C7. Energy Changes

Question 1

What is an exothermic reaction?

Answer 1

A reaction in which energy is transferred from the reacting substances to their surroundings

Question 2

What is an endothermic reaction?

Answer 2

A reaction in which energy transferred to the reacting substances from their surroundings

Question 3

What are some exothermic reactions?

Answer 3

Combustion, many oxidation reactions and neutralisation

Question 4

What are some endothermic reactions?

Answer 4

Thermal decomposition, the reaction of citric acid and sodium hydrogen carbonate

Question 5

What are some everyday uses of exothermic reactioms

Answer 5

Self heating cans and hand warmers

Question 6

What are some everyday uses of endothermic reactions

Answer 6

Instant cold packs

Question 7

What are some endothermic reactions?

Answer 7

Thermal decomposition & the reaction of citric acid and sodium hydrogen carbonate

Question 8

Why do chemical reactions need activation energy to start them?

Answer 8

Reactant molecules must bump into each other, so they must be moving and movement requires energy

Question 9

What is activation energy?

Answer 9

The minimum energy needed for a reaction to take place

Question 10

What is the reaction profile for an exothermic reaction?

Answer 10

The products are at a lower energy level than the reactions which means that when the reactants form the products, energy is transferred to the surroundings

Question 11

What happens to the surroundings in an exothermic reaction?

Answer 11

The surroundings get hotter and hotter and the temperature rises because the difference in energy between the products and the reactants is transferred to the surroumdimgs

Question 12

What happens to the surroundings in an endothermic reaction?

Answer 12

The surroundings get colder because energy is transferred from the surroundings to the reaction mixture

Question 13

What is the reaction profile for an endothermic reaction?

Answer 13

The products are at a higher energy level than the reactants. As the reactants react to form products the energy is transferred from the surroundings to the reaction mixture

Question 14

What is the energy change in terms of balance between bond making and breaking?

Answer 14

Breaking bonds is an endothermic process, energy is taken in from the surroundings
Making bonds is an exothermic process, energy is transferred to the surroundings

Question 15

How does the balance between the energy needed to break bonds and the energy released when new bonds are made decide whether the reaction is exothermic?

Answer 15

The energy released when new bonds are formed (as the products are made) is more than the energy needed to break the bonds in the reactants. These reactions transfer energy to the surroundings

Question 16

How does the balance between the energy needed to break bonds and the energy released when new bonds are made decide whether the reaction is endothermic?

Answer 16

The energy needed to break the bonds in the reactants is more than the energy released when new bonds are formed in the products. These reactions transfer energy to the reacting chemicals

Question 17

How do you calculate the energy change?

Answer 17

1. Work out how much energy change is needed to break the chemical bonds in the reactants
2. Work out how much energy is released when new bonds are formed in the products.

Question 18

What is the general relationship between the difference in reactivity and the voltage?

Answer 18

The greater the difference in reactivity between the two metals used, the higher the voltage produced

Question 19

When happens when two metals are dipped in a solution and joined by a wire?

Answer 19

The more reactive metal will donate electrons to the less reactive metal. This forms a simple electrical cell.

Question 20

What happens to hydrogen in the cathode a hydrogen fuel cell

Answer 20

Hydrogen gas is supplied as a fuel to the cathode. It diffuses through the graphite electrode and reacts with hydroxide ions to form water and provides a source of electrons to an external circuit

Question 21

What is the half equation for hydrogen gas in an hydrogen fuel cell?

Answer 21

2H₂ + 4OH- ➡ 4H₂O + 4e-

Question 22

What happens to oxygen in the anode of a hydrogen fuel cell?

Answer 22

Oxygen gas is supplied to the anode. It diffuses through the graphite and reacts to form hydroxide ions accepting electrons from the external circuit

Question 23

What is the half equation for oxygen in a hydrogen fuel cell?

Answer 23

O₂ + 2H₂O+ 4e- ➡ 4OH-

Question 24

What is oxidised and what is reduced in a hydrogen fuel cell?

Answer 24

Hydrogen is oxidised

Question 25

What is the equation for the oxidation of hydrogen?

Answer 25

2H₂ + O₂ ➡ 2H₂O

Question 26

What are the advantages of hydrogen fuel cells?

Answer 26

Do not need to be electrically charged
No pollutants are produced
Can be a range of sizes for different uses

Question 27

What are the disadvantages of hydrogen fuel cells?

Answer 27

Hydrogen is highly flammable
Hydrogen is sometimes produced for the cell by non-renewable sources
Hydrogen is difficult to store