Thermodynamics and Kinetics
Thermodynamics and Kinetics
This domain tests energy flow, calorimetry, enthalpy, entropy, free energy, reaction rates, and catalysis. Thermodynamics answers whether a change is favored; kinetics answers how fast it occurs.
Thermodynamics
- Enthalpy change delta H is negative for exothermic reactions. Calorimetry uses q = mc delta T; q has the sign of the system. Hess's law adds enthalpy changes.
- Entropy delta S generally increases with gas formation, temperature, and disorder.
- delta G = delta H - T delta S. Negative delta G favors spontaneity at constant temperature and pressure; use kelvin.
- Bond breaking requires energy; bond forming releases energy.
Kinetics
- Rate depends on concentration, temperature, surface area, and catalysts. Collision theory requires sufficient energy and proper orientation.
- Activation energy is the barrier; a catalyst lowers it and speeds both directions but does not change delta H, delta G, or equilibrium position.
- Rate law exponents must be determined experimentally, not from coefficients.
Worked example
A 50.0 g water sample warms from 20.0 to 30.0 degrees Celsius. q = mc delta T = 50.0 g x 4.184 J/(g degree C) x 10.0 degree C = 2092 J, or 2.09 kJ. The water absorbed energy, so the reaction that warmed it released about 2.09 kJ (delta H negative for that reaction). Reporting 2.09 kJ as the water's heat is correct; attaching a negative sign to q for the water is not.
Traps
Using Celsius in delta G; omitting the sign convention; treating delta H as always equal to delta G; assuming a catalyst changes yield; and reading rate-law orders from coefficients.