An endothermic reaction is one that absorbs heat from the surroundings, so the temperature falls and the heat change ΔH is positive.
An endothermic reaction absorbs heat from the surroundings, so the temperature of the mixture falls and the heat change ΔH is positive. Malay: tindak balas endotermik · Chinese: 吸热反应.
In an endothermic reaction the energy taken in to break the bonds in the reactants is greater than the energy given out when new bonds form in the products. To make up the difference, energy is drawn in from the surroundings as heat, so the water, solution or container loses heat and cools down. A thermometer in the mixture reads a lower temperature. On an energy level diagram the products sit higher than the reactants, and the ΔH arrow points upward with a positive sign.
Example. Typical endothermic changes in SPM are the thermal decomposition of carbonates, such as heating copper(II) carbonate, and dissolving certain ammonium salts like ammonium chloride or ammonium nitrate in water, where the beaker becomes noticeably colder. Photosynthesis is another endothermic process, taking in light energy.
Confusion to avoid. Do not swap the signs: a falling temperature means heat is absorbed and ΔH is positive, the opposite of the exothermic case. Students also confuse “cold to the touch” with “no reaction”; a temperature drop is itself the evidence that an endothermic reaction is happening. And do not think endothermic reactions cannot start on their own, many do, drawing heat from their surroundings as they go.
The endothermic/exothermic pair anchors the whole chapter. All four heats of reaction you calculate are exothermic, so an endothermic answer with a negative ΔH is a clear mistake. Our teachers drill the link between a temperature drop and a positive ΔH so that SPM Chemistry answers stay consistent.
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