Chemistry (university) · Part 05
Thermochemistry and thermodynamics
Energy changes in reactions: enthalpy, calorimetry, Hess's law, entropy and Gibbs free energy.
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- First law of thermodynamicsEnergy cannot be created or destroyed, only moved. The change in a system's internal energy equals the heat added to it plus the work done on it: ΔU = q + w. Heat in and work done on the system are positive.
- EnthalpyEnthalpy change (ΔH) is the heat flow at constant pressure, which is how most lab reactions run. ΔH is negative for exothermic reactions (heat released) and positive for endothermic reactions (heat absorbed).
- CalorimetryCalorimetry measures heat by watching a temperature change. The heat absorbed is q = mcΔT, where m is mass, c is specific heat capacity and ΔT is the temperature change. Water has c = 4.184 J/(g·°C).
- Hess's lawHess's law says the total enthalpy change of a reaction is the same whether it happens in one step or many. So you can add the ΔH values of known steps to find the ΔH of a reaction you can't measure.
- Standard enthalpy of formationThe standard enthalpy of formation (ΔHf°) is the heat change when one mole of a compound forms from its elements in their standard states. Elements in their standard state have ΔHf° = 0. Then ΔH°rxn = ΣΔHf°(products) − ΣΔHf°(reactants).
- Bond enthalpyBond enthalpy is the energy needed to break one mole of a particular bond in the gas phase. Breaking bonds costs energy and forming bonds releases it. ΔH ≈ Σ(bonds broken) − Σ(bonds formed). It gives only an estimate.
- EntropyEntropy (S) measures how many ways energy and particles can be arranged, often called disorder. Gases have more entropy than liquids, and liquids have more than solids. Entropy rises when a substance is heated, dissolved or broken into more particles.
- Second law of thermodynamicsIn any spontaneous process, the total entropy of the system plus its surroundings increases: ΔS(universe) > 0. The system's entropy can fall if the surroundings gain more. Heat released raises the surroundings' entropy by ΔS(surr) = −ΔH/T.
- Gibbs free energyGibbs free energy combines enthalpy and entropy: ΔG = ΔH − TΔS. A reaction is spontaneous if ΔG is negative, at equilibrium if ΔG = 0, and non-spontaneous if ΔG is positive. T is in kelvin.