Topic 16 - Energetics
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[edit] 16.1 Standard enthalpy changes of reaction
[edit] 16.1.1
Standard state : 101 kPa, 298 K (or 1 atm, 25 degrees celcuis).
Standard enthalpy change of formation : The enthalpy change when 1 mol of a substance is made from its elements in their standard states. For example C(graphite) + 2H2(g) -> CH4(g). Molecules, like H2 are considered to be 'standard state'. Fractions of mols (i.e. fractions in coefficients), may also be used if necessary as 1 mol must be produced).
[edit] 16.1.2
If a reaction can be expressed in terms of changes of formation (and bond enthalpies as in SL) then add up all the ΔH values to get the ΔH for the reaction.
[edit] 16.2 Lattice enthalpy
[edit] 16.2.1
Lattice enthalpy : The enthalpy change when 1 mol of crystals (i.e. an ionic lattice) is formed from its component particles at an infinite distance apart.
M+(g) + X-(g) -> MX(s)
The value of lattice enthalpy is assumed to be positive for the separation of the lattice, and negative for the formation of the lattice.
[edit] 16.2.2
As above, lattice enthalpies just add another type of reaction to those which can be shown on the Born-Haber cycle.
[edit] 16.2.3
Lattice enthalpy increases with higher ionic charge and with smaller ionic radius (due to increased attraction).
[edit] 16.3 Entropy
[edit] 16.3.1
Factors which increase disorder in a system:
- Mixing of particles.
- Change of state to greater distance between particles (solid -> liquid or liquid -> gas).
- Increased particle movement (temperature).
- Increased number of particles (when more gas particles are produced, this generally outweighs all other factors).
[edit] 16.3.2
Predict the sign of ΔS (the change in entropy) for a reaction based on the above factors. ΔS is positive when entropy increases (more disorder) and negative when entropy decreases (less disorder).
[edit] 16.3.3
The standard entropy change can be calculated by subtracting the absolute entropy of the reactants from that of the products.
[edit] 16.4 Spontaneity of a reaction
[edit] 16.4.1
Reactions which release heat (and so increase stability) tend to occur as do reactions which increase entropy (ΔS is positive). Neither of these can be used to accurately predict spontaneity alone however.
[edit] 16.4.2
When ΔG is negative, the reaction is spontaneous, when it's positive, their reaction is not.
[edit] 16.4.3
ΔG = ΔH - Temperature(in kelvin) x ΔS
Spontaneity depends on ΔH, ΔS and the temperature at which the reaction takes place (or doesn't as the case may be).
[edit] 16.4.4
Substitute values into the equation above. Hopefully that's not too tricky.
[edit] 16.4.5
There are four possibilities:
- ΔG is always negative when ΔH is negative and ΔS is positive.
- ΔG is negative at high temperatures if ΔH is positive and ΔS is positive (i.e. an endothermic reaction is spontaneous when T x ΔS is greater than ΔH).
- ΔG is negative at lower temperatures if ΔH is negative and ΔS is negative (exothermic reactions are spontaneous if ΔH is bigger than T x ΔS).
- ΔG is never negative if ΔH is positive and ΔS negative.
