gibbs free energy
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What is entropy?
The measure of disorder of a system
Symbol for entropy change
ΔS
What affects the entropy of a system?
gases > liquids > solids greater Mr = greater entropy number of species, more species = greater entropy
How to calculate entropy change
ΔS = sum of entropy of products – sum of entropy of reactants
What does a positive (+) ΔS mean?
entropy has increased
How to calculate enthalpy change using ΔH formation data
ΔH = sum of ΔHf products – sum of ΔHf reactants
Gibb’s Free-energy equation
ΔG = ΔH - TΔS T must be in Kelvin (oC +273) ΔS must be in kJ K- mol-1 (/1000)
What does ΔG mean
if ΔG is zero or negative a reaction is feasible
Why do some reactions not occur even if feasible?
the activation energy is too high
Calculate the temperature at which a reaction becomes feasible
ΔG is zero when reaction becomes feasible 0 = ΔH - TΔS, therefore T = ΔH/ΔS T must be in Kelvin (oC +273) ΔS must be in kJ K- mol-1 (/1000)
Using a graph to calculate ΔG
ΔG = ΔH - TΔS ΔG = - ΔS T + ΔH y = -mx + c Plot ΔG vs. T, ΔH = intercept (c), -ΔS = gradient (m)
give the equation for temperature at which reaction just becomes feasible
T = ΔH/ΔS