CHEMISTRY - energy changes

Question 1

What is an exothermic reaction?

Answer 1

A reaction that gives out energy to the surroundings (usually feels hot)

Question 2

What is an endothermic reaction?

Answer 2

A reaction that takes in energy from the surroundings ( usually feels cold)

Question 3

What are some examples of exothermic reactions?

Answer 3

Combustion, Respiration, Neutralisation

Question 4

What are some examples of endothermic reactions?

Answer 4

Photosynthesis, Ice packs, Thermal decomposition

Question 5

What is meant by activation energy?

Answer 5

The minimum amount of energy the reactant particles need in order to collide with eachother and react.

Question 6

Have a look in the book at the energy level diagrams.

Answer 6

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Question 7

What happens to the energy when bonds are broken?

Answer 7

Energy must be put in making the reaction endothermic.

Question 7

What happens to the energy when bonds are made?

Answer 7

Energy is released making the reaction exothermic.

Question 8

What is meant by bond energy?

Answer 8

The amount of energy needed to break 1 mole of a specific bond.

Question 9

What is the relationship between energy and reactions?

Answer 9

If energy is positive, the reaction is endothermic. If energy is negative, the reaction is exothermic.

Question 10

What is an electrolyte?

Answer 10

A substance that dissociates into ions in a solution. They conduct electricity.

Question 11

What is an electrode?

Answer 11

An electrode is an electrical conductor used to make contact with the non-metallic part of the reaction.

Question 12

What factors can affect the potential difference in a chemical cell?

Answer 12

Type of electrode
Type of electrolyte
Temperature
Concentration of electrolyte

Question 13

What’s the difference between a cell and a battery?

Answer 13

A cell is a chemical reaction used to generate electricity whereas a battery is a group of cells as a series circuit.

Question 14

What is the relationship between the difference in reactivity and the voltage in a fuel cell.

Answer 14

The greater the difference in reactivity of the metals used, the higher the voltage.

Question 15

Why should different metals be used for each electrode?

Answer 15

So that one can be oxidized and the other can be reduced.

Question 16

What are the two different types of cells?

Answer 16

Primary - Can only be used once, they last for a long time
Secondary - Rechargeable, current of electricity makes the reaction go in reverse.

Question 17

What are the pros and cons of primary alkaline batteries?

Answer 17

Pros:
- Low cost
- Long lifespan
- No power source needed
- Can be stored for up to 10 years
Cons:
- High environmental damage
- Don’t work in high power devices
- Short lifespan in mobile phones

Question 18

What are the pros and cons of secondary rechargeable batteries?

Answer 18

Pros:
- Long lifespan whatever the device
- Constant output until low charge
- No long-term cost of replacements
Cons:
- High cost
- Power source needed
- Can’t be stored for up to 10 years
- High environmental damage

Question 19

What types of energy are produced by a fuel cell?

Answer 19

Electrical energy
Heat energy

Question 20

What is the balanced chemical equation for the reaction in a hydrogen fuel cell?

Answer 20

2H2 + O2 = 2H20

Question 21

What is the half equation for the reaction at the anode in a hydrogen-oxygen fuel cell?

Answer 21

O2 + 4H+ + 4e- = 2H2O

Question 22

What is the half equation for the reaction at the cathode in a hydrogen-oxygen fuel cell?

Answer 22

H2 = 2H+ + 2e-

Question 23

Fuel cells are an efficient way of converting energy.

Answer 23

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Question 24

What are some of the pros and cons of using a hydrogen fuel cell?

Answer 24

Pros: - Easy to maintain - No moving parts - Small size - No harmful gases are released Cons: - Expensive to manufacture - Constant supply is needed - Availability of hydrogen is limited - Hydrogen is flammable

Question 25

What do fuel cells need to be supplied with?

Answer 25

- Constant supply of suitable fuel - Oxygen

Question 26

What is the equation for bond energy?

Answer 26

Bond energy = energy required to break bonds in the reactants – energy required to make bonds in the product.

Question 27

What's the relationship between voltage and the reactivity of metals at the electrode?

Answer 27

The higher the potential difference, the more reactive the metal in electrode B is compared to the metal in electrode A.

Question 28

What are the steps for the required practical: temperature changes?

Answer 28

1. Add 50cm cubed hydrochloric acid into polystyrene cup 2. Measure temperature and record 3. Add 5cm cubed sodium hydroxide to acid and record temp. 4. Keep adding this and recording temp every 5cm cubed until 40cm cubed has been added 5. Repeat process to calculate the mean 6. Plot graph and estimate neutralization point

Question 29

Why should a polystyrene cup be used in this practical?

Answer 29

This minimizes the loss of thermal energy as it's an insulator.

Question 30

What is the equation at the cathode when hydrogen reacts with hydroxide ions? What is the equation at the anode when oxygen gains electrons and reacts with water to produce hydroxide ions?

Answer 30

1. 2H2 + 4OH- = 4H2O + 4e- 2. O2 + 2H2O + 4e- = 4OH- See page 90