Cambridge O Level Chemistry · Syllabus 5070 · Chemical Energetics
Endothermic Reaction
What is Endothermic Reaction?
A reaction that takes in thermal energy from its surroundings, so the temperature of the surroundings decreases and the enthalpy change of the reaction is positive.
This definition is part of the Chemical Energetics chapter in Cambridge O Level Chemistry.
Endothermic Reaction in context
Chemical energetics is the study of the thermal energy transferred when a chemical reaction takes place. 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 their temperature decreases. The transfer of thermal energy during a reaction is called the enthalpy change, \(\Delta H\): it is negative for an exothermic reaction because energy leaves the system, and positive for an endothermic reaction because energy enters it. The thermometer sits in the surroundings, so whatever it does, the system did the opposite.
Common mistakes with Endothermic Reaction
- 2. “Endothermic reactions cannot happen, because they need energy they do not have.” Repair They take the energy they need from the surroundings, which is precisely why the surroundings get colder. Photosynthesis and thermal decomposition both happen every day. Needing an input is not the same as being impossible.
Questions students ask about Endothermic Reaction
What is the difference between an exothermic and an endothermic reaction?
An exothermic reaction transfers thermal energy to the surroundings, so the temperature of the surroundings increases and \(\Delta H\) is negative. An endothermic reaction takes in thermal energy from the surroundings, so the temperature of the surroundings decreases and \(\Delta H\) is positive. Exothermic does not mean the mixture contains a lot of heat: a chemical does not contain heat. The word describes a transfer, so always say where the energy went, and remember that the thermometer reading is evidence about the surroundings, not the system.