Energy changes

2.3 Rates of reaction

2.5 Reversible reactions

Energy changes

  1. Calculating & explaining energy changes

    1. Calorimetry

      1. When chemical reactions occur, energy is transferred to or from the surroundings
      2. The enthalpy change is denoted by ΔH
      3. The relative amounts of energy released when substances, such as fuels, burn can be measured by simple calorimetry, such as heating water in a glass or metal container
      4. The amount of energy released can be calculated using the equation Q = mcΔT, wherein Q denotes the energy, m the mass, c the specific heat capacity and ΔT the change in the temperature of the heated substance
      5. Energy is normally measured in joules J but for comparison purposes, energy values could be given in kilojoules or calories for a given mass or amount of substance, in grams or moles respectively
      6. The energy change of a chemical reaction in solution, such as solid-with-water or neutralisation reaction, can be calculated from the temperature change of the solution in an insulated container

    2. Energy level diagrams

      1. A simple energy level diagram shows the energy in a reaction as it progresses
      2. The relative energies of the reactants and products, as well as the activation energy, are represented by horizontal lines
      3. The overall energy change over time is represented by a curved arrow

    3. Explanation of energy changes

      1. Energy must be supplied to break bonds and energy is released when bonds are formed
      2. The overall energy change of a reaction is equal to the total amount of energy that would be required to break the bonds in the products minus the total amount of energy that would be required to break the bonds in the reactants
      3. Catalysts provide a different pathway for a chemical reaction that has a lower activation energy

  2. Exothermic & endothermic reactions

    1. Exothermic

      1. An exothermic reaction transfers energy to the surroundings and therefore has a negative energy change
      2. In an exothermic reaction, the energy released from forming new bonds is greater than the energy required to break existing bonds
      3. Examples of exothermic reactions include combustion, many oxidation reactions and neutralisation
      4. Applications of exothermic reactions include self-heating cans and hand warmers

    2. Endothermic

      1. An endothermic reaction takes in energy from the surroundings and therefore has a positive energy change
      2. In an endothermic reaction, the energy released from forming new bonds is lower than the energy required to break existing bonds
      3. Examples of endothermic reactions include thermal decomposition
      4. Applications of endothermic reactions include some sports injury packs

    3. Reversible

      1. If a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction, with the same amount of energy being transferred in each case