C4: Electrolysis and Electrochemical Series Flashcards
(120 cards)
1
Q
What does the word electrolysis mean?
A
Breaking down of substances using electricity
2
Q
in electrolysis, which electrodes will the cations and anions migrate to?
A
3
Q
differences between metallic conduction and conduction in an electrolyte
- Electric current in a metal is carried by (1)_______ , while in an electrolyte it is carried by (2)_________.
- Conduction in a metal results in (3)________ while in an electrolyte it results in (4)________ .
A
- electrons
- mobile ions
- no change
- decomposition of the electrolyte
4
Q
How does electrolysis occur?
A
An electric current is used to break down an ionic substance
5
Q
What is the substance that is broken down by electrolysis called?
A
The electrolyte
6
Q
How is an electrical circuit for electrolysis set up?
A
• two electrodes which are dipped into the electrolyte • one of these is connected to the positive terminal of a power supply (the anode) • the other is connected to the negative terminal (the cathode)
7
Q
What are the electrodes?
A
Conducting rods
8
Q
What are the electrodes usually made of?
A
Inert (unreactive) substances, such as graphite or platinum
9
Q
Why are electrodes made out of inert substances?
A
So that they do not react with the electrolyte or the products made in electrolysis
10
Q
What happens to the ions when they reach the electrode?
A
They lose their charge and become elements
11
Q
What can be given off / deposited at the electrodes?
A
Gases (given off) or metals (deposited)
12
Q
Why do ionic substances not conduct electricity when they are solid?
A
This is because their ions are fixed in their giant lattice
13
Q
What happens to the ions in an ionic substance when it is melted?
A
The ions are free to move around within the liquid and so carry their charge towards the electrodes
14
Q
What is an alternative to melting some ionic substance to free it’s ions?
A
To dissolve it in water, which also enables the ions to become free to move around
15
Q
Why is it more difficult to predict what will be formed when electrolysing ionic compounds in solution?
A
Because water also forms ions so the products at each electrode are not always exactly what should be expected
16
Q
When can covalent compounds be electrolysed?
A
Only when they have reacted (ionized) in water to form ions
17
Q
How do negatively charged ions become neutral atoms?
A
They lose electrons
18
Q
How do positively charged ions become neutral atoms?
A
They gain electrons
19
Q
What is it called when an ion gains electrons?
A
Reduction; the ions have been reduced
20
Q
What is it called when an ion loses electrons?
A
Oxidation; they ions have been oxidised
21
Q
How are the changes at each electrode represented?
A
By using half equations
22
Q
What happens at the cathode (negative electrode)?
A
There is reduction of a positive ion
23
Q
What happens at the anode (positive electrode)?
A
There is oxidation of a negative ion
24
Q
What is the half equation for the reduction of lead (Pb2+)?
A
Pb2+ + 2e- –> Pb
25
What is the half equation for the oxidation of bromine (Br-)?
2Br- –\> Br2 + 2e-
26
If there are two elements that can be produced at an electrode, which one will be formed preferencially?
The less reactive element
27
What two elements will usually be attracted to the cathode in the electrolysis of solutions?
Positively charged metal ions and H+ ions from water
28
What happens when hydroxide ions are discharged?
Oxygen gas is given off at the anode
29
When is oxygen gas (from hydroxide ions) not given off at the anode in aqueous solutions?
When the solution contains a reasonably high concentration of a halide ion (group 7)
30
What element can be produced instead of oxygen gas at the anode in the electrolysis of an aqueous solution?
The halide ion is discharged, which forms the halogen
31
how to describe an electrolytic process
1. what will be the anode and what happens to it
2. what will be the cathode and what happens to it
3. what will be the electrolyte and what happens to it
4. what are the ions and that happens to them
32
The products at the cathode are always (1)\_\_\_\_\_\_\_\_\_\_gas or a (2)\_\_\_\_\_\_\_\_\_\_.
when effervescence is seen at the anode this is usually a (3)\_\_\_\_\_\_\_\_\_ or (4)\_\_\_\_\_\_\_\_.
1. hydrogen
2. metal
3. halogen
4. oxygen
33
what are the 3 main factors affecting the preferential discharge of anions in electrolysis?
* position in the electrochemical series
* concentration
* type of anode
34
what is the only factor affecting the preferential discharge of cations during electrolysis?
their position in the electrochemical series
35
The lower they are in the electrochemical series, both anions and cations are preferentially discharged because\_\_\_\_\_\_\_\_\_\_\_\_\_
they are least stable and will give away or accepted electrons more readily in order to become more stable.
36
Electrochemical series
37
rules to discharge ions in the electrochemical series
38
what happens to the stability of ions and anions as we go up or down the electrochemical series?
the stability increases as we go up and decreases as we go down
39
What process is used to extract aluminum?
Electrolysis
40
What is the compound electrolysed when extracting aluminum?
Aluminum oxide (Al2O3)
41
What ore is aluminum extracted from?
Bauxite
42
What is the first step in the extraction of aluminum?
To separate the aluminum oxide from the ore, which contains a lot of rocky impurities such as iron(III) oxide
43
Why is the aluminum oxide melted before it is electrolysed?
This enables the ions to move to the electrodes
44
What is the disadvantage of having to melt the aluminum oxide?
It has a very high melting point of 2050ºC, which uses a lot of energy
45
What is cryolite and why is it used in the electrolysis of aluminium oxide?
sodium aluminium fluoride, Na3AlF6. It reduces the melting point of aluminium oxide from 2050 deg C to about 950 deg C, therefere utilizing less energy
46
What is the overall reaction in the electrolysis cell when extracting aluminum?
2Al2O3 (l) --\> 4Al (l) + 3O2(g)
47
What happens at the cathodes in the electrolysis of aluminium oxide?
Each aluminium ion (Al3+) gains 3 electrons, which turns the ions into aluminium atoms. Therefore the Al3+ ions are said to have been reduced (as they gain electrons) or form Al atoms
48
What is the half equation at the cathodes for the electrolysis of aluminium oxide?
Al3+ (l) + 3e- ––\> Al (l)
49
What happens to the aluminium metal formed?
It is molten at the temperature of the cell and collects at the bottom, where it is siphoned or tapped off
50
What happens at the anodes in the electrolysis of alumium oxide?
Each oxide ion loses 2 electrons, and oxygen gas is formed
51
What is the half equation at the anodes for the electrolysis of aluminium oxide?
2O2- (l) ––\> O2 (g) + 4e-
52
What are some uses of aluminium or its alloys?
* overhead power cables
* airplanes
* drink cans
* bicycle frames
* car bodies
53
Why is the aluminum oxide melted before it is electrolysed?
This enables the ions to move to the electrodes
54
What process is used to extract aluminum?
Electrolysis
55
Where is the chlorine gas produced?
And the anode (positive electrode)
56
Where is the hydrogen gas produced?
At the cathode (negative electrode)
57
What is the word equation for the electrolysis of brine?
sodium chloride solution ––\> hydrogen gas + chlorine gas + sodium hydroxide solution
58
What happens at the anode in the electrolysis of brine?
The negative chloride ions (Cl-) are attracted to the positive electrode, where they each lose one electrons as they are oxidised. The chlorine atoms bond together in pairs and are given off as chlorine gas (Cl2)
59
What is the half equation at the anode for the electrolysis of brine?
2Cl- (aq) ––\> Cl2 (g) + 2e- This can also be written as: 2Cl- (aq) – 2e- ––\> Cl2 (g)
60
What three products are obtained when an electric current passes through brine?
• chlorine gas • hydrogen gas • sodium hydroxide
61
What two positive ions are attracted to the cathode in the electrolysis of brine?
Hydrogen and sodium
62
What element gets discharged from the cathode in the electrolysis of brine?
Hydrogen ions
63
Why do the sodium ions stay in solution in the electrolysis of brine?
Because they are more reactive than the hydrogen, which is also attracted to the cathode
64
What is the symbol equation for the ionising of water?
H2O (l) ⇌ H+ (aq) + OH- (aq)
65
What is the half equation at the cathode for the electrolysis of brine?
2H+ (aq) + 2e- ––\> H2 (g)
66
Why is the solution (sodium hydroxide) around the cathode alkaline?
Because brine contains aqueous ions of Na+ and Cl- (from salt), and H+ and OH- (from water). The Cl- (aq) and H+ (aq) ions are removed during electrolysis. So this leaves a solution containing Na+ (aq) and OH- ions, which form a solution of sodium hydroxide, NaOH (aq)
67
How can the chlorine from the electrolysis of brine be used?
• making disinfectants (such as bleach) • making plastics (such as PVC)
68
What is an example of hydrogen being used in the food industry?
Margarine can be made by reacting hydrogen with vegetable oils
69
How can the sodium hydroxide from the electrolysis of brine be used?
This is an important alkali in industry, as it is used to make soap and paper as well as bleach (with chlorine)
70
How does electroplating help people's allergies?
Nickel is a metal often used to make cheap jewellery, but people can be allergic to this. By electroplating the object with another metal, they do not have this problem
71
Why does it still make sense to electroplate large metal surfaces so to protect them from rusting and damage?
Even though this makes things expensive too, it can still make economic sense as they do not have to be replaced so often
72
What is the cathode in electroplating?
The metal object to be plated
73
What is the anode in electroplating?
This is made from the plating metal
74
What must be present in the electrolyte when electroplating an object?
ions of the plating metal
75
What is an electroplated object?
An object that has been coated with a thin layer of metal by electrolysis
76
What is used as the electrolyte in an electroplating form of electrolysis?
The metal being used to electroplate another metal in its form of ions, such as nickel sulfate or copper(II) sulfate solution
77
Why are objects electroplated?
• to protect the metal beneath from corroding • to make the object look more attractive • to increase the hardness of a surface and so its resistance to scratching • to save money (and help with allergies)
78
How does electroplating help people's allergies?
Nickel is a metal often used to make cheap jewellery, but people can be allergic to this. By electroplating the object with another metal, they do not have this problem
79
Why does it still make sense to electroplate large metal surfaces so to protect them from rusting and damage?
Even though this makes things expensive too, it can still make economic sense as they do not have to be replaced so often
80
What is the cathode in electroplating?
The metal object to be plated
81
What is an electroplated object?
An object that has been coated with a thin layer of metal by electrolysis
82
pure water is what kind of electrolyte
weak electrolyte
83
When dissolved in water, a strong electrolyte is one which is (1)\_\_\_\_\_\_\_\_\_\_ , while a weak elecrolyte is (2)\_\_\_\_\_\_\_\_\_\_\_\_
1. fully ionised
2. partially ionised
84
examples of non-electrolytes
* some liquids such as kerosene, gasoline, ethanol
* molten wax
* solutions of covalent compounds such as solutions of glucose, or ethanol
85
factors that influence the preferential discharge of anions
* type of anode
* concentration of
* position of the electron in the electrochemical series
86
common solutions used in electrolysis
H2SO4(aq)
NaCl(aq)
Conc. NaCl(aq)
CuSO4(aq)
87
industrial applications of electrolysis
* extraction of metal from their ores
* purification of metals (electrorefining)
* electroplating
* anodising
88
How do we calculate the volume of oxygen produced the anode at rtp when a current of 5.0 A flows through dilute sodium chloride for 3 hours, 51 min and 36s? seconds
1. Write the equation Q = I x t and establish the known values. In this case I and t are known 2. Write the balanced equation for the reaction at the anode to determine the mol ratios: 4OH-(aq) ------\> 2H2O(l) + O2(g) + 4e. from the equation this means 4 mol electron required for 1 mol O2 3. From the mole ratio determine the amount of electricity needed to produce 1 mol O2: 4 x 19500 = 386000C 4. Determine the quantity of electricity (Q) that was used: Q= I xT 5 x 13896 = 69480C 5. Use this value to determine the number of moles of O2 that was produced: 386000C produces 1mol O2 69480C produces 0.18mol O2 6. convert 0.18 mol to volume of O2 produced - 4.32dm3
89
A solution of copper(II) sulphate is electrolysed using inert graphite electrodes. Determine a current of 2.0A must flow to cause the mass of the cathode to increase by 16g.
1. Write the equation Q = I x t and establish the known values. In this case I and t are known 2. Write the equation for the reaction at the cathode: Cu2(aq) + 2e ------\> Cu(s). 2mol electrons give 1 mol Cu 3. From the mol ratio determine the amount of electricity needed to produce 1 mol Cu: 2 x 19500 = 193000C 4. Convert 16g Cu to mol - 0.25mol Cu. So 0.25mol Cu was produced in this process. 5. Since 1mol Cu requires 193000C then 0.25mol Cu requires 48250C 6. Use Q = I x t to find out the time.
90
When electrolysis is used for the extraction of metals from their ores, the metal ore has to be in a (1)\_\_\_\_\_\_\_\_\_\_\_\_\_ state so that (2)\_\_\_\_\_\_\_\_\_\_\_\_\_\_ are free to migrate to the (3)\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_, where the metal ion is formed(4)\_\_\_\_\_\_\_\_\_ to form (5)\_\_\_\_\_\_\_\_\_\_\_\_\_.
1. molten
2. the metal ions
3. cathode
4. reduced
5. metal element
91
Electrorefining defined as (1)\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_. In this process the anode is always the (2)\_\_\_\_\_\_\_\_\_\_\_ while the cathode is made of (3)\_\_\_\_\_\_\_\_\_\_\_\_ and the electrolyte is (4)\_\_\_\_\_\_\_\_\_\_\_\_.
1. the process of purification of metals by electrolysis.
2. the impure metal
3. the pure metal
4. an aqueous solution of the ions of the metal to be purified.
92
In electroplating the anode is made of (1)\_\_\_\_\_\_\_\_\_\_\_ while the cathode is made of (2)\_\_\_\_\_\_\_\_\_\_\_ and the electrolyte is (3)\_\_\_\_\_\_\_\_\_\_
1. pure sample of metal being used for plating
2. the object to be plated
3. solution of the metal being used for plating
93
Anodising is defined as (1)\_\_\_\_\_\_\_\_\_\_\_\_\_\_. It is used mainly to anodize an object made of (2)\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ in order to better protect against (3)\_\_\_\_\_\_\_\_\_\_ and allows them to absorb (4)\_\_\_\_\_\_\_\_\_\_. The anode made of (5)\_\_\_\_\_\_\_\_\_\_ while the cathode is made of (6)\_\_\_\_\_\_\_\_\_\_ and the electrolyte is usually (7)\_\_\_\_\_\_\_\_\_\_\_. The (8)\_\_\_\_\_\_\_\_\_ will go to the (9)\_\_\_\_\_\_\_\_\_\_\_ and react with Al3+ ions on the surface of the aluminium object to form (10)\_\_\_\_\_\_\_\_\_
1. the process used to increase the thickness of an unreactive oxide on the layer of a metal
2. Aluminium
3. corrosion
4. colours
5. the cleaned aluminium object
6. an inert electrode
7. dilute sulphuric acid
8. OH- ions
9. anode
10. Aluminium oxide
94
what metals can be used in electroplating?
only those below hydrogen in the electrochemical series example: Sliver, Nickel, Chromium, Gold
95
In some electrolysis processes such as electrorefining, one of the electrodes has to be active. Which electrode would this be?
the anode
96
What is the importance of the electrochemical series of metals?
It helps to predict which cation will be preferentially discharged at the cathode
97
What kind of reaction occurs in electrolysis?
Redox reactions
98
What is the importance of the electrochemical series of metals?
It helps to predict which anion will be discharged preferentially at the anode
99
Oxidation occurs at the (1)\_\_\_\_\_\_\_\_\_\_ while reduction occurs the (2)\_\_\_\_\_\_\_\_ during electrolysis
1. Anode
2. Cathode
100
What is the Faraday constant?
It is the magnitude of electric charge per mole of electrons and = 96,500 C/mol
101
The ions present in an aqueous solution of dilute sulfuric acid are: (1)\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ . During electrolysis of this solution the ion that will be discharged preferentially at the inert anode is:(2)\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ and that discharged preferentially at the inert cathode is:(3)\_\_\_\_\_\_\_\_\_\_\_\_\_.
(1) hydrogen ions, sulfate ions, hydroxide ions. (2) hydroxide ion (3) hydrogen ion
102
The ions present in an aqueous solution of dilute sodium chloride are: (1)\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ . During electrolysis of this solution the ion that will be discharged preferentially at the inert anode is:(2)\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ and that discharged preferentially at the inert cathode is:(3)\_\_\_\_\_\_\_\_\_\_\_\_\_.
(1)Sodium ions, chloride ions, hydrogen ions, and hydroxide ions (2) Hydroxide ion (3)Hydrogen ion
103
The ions present in an aqueous solution of copper sulfate are: (1)\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ . During electrolysis of this solution the ion that will be discharged preferentially at the inert anode is:(2)\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ and that discharged preferentially at the inert cathode is:(3)\_\_\_\_\_\_\_\_\_\_\_\_\_.
(1) Copper ions, hydrogen ions, hydroxide ions, Sulfate ions (2) hydroxide ions (3)Copper ions
104
The ions present in an aqueous solution of Copper sulfate are: (1)\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ . During electrolysis of this solution the ion that will be discharged preferentially at the copper anode is:(2)\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ and that discharged preferentially at the inert cathode is:(3)\_\_\_\_\_\_\_\_\_\_\_\_\_.
(1)Copper ions, hydrogen ions, sulfate ions, hydroxide ions (2)None(The anode ionizes to form copper ions) (3) copper Ions
105
The ions present in Concentrated sodium chloride: (1)\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ . During electrolysis of this solution the ion that will be discharged preferentially at the inert anode is:(2)\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ and that discharged preferentially at the inert cathode is:(3)\_\_\_\_\_\_\_\_\_\_\_\_\_.
(1)Sodium ions, hydrogen ions, hydroxide ions, chloride ions (2) chloride ions (3)Hydrogen ions
106
What happens to a solution of copper sulfate when it undergoes electrolysis with inert electrodes?
Copper is deposited at the electrodes and the solution becomes more acidic due to the formation of sulfuric acid
107
What happens to a solution of copper sulfate that undergoes electrolysis with an active copper anode and an inert cathode
The solution will remain unchanged as copper will migrate from the anode and deposit on the cathode. The anode will therefore become smaller.
108
What will happen to dilute sulphuric acid when it undergoes electrolysis with inert electrodes?
The solution will become more concentrated because hydrogen and hydroxide ions will be removed leaving hydrogen ions and sulphate ions in excess
109
What will happen to dilute sodium chloride solution when it undergoes electrolysis with inert electrodes?
The solution will become more concentrated because hydrogen ions and hydroxide ions will be discharged leaving hydrogen ions and sulphate ions in excess
110
What will happen to concentrated sodium chloride solution when it undergoes electrolysis with inert electrodes?
The solution will become more alkaline because hydrogen and chloride ions will be removed leaving hydroxide ions and sodium ions in excess
111
Concentrated sodium chloride is also known as (1)\_\_\_\_\_\_\_\_\_\_\_\_\_\_. It is used to manufacture (2)\_\_\_\_\_\_\_\_\_\_\_ and (3)\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ by means of (4)\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_
1. Brine 2. Chlorine 3. Sodium hydroxide 4. Electrolysis
112
What is the size of the charge on one mole of electrons?
96500 coulombs.
113
What is current?
the flow of electrical charge carriers, usually electrons, around a circuit
114
How is current measured and what is it’s unit?
It is measured using an ammeter. It’s unit is A - Amperes(amps)
115
The ampere is: (1)\_\_\_\_\_\_\_\_\_\_\_\_ The unit of Ampere is:(2)\_\_\_\_\_\_\_\_\_\_ The device that measures amperes is: (3)\_\_\_\_\_\_\_\_\_\_\_\_
1.The number of coulombs per second entering an electric circuit. 2. A 3. Ammeter
116
The quantity of electrical charge, or Coulomb, is the amount of electrical current that enters the circuit over a period of time and is calculated by:
Q= I x t
117
The amount of coulombs passing a point in a circuit is dependent on (1)\_\_\_\_\_\_\_\_\_\_\_\_ and (2)\_\_\_\_\_\_\_\_\_\_\_\_\_\_ .
1. Current(I) 2. time (t)
118
Size of electrical charge is known as the (1)\_\_\_\_\_\_\_\_\_\_ and the rate of flow of electrical charge is known as the (2)\_\_\_\_\_\_\_\_\_\_\_\_\_
1. Coulomb 2. Current
119
to determine the quantity of electrical charge (amount of coulombs) needed to discharge an ion the (1)\_\_\_\_\_\_\_\_\_\_\_\_on the ion must be known as this will determine the (2)\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ of electrons needed to discharge each mole of ion. Eg (3)\_\_\_\_\_\_\_\_\_\_\_\_\_ of electrons are needed to discharge I mole of Ca2+ ion and (4)\_\_\_\_\_\_\_\_ of electrons are needed to discharge I mole of Na+ ions. Since 1 mol electrons is equal to (5)\_\_\_\_\_\_\_\_\_\_\_ C, we can now determine the amount of charge needed to discharge these ions which is (6)\_\_\_\_\_\_\_\_\_\_for Ca2+ and (7)\_\_\_\_\_\_\_\_\_\_ for Na+.
1. Charge 2. Number of moles 3. 2 4. 1 5. 96500 6. 96500 x 2 7 96500
120
How do we determine the mass of magnesium produced the cathode when a current of 7.5A flows through molten magnesium chloride for 25min and 44 seconds
1. Write the equation Q = I x t 2. Write the balanced equation for the reaction at the cathode and determine the mole ratios: Mg2+(l) + 2e ------\> Mg(s). 2mol electron give 1 mol Mg 3. From the balanced chemical equation determine the amount of electricity needed to produce 1 mol Mg: 2 x 19500 = 193000C 4. Determine the quantity of electricity (Q) that was used: Q= I xT 7.5 x 1544s = 11580C 5. Use this value to determine the number of moles of Mg that was produced: 193000C produces 1mol Mg 11580C produces 0.06mol Mg 6. Convert 0.6 mol to mass of Mg produced - 1.44g
