17.9
Free Energy Changes for Nonstandard States
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Q1: What is the reaction quotient and how is it calculated?
The reaction quotient (Q) describes the relative amount of products and reactants in a reaction at any moment. For solutions, Q is calculated from the ratio of product to reactant concentrations, each raised to its stoichiometric coefficient. For gases, partial pressures replace concentrations. Q varies as the reaction proceeds toward equilibrium, serving as an indicator of the reaction's status and direction.
Q2: How does the free energy change equation relate standard and nonstandard conditions?
The free energy change under nonstandard conditions equals the standard free energy change plus RT times the natural logarithm of Q. At constant temperature, the standard free energy has a fixed value, but Q varies with reaction composition. This equation allows prediction of spontaneity under any given set of conditions, whether standard or nonstandard.
Q3: Why does ammonia synthesis show different spontaneity under different pressures?
Under standard conditions (1 atm for all gases), ammonia synthesis has ΔG = −32.8 kJ/mol, making it spontaneous. At nonstandard pressures (1.2 atm N₂, 3.6 atm H₂, 0.60 atm NH₃), ΔG = −45.3 kJ/mol, still spontaneous. When ammonia accumulates (4.8 atm), ΔG becomes positive (5.6 kJ/mol), favoring decomposition instead.
Q4: What does a positive free energy change indicate about reaction direction?
A positive free energy change indicates the reverse reaction is energetically favorable. When ΔG > 0, the forward reaction is nonspontaneous, and the system will shift backward toward reactants. This occurs when product concentrations are high relative to equilibrium, as seen when ammonia decomposes under high pressure conditions.
Q5: What is the relationship between Q and equilibrium?
At equilibrium, the free energy change equals zero, and Q reaches a constant value equal to the equilibrium constant. Before equilibrium, Q changes as reactant and product concentrations shift. When Q is initially zero (reactants only), it increases toward equilibrium; when Q is infinite (product only), it decreases toward equilibrium.
Q6: How do partial pressures affect the reaction quotient for gases?
For gaseous reactions, the reaction quotient is calculated using partial pressures instead of concentrations. Each gas's partial pressure is raised to its stoichiometric coefficient. Under standard conditions, all partial pressures equal 1 atm, making Q = 1. Nonstandard pressures produce different Q values, altering the free energy change and reaction spontaneity.
Q7: Why can't standard free energy changes predict spontaneity under all conditions?
Standard free energy changes only apply when reactants and products are at standard states (1 M concentration or 1 bar pressure). Most real reactions occur under nonstandard conditions with different concentrations and pressures. The complete free energy equation incorporating Q is required to predict spontaneity under actual reaction conditions.