9.3 bond enthalpies

Question 1

sometimes it is not possible to measure an enthalpy change directly. This
may be due to:

Answer 1

High activation energy

Slow rate of reaction

More than one reaction occurring at the same time

Question 2

Bond enthalpy (energy) calculations are an ideal way of …….. calculating the
enthalpy change for a given reaction.

Thankfully, Hess’ law allows us to work out enthalpy changes that are impossible to
measure – but can be calculated …………

Answer 2

directly
indirectly

Question 3

Hess’ Law

Answer 3

the enthalpy change for any reaction
is independent of the route taken
provided the initial and final
conditions remain the same.

Question 4

using Hess’ law for enthalpy changes of combustion

Answer 4

If the data in the data in the table is given as enthalpies of combustion then to checkyour Hess’ cycle use:

Combustion

ΔH = Σ ΔcHθ (reactants) - Σ ΔcHθ (products)

Question 5

Using hess’ law for enthalpy change of formation

Answer 5

If the data in the data in the table is given as enthalpies of formation then to check your Hess’ cycle use:

Formation

ΔH = Σ ΔfHθ (products) - Σ ΔfHθ (reactants)