Topic 5 - Energy Changes (No Pracs)

Question 1

What is an exothermic reaction

Answer 1

A reaction which transfers energy to the surroundings (shown by a rise in temperature)

Question 2

What are some examples of exothermic reactions

Answer 2

Combustion, neutralisation, many oxidation reactions

Question 3

What are some everyday uses of exothermic reaction

Answer 3

• hand warmers
• self heating cans of hot chocolate and coffee

Question 4

What’s an endothermic reaction

Answer 4

A reaction which takes in energy from the surroundings (shown by a fall in temperature)

Question 5

What’s an example of an endothermic reaction

Answer 5

Thermal decomposition

Question 6

Are endothermic or exothermic reactions more common

Answer 6

Exothermic

Question 7

What are some everyday uses of endothermic reactions

Answer 7

Sports injury packs

Question 8

Is bond breaking endothermic or exothermic

Answer 8

Endothermic - energy must be supplied

Question 9

Is bond formation endothermic or exothermic

Answer 9

Exothermic - energy is released

Question 10

What is the overall energy change of a reaction in terms of bond energies

Answer 10

The sum of the energies needed to break bonds in the reactants minus the energy released when the new bonds are formed

Question 11

What’s an electrochemical cell

Answer 11

A basic system made up of two different electrodes in contact with an electrolyte

Question 12

How does an electrochemical cell work

Answer 12

1) the two electrodes can conduct electricity
2) the electrolyte contains ions that react with the electrodes
3) the chemical reactions between the electrodes and the electrolyte set up a charge difference between the electrodes
4) if the electrodes are then connected by a wire then charge flows and electricity is produces

Question 13

How do you increase the voltage of an electrochemical cell

Answer 13

1) change the type of electrodes
2) change the electrolyte
3) connect more cells

Question 14

How does changing the electrodes in an electrochemical cells increase the voltage

Answer 14

Different metals with react differently with the same electrolyte - the bigger the difference in reactivity of the electrodes the larger the voltage

Question 15

How does changing the electrolyte in a electrochemical cell change the voltage

Answer 15

Different ions in a solution will react differently with the metal electrodes used

Question 16

What type of reactions does non rechargeable batteries contain

Answer 16

Irreversible

Question 17

How can a reaction in a rechargeable cell be reversed

Answer 17

By connecting it to an external electric current

Question 18

What’s a fuel cell

Answer 18

Is an electrical cell that’s supplied with a fuel and oxygen and uses energy from the reaction between them to produce electrical energy efficiently

Question 19

Whats an example of a fuel cell

Answer 19

Hydrogen - oxygen fuel cell

Question 20

How do fuel cells work

Answer 20

When the fuel enters the cell it becomes oxidised and sets up a potential difference within the cell

Question 21

How do hydrogen oxygen fuel cells work

Answer 21

1) hydrogen goes into the anode compartment and oxygen goes into the cathode compartment
2) hydrogen is oxidised
3) oxygen gains electrons from the cathode and reacts with the H+ ions from the electrolyte to make water
4) the electrons flow through an external circut from the anode to to the cathode- this is the electric current

Question 22

What’s the overall reaction in a hydrogen - oxygen fuel cells

Answer 22

2H2 + O2 > 2H20

Question 23

What are the downsides of conventional fuels

Answer 23

• finite supply
• very polluting

Question 24

What are the advantages of full cell vehicles

Answer 24

• don’t produce as many pollutants
• less expensive than batteries
• need to be recharged less than batteries

Question 25

What are the disadvantages of fuel cell vehicles

Answer 25

• hydrogen is a gas so it takes more space to store
• hydrogen is explosive when mixed with air - hard to store safely
• hydrogen is amde from hydrocarbons or electrolysis- uses fossil fuels