Everything from the 3 lessons of this topic on a few pages: the key idea of each lesson, the diagram to remember, the points to learn and the traps examiners set. Read it, cover it, test yourself.
An exothermic reaction transfers thermal energy to the surroundings, so the temperature of the surroundings increases. An endothermic reaction takes in thermal energy from the surroundings, so the temperature of the surroundings decreases. Extended: the transfer of thermal energy during a reaction is the enthalpy change, ΔH. ΔH is negative for exothermic reactions and positive for endothermic reactions.
In an endothermic reaction the chemicals gain energy, but the thermometer shows a fall, because it measures the surroundings, which have lost energy. Do not say an endothermic reaction "gives out cold": cold is not a form of energy.
On a reaction pathway diagram, if the products are lower in energy than the reactants the reaction is exothermic; if the products are higher it is endothermic. Extended: the enthalpy change ΔH is the difference in energy between reactants and products. The activation energy, Ea, is the minimum energy that colliding particles need in order to react; it is the height of the hill from the reactants to the top.
The activation energy arrow starts at the reactants, not at the bottom axis and not at the products. And ΔH is measured between the reactant and product levels, not from the top of the hill. Arrows drawn from the axis lose the mark.
Extended. Bond breaking is endothermic: energy must be taken in to separate atoms. Bond making is exothermic: energy is released when atoms join. ΔH = energy needed to break bonds − energy released making bonds. If more energy is released making the new bonds than is needed to break the old ones, ΔH is negative and the reaction is exothermic.
Multiply each bond energy by the number of those bonds, including the big numbers in the equation: 2O2 means two O=O bonds, and CO2 has two C=O bonds. And always subtract in the order broken − made. Doing it the other way round gives the right number with the wrong sign.