20:Enthalpy Pt.2

Question 1

What is Hess’ law

Answer 1

The enthalpy change in a reaction is independent of the route taken by the reaction, provided that the initial and final conditions are the same

Question 2

What is bond enthalpy

Answer 2

The enthalpy change when 1 mole of bonds are broken, in molecules in the gaseous state

Question 3

what is average bond enthalpy

Answer 3

bond enthalpy value taken from a range of different compounds

Question 4

How to calculate ΔH using bond enthalpies

Answer 4

1. Write the equation out with displayed formula to show all the bonds
2. Add up the enthalpies of all the bonds that must be broken (left hand side) and the enthalpies of all the bonds that are formed (right hand side).
3. Calculate: ΔH =+(enthalpy of bonds broken) -(enthalpy of bonds formed)

Question 5

Why might ΔH values calculated from average bond enthalpies not be accurate

Answer 5

• Bond enthalpies are average values so they may not be exactly correct for the actual bonds in a particular molecule
• Bond enthalpies are not determined under standard conditions, so they can only give approximate predictions
• Bond enthalpies can only predict gas phase enthalpy changes so if any reactants or products are not gases there will be additional contributions to ΔH