Hess’s law and Hess cycles

Question 1

Hess’s law states that…

Answer 1

If a reaction can be carried out by 2 different pathways then the total enthalpy change for these 2 pathways are equal

Question 2

Why is Hess’s law useful?

Answer 2

If we cannot measure the enthalpy change for a reaction directly due to not taking place in standard conditions we can measure enthalpy change of alternative pathway

Question 3

What are these questions usually focused around?

Answer 3

Finding standard enthalpy change of reaction using standard enthalpy changes of formation
Finding standard enthalpy change of formation using standard enthalpy changes of combustion

Question 4

Common Hess cycle questions

Answer 4

Use enthalpy change value of combustion/formation to find enthalpy change of a reaction too hard to directly find enthalpy change of

Question 5

Using Enthalpy change of formation…

Answer 5

Write constituent elements in standard states, both arrows point upwards to either side of reaction
Enthalpy change = enthalpy change 2-enthalpy change 1

Question 6

Using enthalpy change of combustion

Answer 6

Write the products of combustion (water and carbon dioxide)
Both arrows point downwards to the products
Enthalpy change = enthalpy change 1-enthalpy change 2

Question 7

Finding enthalpy change of hydrating a salt by adding the exact volume of water needed to a salt?

Answer 7

Impossible because we don’t know this exact value
So construct a Hess cycle which points down to aqueos salt in solution