The Factors that Determine the Equilibrium State

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1 The Factors that Determine the Equilibrium State The equilibrium state (or the ratio of products to reactants) is determined by two factors: 1. Energy Systems tend to move toward a state of minimum potential energy (enthalpy) to create products that are more stable. Therefore enthalpy changes favour the exothermic direction of a reaction since these involve products with lower potential energy. A + B C + D + kinetic energy 2. Degree of Randomness Systems tend to move spontaneously toward a state of maximum randomness or disorder; toward maximum entropy. This means that the entropy state favours the side of the reaction which produces more particles since this creates greater disorder. In addition, the states of matter have different degrees of disorder: Most disorder High entropy Least disorder Low entropy Reactions that produce the most gas molecules will be favoured. AB (s) A (g) + B (g) 3. The Compromise The composition of the equilibrium state is a compromise between these two factors: i) minimum potential energy (enthalpy) and ii) maximum randomness (entropy). If both factors favour the products, reactions are usually considered spontaneous. If both factors favour reactants, they are considered non-spontaneous. If these factors oppose each other, a mixture of products and reactants can exist at equilibrium. At very low temperatures, enthalpy tends to be the more important factor. Under such conditions, equilibrium favors the exothermic direction. At very high temperatures, randomness becomes more important. Under such conditions, equilibrium favors the direction that produces the most random distribution of particles without regard to enthalpy.

2 Consider these reactions: 1) H2O2 (l) H2O(l) + O2(g) H = kj/mol _ 2) N2 (g) + 2 O2 (g) kj 2 NO2 (g) _ 3) NaCl (s) NaCl (aq) H = kj/mol _ 4) CO (g) + 2 H2 (g) CH3OH (g) kj 5) CO (g) + ½ O2 (g) CO (g) H = kj/mol 6) H2SO4 (aq) H2SO4 (l) H = kj/mol 7) Pb 2+ (aq) + 2I - (aq) PbI2 (s) H = -585 kj/mol 8) C3H8 (l) kj C3H8 (g) Lower Enthalpy Favours: 9) CS2 (g) + 3 O2 (g) CO2 (g) + 2 SO2 (g) kj Lower Enthalpy Favours:

3 The Factors Affecting Equilibrium The composition of the equilibrium state (i.e. the proportion of reactants and products) is determined by: 1) The Tendency Towards Minimum Potential Energy (Enthalpy or H) H = H products - H reactants H < 0: The forward reaction is exothermic and favoured. H > 0: The forward reaction is endothermic and the reverse reaction is favoured. Most reactions we study are exothermic because these tend to be spontaneous. 2) The Tendency Towards Maximum Randomness (Entropy or S) S = S products - S reactants S > 0: Products are more random and the forward reaction is favoured. S < 0: Products are less random and the reverse reaction is favoured. Reactions favour the direction that produces maximum randomness. At low temperatures, enthalpy change has the greatest influence and exothermic reactions are generally spontaneous. At high temperatures, the random motion of molecules is increased and the entropy factor ( S) has more influence on the equilibrium state.

4 Gibb s Free Energy Change - G Both enthalpy and entropy factors are incorporated into Gibbs Free Energy Change. This concept allows us to predict whether the forward or reverse reaction is favoured. Reactions that favour the products are considered spontaneous. G H T S If G < 0 ( ) If G > 0 ( + ) If G = 0 The forward reaction is spontaneous. The forward reaction is not spontaneous. Equilibrium is reached. Sample Problem: e.g. Calculate the normal boiling point of mercury at kpa. Hg (l) Hg (g) H = kj/mol S = kj/k * At the boiling point, the two phases are at equilibrium ( G = 0)

5 The Factors Affecting Equilibrium 1. For each of these reactions, state whether the entropy increases or decreases in the forward direction: a) 2 H 2 (g) + O 2 (g) 2 H 2 O (g) b) 2 SO 3 (g) 2 SO 2 (g) + O 2 (g) c) MgCO 3 (s) + 2 H 3 O + (aq) Mg 2+ (aq) + 3 H 2 O (l) + CO 2 (g) d) Ag + (aq) + Cl - (aq) AgCl (s) e) Cl 2 (g) 2 Cl (g) f) H 2 O (l) H 2 O (g) g) Mg (s) + 2 H 3 O + (aq) Mg 2+ (aq) + H 2 (g) + 2 H 2 O (l) h) 2 C 2 H 2 (g) + 5 O 2 (g) 4 CO 2 (g) + 2 H 2 O (g) i) NH 3 (g) + HCl (g) NH 4 Cl (s) 2. For each reaction, calculate the value of Gibb s Free Energy ( G) and predict whether the reaction is spontaneous or not spontaneous in the forward direction. H (kj) T S (kj) a) H 2 O (l) H 2 O (g) b) 2 Fe(s) + ½ N 2 Fe 2 N (s) c) C 6 H 14 (g) 6 C(s) + 7 H 2 (g) d) HCl (g) + H 2 O (l) H 3 O + (aq) + Cl - (aq) For each of the following, calculate the value of G and then predict whether there will be more reactants or products at equilibrium at room temperature (298 K). H S a) N 2 (g) + 3H 2 (g) 2 NH 3 (g) kj kj/k b) H 2 O 2 (g) H 2 O (g) + ½ O 2 (g) -106 kj kj/k c) N 2 (g) + 3 F 2 (g) 2 NF 3 (g) -249 kj kj/k d) N 2 (g) + 3 Cl 2 (g) 2 NCl 3 (g) +460 kj kj/k e) N 2 F 4 (g) 2 NF 2 (g) +85 kj kj/k f) 2 H 2 O (l) 2 H 2 (g) + O 2 (g) +572 kj kj/k

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