Endothermic and Exothermic Reactions
Think about striking a match or mixing bicarbonate of soda with vinegar - one heats up, the other cools down. This happens because chemical reactions either release energy or absorb it from their surroundings.
Exothermic reactions are the common ones that transfer energy to the surroundings, usually making things feel warmer. The products end up at a lower energy level than the reactants, which means energy gets released. You'll see this in combustion (burning fuels), neutralisation reactions, oxidation, and dramatic reactions like adding sodium to water.
Endothermic reactions work the opposite way - they take in energy from the surroundings, often making things feel cooler. Here, the products have higher energy than the reactants, so energy must be absorbed. These are less common, but you'll find them in reactions like citric acid with sodium hydrogencarbonate or thermal decomposition processes.
Quick Test Tip: On energy diagrams, if the products are lower than reactants = exothermic (energy out). If products are higher = endothermic (energy in).
Both reaction types need activation energy to get started - think of it as the energy hill that reactants must climb before the reaction can happen.
