Exothermic and endothermic reactions

Breaking bonds absorbs energy -- energy is needed to pull bonded atoms apart. Making new bonds releases energy, as the atoms settle into a more stable, lower-energy arrangement. Candidates very often write the reverse: "breaking bonds releases energy" scores zero however fluently it is argued. Breaking anything takes effort; only the new arrangement gives energy back.

For an exothermic reaction the products line must be drawn lower than the reactants line -- the system has lost energy to the surroundings. Endothermic is the other way round, products above reactants. Drawing the levels the wrong way round contradicts the stated sign of delta-H. Draw a smooth hump between them too: a straight diagonal shows no activation energy.

Classify by the temperature change of the surroundings, not by how hot or violent a reaction looks. Calcium carbonate in a lime kiln must be heated continuously and stops when the heating stops -- it consumes energy, so it is endothermic. Conversely a reaction can be exothermic yet begin and end cold: the DIRECTION of transfer decides, not the absolute temperature.

delta-H is one fixed signed value for a reaction, so "delta-H increases" or "delta-H decreases" is meaningless and scores nothing. Translate the Core observation into the Extended sign instead: temperature of the surroundings RISES, so exothermic, so delta-H is negative; temperature FALLS, so endothermic, so delta-H is positive.

A reaction is exothermic overall only if the energy released making new bonds EXCEEDS that absorbed breaking old bonds (delta-H negative), endothermic when the reverse holds. "More bonds made than broken" is the wrong test: different bonds store different amounts of energy, so a reaction can break fewer bonds than it makes and still be endothermic if those were stronger.

On a reaction-pathway diagram the activation-energy arrow runs from the REACTANTS level UP to the peak -- never from the products level, and never from whichever line happens to be lower. Ea is defined relative to the reactants only, whether the reaction is exothermic or endothermic, and it is always a positive quantity because every reaction needs some input to start.

delta-H is the vertical gap from the reactants level straight to the products level, skipping over the hump in between -- the NET change. Candidates commonly measure it instead as the full height of the curve above the lower line, which is Ea plus the overall change, not delta-H. Two arrows, two different measurements, both taken from the reactants line.