Exothermic and endothermic.

Step 1 of 5

Heat coming out

In the glass chimney a jet of methane burns, the gas in a cooker: $\mathrm{CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O}$⁠. The flame warms the glass and the air around it. That energy was not made from nothing: it was stored in the substances, and the reaction released it.

A reaction that hands energy to its surroundings, usually as heat, is called exothermic. A reaction that takes energy from its surroundings is called endothermic.

Step 2 of 5

The flame’s power

Each mole of methane burned gives out 802 kJ. The flame burns methane at a steady rate, so it gives out heat at a steady rate: a power, in joules per second, that is in watts. Here Gas flow, Oxygen in the air and Water mist are fixed.

Step 3 of 5

A bigger flame

Because every mole gives out the same energy, the heat per second goes in proportion to the gas burned per second. Gas flow is now free.

Step 4 of 5

The sign of ΔH

Chemists describe a reaction’s energy by its enthalpy change, $\Delta H$⁠: the products’ energy minus the reactants’. On the energy profile of an exothermic reaction the products sit below the reactants, so $\Delta H < 0$⁠. In an endothermic reaction they sit higher, and $\Delta H > 0$⁠.

Step 5 of 5

Heat going in

Endothermic reactions and processes are less striking, because they neither glow nor warm anything. But you can feel them: they cool what is around them. Photosynthesis takes in the energy of sunlight; limestone, $\mathrm{CaCO_3}$⁠, breaks down to lime only when it is heated in a kiln; and some salts take in heat as they dissolve in water.