3.1.8 Thermodynamics Quick check
Exam questions
Lattice enthalpy of formation/dissociation
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Sizes of the ions. The larger the ions the weaker the lattice, as the charges are further apart and will have a weaker attractive force between them
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The charges on the ion. The bigger the charge on the ion the stronger the lattice, the greater the attraction between the ions
Perfect ionic model
Theoretical lattice enthalpies assumes a perfect ionic model where the ions are 100% ionic and spherical and the attractions are purely electrostatic/ionic.
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A reaction that has increasing entropy (+ve ∆S) and is exothermic (-ve ∆H ) will make ∆G be negative and will always be feasible
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If ∆G is negative there is still a possibility, however, that the reaction will not occur or will occur so slowly that effectively it doesn’t happen.
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If the reaction has a high activation energy the reaction will not occur.
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If a reaction involves a decrease in entropy (-ve ∆S) then increasing temperature will make it less likely that ∆G is negative and less likely for a reaction to occur.
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If a reaction involves an increase in entropy (+ve ∆S) then increasing temperature will make it more likely that ∆G is negative and more likely that a reaction will occur.
