Representing thermochemical cycles

Question 1

What is the difference between an enthalpy diagram and a thermochemical cycles?

Answer 1

Enthalpy diagrams show the enthalpy changes within the chemical reaction, they show the enthalpy of the products and reactants.

Question 2

What is the enthalpy of all elements in their standard states?

Answer 2

it is taken as zero

Question 3

What are the steps taken to form a thermochemical cycle?

Answer 3

1) write the equation for the reaction
2) write down the elements in the two compounds with the correct quantaties in each
3) put the Δ_fH^θ values with arrows showing the direction - from elements to compounds
4) Put in the arrows to go from the starting materials to products via the elements.
5) Reverse the sign of Δ_fH^θ if the arrow is in the opposite direction.
6) Add up the values of ΔH^θ

Question 4

What are the steps taken to draw an enthalpy diagram?

Answer 4

1. Draw a line at level 0 to represent the elements.
2. Look up the values of Δ_fH^θ for each compound and enter these on the enthalpy diagrams, taking account of the signs, negative values are below 0.
3. Find the difference in levels between the two compounds, this represents the difference in their enthalpies.
4. ΔH^θ is the difference in levels taking account of the direction of change.