Kinetics, Equilibrium, and Thermodynamics
This domain links reaction rate, energy change, entropy, and reversible reactions. Expect questions on collision theory, potential energy diagrams, heat calculations, and Le Chatelier's principle.
Rates and collision theory
Rate rises with temperature (more collisions exceed activation energy), with concentration or gas pressure (more frequent collisions), and with surface area. A catalyst lowers activation energy without being consumed and without changing ΔH.
Energy changes
Negative ΔH means exothermic: products sit lower in energy and heat is released. Positive ΔH means endothermic. Breaking bonds absorbs energy; forming bonds releases it. Activation energy is the barrier between reactants and products. Heat flow is calculated with q = mcΔT; water's specific heat is 4.18 J/(g·K).
Entropy and favorability
Entropy measures disorder and increases for solid → liquid → gas, for dissolving, and when a reaction produces more moles of gas. A process is favored when it releases energy (ΔH < 0) and increases entropy (ΔS > 0).
Equilibrium
At equilibrium the forward and reverse rates are equal and concentrations are constant, though not necessarily equal. Le Chatelier: adding a reactant shifts right; removing a product shifts right; raising pressure shifts toward fewer gas moles; heating shifts in the endothermic direction; a catalyst changes only the time needed to reach equilibrium.
Worked example
For N₂(g) + 3H₂(g) ⇌ 2NH₃(g) + heat, raising temperature shifts the system left, lowering ammonia yield. Raising pressure shifts it right, toward 2 mol of gas from 4 mol.
Traps
- Saying equilibrium means equal concentrations.
- Believing a catalyst raises yield or changes ΔH.
- Writing heat as a product in an endothermic reaction; there it is a reactant.
- Forgetting that q = mcΔT requires the mass of the substance being heated, not the container.