C4 Chemical Change

• Exothermic reaction


  Reaction that transfers energy from the reacting molecules to the surroundings, causing the temperature of the surroundings to increase
• Endothermic reaction


  Reaction that takes in energy from the surroundings, causing the temperature of the surroundings to decrease
• Energy profile diagram


  1. Shows the energy changes that take place in a reaction

  2. Reactants have more energy than products in exothermic reactions

  3. Products have more energy than reactants in endothermic reactions

  4. Difference in energy between reactants and products is the energy released/absorbed
• Uses of exothermic reactions


  • Hand warmers
  • Self-heating cans
• Activation energy


  Minimum amount of energy particles must have in order to react
• Activation energy on energy profile diagram

  Shown as the energy rise from reactants to the peak of the curve
• Electron affinity is the amount of energy transferred to an atom or group of atoms when it gains one electron from its surroundings.
• Exothermic reaction


  Energy is transferred from the chemical to the surroundings
• Endothermic reaction


  Chemicals gain energy from the surroundings
• Calculating energy changes


  1. Look at formed energy when breaking chemical bonds

  2. Get structures of molecules involved

  3. Get energy values for each bond

  4. Break bonds on left side

  5. Make bonds on right side

  6. Add up energy to break bonds

  7. Add up energy to make bonds

  8. Subtract energy to make bonds from energy to break bonds
• Bond energy


  Energy required to break that bond
• Sample reaction


  • Hydrogen + Chlorine → Hydrogen Chloride
• Information needed to calculate energy change

  • Structures of molecules involved
  • Energy values for each bond
• Steps to calculate energy change

  1. Look at bonds broken on left side

  2. Look at bonds made on right side

  3. Add up energy to break bonds

  4. Add up energy to make bonds

  5. Subtract energy to make bonds from energy to break bonds
• Breaking bonds


  Endothermic (energy gained)
• Making bonds


  Exothermic (energy released)
• Final energy change = Energy to break bonds - Energy to make bonds
• The energy released when an atom gains an electron is called the first ionisation energy.