C7: Energy Changes

Question 1

What is an exothermic reaction and give an example

Answer 1

e.g. wood burning (combustion), certain oxidation reactions, neutralisation, hand warmers, self heating cans.
A reaction in which energy is transferred to the surroundings, meaning their temperature increases.

Question 2

What is an endothermic reaction and give an example

Answer 2

e.g. thermal decomposition, cold packs
A reaction in which energy is taken in from the surroundings, causing the temperature to drop.

Question 3

What is the energy profile of an exothermic reaction?

Answer 3

Products have less energy than reactants, as energy has been transferred to surroundings

Question 4

What is the energy profile of an endothermic reaction.

Answer 4

Products have more energy than the reactants, as energy has been taken in. Difference in energy tells us that energy has been taken in by the reaction.

Question 5

What is activation energy?

Answer 5

Reactions can only occur if particles collide with each other with sufficient energy and frequency (collision theory).
The minimum amount of energy particles need to react is called the activation energy.
On energy profiles, it’s the distance between the reactant energy and the peak.

Question 6

Bond breaking is…

Answer 6

Endothermic

Question 7

Bond making is…

Answer 7

Exothermic

Question 8

How can you determine whether a reaction is endothermic or exothermic using bond energies?

Answer 8

Negative value = exothermic
Positive value = endothermic

Question 9

What are cells?

Answer 9

If we take 2 different metals and place them in an electrolyte (a solution that can conduct electricity e.g. an ionic compound) then we can produce electricity. If both metals are connected to a voltmeter, a p.d. will be shown and a current will flow through the wire. Chemical reactions take place between the electrolyte and metals, and this setup is called a cell.

Question 10

What are the properties of cells?

Answer 10

Can only produce electricity for a certain amount of time. Eventually the reactants run out and the reaction stops.
Cells only produce electricity if metals with different reactivities are used.

Question 11

How does the reactivity of metals used influence the cell?

Answer 11

The bigger the difference in reactivity between the metals, the bigger p.d. is produced.
(Electrolyte also affects p.d.)

Question 12

What is a battery?

Answer 12

2 or more cells connected in series, producing a greater voltage.

Question 13

What is a rechargeable cell/ battery?

Answer 13

In alkaline batteries, the reactants run out at some point - this reaction is non-reversible, so the battery is non-rechargeable.
Rechargeable batteries can be recharged because the chemical reactions can be reversed when an electrical current is supplied.

Question 14

What is a fuel cell?

Answer 14

Reacting a fuel (such as hydrogen) with a reactant (such as oxygen or air). This produces an electric current.

Question 15

What are the waste products of hydrogen fuel cells?

Answer 15

Water

Question 16

What is the reaction at the negative electrode of a fuel cell?

Answer 16

2H₂ → 4H+ + 4e-

Question 17

What is the reaction at the positive electrode of a fuel cell?

Answer 17

O₂ + 4H+ + 4e- → 2H₂O

Question 18

What is the overall equation for the hydrogen fuel cell?

Answer 18

2H₂ + O₂ → H₂O
Hydrogen is oxidised

Question 19

What are the advantages of using hydrogen fuel cells?

Answer 19

Will provide electricity for as long as you provide hydrogen, whereas rechargeable batteries will run out and need to be recharged.
Do not get the less efficient the more they run, whereas rechargeable batteries store less electricity the longer they run, and eventually need to be replaced.
They can be a source of drinkable water e.g. in spacecraft

Question 20

What are the disadvantages of using hydrogen fuel cells?

Answer 20

Hydrogen is highly flammable and is difficult to store.
Do not produce a large p.d. so several are needed together.