Breaking bonds takes energy in and making bonds gives it out. Whether a reaction warms or cools its surroundings comes down to which of those is larger.
Energy is conserved in a chemical reaction: it is transferred between the reacting substances and their surroundings, not created or destroyed.
An exothermic reaction transfers energy to the surroundings, so their temperature rises. Combustion, neutralisation and many oxidation reactions are exothermic, and hand warmers and self-heating cans use the effect.
An endothermic reaction takes energy from the surroundings, so their temperature falls. Thermal decomposition and the reaction of citric acid with sodium hydrogencarbonate are endothermic; sports injury cold packs use it.
A reaction profile plots energy against the progress of the reaction. In an exothermic reaction the products are at a lower energy than the reactants; in an endothermic reaction they are at a higher energy.
The difference between the reactant energy level and the peak of the profile is the activation energy, the minimum energy colliding particles need for the reaction to happen at all. A catalyst lowers the activation energy by providing an alternative pathway, but it does not change the energy levels of the reactants or the products.
Breaking bonds is endothermic, so energy must be supplied. Making bonds is exothermic, so energy is released.
The overall energy change is energy in to break bonds − energy out from making bonds. A negative answer means more energy came out than went in, so the reaction is exothermic; a positive answer means it is endothermic.
A simple cell is made by putting two different metals in an electrolyte. A potential difference is produced, and metals further apart in the reactivity series often give a larger one.
A battery is two or more cells joined in series. In a non-rechargeable cell the reaction stops once one reactant is used up; in a rechargeable one the reaction can be reversed by applying an external current.
A fuel cell is supplied with fuel and oxygen continuously and produces a potential difference for as long as the supply lasts. The hydrogen fuel cell combines hydrogen and oxygen to produce water: 2H2 + O2 → 2H2O.
The advantages over a rechargeable battery are that it does not run down or need recharging, and the only chemical product at the cell is water, though there is waste heat, and producing the hydrogen may cause emissions elsewhere. The difficulties are storing and transporting hydrogen safely, and the fact that most hydrogen is currently made from fossil fuels.
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