Booklet 5 - Exothermic and Endothermic Reactions

Question 1

What are exothermic reactions?

Answer 1

Reactions that transfer energy to the surroundings so the temperature of the surroundings increases

Question 2

What are endothermic reactioms?

Answer 2

Reactions that takes in energy from the surroundings so the temperature of the surroundings decreases

Question 3

What other reactions are there of exothermic reactions?

Answer 3

Combustions neutralisations and some oxidations

Question 4

Where can endothermic reactions be used in everyday life?

Answer 4

Instant ice packs, thermal decompositionz and reaction of citic acid and sodium hydrogencarbonate

Question 5

What happens in the hand warmers and ice packs?

Answer 5

They are kept apart by a membrane, which when broken allows chemicals to mix, react and produce a warming/cooling effect

Question 6

What is the advantage of hand warmers/ice packs?

Answer 6

They’re portable + convenient

Question 7

What is the disadvantage of the hand warmers and ice packs?

Answer 7

Once the reaction has occured it cannot be reversed easily

Question 8

For particles to react what needs to happen?

Answer 8

They need to collide with each other, but not only that, they must collide with sufficient energy cause a reaction

Question 9

What the mimimal amount of energy needed for particles to collide with?

Answer 9

Activation energy, EA

Question 10

What happens if particles do not posses energy needed?

Answer 10

If the dont posses energy equal or greater than, the activation energy cant occur no matter how many times particles collide

Question 11

In a chemical reaction, what do exisiting bonds in reactants must do?

Answer 11

They must be broken before the new bonds of the products can be formed

Question 12

What do you have to supply to a chemical reaction?

Answer 12

Supply energy to the bonds if you want to break them and hence start the reaction -> activation energy

Question 13

What does a reaction profile show?

Answer 13

Can be used to show the relative energies of reactions and products, the activation energy and the overall energy change of a reaction

Question 14

What happens when bonds in the product are formed?

Answer 14

Energy is released

Question 15

What can be calculated from bond energies?

Answer 15

The energy needed to break bonds and the energy released when bonds are formed

Question 16

What is the overall energy change of a reaction?

Answer 16

The difference between the sum of the energy needed to break bonds in the reactants and the sum of energy released when bonds in the products are formed

Question 17

What is the energy level like in a exothermic reaction?

Answer 17

The energy released from forming new bonds is greater than the energy needed to break existing bonds

Question 18

What is the energy level like in a endothermic reaction?

Answer 18

The energy needed to break exisiting bonds is greater than the energy released from forming new bonds

Question 19

What do cells contain?

Answer 19

Contain chemicals which react to produce electricity

Question 20

What is the voltage produced by a cell dependent on?

Answer 20

The type of electrode and electrolyte

Question 21

What can a simple cell be made by?

Answer 21

Connecting two different metals in contact with electrolysis

Question 22

What are batteries?

Answer 22

Consist of two or more cells connected together in series to provide a greater voltage

Question 23

What happens in non-rechargable cells and batteries?

Answer 23

The chemical reactions stop when one of the reactants has been used up. Eg alkaline batteries

Question 24

What happens in rechargeable batteries?

Answer 24

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

Question 25

What are fuel cells?

Answer 25

They are supplied by an external source of fuel eg hydrogen and oxygen in air. the fuel is oxidised electromagnetically within the fuel cell to produce a potential difference

Question 26

What does the overall reaction in a hydrogen fuel cell involve?

Answer 26

Involves the oxidation of hydrogen to produce water

Question 27

What do hydrogen fuel cells do?

Answer 27

Offer a potential difference alternative to rechargeable cells and batteries

Question 28

What are advantages of hydrogen fuel cells?

Answer 28

• dont need to be electrically recharged
• no pollutant gases are produced
• can be a range of different sizes for different uses

Question 29

What are dis-advantages of hydrogen fuel cells?

Answer 29

• hydrogen is highly flammable
• hydrogen is produced for the cells by non renewable sources
• hydrogen is difficult to store

Question 30

What type of reaction is it when energy released from forming new bonds is greater than energy needed to break existing bonds?

Answer 30

Its an exothermic reaction

Question 30

Why type of reaction is it when energy needed to break exisiting bonds is greater than energy released from forming new bonds?

Answer 30

An endothermic reaction

Question 31

What is an electrolyte?

Answer 31

A liquid that contains ions which react with the electrodes

- the chemical reaction between the electrodes and electrolyte set up a charge difference between the electrodes

Question 32

What does it mean the bigger the difference in reactivity?

Answer 32

The bigger the voltage of the cell

Question 33

what is the electrolyte made of in a hydrogen-oxygen fuel cell?

Answer 33

often a solution of potassium hydroxide, the electrodes are often porous carbon with a catalyst

Question 34

what happens at the positive electrode in a hydrogen-oxygen fuel cell?

Answer 34

hydrogen loses electrons to produce H+ ions, this is oxidation
- the H+ ions in the electrolyte move to the cathode
= H2 -> 2h+ + 2e

Question 35

what happens at the negative electrode in a hydrogen-oxygen fuel cell?

Answer 35

oxygen gains electrons from the cathode and reacts with H+ ions to make water, this is reduction
- O2 + 4H+ + 4e -> 2H20

Question 36

what are the pros and cons of batteries?

Answer 36

PROS:
- batteries in electric vehicles are rechargeable but there’s a limit to how many times they can be recharged before replacing
CONS:
- they store less energy so would need to be recharged
- they’re more expensive than fuel cells
- they are polluting to dispose of as they are made from toxic metal compounds

Question 37

what are the pros and cons of fuel cells?

Answer 37

PROS:

- don’t produce as many pollutant