Mendeleev · Science

Gibbs free energy and spontaneity

At constant temperature and pressure, ΔG = ΔH − TΔS helps predict the thermodynamically favored direction of a change. ΔG < 0 indicates spontaneity under those conditions, not necessarily speed.

Core idea

At constant temperature and pressure, ΔG = ΔH − TΔS helps predict the thermodynamically favored direction of a change. ΔG < 0 indicates spontaneity under those conditions, not necessarily speed.

A concrete example

A change can be thermodynamically favorable yet extremely slow if a large kinetic barrier must be crossed.

Why it matters

This concept connects symbolic scientific language with what actually happens at atomic, molecular or experimental scales. Before applying a formula, always check the quantities, units and assumptions of the model.

Check it yourself

Restate the idea without technical vocabulary, then repeat the reasoning with a simple value. If your explanation and calculation tell different stories, revisit the assumptions.

A change can be thermodynamically favorable yet extremely slow if a large kinetic barrier must be crossed.

Understand it before converting

Energy is not just a classroom quantity: it appears constantly in electricity, food, heating, mechanics and physics. Units are chosen for practical reasons. Some are convenient in everyday life, while others dominate scientific or technical work. This page deliberately focuses on familiar units — joules, kilojoules, watt-hours, kilowatt-hours, calories and kilocalories — instead of presenting a long catalogue of rarely used formats. Every pair links to a dedicated conversion page with the calculation rule, worked examples and useful reference values. The aim is to get the answer quickly while still understanding what is being converted and whether the result makes physical sense.