TOPIC 5

• Chemical reactions

  Elements can achieve a more stable energy state by gaining a full outer shell of electrons
• Chemical bonding

  1. Old bonds are broken

  2. New bonds are formed
• System

  What happens in the chemical reaction
• Surroundings

  The environment outside the chemical reaction
• Chemical energy

  Stored in chemical bonds
• Heat energy

  The most common form of energy released in reactions
• Physical processes can also involve heat exchange, such as freezing or melting which involve a change in state
• Exothermic reactions

  Energy is given out to the surroundings, increasing the temperature of the surroundings
• Exothermic reactions

  • Combustion
  • Oxidation
  • Neutralisation
• Hand warmers and self-heating cans use exothermic reactions
• Endothermic reactions

  Energy is taken in from the surroundings, decreasing the temperature of the surroundings

  
• Endothermic reactions

  • Electrolysis
  • Thermal decomposition
  • First stages of photosynthesis
  • Reaction between citric acid and sodium hydrogencarbonate
• Sports injury treatment often uses cold packs based on endothermic reactions to take heat away from the injured area
• The overall amount of energy does not change as energy is conserved in reactions
• If energy is transferred to the surroundings during a chemical reaction, then the products formed must have less energy than the reactants by the same amount as that transferred
• Examples of heat changes in reactions

  • Neutralisation reactions always give energy out
  • Displacement reactions can either take energy in or give it out
  • Combustion reactions always give energy out

  
• In exothermic reactions, the temperature of the surroundings increases and the heat content of the system falls

  
• In endothermic reactions, the temperature of the surroundings falls and the heat content of the system increases

  
• You may be asked to deduce whether a reaction is exothermic or endothermic given experimental results involving temperature changes

  
• The temperature change of the surroundings can be used to decide whether a reaction is exothermic or endothermic

  
• Required Practical 4: Investigating Temperature Changes


  • Objective: To perform a calorimetry study of the reaction between HCl and NaOH
  • Hypothesis: The temperature change will vary depending on the amounts of solutions involved

  
• Required Practical 4 method

  1. Place 25 cm3 of NaOH solution into the calorimeter

  2. Measure and record the temperature of the solution

  3. Add 5 cm3 of dilute HCl and stir

  4. Measure and record the highest temperature reached by the mixture

  5. Repeat increasing the amount of acid by 5 cm3 each time
• The larger the difference in the temperature the more energy is absorbed or released
• Activation energy


  The minimum amount of energy required for atoms or particles to react in a collision
• All reactions have an activation energy as the chemical bonds in the reactant molecules have to be broken first
• Reaction profiles

  Graphical representations of the relative energies of the reactants and products in chemical reactions

  
• The initial increase in energy, from the reactants to the peak of the curve, represents the activation energy required to start the reaction
• The greater the initial rise then the more energy that is required to get the reaction going, e.g. more heat needed
• In exothermic reactions, the energy of the products is lower than the energy of the reactants, so the change in energy is negative
• In endothermic reactions, the energy of the products is higher than the energy of the reactants, so the change in energy is positive

  
• You should be able to draw clear and fully labelled reaction profile diagrams for both exothermic and endothermic reactions, and identify a reaction from its reaction profile

  
• Bond breaking

  An endothermic process that absorbs energy from the reaction surroundings
• Bond forming


  An exothermic process that releases energy to the surroundings
• Whether a reaction is overall endothermic or exothermic depends on the difference between the sum of the exothermic steps and the sum of the endothermic steps
• Endothermic reactions


  More energy is required to break the bonds than is released from making the new bonds, so the change in energy is positive and the products have more energy than the reactants
• Exothermic reactions

  More energy is released when new bonds are formed than energy required to break the bonds in the reactants, so the change in energy is negative and the products have less energy than the reactants
• Bond breaking is endothermic and results in the end of the bond
• Bond energy


  The amount of energy required to break a bond or the amount of energy given out when the bond is formed
• Calculating the energy change of a reaction using bond energies

  1. Add together all the bond energies for all the bonds in the reactants - this is the 'energy in'

  2. Add together the bond energies for all the bonds in the products - this is the 'energy out'

  3. Calculate the energy change using the equation: Energy change = Energy taken in - Energy given out
• For bond energy questions, it is helpful to write down a displayed formula equation for the reaction before identifying the type and number of bonds, to avoid making mistakes