An exothermic reaction is one that releases heat to the surroundings, so the temperature rises and the heat change ΔH is negative.
An exothermic reaction releases heat to the surroundings, so the temperature of the mixture rises and the heat change ΔH is negative. Malay: tindak balas eksotermik · Chinese: 放热反应.
In an exothermic reaction the energy given out when new bonds form in the products is greater than the energy taken in to break the bonds in the reactants. The extra energy leaves the reacting system as heat, and the surroundings, the water, the solution and the container, gain it. That is why a thermometer placed in the mixture reads a higher temperature. On an energy level diagram the products sit lower than the reactants, and the ΔH arrow points downward with a negative sign.
Example. The commonest exothermic reactions in SPM are combustion, neutralisation of an acid by an alkali, respiration, and the reaction of a reactive metal with an acid. Burning methane, for instance, warms everything around the flame; adding sodium hydroxide solution to hydrochloric acid makes the beaker noticeably warmer, and that temperature rise is what you feed into Q = mcθ.
Confusion to avoid. Do not confuse the direction of heat flow with the sign of ΔH, they seem opposite. Heat flows out of the reaction, yet ΔH is negative, because ΔH describes the reacting substances losing energy, not the surroundings gaining it. Also do not assume every exothermic reaction feels hot instantly; a slow one releases the same heat over a longer time, so the temperature rise is gentler.
The exothermic/endothermic pair is the backbone of the chapter, and all four heats of reaction you meet later, neutralisation, displacement, precipitation and combustion, are exothermic. Recognising the temperature rise and writing ΔH as negative is exactly what SPM Chemistry rewards.
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