topic 5 - energy changes

• How does the principle of energy conservation apply to chemical reactions?

  Energy is conserved in chemical reactions. The amount of energy in the universe at the end of a chemical reaction is the same as before the reaction takes place. If a reaction transfers energy to the surroundings the product molecules must have less energy than the reactants, by the amount transferred.
• What is a exothermic reaction?

  Transfers energy to the surroundings so the temperature of the surroundings increases e.g. self-heating cans and hand warmers
• What are examples of exothermic reactions?

  Combustion, many oxidation reactions and neutralisation
• What is a endothermic reaction?

  Takes in energy from the surroundings so the temperature of the surroundings decreases
• What are examples of endothermic reactions?

  Thermal decompositions and the reaction of citric acid and sodium hydrogen carbonate
• What is activation energy?

  Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy, the minimum amount of energy that particles must have to react is the activation energy
• What are reaction profiles used to show?

  The relative energies of reactants and products, the activation energy and the overall energy change of a reaction
• How do reaction profiles for exothermic and endothermic reactions look?

  
• What happens during a chemical reaction?

  • energy must be supplied to break bonds in the reactants
  • energy is released when bonds in the products are formed
• What determines the overall energy change of a chemical reaction?

  The difference between the sum of the energy needed to break bonds in the reactants and the sum of the energy released when bonds in the products are formed is the overall energy change of the reaction.
• What can be calculated using bond energies?

  The energy needed to break bonds and the energy released when bonds are formed
• What is the bond energy like in a exothermic reaction?

  The energy released from forming new bonds is greater than the energy needed to break existing bonds.
• What is the bond energy like in a endothermic reaction?

  The energy needed to break existing bonds is greater than the energy released from forming new bonds.