Driving Forces for Reactions

Reaction Energy
and Reaction
Driving Force of
1. Explain the relationship between enthalpy change
and the tendency of a reaction to occur.
2. Explain the relationship between entropy and the
tendency of a reaction to occur.
3. Define free energy and explain how the quantity is
calculated and interpreted.
4. Describe the use of free energy change to determine
the tendency of a reaction to occur.
Enthalpy and Reaction Tendency
• A majority of the reactions that occur in nature are
• Reactions tend to proceed in a direction of a lower
energy state.
• However, some endothermic reactions occur
Entropy and Reaction Tendency
• Melting is an endothermic process that happens
2 NH4NO3 (s)  2 N2(g) + 4 H2O(l) + O2(g)
• Processes tend to occur in nature in a direction of
increasing disorder.
• Entropy(S) – measure of the degree of randomness of
the particles of a system.
• In general, entropy increases going from solid to
liquid to gas.
• Solution formation is also results in an increase in
• At absolute zero, the particles of a solid have an
entropy of 0 kJ/mol*K.
• Entropy increases with temperature.
• Change in entropy (ΔS) = entropy of the products entropy of the reactants
• Increase in entropy (+ΔS)
• Decrease in entropy (-ΔS)
Free Energy
• Processes occur in nature in the direction of
decreasing enthalpy and/or increasing entropy.
Free energy (G) – a function combining the entropy
and enthalpy of a system
Free energy change (ΔG) – Difference between the
change in enthalpy and the product of the Kelvin
temperature and entropy change.
ΔGo = ΔHo - TΔSo
Free Energy
• If ΔG is negative, a reaction can occur spontaneously.
• If ΔG is positive, a reaction will not occur naturally.
C2H4(g)+ H2(g)  C2H6(g)
ΔHo= -136.9 kJ/mol
ΔSo = -0.1207 kJ/mol*K
• At room temperature, ΔG is negative  spontaneous
Free Energy
CH4(g)+ H2O(g)  CO(g) + 3 H2(g)
ΔHo= +206.1 kJ/mol
ΔSo = +0.215 kJ/mol*K
• At room temp. (298K), ΔG is positive. The reaction
will not occur.
1. Will the following reaction be spontaneous at 298
Cu2S(s) + S(s)  2 CuS(s)
ΔHo= -26.7 kJ/mol
ΔSo = -19.7 J/(mol*K)
2. For the vaporization of Bromine, ΔHo = 31.0 kJ/mol
and ΔSo = 93.0 J/(mol*K). At what temperature will
this process be spontaneous?

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