9.3 bond enthalpies Flashcards
sometimes it is not possible to measure an enthalpy change directly. This
may be due to:
High activation energy
Slow rate of reaction
More than one reaction occurring at the same time
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 …………
directly
indirectly
Hess’ Law
the enthalpy change for any reaction
is independent of the route taken
provided the initial and final
conditions remain the same.
using Hess’ law for enthalpy changes of combustion
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)
Using hess’ law for enthalpy change of formation
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)