topic 5- energy changes

• Chemical reactions


  When they occur, there are changes in energy
• When chemicals react, they can heat up or cool down
• Energy


  It is moved around in chemical reactions
• Chemicals store a certain amount of energy
• If the products of a reaction store more energy than the reactants


  The difference in energy is taken from the surroundings during the reaction
• If the products store less energy than the reactants

  The excess energy was transferred to the surroundings during the reaction
• The overall amount of energy doesn't change, as energy is conserved in reactions - it can't be created or destroyed, only moved around
• Exothermic reaction


  A reaction that transfers energy to the surroundings, usually by heating, shown by a rise in temperature
• Examples of exothermic reactions

  • Burning fuels (combustion)
  • Neutralisation reactions (acid + alkali)
  • Many oxidation reactions
• Exothermic reactions have lots of everyday uses, such as in hand warmers and self-heating cans
• Endothermic reaction


  A reaction that takes in energy from the surroundings, shown by a fall in temperature
• Examples of endothermic reactions

  • Reaction between citric acid and sodium hydrogencarbonate
  • Thermal decomposition (e.g. heating calcium carbonate)
• Endothermic reactions also have everyday uses, such as in sports injury packs
• Measuring energy transfer in a chemical reaction

  1. Take the temperature of the reagents before and after mixing in a polystyrene cup

  2. Reduce energy lost to surroundings by insulating the cup

  3. Can investigate effect of variables like mass or concentration
• Reaction profile


  A diagram that shows the relative energies of the reactants and products, and how the energy changes over the course of the reaction
• Exothermic reaction profile


  • Products are at a lower energy than reactants
  • Difference in height represents the overall energy change (energy given out) per mole
  • Initial rise in energy represents the activation energy needed to start the reaction
• Endothermic reaction profile


  • Products are at a higher energy than reactants
  • Difference in height represents the overall energy change (energy taken in) per mole
• Energy must be supplied to break chemical bonds, as bond breaking is an endothermic process
• Energy is released when new chemical bonds are formed, as bond formation is an exothermic process
• In exothermic reactions, the energy released by forming bonds is greater than the energy used to break them
• In endothermic reactions, the energy needed to break bonds is greater than the energy released by forming them
• Bond energy


  The particular energy associated with each chemical bond, which varies slightly depending on the compound
• Calculating the overall energy change of a reaction using bond energies

  1. Find the energy required to break the original bonds

  2. Find the energy released by forming the new bonds

  3. Overall energy change = energy to break bonds - energy released by forming bonds
• You can't compare the energy changes of different reactions unless you know the numerical differences in the bond energies
• Electrochemical cell


  A basic system made up of two different electrodes in an electrolyte solution, which produces electricity from chemical reactions
• /mol


  +242 kJ/mol
• 678 kJ/mol


  Energy released by forming the new bonds
• Find the energy released by forming the new bonds

  2 H-Cl 2 431 kJ/mol - 862 kJ/mol
• Find the overall energy change for the reaction

  1. Overall energy change-energy required to break bonds - energy released by forming bonds

  2. -678 kJ/mol-862 kJ/mol -184 kJ/mol
• You can't compare the energy changes of reactions unless you know the numerical differences in the bond energies
• Chlorine and bromine react with hydrogen

  Br-Br bonds are weaker than Cl-Cl bonds and H-B bonds are weaker than H-Cl bonds
• Less energy is needed to break the bonds in the reaction with bromine


  But less energy is released when the new bonds form
• Unless you know the exact difference, you can't say which reaction releases more energy
• Electrochemical cell


  A basic system made up of two different electrodes in contact with an electrolyte
• Electrodes


  • They must be able to conduct electricity and are usually metals
• Electrolyte


  A liquid that contains ions which react with the electrodes
• Chemical reactions between the electrodes and the electrolyte

  1. Set up a charge difference between the electrodes

  2. If the electrodes are then connected by a wire, the charge is able to flow and electricity is produced

  3. A voltmeter can also be connected to the circuit to measure the voltage of the cell
• Voltage of a cell


  • Depends on the type of electrodes used
  • Depends on the difference in reactivity of the electrodes
  • Can be predicted from information about the voltages of other cells
• Voltages of two simple cells with the same electrolyte and different electrodes


  • Iron/iron cell: -0.44V
  • Iron/tin cell: -0.43V
• If the metal used for Electrode A is less reactive than the other metal electrode, the voltage is positive and vice versa