8: Energetics I

Question 1

What are standard conditions?

Answer 1

100 kPa
298 K

Question 2

State what is meant by the enthalpy change?

Answer 2

The heat energy change measured at constant pressure

Question 3

Define standard enthalpy change of reaction

Answer 3

The enthalpy change when
- Substances react together
- In the mole quantities given in the equation
- Under standard conditions (100 kPa, 298 K)

Question 4

Define standard enthalpy change of formation

Answer 4

The enthalpy change when
- 1 mole of a substance is formed
- from its elements in their standard states
- Under standard conditions (100 kPa, 298 K)

Question 5

Define standard enthalpy change of combustion

Answer 5

The enthalpy change when
- 1 mole of a substance
- is burnt completely in excess oxygen
- Under standard conditions (100 kPa, 298 K)

Question 6

Define standard enthalpy change of neutralisation

Answer 6

The enthalpy change when
- 1 mole of water is formed
- from the reaction between an acid and an alkali
- Under standard conditions (100 kPa, 298 K)

Question 7

When measuring standard enthalpy change of combustion, what reasons could cause a difference in the experimental value and the theoretical value?

Answer 7

Heat losses to the surroundings
Incomplete combustion

Question 8

What assumptions are made when measuring enthalpy change?

Answer 8

• No energy is lost to the surroundings
• The density of a solution is 1.00 g cm^-3
• The SHC of a solution is 4.18

Question 9

State Hess’s Law.

Answer 9

The enthalpy change for a reaction is independent of the route taken.

Question 10

What is bond enthalpy?

Answer 10

The energy needed to break a bond
(The same as the energy released when the bond is formed)

Question 11

How do you calculate the enthalpy change of a reaction using the mean bond enthalpies?

Answer 11

Enthalpy change = bonds broken - bonds formed