Energy Transfer in Reactions

Question 1

What is an exothermic reaction?

Answer 1

A reaction that releases energy to the surroundings, usually as heat. It causes the temperature to rise.
Example: Combustion.

Question 2

What is an endothermic reaction?

Answer 2

A reaction that absorbs energy from the surroundings, usually as heat. It causes the temperature to fall.

Question 3

What is enthalpy change (ΔH)?

Answer 3

• The overall energy change in a reaction
• Units: kJ/mol
• Negative ΔH = exothermic
• Positive ΔH = endothermic

Question 4

What does a reaction profile show?

Answer 4

It shows how the energy of reactants and products changes during a reaction.

Question 5

What does an exothermic reaction profile look like?

Answer 5

• Products are at a lower energy level than reactants
• ΔH is negative
• A peak shows activation energy (minimum energy to start the reaction)

Question 6

What does an endothermic reaction profile look like?

Answer 6

• Products are at a higher energy level than reactants
• ΔH is positive
• Activation energy is also shown as a peak

Question 7

What is calorimetry used for?

Answer 7

To measure energy transferred during chemical reactions.

Question 8

How can you measure enthalpy change in dissolving, displacement, or neutralisation reactions?

Answer 8

• Use a polystyrene cup with a lid
• Take initial temps of both reactants (ensure they’re the same)
• Mix them in the cup
• Record temperature at regular intervals
• Use maximum temperature change to calculate energy transfer

Question 9

How can heat loss be reduced in calorimetry experiments?

Answer 9

• Use a lid
• Place the polystyrene cup inside a beaker with cotton wool for insulation

Question 11

How do you measure enthalpy change in a combustion reaction?

Answer 11

1. Put 50 g water in a copper can and record its temperature
2. Weigh spirit burner
3. Place burner under can and light the wick
4. Stir water until temp reaches ~50 °C
5. Extinguish flame, record final water temp
6. Reweigh burner and calculate fuel used

Question 12

Why is copper used in combustion calorimetry?

Answer 12

It conducts heat very well, helping heat transfer to water.

Question 13

How do you minimise heat loss in combustion calorimetry?

Answer 13

Use a draught excluder and avoid doing the experiment near open windows.