What is thermodynamics? — The study of energy and its transformations, particularly the relationship between heat, work, and chemical reactions. It determines whether a reaction is spontaneous.

What is the First Law of Thermodynamics? — Energy cannot be created or destroyed — only converted from one form to another. ΔU = q + w (change in internal energy = heat absorbed + work done on system). Conservation of energy.

What is the Second Law of Thermodynamics? — The total entropy of an isolated system always increases in a spontaneous process. The universe tends toward disorder. No process can be 100% energy-efficient.

What is the Third Law of Thermodynamics? — The entropy of a perfect crystalline substance is zero at absolute zero (0 K / −273.15°C). This sets the absolute reference point for entropy measurements.

What is enthalpy (H)? — A measure of the total heat content of a system at constant pressure. ΔH = heat absorbed or released in a reaction. ΔH < 0 = exothermic. ΔH > 0 = endothermic.

What is the standard enthalpy of formation (ΔHf°)? — The enthalpy change when 1 mole of a compound is formed from its elements in their standard states at 298 K and 1 atm. ΔHf° of any pure element in its standard state = 0.

What is Hess's Law? — The total enthalpy change of a reaction is the same regardless of the route taken — it depends only on initial and final states. Allows calculation of ΔH for reactions that cannot be measured directly.

How do you apply Hess's Law? — Add the enthalpy changes of individual steps that sum to the target reaction. If a step is reversed, change the sign of ΔH. If multiplied, multiply ΔH by the same factor.

What is entropy (S)? — A measure of the disorder or randomness of a system. ΔS > 0 = increase in disorder. ΔS < 0 = decrease in disorder. Gases have higher entropy than liquids, which have higher than solids.

What factors increase entropy? — 1. Phase change from solid → liquid → gas. 2. Dissolving a solid in solution. 3. Increasing temperature. 4. Increasing moles of gas in a reaction. 5. Mixing different substances.

What is Gibbs free energy (ΔG)? — The energy available to do useful work. Determines spontaneity: ΔG = ΔH − TΔS. If ΔG < 0 → spontaneous. ΔG > 0 → non-spontaneous. ΔG = 0 → equilibrium.

What are the four spontaneity combinations for ΔH and ΔS? — ΔH− ΔS+: Always spontaneous (ΔG always negative). ΔH+ ΔS−: Never spontaneous (ΔG always positive). ΔH− ΔS−: Spontaneous at low temperature. ΔH+ ΔS+: Spontaneous at high temperature.

What is specific heat capacity (c)? — The amount of heat needed to raise the temperature of 1 gram of a substance by 1°C (or 1 K). Water has an unusually high specific heat: 4.18 J/g·°C.

What is the heat equation (q = mcΔT)? — q = mcΔT. q = heat energy (J), m = mass (g), c = specific heat capacity (J/g·°C), ΔT = temperature change (°C or K). Used in all calorimetry calculations.

What is calorimetry? — Experimental measurement of heat flow in chemical reactions. A calorimeter measures temperature changes to calculate heat absorbed or released. Assumes no heat loss to surroundings.

What is a bomb calorimeter? — A constant-volume calorimeter used to measure the heat of combustion reactions. The reaction occurs in a sealed steel container submerged in water. Measures ΔU (internal energy) not ΔH.

What is bond enthalpy? — The energy required to break one mole of a specific bond in gaseous molecules. Breaking bonds = endothermic (absorbs energy). Forming bonds = exothermic (releases energy).

How do you calculate ΔH from bond enthalpies? — ΔH = Σ(bonds broken) − Σ(bonds formed). Sum the energies of bonds broken in reactants (positive) and subtract the sum of bonds formed in products (negative).

What is lattice enthalpy? — The energy required to separate one mole of an ionic solid into its gaseous ions. Large lattice enthalpy = strong ionic lattice = high melting point. Related to ion charge and size.

What is a Born-Haber cycle? — A thermochemical cycle applying Hess's Law to calculate lattice enthalpy of an ionic compound. Uses enthalpy of formation, atomization, ionization energies, and electron affinity values.

What is enthalpy of combustion (ΔHc°)? — The enthalpy change when 1 mole of a substance is completely burned in excess oxygen under standard conditions. Always exothermic (negative ΔH). Example: CH₄: ΔHc° = −890 kJ/mol.

What is enthalpy of neutralization? — The heat released when an acid and base react to form 1 mole of water. For strong acid + strong base it is approximately −57 kJ/mol regardless of the specific acid/base used.

What is endothermic vs exothermic in terms of energy diagrams? — Exothermic: Products at lower energy than reactants. ΔH = negative. Energy is released as heat. Endothermic: Products at higher energy than reactants. ΔH = positive. Energy is absorbed as heat.

What is activation energy (Ea)? — The minimum amount of energy required for a reaction to occur. A catalyst lowers the activation energy — increasing reaction rate without being consumed.

What is the relationship between ΔG and equilibrium constant (K)? — ΔG° = −RT ln K. If K > 1 → ΔG° < 0 → products favored. If K < 1 → ΔG° > 0 → reactants favored. If K = 1 → ΔG° = 0 → equilibrium.