Summary Chemical energetics and entropy CIE a level chemistry study notes

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Ch 23: Chemical Energetics &
Entropy

💡 Keywords: Enthalpy change of atomisation, lattice energy, solution,
hydration, electron affinity, Hess law, entropy, Gibb’s free energy,
feasibility



Formulas
ΔHsol ꝋ = −ΔH
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latt ꝋ + ΔH
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hyd ꝋ
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ΔSsystem ꝋ = ΣS
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products ꝋ − ΣS
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reactants ꝋ
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ꝋ ꝋ ꝋ
ΔG = ΔH − T ΔS
reaction system
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Enthalpy change
What is enthalpy change of atomisation?
Enthalpy change when 1 mole of gaseous atom is formed from its elements
under standard conditions. (its elements is its original form)
Endothermic (Positive enthalpy)




Ch 23: Chemical Energetics & Entropy 1

, H2 (g) − − > 2H (g)
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What is enthalpy change of lattice energy?
Enthalpy change when 1 mole of an ionic compound is formed from its
gaseous ions under standard conditions.
Exothermic (Negative, high enthalpy)

+ −
Na (g) + C l (g) − − > N aCl(s)



What are trends in lattice energy & how it it be determined?
The more negative the value, the more stable its ionic compound over
gaseous ion.
—> Need many experiments (requires energy cycle)

What is first electron affinity & successive affinities?
>First: Enthalpy change when 1 mole of electrons is added to 1 mole of
gaseous atoms to form 1 mole of gaseous ions each with a single negative
charge under standard conditions.
Usually exothermic (usually negative): stronger attraction, higher
exothermic

>Second, third: Formation with 2- and 3- charges, usually endothermic
Why? incoming electron added to negative ion, energy required to
overcome repulsive forces




What are factors affecting electron affinity?

1. Nuclear charge: The greater the nuclear charge, the stronger the
attractive forces between incoming electron and nucleus



Ch 23: Chemical Energetics & Entropy 2