Energy in chemical reactions

Burning one mole of methane, 16 g, releases 890.8 kJ of heat, enough to heat about 2.7 litres of water from 20 °C to 100 °C.

Some reactions get hot, like a gas flame. Others take heat in from their surroundings. The difference comes from bonds. Breaking a chemical bond always takes energy, and making one always releases energy. The hydrogen molecule, for example, needs 436 kJ per mole to break apart. Whether a reaction as a whole gives out heat or takes it in depends on which is larger: the energy spent breaking the old bonds or the energy released forming the new ones.

Exothermic and endothermic

Chemists measure the heat of a reaction at constant pressure as the enthalpy change, ΔH. If the products have less enthalpy than the reactants, the difference is released as heat, ΔH is negative and the reaction is exothermic. If the products have more, heat must be taken in from the surroundings, ΔH is positive and the reaction is endothermic. Reverse a reaction and the sign of ΔH flips: making water from hydrogen and oxygen releases 571.6 kJ for every two moles of water, so splitting two moles of liquid water back into the elements needs 571.6 kJ.

Fuels are substances with large, negative enthalpies of combustion. Measured values include −890.8 kJ per mole for methane, −285.8 kJ for hydrogen, −1366.8 kJ for ethanol and −726.1 kJ for methanol. Per gram the picture changes. Methane gives about 55.5 kJ per gram, ethanol about 29.7 kJ per gram, and hydrogen, being so light, about 141.8 kJ per gram. OpenStax estimates that burning one litre of isooctane, a common component of petrol, releases 33,100 kJ. Not every useful reaction is exothermic. Ozone forms from oxygen high in the atmosphere by an endothermic process driven by ultraviolet light, and nitrogen and oxygen take in 180.5 kJ to form two moles of nitrogen monoxide, which is why that reaction happens mainly in the very hot cylinders of engines.

Why fuels need a spark

If burning methane releases so much energy, why does the gas in a kitchen pipe not catch fire on its own? Because the reaction first has to break bonds before it can make new ones. Molecules must collide with at least a minimum energy, the activation energy Eₐ, to reach the unstable arrangement at the top of the hump, called the transition state. At room temperature hardly any collisions are that energetic. A spark or a match supplies the energy for the first few reactions, and the heat they release then keeps the rest going.

Slide the dot along the curve in the lab. On the way up, bonds stretch and the energy rises. At the top the old bonds are half broken and the new ones half formed. On the way down, the new bonds finish forming and energy pours out. The height of the hump decides how fast the reaction goes, which is the subject of the next unit; the difference between the two ends decides how much heat comes out. A catalyst, switched on with the button, gives the reaction an easier path with a lower hump, but both ends stay where they are, so ΔH does not change.