Thermochemistry, Kinetics, Equilibrium, and Electrochemistry
This domain covers energy flow, reaction rates, equilibrium, and electrochemical cells. Questions ask you to interpret signs, graphs, and equilibrium expressions rather than simply recall definitions.
Core rules
- Enthalpy: negative delta H is exothermic; calorimetry uses q = mc delta T; Hess's law adds stepwise enthalpy changes.
- Spontaneity: delta G = delta H - T delta S; negative delta G means spontaneous under the stated conditions.
- Rates depend on concentration, temperature, surface area, and catalysts. Rate-law exponents come from experiment, not from coefficients.
- Le Chatelier: adding reactant shifts right; increasing pressure shifts toward fewer gas moles; a catalyst changes rate but not K.
- Equilibrium expressions exclude pure solids and liquids. Ksp and Ka use molarity as an approximation of activity.
- Galvanic cells: oxidation at the anode, reduction at the cathode; E cell = E cathode - E anode; positive E cell means spontaneous.
Worked example
A 50.0 g water sample warms from 20.0 degrees C to 30.0 degrees C. q = mc delta T = 50.0 x 4.184 x 10.0 = 2.09 x 10^3 J absorbed by the water. If the reaction released that heat, delta H for the reaction as written is negative, with magnitude depending on the moles reacted. For an equilibrium with K = 10 at 298 K, delta G standard = -RT ln K = -(8.314)(298)(ln 10) = -5.71 kJ/mol, consistent with a spontaneous forward reaction under standard conditions.
Traps that cost marks
- Reversing the sign of q for the surroundings.
- Treating a catalyst as a shift in equilibrium position.
- Including solids in equilibrium constant expressions.
- Swapping anode and cathode in cell notation.