C3 Energy

Question 1

What are the units of energy?

Answer 1

Joules, J, or kilojoules, kJ. Calories are used for food, one calorie is 4.2 Joules.

Question 2

How would you measure the energy change from burning a fuel?

Answer 2

The energy released when fuels burn can be measured by simple calorimetry, e.g. by heating water in a glass or metal container.

Question 3

What equation would you use to find a value for the energy change in burning a fuel?

Answer 3

Q = mc ΔT

Energy change = mass x specific heat x temp change

Question 4

To what does m refer, in Q = mc ΔT?

Answer 4

m is the mass of the liquid being heated

Question 5

How would you measure the energy change during a neutralisation reaction between an acid and an alkali?

Answer 5

The energy change can be calculated from the measured temperature change of the solution when the reagents are mixed in an insulated container.

Question 6

Draw the simple energy level diagram for an exothermic reaction

Answer 6

Question 7

What is activation energy? (Have you shown it on the diagrams?)

Answer 7

The minimum amount of energy needed to start the reaction. It is shown as the distance between the energy of the reactants and the highest energy shown on the curve.

Question 8

Draw the simple energy level diagram for an endothermic reaction.

Answer 8

Question 9

What is needed to break bonds in a chemical reaction?

Answer 9

Energy must be supplied to break bonds, i.e. bond breaking is endothermic.

Question 10

What is released when bonds are formed in a chemical reaction?

Answer 10

Energy is released when bonds are formed, i.e. bond making is exothermic.

Question 11

Use the idea of bond breaking and bond making to explain an exothermic reaction.

Answer 11

In an exothermic reaction, the energy given out by making new bonds is greater than the energy needed to break existing bonds.

Question 12

Use the idea of bond breaking and bond making to explain an endothermic reaction.

Answer 12

In an endothermic reaction, the energy needed to break existing bonds is greater than the energy released from forming new bonds.

Question 13

Can you calculate the energy change, ΔH, for the reaction of hydrogen and chlorine;

Hydrogen + chlorine àhydrogen chloride.
H_2(g) + Cl_2(g) à 2HCl_(g)

given;

Bond

Bond Energy (in kJ per mole)

H—H

436

Cl—Cl

242

H—Cl

431

Answer 13

You must break;
1 mole of H—H bonds (+436) and

1 mole of Cl—Cl bonds (+242)
So energy needed is 436 + 242kJ = +678 kJ.

You must make;

2 moles of H—Cl bonds (-431)

So energy given out is 2 x 431 kJ = -862 kJ.

So energy change is +678 – 862 = -184 kJ mol^-1

Energy change is negative so it is exothermic.

Question 14

Explain how catalysts work.

Answer 14

Catalysts provide a different pathway for a chemical reaction that has a lower activation energy.

Question 15

Show how a catalyst works using an energy level diagram

Answer 15

Question 16

In modern cars hydrogen gas can be burned as a fuel. Name the reaction and provide the equation.

Answer 16

Combustion.

Hydrogen + oxygen → water

2H₂ + O₂ →2H₂O

Question 17

Why is hydrogen preferred to petrol?

Answer 17

The only waste product is water, whereas petrol produces carbon dioxide and water.

Question 18

Hydrogen can also be used in fuel cells, how does this compare to combustion?

Answer 18

The fuel cell produces electricity and drives an electric motor. This is cleaner than petrol combustion and the electric motor has fewer parts than an internal combustion engine.