Topic 8 - Energetics

Question 1

What are the standard conditions of a reaction?

Answer 1

100kPa (1 atmosphere) and 298K (25*C)

Question 2

Define enthalpy change:

Answer 2

The heat energy change measured at constant pressure

Question 3

What’s a difference between an enthalpy level diagram and a reaction profile diagram?

Answer 3

Activation energy is not shown in enthalpy level diagrams but is shown in reaction profile diagrams

Question 4

Define enthalpy change of reaction:

Answer 4

This is the enthalpy change when substances react under standard conditions in quantities given by the equation for the reaction.

Question 5

Define enthalpy change of formation:

Answer 5

Enthalpy change when one mole of a substance is formed from its constituent elements with all substances in their standard states (100kPa and 298K)

Question 6

Define standard enthalpy change of combustion:

Answer 6

Enthalpy change when one mole of a substance undergoes complete combustion in oxygen with all substances in standard states (298K and 100kPa)

Question 7

Define enthalpy of neutralisation:

Answer 7

Enthalpy change when one mole of water is formed in a reaction by an alkali and acid under standard conditions (100kPa and 298K)

Question 8

What is the equation using SHC?

Answer 8

Energy transferred = mass x SHC x temperature change (Q=m x c x (change in) T)

Question 9

What are potential errors and some solutions that could occur in when mixing substances in an insulated container (calorimetry)?

Answer 9

• Heat loss to surroundings so the measured temperature change will be lower than expected
• Incomplete reaction/dissolution so if the reaction doesn’t go to completion, the heat change won’t reflect the full enthalpy change.
• Neglecting Heat absorbed by the container as the calorimeter (e.g. polystyrene cup) may absorb some heat.
• Assumption of constant specific heat capacity
• Use a lid and a well-insulated container to minimise heat loss
• Stir the mixture well to ensure even temperature distribution
• Use a temperature probe instead of a thermometer for more precise readings

Question 11

What are potential errors and some solutions that could occur in the enthalpy change of combustion (burning alcohol in a spirit burner to heat water)?

Answer 11

• Heat loss to surrounding
• Incomplete combustion due to limited oxygen
• Evaporation of alcohol before combustion begins which leads to an overestimation of fuel used
• Heat absorption by equipment (metal calorimeter and burner)
• Flame height variability due to drafts affecting heat transfer efficiency
• Use a draft shield to minimise heat loss
• Ensure sufficient oxygen supply to promote complete combustion
• Minimise alcohol evaporation by covering the burner when not in use
• Use a bomb calorimeter for more accurate measurements

Question 12

What are potential errors and some solutions that could occur in the indirect measurement of enthalpy changes (determining enthalpy change using Hess’ Law as direct measurement isn’t possible)?

Answer 12

• Heat loss in Intermediate steps
• Assumptions about reaction conditions as there might be variations in the assumed standard conditions of 298K and 100kPa
• Conduct experimentsn well insulated environments
• Use high-precision instruments (e.g., digital thermometers)

Question 13

What’s the unit used for enthalpy change?

Answer 13

kJ mol^-1