chemical energetics and reactions

Question 1

what happens in a chemical reaction

Answer 1

bonds break and new bonds form

Question 2

what do collisions do in a chemical reaction

Answer 2

they provide the energy for chemical reactions

Question 3

What is activation energy

Answer 3

the minimum amount of energy particles must collide with in order to break bonds

Question 4

what is collision theory

Answer 4

You need a collision to have a reaction. ​
Collisions provide the energy required to break bonds.​
Most collisions are not successful.​
A successful collision is very important for any chemical reaction.

Question 5

exothermic reaction equation

Answer 5

reactants -> products + energy

Question 6

endothermic reaction equation

Answer 6

energy + reactants -> products

Question 7

describe exothermic reactions

Answer 7

• energy is released by reaction
• energy level of reactants is higher than products
• activation energy is achieved earlier (peaks earlier)

Question 8

describe endothermic reactions

Answer 8

• energy is absorbed by reaction
• energy level of reactants is lower than products
• activation energy is achieved later (peaks later)

Question 9

how does concentration affect rate of reaction and why

Answer 9

Increase in the concentration of a solution, the rate of reaction will increase
This is because there will be more reactant particles in a given volume, allowing more frequent and successful collisions per second, increasing the rate of reaction

Question 10

how does surface area affect rate of reaction and why

Answer 10

Increase in the surface area of the solid, the rate of reaction will increase
This is because more surface area particles will be exposed to the other reactant so there will be more frequent and successful collisions per second, increasing the rate of reaction

Question 11

how does temperature affect rate of reaction and why

Answer 11

Increase in the temperature, the rate of reaction will increase
This is because the particles will have more kinetic energy than the required activation energy, therefore there will be more frequent and successful collisions per second, increasing the rate of reaction

Question 12

how do catalysts affect rate of reaction and why

Answer 12

Catalysts reduce the activation energy as they create alternative pathways requiring lower activation energy, allowing more successful and frequent collisions
This shows that when a catalyst is used, the rate of reaction will increase

Question 13

what is explosive combustion

Answer 13

Explosive combustion occurs when there are many fine particles in the air
Many industrial processes such as metal working, coal mining or flour milling produce very fine and tiny particles
These particles have a very large surface area and are combustible in air
Even a small spark may cause them to ignite and since the surface area is so large, the rate of reaction can be incredibly fast, hence they are explosive
Methane gas mixed with air in coal mines can also form an explosive mixture

Question 14

investigation for surface area

Answer 14

Method:
Add dilute hydrochloric acid into a conical flask
Use a capillary tube to connect this flask to a measuring cylinder upside down in a bucket of water (downwards displacement)
Add calcium carbonate chips into the conical flask and close the bung
Measure the volume of gas produced in a fixed time using the measuring cylinder
Repeat with different sizes of calcium carbonate chips (solid, crushed and powdered)
Result:
Smaller sizes of chips causes an increase in the surface area of the solid, so the rate of reaction will increase
This is because more surface area of the particles will be exposed to the other reactant so there will be more frequent and successful collisions, increasing the rate of reaction

Question 15

investigation for concentration

Answer 15

Method:
Measure 50 cm3 of Sodium Thiosulfate solution into a flask
Measure 5 cm3 of dilute Hydrochloric acid into a measuring cylinder
Draw a cross on a piece of paper and put it underneath the flask
Add the acid into the flask and immediately start the stopwatch
Look down at the cross from above and stop the stopwatch when the cross can no longer be seen
Repeat using different concentrations of Sodium Thiosulfate solution (mix different volumes of sodium thiosulfate solution with water to dilute it)
Result:
With an increase in the concentration of a solution, the rate of reaction will increase
This is because there will be more reactant particles in a given volume, allowing more frequent and successful collisions, increasing the rate of reaction

Question 16

investigation for temperature

Answer 16

Method:
Dilute Hydrochloric acid is heated to a set temperature using a water bath
Add the dilute Hydrochloric acid into a conical flask
Add a strip of Magnesium and start the stopwatch
Stop the time when the Magnesium fully dissolves
Repeat at different temperatures and compare results
Result:
With an increase in the temperature, the rate of reaction will increase
This is because the particles will have more kinetic energy than the required activation energy, therefore more frequent and successful collisions will occur, increasing the rate of reaction

Question 17

graph for higher concentration

Answer 17

Compared to a reaction with a reactant at a low concentration, the graph line for the same reaction but at a higher concentration has a steeper gradient at the start and becomes horizontal sooner
This shows that with increased concentration of a solution, the rate of reaction will increase

Question 18

graph for particle size/surface area

Answer 18

Compared to a reaction with lumps of reactant, the graph line for the same reaction but with powdered reactant has a steeper gradient at the start and becomes horizontal sooner
This shows that with increased surface area of the solid, the rate of reaction will increase

Question 19

graph for catalyst

Answer 19

The diagram shows that when a catalyst is used, the activation energy is reduced as it creates an alternative pathway requiring lower activation energy, allowing more successful and frequent collisions
This shows that when a catalyst is used, the rate of reaction will increase

Question 20

graph for temperature

Answer 20

Compared to a reaction at a low temperature, the graph line for the same reaction but at a higher temperature has a steeper gradient at the start and becomes horizontal sooner
This shows that with increased temperature, the rate of reaction will increase

Question 21

photochemical reactions + properties

Answer 21

Light wave energy (UV or Radiation = α,β,γ)acts as electromagnetic waves
considered as an energy ‘bullets’ called photons ​
have sufficient energy to overcome the activation energy to break chemical bonds.
greater the intensity of light , the more reactant molecules are likely to gain the required energy (activation energy) - reaction speed increases.​
not a catalyst - the photons of energy are used up in the chemical changes they induce, the photons cannot be recycled

Question 22

photosynthesis characterisitc

Answer 22

Requires the input of sunlight energy and the green chlorophyll molecules absorb the photon energy packets of light and initiate the chemical changes.
> Intensity Of Light, > Rate Of Photosynthesis.
Factors affecting the rate - light, temperature & CO2 concentration

Question 23

photography characteristic

Answer 23

Each silver halide (AgCl, AgBr, AgI) salt has a different sensitivity to light (photosensitive).​
All three are used in the production of various types of photographic film to detect visible light and beta and gamma radiation from radioactive materials.
AgI is the least sensitive and used in X-ray radiography, AgCl is the most sensitive and used in ‘fast’ film for cameras.

Question 24

equation for photography

Answer 24

When radiation hits the film the silver ions in the salt are reduced by electron gain to silver​:
Ag+ + e- → Ag​
Halide ion is oxidized to the halogen molecule by electron loss​:
2X- → X2 + 2e- (X = halogen atom, Cl, Br or I)​
Balance Equation :
2AgX → 2Ag + X2

Question 25

what are NPK fertilisers

Answer 25

Fertilisers contain nitrogen, potassium and phosphorus Nitrogen promotes healthy leaves, potassium promotes growth and healthy fruit and flowers and phosphorus promotes healthy roots

Question 26

what do fertiliser compounds contain

Answer 26

Fertiliser compounds contain the following water-soluble ions: Ammonium ions, NH4+ and nitrate ions, NO3–, are sources of soluble nitrogen Phosphate ions, PO43- are a source of soluble phosphorus Most common potassium compounds dissolve in water to produce potassium ions, K+

Question 27

what do fertiliser compounds contain

Answer 27

Fertiliser compounds contain the following water-soluble ions: Ammonium ions, NH4+ and nitrate ions, NO3–, are sources of soluble nitrogen Phosphate ions, PO43- are a source of soluble phosphorus Most common potassium compounds dissolve in water to produce potassium ions, K+

Question 28

what is displacement of ammonia

Answer 28

Ammonia can be displaced from its salts by the addition of an alkali substance

Question 29

use of ammonia displacement

Answer 29

Farmers regularly add basic substances such as calcium hydroxide to their soil to neutralise any excess soil acidity If too much of the basic substance is added or if it has been added too soon after fertiliser has been added, then an ammonia displacement reaction may occur This involves the loss of nitrogen from the fertiliser, nullifying its effectiveness as a fertiliser For example, the salt ammonium chloride is used extensively in fertilisers and reacts with calcium hydroxide: 2NH4Cl + Ca(OH)2 → CaCl2 + 2NH3 + 2H2O

Question 30

what are the sources of sulfur

Answer 30

- Sulfur is found in its elemental state underground in the USA, Mexico and Poland - It is also a by-product from the removal of sulfur from petroleum and natural gas - Sulfur can also be obtained from sulfide ores

Question 31

what are the uses of sulfur

Answer 31

- The main use of sulfur is in making sulphuric acid which is a very important chemical used in many industries - It is also used extensively in making rubber tyres more flexible (vulcanising), where the rubber is heated with sulfur

Question 32

making of sulfur dioxide

Answer 32

Sulfur dioxide can be made by the direct combination of sulphur with oxygen This is the method used in the first stage of the manufacture of sulfuric acid: S + O2 → SO2

Question 33

how is sulfuric acid manufactured

Answer 33

Sulfuric acid is synthesised by the Contact process which use sulfur and oxygen from air and is done in three distinct stages

Question 34

stages of sulfuric acid

Answer 34

Stage 1: The first stage is the oxidation of sulfur: S + O2 → SO2 Stage 2: The main stage is the oxidation of sulfur dioxide to sulfur trioxide using a V2O5 catalyst: 2SO2 + O2 ⇌2SO3 The conditions for the main stage of production of sulfur trioxide need to be considered: Temperature: 450ºC The reaction is exothermic, so increasing the temperature shifts the position of equilibrium to the left in the direction of the reactants Therefore the higher the temperature, the lower the yield of sulfur trioxide The optimum temperature is a compromise between a higher rate of reaction at a higher temperature and a lower equilibrium yield at a higher temperature Pressure: 2 atm An increase in pressure shifts the position of equilibrium to the right in the direction of a smaller number of gaseous molecules However the position of equilibrium lies far to the right (the equilibrium mixture contains about 96% sulfur trioxide) So the reaction is carried out at just above atmospheric pressure because: a) it is not worth spending the extra energy or money required to produce high pressures b) a higher pressure would increase the problems of dealing with the corrosive mixture of gases Stage 3: Once stage 2 is completed, the sulfur trioxide is absorbed into a solution of 98% sulphuric acid to produce a thick liquid called oleum: SO3 + H2SO4 → H2S2O7 It is not absorbed into water because a fine mist of sulfuric acid would be produced and this would be difficult to condense and is also highly dangerous Oleum is added to water to form concentrated sulfuric acid: H2S2O7 + H2O → 2H2SO4

Question 35

properties of sulfuric acid

Answer 35

Sulfuric acid is a strong dibasic acid as two of its hydrogen atoms can be replaced by a metal It reacts in a similar way to other acids with metal carbonates, oxides, hydroxides (and ammonia) and metals, e.g: ZnO + H2SO4 → ZnSO4 + H2O Mg + H2SO4 → MgSO4 + H2 Na2CO3 + H2SO4 → Na2SO4 + CO2 + H2O Concentrated sulphuric acid is corrosive and a powerful oxidising agent Concentrated sulphuric acid is also a very powerful dehydrating agent and is very good at removing water from other substances For example, if mixed with sugar (C6H12O6), concentrated H2SO4 will remove water molecules and leave behind carbon in a spectacular looking reaction that produces a tower of pure carbon

Question 36

what are reversible reactions

Answer 36

In reversible reactions, the product molecules can themselves react with each other or decompose and form the reactant molecules again It is said that the reaction can occur in both directions: the forward reaction (which forms the products) and the reverse direction (which forms the reactants)

Question 37

what are hydrated salts

Answer 37

Hydrated salts are salts that contain water of crystallisation which affects their molecular shape and colour Water of crystallisation is the water that is stoichiometrically included in the structure of some salts during the crystallisation process

Question 38

what are anhydrated salt

Answer 38

Anhydrous salts are those that have lost their water of crystallisation, usually by heating, in which the salt becomes dehydrated

Question 39

Dehydration of Hydrated Copper (II) Sulfate:

Answer 39

Hydrated Copper (II) Sulfate ⇌ Anhydrous Copper (II) Sulfate + Water When anhydrous copper (II) sulfate crystals are added to water they turn blue and heat is given off (exothermic); this reaction is reversible When Copper (II) Sulfate crystals are heated in a test tube, the blue crystals turn into a white powder and a clear, colourless liquid (water) collects at the top of the test tube The form of Copper (II) Sulfate in the crystals is known as Hydrated Copper (II) Sulfate because it contains water of crystallisation When Hydrated Copper (II) Sulfate is heated, it loses its water of crystallisation and turns into anhydrous Copper (II) Sulfate: CuSO4.5H2O (s) ⇌ CuSO4 (s) + 5H2O (l)

Question 40

Dehydration of Hydrated Cobalt (II) Chloride:

Answer 40

Hydrated Cobalt (II) Chloride ⇌ Anhydrous Cobalt (II) Chloride + Water When anhydrous blue cobalt(II) chloride crystals are added to water they turn pink and the reaction is reversible When the cobalt(II) chloride crystals are heated in a test tube, the pink crystals turn back to the blue colour again as the water of crystallisation is lost The form of cobalt(II) chloride in the crystals that are pink is known as hydrated cobalt (II) chloride because it contains water of crystallisation When hydrated cobalt(II) chloride is heated, it loses its water of crystallisation and turns into anhydrous cobalt(II) chloride: CoCl2.6H2O (s) ⇌ CoCl2 (s) + 6H2O (l)

Question 41

what is a state of equilibrium

Answer 41

When during the course of reaction, the rate of the forward reaction equals the rate of the reverse reaction, then the overall reaction is said to be in a state of equilibrium

Question 42

characteristics of equilibrium

Answer 42

It is dynamic eg the molecules on the left and right of the equation are changing into each other by chemical reactions constantly and at the same rate The concentration of reactants and products remains constant (given there is no other change to the system such as temperature and pressure) It only occurs in a closed system so that none of the participating chemical species are able to leave the reaction vessel

Question 43

The reaction between H2 and N2 in the Haber process

Answer 43

When only nitrogen and hydrogen are present at the beginning of the reaction, the rate of the forward reaction is at its highest, since the concentrations of hydrogen and nitrogen are at their highest As the reaction proceeds, the concentrations of hydrogen and nitrogen gradually decrease, so the rate of the forward reaction will decrease However, the concentration of ammonia is gradually increasing and so the rate of the backward reaction will increase (ammonia will decompose to reform hydrogen and nitrogen) Since the two reactions are interlinked and none of the gas can escape, the rate of the forward reaction and the rate of the backward reaction will eventually become equal and equilibrium is reached: 3H2 (g) + N2 (g) ⇌ 2NH3 (g)

Question 44

what is the position of equilibrium

Answer 44

- Equilibrium position refers to the relationship between the concentration of reactants and products at the equilibrium state - When the position of equilibrium shifts to the left, it means the concentration of reactant increases - When the position of equilibrium shifts to right, this means the concentration of product increases

Question 45

what is the effect of catalysts on equilibrium positions

Answer 45

The presence of a catalyst does not affect the position of equilibrium but it does increase the rate at which equilibrium is reached This is because the catalyst increases the rate of both the forward and backward reactions by the same amount (by providing an alternative pathway requiring lower activation energy) As a result, the concentration of reactants and products is nevertheless the same at equilibrium as it would be without the catalyst

Question 46

what is le chatelier's principle

Answer 46

Le Chatelier’s Principle states that when a change is made to the conditions of a system at equilibrium, the system automatically moves to oppose the change. The principle is used to predict changes to the position of equilibrium when there are changes in temperature, pressure or concentration.

Question 47

effect of temperature in equilibrium

Answer 47

increase in temperature: equilibrium moves in the endothermic direction to reverse the change decrease in temperature: equilibrium moves in the exothermic direction to reverse the change

Question 48

effect of pressure in equilibrium

Answer 48

increase in pressure: equilibrium shifts in the direction that produces the smaller number of molecules of gas to decrease the pressure again decrease in pressure: equilibrium shifts in the direction that produces the larger number of molecules of gas to increase the pressure again

Question 49

effects of concentration in equilibrium

Answer 49

increase in concentration: equilibrium shifts to the right to reduce the effect of increase in the concentration of a reactant decrease in concentration: equilibrium shifts to the left to reduce the effects of a decrease in reactant (or an increase in the concentration of a product)

Question 50

what is a fuel

Answer 50

A fuel is a substance which releases energy when burned

Question 51

combustion reaction with hydrocarbon fuels what r the products

Answer 51

When the fuel is a hydrocarbon then water and carbon dioxide are produced in combustion reactions

Question 52

what is fuel efficiency

Answer 52

The efficiency of a fuel refers to how much energy is released per unit amount We can measure the efficiency of fuels by calorimetry

Question 53

how to find a fuels heat of combustion

Answer 53

A known mass of the fuel is combusted and used to heat up a known mass of water to calculate its heat of combustion Different fuels heat the water by different amounts and they can be analysed and compared in this way

Question 54

calorimetry experiment - combustion

Answer 54

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 Calculation: 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

Question 55

hydrogen fuel advantages disadvantages

Answer 55

- Hydrogen is used in rocket engines and in fuel cells to power some cars Advantages: - 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 Disadvantages: - 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 pressurized containers)

Question 56

radioactive isotopes as fuels

Answer 56

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 Nuclear fuel energy is clean as it does not produce pollutants such as CO2 or oxides of nitrogen or sulfur 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

Question 57

what is a fuel cell

Answer 57

A fuel cell is an electrochemical cell in which a fuel donates electrons at one electrode and oxygen gains electrons at the other electrode These cells are becoming more common in the automotive industry to replace petrol or diesel engines

Question 58

hydrogen fuel cells reactions

Answer 58

H2 and O2 are pumped through two porous electrodes where the half-reactions occur The following reaction occurs at the anode: 2H2 → 4H+ + 4e- At the cathode the following reaction takes place: 4H+ + O2 + 4e- → 2H2O The overall reaction is: 2H2 + O2 → 2H2O The electrons move around the external circuit from the cathode to the anode This movement of electrons is used to drive an electric motor

Question 59

advantages of fuel cells

Answer 59

Advantages 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

Question 60

disadvantages of fuel cell

Answer 60

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

Question 61

what are physical changes

Answer 61

``` Physical changes (such as melting or evaporating) do not produce any new chemical substances These changes are often easy to reverse and mixtures produced are usually relatively easy to separate Chemical change ```

Question 62

what are chemical changes

Answer 62

In chemical reactions, new chemical products are formed that have very different properties to the reactants Most chemical reactions are impossible to reverse Energy changes also accompany chemical changes and energy can be given out (exothermic) or taken in (endothermic) The majority of chemical reactions are exothermic with only a small number being endothermic

Question 63

what are redox reactions

Answer 63

Oxidation and reduction take place together at the same time in the same reaction These are called redox reactions

Question 64

what are oxidation reactions

Answer 64

Oxygen is added to an element or a compound An element, ion or compound loses electrons The oxidation state of an element is increased

Question 65

what are reductions

Answer 65

Oxygen is removed from an element or a compound An element, ion or compound gains electrons The oxidation state of an element is decreased

Question 66

what is an oxidising agent

Answer 66

A substance that oxidises another substance, in so doing becoming itself reduced Common examples include hydrogen peroxide, fluorine and chlorine

Question 67

what is a reducing agent

Answer 67

A substance that reduces another substance, in so doing becoming itself oxidised Common examples include carbon and hydrogen The process of reduction is very important in the chemical industry as a means of extracting metals from their ores

Question 68

how to identify redox reactions

Answer 68

Redox reactions can be identified by the changes in the oxidation states when a reactant goes to a product

Question 69

identify redox reactions by color change

Answer 69

The tests for redox reactions involve the observation of a colour change in the solution being analyse Two common examples are acidified potassium manganate(VII), and potassium iodide Potassium manganate (VII), KMnO4, is an oxidising agent which is often used to test for the presence of reducing agents When acidified potassium manganate (VII) is added to a reducing agent its colour changes from pink-purple to colourless Potassium iodide, KI, is a reducing agent which is often used to test for the presence of oxidising agents When added to an acidified solution of an oxidising agent such as aqueous chlorine or hydrogen peroxide, the solution turns a brown colour due to the formation of iodine