Thermodynamics and kinetics

What the exam asks

The highest-value item is the four-quadrant Gibbs question: you are handed the signs of ΔH and ΔS and asked at what temperature the reaction goes. Do not memorise the table, rebuild it. Write ΔG = ΔH - TΔS, notice that only the second term carries a T, and ask what happens to it when T is large. The trap underneath it is reading spontaneous as fast, and the exam sets it deliberately by describing a reaction that is thermodynamically favourable and does not proceed. The answer is always a kinetic one. On the equilibrium half, the trap is the catalyst: it appears in Le Chatelier questions purely to see whether you will shift the position. It cannot, because it speeds both directions equally, and the same reasoning tells you it cannot change ΔH either. Remember that equilibrium is examined under this topic even though the checklist gives it no heading of its own.

This topic asks two questions about a reaction, and keeps them apart. Will it go, which is thermodynamics, and how fast, which is kinetics. They have almost nothing to do with each other. Diamond turning into graphite releases free energy and is entirely spontaneous, and it will not happen to your ring. Spontaneous means allowed, not soon.

The going question is settled by ΔG = ΔH - TΔS. Enthalpy asks whether energy is released, entropy asks whether disorder increases, and temperature decides which of the two gets the casting vote. That single equation generates the four-quadrant logic that most of the marks in this topic sit on, and it is worth being able to reconstruct it rather than recall it.

Equilibrium is taught here. The checklist has no separate equilibrium topic, so Le Chatelier, equilibrium constants and solubility equilibria are all tagged to this one, and questions on them will route you back to this page. That is not a filing accident, and it is not a stretch either: an equilibrium constant is a statement about free energy, and Le Chatelier is what a system does when you disturb one. Expect them on the exam and learn them here.

What to hold

• ΔG = ΔH - TΔS, and a negative ΔG means the reaction is spontaneous in the forward direction as written.
• Spontaneous says nothing about speed: a reaction can have a large negative ΔG and be far too slow to observe, because the rate is set by the activation barrier and not by ΔG.
• When the two terms agree, temperature cannot overrule them: ΔH negative with ΔS positive is spontaneous at every temperature, and ΔH positive with ΔS negative is spontaneous at none.
• If ΔH and ΔS are both negative, the reaction is spontaneous only at low temperature, because a large T lets the unfavourable entropy term overrun the favourable enthalpy.
• If ΔH and ΔS are both positive, the reaction is spontaneous only at high temperature, because only a large T makes TΔS big enough to beat the unfavourable enthalpy.
• The temperature where the sign flips is T = ΔH/ΔS, the point at which ΔG is zero and the two terms exactly cancel.
• Entropy rises when the number of moles of gas rises, and that comparison dominates any other entropy argument you can make from a balanced equation.
• Enthalpy is a state function, so ΔH depends only on the start and end points and not the route, which is what makes Hess's law work: reverse a step and the sign flips, double a step and ΔH doubles.
• From formation enthalpies, ΔH is the sum for the products minus the sum for the reactants; from bond enthalpies, it is the bonds broken minus the bonds formed, because breaking costs energy and forming releases it.
• The equilibrium constant includes only gases and aqueous species, because a pure solid or pure liquid has no concentration that can change, and comparing Q with K gives the direction: Q below K runs forward, Q above K runs back.
• Le Chatelier: a disturbed equilibrium shifts to oppose the disturbance, so adding a reactant pushes it forward and compressing it pushes it towards whichever side has fewer moles of gas, but only a temperature change alters the value of K itself.
• A catalyst changes neither the equilibrium position nor ΔH, because it lowers the barrier for the forward and reverse reactions by the same amount and leaves the energies of reactants and products untouched.