Gibbs and the Law of Entropy
• In chemical reactions, reactants and products will
always try to reach their lowest energy state. This
low energy state relates to the chemical's
minimum potential energy.
– Free energy (DG) is the measure of the energy stored
in a substance and is representative of the work that
can be done by that substance.
– The enthalpy (DH) is the numerical representation of
the potential energy of the of the products compared
to the potential energy of the reactants.
Enthalpy
Enthalpy
• DH for an endothermic reaction is positive
• DH for an exothermic reaction is negative
– After the activation energy has been generated, most
spontaneous reactions are exothermic. The energy
required to move the reaction forward is provided by the
formation of the new bonds.
– reactions that generate a large amount of heat are always
spontaneous
• C3H8(g) + 5O2(g) g 3CO2
(g)
• H2O(s) g
DH = +6.01kJ
+ 4H2O(g)
DH = -2043kJ
– Not ALL spontaneous reactions are exothermic
H2O (l)
Entropy
• In spontaneous reactions there is a tendency for things to
become less ordered. This increasing amount of disorder is
referred to as Entropy.
– entropy is the quantitative measure of disorder (randomness) in a
system.
– entropy is given the symbol S
• DS = Sproducts - Sreactants
– when Sproducts > Sreactants the entropy is positive and the amount of
disorder increases in the reaction as the products are more
disordered than the reactants
– Melting ice makes a perfect example of entropy. As ice the individual
molecules are fixed and ordered. As ice melts the molecules become
free to move therefore becoming disordered. As the water is then
heated to become gas, the molecules are then free to move
independently through space.
Examples of Entropy
• Here are some examples of reactions that would have positive entropy.
– Formation of gases
• CO2(s) g CO2(g)
• H2O(l) g H2O(g)
– Formation of ions in solution
• NaCl(s) g Na+(aq) + Cl-(aq)
– More molecules of product than reactants
• 2NH3(g) g N2(g) + 3H2(g)
• NH4Cl(s) g NH3(g) + 3HCl(g)
– The temperature of a substance is increased.
• H2O(l, 25oC) g H2O(l, 50oC)
• The second law of thermodynamics is that ALL things lead to entropy.
What this means is that with all spontaneous reactions, entropy increases.
– Within you, your community, country, world, solar, system, and universe
things are speeding toward increasing disorder (absolute chaos!).
– It requires a constant input of energy to counteract Entropy.
Gibbs Free Energy
• Because not all exothermic reactions are spontaneous, an American
mathematician named J. Willard Gibbs related enthalpy and
entropy to determine whether reactions would be spontaneous or
not. The information was related in the Gibbs Free Energy equation.
– DG = DH - TDS
• If DG is negative
– the reaction is spontaneous (favors the formation of products)
– energy is released
• If DG is positive
– the reaction is not spontaneous and requires additional energy to
proceed
– energy is absorbed
• If DG is zero, the reaction is at equilibrium