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CSEC Chemistry: Energetics

Chemical reactions involve the breaking of old bonds and the formation of new bonds. These changes experienced in materials and chemical reactions are all caused by energy. Energetics is the study of chemical changes caused by energy. Chemical reactions will either give off energy or absorb energy, usually in the form of heat.

Exothermic reactions are chemical changes that result in an increase in the temperature of their surroundings (they release energy). The energy released is usually due to the formation of new bonds. What happens is that the reactants lose energy to form the products. In an exothermic reaction, the container and the contents tend to get hotter. Examples of such reactions include:

  • dissolving sodium hydroxide in water

  • burning propane gas

  • respiration

Endothermic reactions are those chemical changes that cause a decrease in the temperature of their surroundings (they absorb energy). In these reactions, the reactants gain energy to form the products. Energy is absorbed in order to break old bonds in the reactants. These are far rarer than exothermic reactions, but examples include:

  • photosynthesis

  • reaction between steam and carbon

  • dissolving some substances in water, like urea and sodium thiosulphate

Where does this energy for reaction come from? Well, chemicals inherently have energy:

  • the particles in chemicals have kinetic energy due to constant movement

  • chemicals store energy in their bonds

  • less important in chemical reactions, energy is stored in the nuclei of atoms, and does not change during reactions

Kinetic energy and chemical energy together constitute the energy of a chemical (also called its heat content or enthalpy at constant pressure).

Energy changes occur naturally during chemical reactions, as reactants form new products. Energy change is represented as ∆H.

∆H = H of products - H of reactants

∆H = Hᴘ - Hʀ

Enthalpy change (or energy change) is the total heat content of the products minus the total heat content of the reactants.


If Hᴘ is less than Hʀ, then ∆H is negative and the reaction is exothermic.

If Hᴘ is more than Hʀ, then ∆H is positive and the reaction is endothermic.


Enthalpy is measured is J/mol (joules per mole) or kJ/mol.


The energy changes that occur during chemical reactions can be represented on graphs called 'energy profile diagrams.'



These diagrams a re simplified representations of the net energy changes in a chemical reaction.

Energy, as you can see, is released during the formation of the products, that is, the activation energy is always higher than the final energy of the products.

More energy is released than supplied in exothermic reactions. More energy is supplied than is released for endothermic reactions.

The activation energy is the minimum energy which must be supplied before the reaction proceeds.

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