DF2 - How much energy?

Question 1

In enthalpy change of combustion, do the arrows point towards or away from the reactants + products (up or down)?

Answer 1

The arrows point down (away from the products + reactants in the top line)

Question 2

What is always at the bottom of the cycle (triangle) in enthalpy change of combustion?

Answer 2

CO2 + H2O

Question 3

In enthalpy change of formation, do the arrows point towards or away from the reactants + products (up or down)?

Answer 3

The arrows point up (towards products + reactants in the top line)

Question 4

What is always at the bottom of the cycle (triangle) in enthalpy change of formation?

Answer 4

The elements that make up reactants/products in their standard states (include state symbols + diatomic molecules)

Question 5

Define Hess’s Law

Answer 5

Enthalpy change of a process is independent from a pathway taken assuming initial/end conditions are the same

Question 6

What happens to the sign when you go against the arrow?

Answer 6

You change the sign e.g + to -

If you go the same way as the arrow is pointing the sign stays the same

Question 7

In the calculation for enthalpy change of formation, what happens if there are the same number of moles of e.g O2 in the reactants and elements at standard states?

Answer 7

You ignore it and do not include it in the calculation

Question 8

What do you need to remember when calculating enthalpy change of formation/combustion?

Answer 8

• Multiply the enthalpy change by the no. of moles of the compound for all reactants and products in the top line of the enthalpy cycle
  (unless e.g there are same moles of O2 in reactants as elements in standard states in which case you ignore)
• make sure your answer is in kJ/mol (e.g using 2 moles to make something you need to remember to divide the final answer by 2)