Topic 8: Energetics

Question 1

Standard conditions

Answer 1

• 100 kPa

* 298 K

Question 2

Enthalpy change

Answer 2

•Heat energy change measured at a constant pressure

Question 3

Standard enthalpy change of reaction

Answer 3

• Enthalpy change at 100kPa & 298K

* When the number of moles of substances in the equation (as written) react

Question 4

Standard enthalpy change of combustion

Answer 4

• Enthalpy change when one mole of a substance is completely burned in oxygen
• 100kPa & 298K

Question 5

Standard enthalpy change of neutralisation

Answer 5

• Enthalpy change when one mole of water is formed when an acid and an alkali react
• 100kPa & 298K

Question 6

How to measure the enthalpy change of neutralisation

Answer 6

• React acid and alkali in a polystyrene cup in a beaker
• Make sure the temperature of the acid and alkali are the same
• Measure the change in temperature

Question 7

Standard enthalpy change of formation

Answer 7

• Enthalpy change when one mole of a substance is formed from its elements
• 100kPa & 298K

Question 8

Hess’s law

Answer 8

Enthalpy change of a reaction is independent of the path taken

Question 9

Bond enthalpy

Answer 9

•enthalpy change when one mole of a bond in the gaseous state is broken
(Cl2(g) -> 2Cl (g))

Question 10

Mean bond enthalpy

Answer 10

•Enthalpy change when one mole of a bond is broken in a gaseous state over many different molecules

Question 11

Experiment to measure enthalpy change of combustion

Answer 11

1.Weigh the liquid in the burner (alcohol)
2.Light a spirit burner under a beaker containing water
3.Measure the initial temp
4.When the temperature of the water has increased by 20 C extinguish the flame
5.Measure the final temperature
6.Enthalpy change can not be measured directly
(Q=mcΔT)

Question 12

The equation for enthalpy change

Answer 12

(Q=mcΔT)