that 0.90g of fuel produces enough energy to heat up 50g of water by 31 degrees and it takes 4.2 joules of energy to heat up 1g of water by 1 degrees)
4.2 x 50 x 31 = 6510 joules
179
So if 0.9 of methylated produces 6510 joules of energy, how much does 1g of methylated produced?
6510/0.9 = 7233J or 7.233 KJ
180
How do you work out the energy per mole of a substance?
Calculate the amount of energy transferred
Work out how many moles of fuel produced the heat (moles=mass/mr)
Work out the heat produced by 1 mole
181
Work out the energy per mole of methylated spirit
Total energy produced = 6.510 kj
Mr = 44.6
Mass of meths burnt = 0.90g
Moles = 0.90/44.6 = 0.020 moles
-6.510/0.020 = -325.5 kj/mol
182
Why does the total energy produced in the methylated spirit turn into a negative number?
Combustion is a exothermic reaction
183
What happens during a chemical reaction?
Old bonds are broken and new bonds are formed
184
What process describes breaking bonds?
Endothermic - energy must be supplied to break existing bonds
185
What process describes the formation of bonds?
Exothermic - energy is releases when new bonds are formed
186
In terms of energy what happens in an exothermic reaction?
The energy released in bond formation is greater than the energy used in breaking old bonds
187
What is an exothermic reaction?
One which gives out energy to the surroundings usually in the form of heat shown by a rise in temperature
188
In terms of energy what happens in an endothermic reaction?
The energy required to break bonds is greater than the energy released when new bonds are formed
189
What is an endothermic reaction?
One which takes in energy from the surroundings usually in the form of heat shown by a fall in temperature
190
What is the overall change in energy in a reaction called?
The enthalpy change (symbol = ΔH)
191
What are the units of ΔH?
KJ/mol - its the amount of energy in kilojoules per mole of reactant
192
What value can the enthalpy change be?
Either positive or negative
193
If the reaction is exothermic, what is the value of the enthalpy change?
Negative because the reaction is giving out energy
194
If the reaction is endothermic, what is the value of the enthalpy change?
Positive because the reaction takes in energy
195
If the reaction is endothermic, what is the value of the enthalpy change?
Positive because the reaction takes in energy
196
What do energy level diagrams show?
How the energy levels of the reactants change when they react to form the products (shows if its exothermic or endothermic)
197
What do exothermic reactions look like on an energy level diagram?
The products are at a lower energy than the reactants
198
What does the difference in height bwteen the reactants and products represent?
The energy given out in the reaction (per mole) - the enthalpy change is -ve
199
What does the initial rise in the line represent?
The energy needed to break the old bonds (activation energy)
200
What do endothermic reactions looks like on energy level diagrams?
The products are at a higher energy than the reactants, so thr enthalpy change is +ve
201
What is activation energy?
The minimum energy needed by reacting particles for the reaction to occur
202
What does a catalyst do in reactions?
Makes them faster by providing an alternative reaction pathway with a lower activation energy
203
What curve represents the activation energy on this graph?
The lower curve as it shows the less initial energy needed for the reaction to begin
204
What remains the same in the reaction with a catalyst?
The overall energy change - catalysts are not used up during reactions
205
How can you calculate the enthalpy change for a reaction?
Look at the bonds that are made and broken
206
What is bond energy?
The amount of energy in a bond
207
What does each type of chemical bond have?
A particular bond energy associated with it
208
What bonds do you add up?
The energy of the bonds that are broken
209
What bonds do you subtract?
The energy of the bonds that are made
210
Using the bond energies for the following reaction H2 + Cl2 —> 2HC
H-H: +436 kj/mol
Cl-Cl: +242 kj/mol
H-Cl: +431 kj/mol
Work out the energy needed for breaking the bonds
436 + 242 = 678 kj
211
Using the bond energies for the following reaction H2 + Cl2 —> 2HC
H-H: +436 kj/mol
Cl-Cl: +242 kj/mol
H-Cl: +431 kj/mol
Work out the energy needed to form the bonds
2 x 431 = 862kj
212
What formula can you use to calculate the differences in the energy calculated in the formation of HCl?
Enthalpy change = Total energy absorbed to break bonds - total energy released in making bonds
213
What is the enthalpy change in the formation of HCl (breaking=678 and forming=862)?
678 - 862 = -184 kj/mol
214
What does the enthalpy change of the formation of HCl show?
The value is negative so it in an exothermic reaction
215
What is electrolysis?
breakdown of an ionic compound (when molten or in aqueous solution) by passage of electricity
216
What is an electrolyte?
Aqueous solution of ionic substance or molten ionic salt
217
Why does an electrolyte conduct electricity?
Conducts electricity due to the presence of mobile ions
218
What are electrodes?
Rods which help current enter the electrolyte
219
What are the products formed from the electrolysis of Molten lead(II) bromide?
At cathode: lead
At anode: bromine
220
What are the products formed from the electrolysis of Concentrated hydrochloric acid?
At cathode: Hydrogen
At anode: chlorine
221
What are the products formed from the electrolysis of Concentrated aqueous sodium chloride?
At cathode: hydrogen
At anode: chlorine
222
What is formed at the cathode?
Metal/Hydrogen
223
What is formed at the anode?
Non-metal/Oxygen (OH-)
224
What is electroplating?
Coating one metallic object with another metal using electrolysis
225
What are the materials needed for electroplating? (3)
Anode made of metal you want to electroplate object with
Ions of same metal as anode in solution
Object to be plated at cathode
226
What are the uses of electroplating? (3)
Improving metal appearance
Galvanise
Prevent corrosion
227
Why is aluminum used for electricity cables? (4)
Light
Non-corrosive
Good conductor
Cheaper than copper
228
Why is copper used in electrical wiring? (2)
Good conductor
Ductile
229
Why are plastics & ceramics used as insulators?
Don’t conduct electricity
Conduct heat poorly
230
How is copper purified?
By using an impure copper anode and a pure copper cathode
Using inert electrodes
231
What are the materials needed for the purification of copper with copper electrodes?
Anode: Impure copper
Cathode: Pure copper
Copper sulfate solution
232
What happens during electrolysis using copper electrodes?
the metal atoms at the anode lose electrons to form positive ions
233
What are the materials needed for the purification of copper with inert electrodes?
Inert anode: Graphite
Inert cathode: Graphite
Copper sulfate solution
234
What happens during electrolysis using inert electrodes?
Negative ions in the electrolyte lose electrons to anode
235
What happens at the anode?
Reduction of positive ions
236
What is the main ore of aluminum?
Bauxite
237
What are the electrodes used in the extracting aluminum made of?
Carbon
238
What is the electrolyte in the cell?
molten aluminium oxide dissolved in molten cryolite
239
Why is aluminum oxide dissolved in cryolite?
Decrease melting point, improve conductivity
240
Describe the process of extracting aluminium
1) Bauxite treated w/ sodium hydroxide to form sodium aluminate and heated to form aluminum oxide
2) Aluminum oxide dissolved in cryolite
3) Electrolysis using carbon electrodes
4) At cathode aluminum ions gain electrons, reduced to aluminum
5) At anode, oxide ions are oxidised to oxygen
6) Oxygen reacts w/ carbon
241
What are the products in extracting aluminum?
At cathode: aluminum
At anode: oxygen
242
What is brine?
Concentrated aqueous solution of sodium chloride
243
Identify the ions present in brine (4)
Na+ , Cl-, H+ and OH-
244
Where is brine electrolyzed?
In a diaphragm cell
245
What are the products electrolysis of brine? (3)
At cathode: Hydrogen
At anode: Chlorine
Sodium hydroxide forms in solution
246
What are the uses of chlorine? (2)
treating drinking water
- making bleach
247
What are the uses of hydrogen? (2)
making ammonia
- making margarine
248
What is the use of sodium hydroxide?
soap
249
Explain ionic movement in electrolysis
In a molten ionic compound, ions able to move
When current applied, cations move towards cathode - anions move towards anode
Cations reach cathode and gains electrons (reduction)
Anions reach anode and lose electrons (oxidation)
250
Where does reduction occur?
Cathode
251
Where does reduction occur?
Cathode
252
What is copper refining?
When copper is purified by electrolysis
253
In copper refining, what electrodes are used?
Anode: Impure copper
Cathode: Pure copper
254
During electrolysis using inert electrodes, what happens to the negative ions in the electrolyte?
They lose their electrons to the anode
255
In copper refining using inert electrodes, why does the eletrolyte lose its blue colour?
Copper ions in solution are turning into copper atoms at the cathode
256
In copper refining using 2 copper electrodes, what happens to the anode?
As the anode is not inert, it loses electrons and copper ions go into solution - anode gets smaller
257
In copper refining using 2 copper electrodes, why does the electrolyte remain the same deep blue colour?
Copper ions removed from solution at cathode are replaced in solution by copper ions formed at the anode - going in to solution
258
What is the overall equation for the electrolysis of aluminium oxide using electrolysis?
2Al2O3 => 4Al + 3O2
259
What are exothermic reactions?
Chemical reactions in which heat is given out
260
What is bond forming an example of?
Exothermic reaction
261
What are the energy changes that occur during an exothermic reaction?
Energy released is more than the energy absorbed
262
Bond forming __ energy
releases
263
What happens to heat during an exothermic reaction?
Heat is given out and transferred from the chemical to surrounding
264
What happens to the temperature of the mixture during an exothermic reaction?
Temperature of mixture will rise until the highest temperature is reached, then fall to room temp.
265
What are the energy transformations that occur during an exothermic reaction?
Transfer chemical to heat energy
266
What are three common examples of exothermic reactions?
Combustion of fuels
Rusting iron
Metal corroding
Neutralisation between acid and alkali
Respiration
267
What are endothermic reactions?
Chemical reactions in which heat is absorbed/taken in
268
What is bond breaking an example of?
Endothermic reaction
269
What are the energy changes that occur during an endothermic reaction?
Energy absorbed is more than the energy released
270
Bond breaking __ energy
requires
271
What happens to heat during an endothermic reaction?
Heat is absorbed and transferred from surroundings to reactants
272
What happens to heat during an endothermic reaction?
Heat is absorbed and transferred from surroundings to reactants
273
What happens to the temperature of the mixture during an endothermic reaction?
Temperature of mixture falls until lowest temp is reached, then rise until room temp
274
What are the energy transformations that occur during an exothermic reaction?
Transfer heat to chemical energy
275
What are common examples of endothermic reactions? (2)
Photosynthesis
Thermal decomposition
276
What is a fuel?
substance that can be burned to release energy
277
What happens when fuel is burned/ the combustion of fuel? (2)
Energy is stored inside the fuel, burning the fuel releases it
Combustion reactions turns fuel and oxygen into water and carbon dioxide (and releases energy)
278
Why is hydrogen used as a fuel?
Its environmentally friendly
279
What happens in a hydrogen fuel cell?(2)
In hydrogen fuel cell, hydrogen reacts with oxygen without burning
Energy released is used to generate electricity (which is used to drive an electric motor)
280
What are three advantages of fuel cells?
Lightweight
High efficiency
Only water is made – no additional pollutants
281
How are radioisotopes used a source of energy?
These isotopes are used to produce nuclear fuels
282
What are 2 common radioisotopes used as a source of energy?
Urainium235
Plutonium239
283
What are the products that would be released if petrol and natural gas (methane) were used in energy production?
Carbon dioxide and water
284
What is the equation for the reaction that takes part in a fuel cell?
2H2 + O2 –> 2H2O
285
What is an exothermic reaction?
Reaction in which energy is given out to surroundings
286
Give examples of exothermic reactions
Combustion of fuels
Reaction of acids and metals
Neutralisation
287
What is an endothermic reaction?
Reaction in which energy is taken in from surroundings
288
Give examples of endothermic reactions
Photosynthesis
Thermal decomposition of carbonates
Electrolysis
289
What is endothermic?
bond breaking positive change in temperature value is positive
290
What is exothermic?
Bond forming negative change in temperature value is negative
291
How can you calculate the energy of a reaction
Add together all bond energies for all the bonds in reactants- energy in
Add together all bond energies for all the bonds in products- energy out
Calculate the energy change by energy in- energy out
292
What is the equation for energy change?
Energy taken in - Energy given out
293
What is a fuel?
A substance which releases energy when burned
294
What does a fuel release?
Carbon dioxide and water
295
How can we measure the efficiency of fuels?
By a calorimeter
296
Where is hydrogen used?
In rocket engines and in fuel cells to power some car
297
What are the advantages of fuel cells?
Releases more energy/kilogram
It does not pollute
298
What are the disadvantages of fuel cells?
Expensive to produce and requires energy for production process
Difficult to store and move around
Affected by low temps
299
How does Uranium-235 release energy?
Undergoes decay and gives off heat energy
300
What are the advantages of using Uranium-235?
It is clean as it does not release polluting gases
301
What are the disadvantages of using Uranium-235?
Nuclear power plants are expensive and radioactive materials may be released by accident
302
How does a hydrogen fuel cell work?
Donates electrons at one electrode and oxygen gains electrons at the other electrode
303
What happens at the anode in a fuel cell?
2H2 –> 4H+ +4e-
304
What happens at the cathode in a fuel cell?
4H+ +02 +4e- –> 2H2O
305
What is the overall reaction in a fuel cell?
2H2 + O2 –> 2H2O
306
define electrolysis
the decomposition of an
ionic compound, when molten or in
aqueous solution, by the passage of an electric current
307
which is the positive electrode?
anion
308
which is the negative elctrode?
cathode
309
What is the the electrolyte ?
the molten or aqueous substance that undergoes electrolysis
310
Why are metal objects are electroplated?
to improve their appearance and resistance to corrosion
311
State the use of hydrogen–oxygen fuel cell
uses hydrogen and oxygen to produce electricity with water as the only chemical product
312
How are metals electroplated
313
Describe the disadvantages of
using hydrogen–oxygen fuel cells in comparison with gasoline /petrol engines in vehicles
314
Describe the advantages of
using hydrogen–oxygen fuel cells in comparison with gasoline /petrol engines in vehicles
315
what is an exothermic reaction?
reaction that transfers
thermal energy to the surroundings leading to an increase in the temperature of the surroundings
316
What is an endothermic reaction?
reaction that takes in
thermal energy from the surroundings leading to a decrease in the temperature of the surroundings
317
What is the transfer of thermal energy during a reaction called?
the enthalpy change ∆H
318
is ∆H negative or positive in exothermic reactions
negative
319
is ∆H negative or positive in endothermic reactions
positive
320
define activation energy
minimum energy that colliding particles must have to react
321
what is the symbol for activation energy
Ea
322
is bond breaking an endothermic or exothermic reaction?
endothermic
323
is bond making an edothermic or exothermic reaction?
exothermic
324
what is an example of an exothermic reaction?
combustion
neutralisation
325
what is an example of an edothermic reaction?
hand warmers
self heating cans
326
How can we calculate the enthalpy change of a reaction using bond energies
Add together all the bond energies for all the bonds in the reactants – this is the
‘energy in’.
Add together the bond energies for all the bonds in the products – this is the
‘energy out’.
Calculate the energy change: energy in – energy out
327
What does it mean if If the energy out > energy in?
the energy change will be negative showing an exothermic reaction
328
What does it mean if the energy out
will be positive showing
an endothermic reaction.
329
What can Reaction profiles be used to show?
an be used to show the relative energes of
reactants and products, the activation energy and the overall
energy change of a reaction.
