3.1.4 Energetics

Question 1

What is Enthalpy change?

Answer 1

It is the energy change measured in a reaction under conditions of constant pressure

Question 2

What are standard conditions?

Answer 2

100 kPa (pressure) and 298K (temperature)

Question 3

Define Standard enthalpy of combustion

Answer 3

The enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions

Question 4

Define Standard enthalpy of formation

Answer 4

The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions

Question 5

How to calculate the heat change (q)?

Answer 5

q = mc ∆ T

m = mass
c = specific heat capacity
∆ T = temperature change

Question 6

Define Hess’ Law

Answer 6

The total enthalpy change of a reaction remains the same independent of the route taken

Question 7

Define Mean Bond Enthalpy

Answer 7

The average enthalpy change required to break one mole of a particular type of bond in a gaseous molecule, averaged over a range of different molecules

Question 8

Why are calculations from mean bond enthalpy values different from those determined from Hess’ Law?

Answer 8

This is because mean bond enthalpy values are average values taken from multiple compounds
Whereas Hess’ law determines enthalpy change specific to the actual substance