Entropy, Gibbs Free Energy & Feasibility (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about entropy, Gibbs free energy and reaction feasibility.

Enthalpy alone can't explain why endothermic reactions happen — entropy is the missing piece, and Gibbs free energy combines both to determine whether a reaction is actually feasible.

Enthalpy change alone isn't enough to explain why some reactions happen, because it makes no sense for a system to end up in a less stable, higher-energy state in an endothermic reaction. Entropy is the missing piece: it measures how disordered a system is, and a more disordered system is energetically more favourable. Gibbs free energy combines both enthalpy and entropy into a single equation, delta G equals delta H minus T delta S, which determines whether a reaction is feasible.

This video starts with entropy and what it measures, then covers Gibbs free energy and how it combines enthalpy and entropy, and finishes with reaction feasibility — what the sign of delta G tells you, and how temperature can change the outcome.

The entropy of a system is the number of possible arrangements of its particles and their energy — in other words, a measure of how disordered or chaotic it is. When a system becomes more disordered, its entropy increases, and a system with higher entropy is energetically more favourable. This is why endothermic reactions can still happen: the second law of thermodynamics states that the entropy of the universe is always increasing, so a reaction can be feasible even if it takes in energy, as long as it increases the total disorder. Entropy is measured in joules per kelvin per mole, and the standard entropy change of a reaction is calculated as the sum of the entropies of the products minus the sum of the entropies of the reactants.

The thermodynamic feasibility of a reaction depends on both its enthalpy change and its entropy change, and these are combined in the Gibbs equation: delta G equals delta H minus T delta S. Because enthalpy is usually given in kilojoules per mole and entropy in joules per kelvin per mole, the entropy value has to be converted to kilojoules per kelvin per mole before it's substituted in. Delta G can also be calculated from the delta G values of all the substances present, as the sum of the delta G of the products minus the sum of the delta G of the reactants.

A reaction is feasible when delta G is zero or negative. Whether that happens depends on the signs of delta H and delta S: an exothermic reaction with a positive entropy change is always feasible, and an endothermic reaction with a negative entropy change is never feasible. But when the signs of delta H and delta S work against each other, temperature decides the outcome. Because the reaction becomes feasible when delta G is zero the equation can be rearranged to find the exact temperature at which a reaction becomes feasible. This is why some reactions that aren't feasible at room temperature, such as certain metal extractions, become feasible at much higher temperatures.

Free energy is what's left over to do useful work once bonds have been broken and formed. Converting delta S to kilojoules before substituting it into the Gibbs equation is the single most commonly missed mark — mark schemes award a specific mark just for dividing delta S by 1000.

Entropy measures how disordered a system is, and a more disordered system is energetically more favourable. Gibbs free energy, delta G equals delta H minus T delta S, combines enthalpy and entropy into one value. A reaction is only feasible when delta G is zero or negative, and where the signs of delta H and delta S oppose each other, temperature can change whether that's the case.

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