Exothermic and Endothermic Reactions

• Energy transfers

  When chemical reactions occur, energy is transferred from the chemicals to or from the surroundings
• Many reactions are accompanied by a temperature change
• Exothermic reactions

  • Accompanied by a temperature rise
  • Transfer heat energy from the chemicals to the surroundings
  • Give out heat energy
• Exothermic reactions

  • Combustion
  • Rusting
  • Neutralisation
• Products using exothermic reactions

  • Self-heating cans (for coffee)
  • Hand warmers
• Endothermic reactions

  • Accompanied by a fall in temperature
  • Heat energy is transferred from the surroundings to the chemicals
  • Take in heat energy
• Endothermic reactions

  • Thermal decomposition
  • Reaction between citric acid and sodium hydrogen carbonate
• Products using endothermic reactions

  • Sports injury packs
• If a reversible reaction is exothermic in one direction

  It is endothermic in the opposite direction
• The same amount of energy is transferred in each case
• Key Point

  . Exothermic reactions are accompanied by a temperature rise

  . Endothermic reactions are accompanied by a fall in temperature
• Investigate the variables that affect temperature changes in reacting solutions

  1. Set up the equipment

  2. Take the temperature of the acid

  3. Add the metal powder and stir

  4. Record the highest temperature the reaction mixture reaches

  5. Calculate the temperature change for the reaction

  6. Repeat the experiment using a different metal
• Reactivity of the metal

  Temperature change
• The more reactive the metal, the greater the temperature change
• When a measurement is made there is always some uncertainty about the results obtained so work out the mean
• Variables

  • Independent variable: the metal used
  • Dependent variable: the temperature change
  • Control variables: the type, concentration and volume of acid
• Eye protection must be used
• Key Point

  . An accurate measurement is one that is close to the true value. If the experiment was repeated the results could be analysed to see if they are precise. The results are precise if they are all close together

  . Measurements are repeatable if the experimenter repeats the experiment and gets similar results. Results are reproducible if similar results are obtained by different experimenters using different equipment
• Chemical reactions

  Atoms are rearranged as old bonds are broken and new bonds are formed
• Bonds being broken

  • Reacting particles must collide with sufficient energy
• Activation energy

  The minimum amount of energy that the particles must have for a reaction to take place
• Energy level diagram or reaction profile

  Shows the energy changes in a chemical reaction
• Exothermic reaction

  • Energy is given out to the surroundings
  • The products have less energy than the reactants
• Endothermic reaction

  • Energy is being taken in from the surroundings
  • The products have more energy than the reactants
• Catalyst

  Reduces the activation energy needed for a reaction, making the reaction go faster
• Key Point

  A catalyst makes a reaction go faster by lowering the activation energy