Chapter C7 Energy Changes

Question 1

What is an exothermic reaction?

Answer 1

Energy is transferred from the reaction to the surroundings

Question 2

What is an endothermic reaction?

Answer 2

Energy is transferred from tens surroundings to the reaction

Question 3

What happens to the temperature of surroundings in endothermic and exothermic reactions?

Answer 3

Endo- gets colder

Exo- gets warmer

Question 4

What energy content do the products of an exothermic reaction contain?

Answer 4

A lower energy content than the reactions as energy was transferred to the surroundings

Question 5

What energy content do the products of an endothermic reaction contain?

Answer 5

A higher energy content than the reactants

Question 6

Give examples of uses of exothermic reactions

Answer 6

Hand and body warmers

Self heating cans

Question 7

How do reusable hand warmers work?

Answer 7

Salt crystals are dissolved in hot water, and then allowed to cool
A metal disc is inside the solution, which when clicked start off the crystallisation process. The crystals spread throughout the solution resulting in exothermic energy. They can then be reused by redissolving the crystals

Question 8

Give examples of endothermic reactions

Answer 8

Chemical cold packs

Question 9

How does a chemical cold pack work?

Answer 9

The ammonium nitrate and water are in a bag but separated. When the bag is squeezed, the ammonium nitrate is released, taking in temperature from the surroundings and resulting in a cold compress.

Question 10

Describe the exothermic reaction profile

Answer 10

Reactants are high on the left, and the products are low on the right.
This means that when the reactants form the products energy is lost to the surroundings

Question 11

Draw the reaction profile of an endothermic reaction

Answer 11

Products 
Activation  [                    /\
      Energy [                  /   \
    Required[                /      \\_\_\_\_\_\_\_\_\_\_\_\_
                   [               |
                   [               |
                   [               |
                   [               |
                   [               | 
 \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_|
Reactants

Question 12

Describe the reaction profile of an endothermic reaction

Answer 12

Reactants are low on the left, whilst the products are high on the right. As the reactants form products, energy is transferred from the surroundings to the reaction mixture

Question 13

What is activation energy?

Answer 13

Minimum amount of energy needed before colliding particles of reactants can cause a reaction

Question 14

What is breaking bonds?

Answer 14

An endothermic process- energy has to be supplied to break bonds

Question 15

What is making bonds?

Answer 15

Exothermic process- energy is transferred to surroundings

Question 16

Do bind breaking and making processes happen at the same time?

Answer 16

Yes

Question 17

What is bond energy?

Answer 17

Energy needed to break between two atoms

Question 18

What are bond energies measured in?

Answer 18

KJ/mol

Question 19

What do bond energies show?

Answer 19

Work out energy changes for chemical reactions

Question 20

What similarities do bond energies have?

Answer 20

Braking and making bonds require the same amount of energy

Question 21

What does an exothermic bond energy calculation always have?

Answer 21

A negative final result

Question 22

What is a redox reaction?

Answer 22

Change in charge of ion

Question 23

What is an electric current?

Answer 23

A flow of electrons

Question 24

What does a greater difference in reactivity between two metals result in?

Answer 24

A higher voltage produced

Question 25

Why does a primary cell run out?

Answer 25

Yeh reactants are used up

Question 26

What happens in the recharging process of a battery?

Answer 26

Reversed reactions that occur at the electrode and so regenerates initial reactants

Question 27

Why is hydrogen a good fuel?

Answer 27

It burns well and produces no pollutants

Question 28

What is an advantage to hydrogen as a fuel

Answer 28

Produces no pollutants as so reduces impact on global warming.

Question 29

What is a disadvantage to hydrogen as a fuel?

Answer 29

Proteins with safety and storage

Supplying the hydrogen through electrolysis burns fossil fuels so carbon dioxide is still produced

Question 30

What is a fuel cell?

Answer 30

Cells fed with hydrogen and oxygen and produce water- energy released transferred to electrical energy

Question 31

What is a challenge of hydrogen fuel cells?

Answer 31

They just match the performance, convenience and price of diesel powered alternatives

Question 32

Explain how a hydrogen fuel cell works

Answer 32

Hydrogen gas is supplied at the cathode. It diffuses through the graphite electrode and reacts with hydroxide ions to form water and provide electrons to the external circuit (as hydroxide is negatively charged ion)
Oxygen gas is supplied at the anode. It diffuses through the graphite and reacts to form hydroxide ions, accepting electrons from the external circuit (provided by the hydrogen and hydroxide reaction)
When added, these reactions cancel out and so you are left with the overall change in the hydrogen cell, which is the oxidation of hydrogen, with a wasted product of water

Question 33

Advantages of hydrogen fuel cells

Answer 33

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

Question 34

Disadvantages of hydrogen fuel cells

Answer 34

Hydrogen is highly flammable
The hydrogen needed is produced via non renewable sources
Hydrogen is difficult to store
Hydrogen needs to be compressed- requires large tank
Doesn’t work at low temperatures

Question 35

Energy calculations

Answer 35

Normally do reactants - product

Question 36

What is a battery also called?

Answer 36

A cell

Question 37

By product of a fuel cell?

Answer 37

Water