Energetics

Basics of Energetics

  • Energetics refers to the study of energy transfer in chemical reactions.
  • Chemical reactions either absorb energy from their surroundings (endothermic reactions) or release energy into their surroundings (exothermic reactions).
  • The energy is usually transferred as heat, but can also be in the form of light or electricity.

Exothermic Reactions

  • Exothermic reactions are those that give out energy to their surroundings, typically in the form of heat.
  • Examples include combustion reactions, many oxidation reactions and neutralisation reactions in chemistry.
  • These reactions often occur spontaneously as they result in the lowering of energy, making them more stable.

Endothermic Reactions

  • Endothermic reactions are reactions that absorb energy from their surroundings, again, typically in the form of heat.
  • Examples include thermal decomposition, the process of photosynthesis in plants, and the reaction between citric acid and sodium hydrogencarbonate.
  • These reactions are less common than exothermic reactions, as they result in an increase in energy.

Energy Profile Diagrams

  • Energy changes in reactions can be represented graphically using energy profile diagrams.
  • These diagrams show the energy of the reactants and products, and the activation energy of the reaction.
  • In an exothermic reaction, the energy of the products will be lower than that of the reactants. In an endothermic reaction, the energy of the products will be higher.

Activation Energy

  • The activation energy is the minimum amount of energy that particles must have in order to react.
  • It can also be defined as the difference in energy between the reactants and the top of the energy barrier on an energy profile diagram.
  • Catalysts lower the activation energy for a reaction, allowing it to proceed at a faster rate.

Hess’ Law

  • Hess’ law states that the total enthalpy change for a reaction remains constant regardless of the route taken.
  • This is particularly useful for calculating enthalpy changes that are difficult to measure directly. It involves breaking down the overall reaction into a number of steps for which enthalpy changes are known.

Energy and Reversibility

  • For a reversible reaction, the enthalpy change in the forward direction is equal and opposite to the enthalpy change in the reverse direction.
  • In terms of energy profile diagrams, this suggests that the height of the energy barrier in the forward direction is the same as the height of the energy barrier in the reverse direction.

The Role of Energetics in Everyday Life

  • Understanding energetics is important in predicting the behaviour of chemical systems.
  • This includes everything from energy production and industrial processes, to biological systems and environmental processes, making it a crucial aspect of a large number of applications.