C3.3 Energy Change

Question 1

What is an exothermic reaction?

Answer 1

More energy is being released from bond making than is being absorbed from bond breaking - this increases the temperature of the surrounding

Question 2

What is an endothermic reaction?

Answer 2

More energy is being absorbed from bond breaking than is being transferred from bond making - this decreases the temperature of the surrounding

Question 3

Give examples of endo- and exo- thermic reactions

Answer 3

Endothermic: when a substance thermally decomposes
Exothermic: when a fuel burns

Question 4

What happens when bonds are being broken?

Answer 4

Energy is supplied to break bonds, making bond breaking an endothermic process

Question 5

What happens when bonds are being made?

Answer 5

Energy is released when bonds are made, making bond making an exothermic process

Question 6

What is bond energy?

Answer 6

How much energy is required to break one bond of that particular substance

Question 7

How can you use bond energies to calculate the overall energy change for a reaction?

Answer 7

1. Draw out the display formula of all the molecules present in the reaction e.g. H-H + Cl-Cl => H-Cl, H-Cl
2. Add up all the bond energies from the reactants
3. Add up all the bond energies from the products
4. Use this formula: Energy change =
   Energy of bond breaking - Energy of bond making or in other words: Reactants - Products

Question 8

From bond energy calculations, if the final value is negative what does this mean?

Answer 8

The reaction is exothermic - more energy was released from bond making than bond breaking

Question 9

From bond energy calculations, if the final value is positive what does this mean?

Answer 9

The reaction is endothermic - more energy was absorbed from bond breaking than bond making

Question 10

What is an energy level diagram?

Answer 10

A graph that shows how the energy in a reaction changes as the reaction progresses

Question 11

How is an energy level diagram drawn?

Answer 11

The graph starts at the energy level of the reactants and finishes at the energy level of the products - these 2 points are joined by a smooth curve

Question 12

How do you find the overall energy change of a reaction from an ELD?

Answer 12

The difference in height between the energy level of the products and the energy level of the reactants

Question 13

How can you tell if a reaction is endothermic from an ELD?

Answer 13

If the reaction is endothermic, the reactants in the ELD will be lower than the products (as they have less energy)

Question 14

How can you tell if a reaction is exothermic from an ELD?

Answer 14

If the reaction is exothermic, the reactants in the ELD will be higher than the products (as they have more energy)

Question 15

What is the minimum amount of energy needed by reacting particles to break their bonds called?

Answer 15

The activation energy

Question 16

How can you tell what the activation energy is of an ELD?

Answer 16

It is the difference between where the curve starts and its highest point

Question 17

How will an EDL change when a catalyst is added to the same reaction?

Answer 17

It creates a different pathway to the original - it has a lower activation energy, but the energy level of the reactants and products are the same

Question 18

What is energy transfer measured in?

Answer 18

Kilojoules per gram (kJ/g) or kilojoules per mole of reactant (kJ/mol)

Question 19

How can you measure the energy transfer of a reaction in a solution?

Answer 19

By taking the original temperature, and then adding the reactant(s) and then taking the temperature at the end of the solution - this method works for reactions where you mix two solutions like neutralisation reactions

Question 20

How will you know if a reaction is endo/exothermic after taking its original and final temp?

Answer 20

Exo - temperature of solution will increase, endo - temperature of solution will decrease

Question 21

What method can you use to measure how much energy is released from a fuel?

Answer 21

Calorimetry

Question 22

How can you work out the energy transferred of a reaction?

Answer 22

Q = m x c x ΔT ==> energy transferred (J) = mass of water (g) x specific heat capacity of water (4.2) x temperature change (°C)

Question 23

What problems are there with burning fuels? (2)

Answer 23

Releases greenhouse gases causing global warming and other forms of climate change, and crude oil is running out

Question 24

Why can hydrogen be used as a fuel?

Answer 24

Because the reaction is exothermic when it reacts with oxygen

Question 25

How can you harness the energy released from burning hydrogen?

Answer 25

By reacting hydrogen and oxygen in a fuel cell

Question 26

Why is hydrogen very clean as a fuel?

Answer 26

Because it only produces water when burnt

Question 27

What is the word and symbol equation for the combustion of hydrogen?

Answer 27

Hydrogen + oxygen ==> water

2H2* + O2* ==> 2H2*O

Question 28

What are the disadvantages of using hydrogen as a fuel? (3)

Answer 28

The fuel cell which hydrogen needs to be able to be used as a fuel is very expensive, the obtaining of hydrogen still requires another source of energy and it’s hard to store as it is very explosive

Question 29

What is a fuel cell?

Answer 29

An electrical cell that’s supplied with a fuel and oxygen and uses energy from the reaction between them to generate electricity

Question 30

What benefits do hydrogen-oxygen fuel cells have over a battery?

Answer 30

A fuel cell doesn’t run down or need recharging from the mains

Question 31

What energy does a fuel cell produce? (2)

Answer 31

It’ll produce energy in the form of electricity and heat as long as fuel is supplied

Question 32

What is the main benefit of a fuel cell?

Answer 32

It doesn’t produce any conventional pollutants

Question 33

Why doesn’t hydrogen replace crude oil as the main source of fuel? (3)

Answer 33

Because it is a gas, it takes up much more space than liquid fuels like petrol, hydrogen is extremely explosive and is hard to store safely, and the hydrogen is usually obtained from either hydrocarbons, or the electrolysis of water - which both need energy which usually comes from a fossil fuel