9.4 Hess' law and enthalpy cycles

Question 1

What does Hess’ law allow

Answer 1

Allows enthalpy changes to be determined indirectly

Question 2

What is Hess’ law

Answer 2

States that if a reaction can take place by two routes and the starting and finishing conditions are the same, the total enthalpy change is the same for each route

Question 3

Where does Hess’ law come from

Answer 3

The idea of the conservation of energy

Can easily be visualised with a diagram

Question 4

In what situations can the principle of Hess’ law be used

Answer 4

Any number of enthalpy changes provided that all enthalpy changes are known except for one, the unknown enthalpy change can always be determined

Question 5

What is the ΔfHΦ value for elements

Answer 5

ZERO

Question 6

Rules to use to calculate using enthalpy changes of formation and combustion:

1. Using enthalpy changes of formation ΔfH
2. Using enthalpy changes of combustion ΔcH

Answer 6

1. ΔrH = ΣΔfH products - ΣΔfH reactants

2. ΔrH = ΣΔcH reactants - ΣΔcH products