Thermodynamics is the study of energy changes during chemical reactions and physical processes. It tells you whether a reaction will happen on its own (spontaneous), how much energy it releases or absorbs, and how changes in temperature affect the outcome. Three quantities are central to chemical thermodynamics: enthalpy (H), entropy (S), and Gibbs free energy (G).
Enthalpy: Heat of Reaction
Enthalpy change (delta H) measures the heat absorbed or released during a reaction at constant pressure. A negative delta H means the reaction is exothermic — it releases heat, like burning wood or dissolving sodium hydroxide in water. A positive delta H means the reaction is endothermic — it absorbs heat, like melting ice or photosynthesis. You can calculate delta H from standard enthalpies of formation using Hess's Law: delta H = sum of products minus sum of reactants. Our Enthalpy Calculator does this calculation for you.
Entropy: Disorder and Randomness
Entropy (S) is a measure of disorder or randomness in a system. Reactions tend to favor states of higher entropy. A positive delta S means the system becomes more disordered (gases have more entropy than liquids, which have more than solids). Melting ice increases entropy because the ordered crystal structure gives way to the more random arrangement of liquid water. However, entropy alone does not determine spontaneity — it needs to be combined with enthalpy through Gibbs free energy.
Gibbs Free Energy: The Spontaneity Predictor
The Gibbs free energy equation, delta G = delta H - T delta S, is the master equation for predicting whether a reaction will proceed spontaneously. If delta G is negative, the reaction is spontaneous under the given conditions. If delta G is positive, it is non-spontaneous (it will not happen without an input of energy). If delta G is zero, the system is at equilibrium. Temperature plays a crucial role here: a reaction that is non-spontaneous at room temperature might become spontaneous at higher temperatures if delta S is positive. The Gibbs Free Energy Calculator lets you explore these relationships interactively.
Heat Capacity and Calorimetry
Heat capacity is the amount of heat required to raise the temperature of a substance by one degree. Specific heat capacity (c) is the heat capacity per gram. Water has a high specific heat capacity (4.184 J/g K), which is why it takes a long time to heat up and cool down — and why it is such an effective coolant. The equation q = mc delta T connects heat (q), mass (m), specific heat (c), and temperature change (delta T). Use the Heat Capacity Calculator for quick calorimetry calculations.