What is Hess' law?

Hess' law states that the overall enthalpy change for a reaction is independent of the route taken.

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What enthalpy changes do the arrows A-D represent?

AEnthalpy of atomisation.

BIonisation energy.

CLattice enthalpy.

DEnthalpy of formation.

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What do the directions of the arrows tell you about the enthalpy changes they represent?

Arrows pointing up represent endothermic (positive) enthalpy changes.

Arrows pointing down represent exothermic (negative) enthalpy changes.


The total enthalpy change for the formation of the ionic lattice is the same for whichever route (red or blue) is taken.

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What does the vertical scale of a Born-Haber cycle represent?

Enthalpy (kJ mol-1)

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How does a larger difference between experimental and theoretical lattice enthalpies indicate the nature of bonding?

A larger difference suggests stronger polarisation and more covalent character in the ionic bonding.

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What accounts for the difference between lattice enthalpies calculated using Born-Haber cycles and those from calculations based a perfect ionic model?

The difference in lattice enthalpies is due to the presence of covalent character in certain ionic compounds.

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What is the effect of ion polarisation on the ionic bonding?

Ion polarisation leads to some covalent character in the ionic bonding due to the distortion of the anion's electron cloud.

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What is ion polarisation?

Ion polarisation is the distortion of an anion's electron cloud caused by the attraction from a cation's positive charge in an ionic lattice.

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How does a smaller difference between experimental and theoretical lattice enthalpies indicate the nature of bonding?

A smaller difference indicates a more purely ionic compound with minimal covalent character.

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