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.

EnthalpyEnthalpy• 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) + 4H2O(g) DH = -2043kJ

– Not ALL spontaneous reactions are exothermic • H2O(s) g H2O (l) DH = +6.01kJ

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