Energy Transfer

Energy Transfer in Chemical Reactions

  • Energy is either taken in or released during a chemical reaction. The substances that react together are changed into new substances.
  • If a reaction takes in energy primarily in the form of heat, it is called endothermic.
    • Examples of endothermic reactions include photosynthesis in plants, and the reaction between citric acid and sodium hydrogen carbonate.
  • If a reaction releases energy, primarily in the form of heat, it is called exothermic.
    • Examples of exothermic reactions include neutralisation reactions (acid + base), combustion reactions, and many oxidation reactions.

Measuring Energy Changes

  • Energy changes in a chemical reaction can be measured using a bomb calorimeter or a simple calorimeter.
  • The rise or fall in temperature of a known mass of a substance (water, in many cases) is used to calculate the amount of heat energy absorbed or released during the reaction.

Energy Level Diagrams

  • Energy level diagrams are used to represent energy changes in a chemical reaction. They show the difference in energy between the reactants and the products.
  • In endothermic reactions, the products have more energy than the reactants. The energy level of the products will be higher than the reactants on the energy level diagram, with the energy change (ΔH) being positive.
  • In exothermic reactions, the products have less energy than the reactants. The energy level of the products will be lower than the reactants on the energy level diagram, with the energy change (ΔH) being negative.

Energy and the Rate of Reaction

  • Energy is required to break the bonds in the reactant molecules. This is known as the activation energy.
  • An energy barrier must be overcome before a reaction can proceed. Reactant particles must collide with each other with enough energy to overcome this barrier. This is depicted in the hump in the middle of the energy level diagrams.
  • A catalyst can be used to speed up a reaction by providing an alternative reaction pathway with a lower activation energy.

Calculating Enthalpy Change

  • In chemistry, the heat flow in or out of a chemical reaction at constant pressure is called the enthalpy change (ΔH).
  • It can be determined experimentally by calorimetry, or calculated using the Hess’s Law which states that the overall enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place. It is calculated by subtracting the enthalpy of the reactants from that of the products.

Energetics and Stoichiometry

  • Stoichiometric coefficients in a balanced chemical equation also indicate the number of moles of heat energy involved in the reaction.
  • This energy is often provided or consumed in the form of heat. This can be represented as an additional reactant or product in the balanced chemical equation.

Experimentally Determining ΔH

  • The heat absorbed or released during a reaction can be determined by measuring temperature changes in a known mass of water surrounding the reaction (since the specific heat capacity of water is known).
  • The enthalpy change for the reaction in joules can be determined from the formula: ΔH = mass of water x specific heat capacity of water x change in temperature.