Topic 5 - Energy Changes

Question 1

What is an exothermic and endothermic reaction?

Answer 1

Exothermic - transfers energy to the surroundings (rise in temp)
Endothermic - takes in energy from the surroundings (fall in temp)

Question 2

What is combustion?

Answer 2

Burning fuels - exothermic reaction

Question 3

Give some examples of how exothermic reactions can have everyday uses.

Answer 3

• Hand warmers

- Self heating cans of coffee/hot chocolate

Question 4

Give an example of how endothermic reactions have everyday uses.

Answer 4

• Sport injury packs

Question 5

What are the reaction profiles for endo / exo thermic reactions?

Answer 5

Endo: Energy begins low in the reactants, rises for the products.
Exo: Energy begins high in the reactants, decreases for the products.

Question 6

How do you find the overall energy change in a reaction?

Answer 6

Energy required to break bonds - Energy required to form bonds

Question 7

What is an electrochemical cell?

Answer 7

Two electrodes are in contact with an electrolyte and produce a flow and voltage.

Question 8

What is the difference between rechargeable and non-rechargeable batteries?

Answer 8

Non-rechargeable - they’re irreversible so once a reactant is used up, they can no longer produce electricity.
Rechargeable - the reaction is reversible and can be reversed by connecting it to an external electric current.

Question 9

What is a fuel cell?

Answer 9

An electrical cell that’s supplied with a fuel and oxygen. It uses energy from the reaction between the two to produce electrical energy.

Question 10

What is a hydrogen-oxygen fuel cell?

Answer 10

The cell combines hydrogen and oxygen to produce water and release energy. Hydrogen begins at the anode and loses electrons to become a positive ion - oxidation. Oxygen begins at the cathode and gains electrons, it reacts with the H+ ions to produce water. The electrons flow through an external circuit to make an electric current.

Question 11

Why are fuel cells better than batteries for use in electric cars?

Answer 11

• less polluting to dispose of
• batteries have a limit to how many times they can be recharged ( they’ll need replacing )
• fuel cells are cheaper to make
• fuel cells store more energy

Question 12

What is the half equation for hydrogen-oxygen fuel cells at the anode?

Answer 12

H2 -> 2H+ + 2e-

Question 13

What is the half equation for hydrogen-oxygen fuel cells at the cathode?

Answer 13

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