9.3 bond enthalpies Flashcards

1
Q

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

A

High activation energy

Slow rate of reaction

More than one reaction occurring at the same time

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2
Q

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 …………

A

directly
indirectly

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3
Q

Hess’ Law

A

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

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4
Q

using Hess’ law for enthalpy changes of combustion

A

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)

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5
Q

Using hess’ law for enthalpy change of formation

A

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)

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