Delta G equals delta G standard plus RT ln Q gives direction under nonstandard conditions.
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The Gibbs energy change under nonstandard conditions is given by delta G equals delta G standard plus R T ln Q, where Q is the reaction quotient evaluated with actual concentrations or pressures. At equilibrium Q equals K and delta G is zero, while away from equilibrium the sign of delta G shows the direction in which the reaction will proceed. If Q is less than K, delta G is negative and the reaction proceeds forward, forming more products. This relation explains how concentrations and pressures can reverse the spontaneity suggested by standard values alone. Industrial chemists use it to control reaction conditions and maximize yields.
Key ideas: Compute Q from current concentrations or pressures using the same form as K, then evaluate delta G equals delta G degree plus RT ln Q. If Q is below K the reaction runs forward, and if above K it runs backward. At equilibrium delta G is zero and Q equals K.
Common mistakes and connections: Pure solids and liquids are left out of Q. Students often confuse delta G degree with delta G or use concentrations in the wrong units. It links to Le Chatelier's principle and to the Nernst equation in electrochemistry.
Delta G = RT ln(Q/K) at 298 K for K = 100. It is negative when Q is below K, zero at equilibrium, and positive when Q exceeds K. The interactive lesson shows this as a live animation or chart.
Removing product from a reaction vessel keeps Q small, so delta G stays negative and the reaction continues forward.
Delta G equals delta G standard plus RT ln Q gives direction under nonstandard conditions.
Reaction Quotient and Free Energy is a advanced topic in Thermodynamics & Energetics. It builds on beginner and intermediate topics, so review those first if needed.
Study these earlier topics in Thermodynamics & Energetics first: Temperature Dependence of Spontaneity, Standard Free Energy of Formation, Free Energy and Equilibrium.
The next topics in Thermodynamics & Energetics are Free Energy and Work and Helmholtz Free Energy.
Yes. The interactive lesson includes a self-check question on Reaction Quotient and Free Energy, and the Thermodynamics & Energetics quiz draws on every topic in this area.
Reaction Quotient and Free Energy is topic 42 of 91 in Thermodynamics & Energetics.