Chemistry C7 Energy Changes

• Reaction profile


  Diagram showing the relative amounts of energy contained in the reactants and the products, measured in kilojoules per mole (kJ/mol)
• Reaction profile


  • A curved line, drawn from reactants to products, shows the course of the reaction
  • The difference in energy between the reactants and the peak of the curve indicates the energy input required for the reaction to take place
• Exothermic reaction


  Products are at a lower energy level than the reactants, so energy is transferred to the surroundings
• Endothermic reaction


  Products are at a higher energy level than the reactants, so energy is transferred from the surroundings to the reaction mixture
• Activation energy


  The minimum amount of energy needed before colliding particles of reactants have sufficient energy to cause a reaction
• If colliding particles of reactants collide with less energy than the activation energy, they will just bounce off each other
• Breaking chemical bonds


  An endothermic process, energy is taken in from the surroundings
• Forming new chemical bonds

  An exothermic process, energy is transferred to the surroundings
• In reality, the bond breaking and making processes happen at the same time, but comparing the energy required to break the bonds with the energy released when the new bonds form gives a good guide to the overall energy change
• Exothermic reaction


  The energy released when new bonds are formed (as the products are made) is more than the energy needed to break the bonds in the reactants, so energy is transferred to the surroundings
• Endothermic reaction


  The energy needed to break the bonds in the reactants is more than the energy released when new bonds are formed in the products, so energy is transferred from the surroundings to the reacting chemicals
• Bond energy


  The energy needed to break the bond between two atoms, measured in kJ/mol
• You can use bond energies to work out the energy change for many chemical reactions
• To calculate the energy change for a chemical reaction


  1. Work out how much energy is needed to break the chemical bonds in the reactants

  2. Work out how much energy is released when the new bonds are formed in the products
• The data on bond energies is the energy required to break the bond, not the energy released when the bond is formed
• Bond energy


  The energy needed to break the bond between two atoms
• Bond energies are measured in kJ/mol
• The data in the table is the energy required for breaking bonds
• When bonds are formed, the energy involved is the same as when they are broken
• If the energy required to break bonds is greater than the energy released when new bonds are formed, the reaction is endothermic
• If the energy released when new bonds are formed is greater than the energy required to break bonds, the reaction is exothermic
• The value for the energy change is an approximation of the true value
• Oxidation reactions


  Exothermic - release energy
• Exothermic reactions have lots of everyday uses
• Exothermic reactions with everyday uses

  • Hand warmers using exothermic oxidation of iron in air
  • Self heating cans of hot chocolate and coffee using exothermic reactions between chemicals
• Endothermic reaction


  Reaction that takes in energy from the surroundings, shown by a fall in temperature
• Endothermic reactions


  • Reaction between citric acid and sodium hydrogencarbonate
  • Thermal decomposition of calcium carbonate
• Endothermic reactions have everyday uses
• It's important to know how much energy is absorbed or released in a reaction, not just if it is endothermic or exothermic</b>
• Measuring energy released by a chemical reaction

  1. Take temperature of reagents, mix in polystyrene cup, measure final temperature

  2. Reduce energy lost to surroundings by insulating cup and using lid
• This method works for neutralisation reactions, reactions between metals and acids, or carbonates and acids
• Testing effect of acid concentration on energy released in a neutralisation reaction

  Use 0.25, 0.5 and 1 mol/dm3 hydrochloric acid, measure temperature change
• Reaction profile


  Diagram showing relative energies of reactants and products, and how energy changes over the course of the reaction
• Exothermic reaction profile


  • Products are at lower energy than reactants, difference in height represents overall energy change
  • Initial rise in energy represents activation energy needed to start reaction
• Endothermic reaction profile


  • Products are at higher energy than reactants, difference in height represents overall energy change
  • Activation energy is the minimum energy needed for reactants to collide and react
• Energy must be supplied to break bonds, as bond breaking is endothermic
• Energy is released when new bonds are formed, as bond forming is exothermic
• In exothermic reactions, energy released by forming bonds is greater than energy used to break them. In endothermic reactions, energy used to break bonds is greater than energy released by forming them
• Calculating overall energy change of a reaction using bond energies

  Sum energy needed to break bonds in reactants minus energy released when new bonds form in products
• Chlorine and bromine react with hydrogen in a similar way, but bromine requires less energy to break bonds and releases less energy when forming new bonds