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15.1.1:
Define and apply the terms standard
state, standard enthalpy change of
formation ( ∆Hf Ö ) and standard
enthalpy change of combustion
( ∆Hc Ö ) .
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He standard enthalpy change of formation (∆Hfѳ) is the
amount of energy evolved or absorbed in the formation
of one mole of the compound, in its standard state, from
its constituent elements in their standard states.
Formation: P-R Standard state refers to the form normally found at a temperature of 25 °C (298 K) and a pressure of 101.3 kPa (1 atmosphere pressure). The standard enthalpy change of combustion (∆Hѳcomb) is the enthalpy change when one mole of the compound undergoes complete combustion in excess oxygen under standard conditions. Combustion: R-P |
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15.2.1:
Define and apply the terms lattice
enthalpy and electron affinity.
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attice enthalpy is the energy required to convert
one mole of the solid compound into gaseous ions.
The electron affinity is the enthalpy change when one
mole of gaseous atoms or anions gains electrons to form a
mole of negatively charged gaseous ions.
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15.2.2:
Explain how the relative sizes
and the charges of ions affect the lattice enthalpies of different ionic compounds. |
The relative value of the theoretical lattice enthalpy
increases with higher ionic charge and smaller ionic
radius due to increased attractive forces.
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15.2.3:
Construct a Born–Haber cycle for
group 1 and 2 oxides and chlorides,
and use it to calculate an enthalpy
change.
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 For example: NaCl Born-Haber cycle: The formation of an ionic compound can be considered as the sum of a number of individual process – converting the elements from their standard states into gaseous atoms, losing and gaining electrons to form the cations and anions and finally the coming together of these ions into the solid compound. The diagrammatic representation of this is known as the Haber cycle. he standard enthalpy change of atomisation is the enthalpy change required to produce one mole of gaseous atoms of an element from the element in the standard state. |
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15.2.4:
Discuss the difference between
theoretical and experimental lattice
enthalpy values of ionic compounds
in terms of their covalent character.
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A significant difference between the two values
indicates covalent character.
The Born–Haber cycle provides a way in which lattice enthalpies can be indirectly measured through experimental techniques (an empirical value). It is also possible to calculate theoretical lattice enthalpies for ionic compounds. This is interpreted as evidence for a significant degree of covalent character in the bonding of such compounds (difference in electronegativities < ~1.7). The presence of covalent character in a bond always leads to an increase in the lattice enthalpy. |
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15.3.1:
State and explain the factors that
increase the entropy in a system.
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Entropy: The disorder of a system is measured as its entropy. Scattered marbles have a higher entropy
than gathered marbles.
Entropy change: The change in disorder of a system. When going from more moles of gas to less
moles there is a decrease in entropy, and vice versa.
Increase Entropy: Increase VolumeDecrease PressureSolid -> Liquid -> GasIncrease TemperatureIncrease Number of Particles / Moles Decrease Entropy: Decrease VolumeIncrease PressureGas -> Liquid - > Solid more compactDecrease tempDecrease Number of Particles / Moles |
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15.3.2:
Predict whether the entropy change
(ΔS) for a given reaction or process is
positive or negative.
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The change in entropy, ΔS = Sp – SR. For a reaction, ΔSo = Σ Soproduct – Σ Soreactants. By definition, ΔS, the entropy change is positive for increasing disorder.
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15.3.3:
Calculate the standard entropy
change for a reaction ( ∆S Ö ) using
standard entropy values (SÖ ) .
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Gibb’s free energy: Given by the formula ∆G = ∆H − T∆S . A measure of the spontaneouty of the
reaction. If ∆G is negative, the reaction is spontaneous.
ΔG is a measure of the driving force of a reaction, i.e., its tendency to proceed spontaneously. A reaction or physical process can occur spontaneously if ΔG is negative. For a non-spontaneous reaction (or spontaneous in the reverse direction) ΔG is positive and for an equilibrium process ΔG = 0. |
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15.4.1:
Predict whether a reaction or process
will be spontaneous by using the sign
of ∆G Ö .
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Positive = non sponNegative = Spon
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15.4.3:
Predict the effect of a change in
temperature on the spontaneity of a
reaction using standard entropy and
enthalpy changes and the equation
∆GÖ=∆HÖ−T∆SÖ .
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 See image. |
