Topic 5-Energy Changes

• What always changes when chemical reactions occur?

  Change in energy
• What must of have happened in a reaction if the products of a reaction store more energy than the original reactants?

  Energy must have been taken in from the surroundings
• It the products store less energy than the reactants, then what must have happened?

  Excess energy was transferred to the surroundings during the reaction
• What is an exothermic reaction?

  When energy is transferred to the surroundings, usually by heating
• How can you tell an exothermic reaction has taken place?

  Rise in temperature
• Give 3 examples of an exothermic reaction

  Combustion (burning feuIs), neutralisation, oxidation
• What are the everday uses of exothermic reactions?

  .Hand warmers, self heating cans
• What is an endothermic reaction?

  When energy is taken in from the surroundings, shown by a fall in temperature
• Give 2 examples of endothermic reactions

  The reaction between citric acid and sodium hydrogencarbonate, thermal decomposition
• How can you measure the amount of energy released by a chemical reaction?

  Taking a temperature of the reagents and massuring the temperature of the solution at the end of the reaction
• What is the biggest problem with energy measurements?

  Energy lost to the surroundings
• How can you reduce the amount of energy lost?

  1. Using a polystyrene cup to give more insulation
  2. Putting a lid on the ctup to reduce energy lost by evaporation
• How would you test the effect of acid concentration on the energy released in a neutralisation.reaction between hydrochloric acid and sodium hydroxide?

  1. Put 25cm³of 0.25mol/dm³ of hydrochloric acid and sodium hydrotide in seperate beakers
  2. Place the beakers in a water bath set to 25° C until they are both the same temp.
  3. Add them together to a polystyrene cup with a lid
  4. Take the temp of the mixture every 30 seconds
  5. Repeat steps 1-4 using 0.5 mol/dm³ and then 1mol/dm³
• What are reaction profiles?

  Graphical representations of changes in energy to the reactants and products during a chemical reaction
• Reaction Profile for an exothermic reaction

  The products are at a lower energy than the reactants

  The difference in height represents the overall energy change in the reaction per mole

  
• What does the initial rise in energy represent?

  The energy needed to start the reaction, activation energy
• What is the activation energy?

  The minimum amount of energy the reactants need to collide with each other and react
• Reaction Profile for endothermic reaction

  The products are at a higher energy than the reactants

  
• What happens in a chemical reaction in relation to bonds?

  Old bonds are broken and new bonds are formed
• What has to be supplied in order to break existing bonds?

  Energy
• What type of process is bond breaking?

  Endothermic
• What is released when new bonds are formed?

  Energy
• What type of process is bond formation?

  Exothermic
• What is associated with every chemical bond?

  Particular bond energy
• How do you calculate the overall change in energy?

  sum of the energies needed to break the bonds minus the energy released when the new bonds are formed in the products