Topic 5-Energy Changes

Question 1

What is an exothermic reaction?

Answer 1

It is one that loses energy to the surrounding so the temperature of the surroundings increases

Question 2

What are some examples of exothermic reactions?

Answer 2

• Combustion
• Many oxidisation reactions
• Neutralisation
• Self heating cans
• Hand warmers

Question 3

What is an endothermic reaction?

Answer 3

It is one that takes in energy from the surroundings so the temperature of the surroundings decreases
(product molecules must have more energy than reactants)

Question 4

What are some examples of endothermic reactions?

Answer 4

• Thermal decomposition
• Citric acid and sodium hydrogen carbonate
• Some sport injury packs

Question 5

What is activation energy?

Answer 5

The minimum amount of energy that particles must have to react

Question 6

Describe exothermic and endothermic reactions in terms of bond breaking/forming

Answer 6

Energy is supplied to break bonds and energy is released when bonds are made

Exothermic-energy released from forming bonds is greater than that needed to break the bonds

Endothermic- energy needed to break bonds is greater than energy released making them

Question 7

What is the equation to find enthalpy change in terms of bond energies

Answer 7

Energy of reaction= sum of bonds broken-sum of bonds made

Question 8

What is a cell?

Answer 8

A cell is composed of two electrodes dipped in an electrolyte solution.

It produces electricity from a chemical reaction

Question 9

What is a battery?

Answer 9

A battery consists of two or more cells connected in series

Question 10

What determines the voltage obtained from a cell?

Answer 10

Identities of metals used as electrodes and the identity and concentration of an electrolyte

Question 11

State the advantages and disadvantages of using cells and batteries

Answer 11

+relatively cheap, some are rechargeable, a convenient source of electrical energy

-harmful chemicals

Question 12

Describe rechargeable cells

Answer 12

Can be recharged because the chemical reactions are reversed when an external electrical current is supplied

Question 13

Describe non-rechargeable cells

Answer 13

Chemical reactions stop when one of the reactants has been used up

Alkaline batteries are non-rechargeable

Question 14

What is a fuel cell?

Answer 14

Fuel cells are supplied by fuel and oxygen to oxidise the fuel to generate electricity

Question 15

What is the overall reaction in a hydrogen fuel cell?

Answer 15

2H₂+O₂ –> 2H₂O

Question 16

What are the half equations at each electrode

Answer 16

Cathode: 2H₂ –> 4H+ +4e-

Anode: O₂ + 4H+ +4e- –> 2H₂O

Question 17

What are the advantages and disadvantages of hydrogen fuel cells?

Answer 17

Advantages: no pollutants, no recharging

Disadvantages: flammable, H₂ is difficult to store and transport (needs high pressure), toxic chemicals, expensive production of H₂ by electrolysis

Question 18

What are the advantages and disadvantages of rechargeable cells and batteries?

Answer 18

-can be recharged by reversing reaction, so fuel doesn’t need to keep being supplied

Hard to dispose of since it is non-biodegradable

It will eventually stop working

Question 19

Why do hydrogen fuel cells produce steam?

Answer 19

Water is produced and released as steam because hydrogen is oxidised