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A-Level Chemistry Flashcards: Physical Chemistry

A-Level Chemistry — Physical Chemistry Flashcards

20 flashcards with spaced repetition. Press Space to flip, then rate your recall (1-4).

What These Flashcards Cover

Physical chemistry links the mathematical and conceptual frameworks of chemistry. These flashcards cover energetics, reaction rates, chemical equilibrium, acid-base chemistry, and redox processes.

Key areas:

  • Enthalpy Changes: ΔH\Delta H is the enthalpy change at constant pressure. Exothermic (ΔH<0\Delta H < 0): releases heat. Endothermic (ΔH>0\Delta H > 0): absorbs heat. Hess’s law: the total enthalpy change is independent of the route taken. Born-Haber cycles combine multiple enthalpy changes to find lattice energy.
  • Bond Enthalpies: Average bond enthalpy is the energy to break one mole of bonds in the gaseous state. ΔH≈∑(bonds broken)−∑(bonds formed)\Delta H \approx \sum(\text{bonds broken}) - \sum(\text{bonds formed}). This is an estimate because average bond enthalpies vary between molecules.
  • Reaction Rates: Rate =k[A]m[B]n= k[A]^{m}[B]^{n} where kk is the rate constant and mm, nn are orders. Zero order: rate is constant. First order: rate ∝\propto concentration. Second order: rate ∝\propto concentration2^{2}. The rate-determining step is the slowest step in a multi-step mechanism.
  • Collision Theory: For a reaction to occur, particles must collide with sufficient energy (activation energy EaE_{a}) and correct orientation. Factors affecting rate: concentration, temperature, surface area, catalysts.
  • Equilibrium: At equilibrium, forward and reverse rates are equal. The equilibrium constant Kc=[products][reactants]K_{c} = \frac{[\text{products}]}{[\text{reactants}]} (each raised to stoichiometric coefficients). KpK_{p} uses partial pressures for gas equilibria. Le Chatelier’s principle: a system at equilibrium opposes any imposed change.
  • pH and Acids: pH=−log⁡[H+]\text{pH} = -\log[H^{+}]. Strong acids fully dissociate. Weak acids partially dissociate: Ka=[H+][A−][HA]K_{a} = \frac{[H^{+}][A^{-}]}{[HA]}. Buffer solutions resist pH changes. Titration curves show how pH changes as acid is added to base (or vice versa).
  • Redox Reactions: Oxidation is loss of electrons; reduction is gain of electrons (OIL RIG). Oxidation state rules allow you to identify which species is oxidised and which is reduced. Half-equations separate the oxidation and reduction processes.

Intuition

Think of enthalpy as the “heat content” of a system. When bonds break, energy is absorbed (endothermic). When bonds form, energy is released (exothermic). The overall enthalpy change is the balance between these two.

Kinetics is about speed. A catalyst lowers the activation energy, like building a tunnel through a mountain instead of climbing over it. The path is shorter, so more particles have enough energy to react.

Equilibrium is a dynamic balance. The forward and reverse reactions are both happening, but at the same rate, so there is no net change. Le Chatelier’s principle is like a thermostat: if you disturb the system, it adjusts to counteract the disturbance.

Common Pitfalls

  1. Confusing ΔH\Delta H with activation energy. ΔH\Delta H is the overall energy change (products minus reactants). EaE_{a} is the energy barrier that must be overcome for the reaction to proceed. A reaction can be exothermic but have a high activation energy.
  2. Misapplying Le Chatelier’s principle. Adding a catalyst does NOT shift the equilibrium position. It speeds up both the forward and reverse reactions equally, so equilibrium is reached faster but the position does not change.
  3. Forgetting that KcK_{c} only includes aqueous and gaseous species. Pure solids and liquids are omitted from the equilibrium expression because their concentrations are constant.
  4. Sign errors in pH calculations. pH=−log⁡[H+]\text{pH} = -\log[H^{+}]. A common error is forgetting the negative sign, or using [OH−][OH^{-}] instead of [H+][H^{+}].

Cross-References