Module 3.2- Hess' Law

Question 1

What is Hess’ law?

Answer 1

If a reaction can take place by more than one route, the enthalpy change for the reaction is the same, irrespective of the route taken

Question 2

How do you represent two equal paths of enthalpy change??

Answer 2

🔼H1= 🔼H2 + 🔼H3

🔼H1 is a direct route, and the other way is an indirect route with two stages

Question 3

What do you include in a enthalpy change of combustion cycle??

Answer 3

The original reaction you are told goes at the top, and is labelled as 🔼H1.

The reactants and products have arrows pointing down, to the products and combustion, which is CO2 and H2O

You need to balance the carbons and hydrogen’s, and can ignore the oxygens.

Use the two different pathways to work out what you need

Question 4

What is standard enthalpy of formation??

Answer 4

Looks at the formation of a compound from its elements, and also from a different compounds reaction

Question 5

How do you draw and work out a enthalpy of formation cycle

Answer 5

You’re given enthalpies of formation values, so arrows point up because you’re making one mole of a substance. ( if you make 2 moles or more, you multiply the enthalpies by the number of moles)

You write out the equation you’re given and label it as 🔼H1, and then you have arrows pointing up from the elements involved to both products and reactants, labelled as 🔼H2 and 🔼H3

If an element involved in formation remains unchanged eg O2 remains as O2 in both products and reactants, you ignore it