Lesson 5
Gibbs free energy (∆G)
Enthalpy change (∆H)
Entropy change (∆S)
the heat released or absorbed from a system
the reaction with heat released from a system to surrounding is exothermic reaction
As the heat is giving out from a system, the temperature of the system will decrease
the reaction with heat absorbed from a system to surrounding is endothermic reaction
As the heat is taking in from surrounding, the temperature of the system will increase
As the heat energy of the system decreases, ∆H is in negative value
As the heat energy of the system increases, ∆H is in positive value
the degree of disorder in a system
∆S is in negative value if there is an decrease in disorder
∆S is in positive value if there is an increase in disorder
the factors that affect the ∆S
The degree of disorder, the highest entropy to lowest entropy is
gas,
aqueous,
liquid then
solid
the number of mole of the molecules
For example,
2SO3(g) -> O2(g) + 2SO2(g)
2 mole of gas to 3 mole of gas
hence, the ∆S is positive
For example,
2Na(s) + 2H20(l) -> 2NaOH(aq) + H2(g)
2 mole of solid and 2 mole of liquid to 2 mole of aqueous and 1 mole of gas
hence, the ∆S is positive
The Gibbs free energy of a system is defined as the enthalpy change of the system minus the product of the temperature and the entropy change of the system.
∆G=∆H-T∆S
∆S is entropy change
unit is J/(mol*K)
∆H is the enthalpy change
unit is kJ/mol
T is temperature
unit is K
∆G is the Gibbs free energy
unit is kJ/mol
∆G = negative value
the reaction occurs spontaneously
∆G = positive value
the reaction occurs non-spontaneously
negative ∆H, negative ∆S
react spontaneously at low temperature
negative ∆H, positive ∆S
react spontaneously at all temperature
positive ∆H, positive ∆S
react spontaneously at high temperature
positive ∆H, negative ∆S
react non-spontaneously at all
∆G (+ve) = ∆H(+ve) - T∆S(-ve)
∆G (-ve) = ∆H(-ve) - T∆S(-ve)