energy changes

• What are the two types of chemical reactions discussed?

  Exothermic and endothermic reactions
• What happens to energy during a chemical reaction?

  Energy is moved around in a chemical reaction
• What occurs if the products store less energy than the reactants?

  Excess energy is transferred to the surroundings
• What principle states that energy cannot be created or destroyed?

  Energy conservation principle
• What type of reactions are neutralization reactions classified as?

  Exothermic reactions
• What type of process is breaking existing bonds?

  Endothermic process
• What happens when new bonds are formed?

  Energy is released
• In endothermic reactions, what is true about the energy used and released?

  Energy used is greater than energy released
• 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.
• An exothermic reaction is one that transfers energy to the surroundings so the temperature of the surroundings increases.
• Exothermic reactions include combustion, many oxidation reactions and neutralisation.
• An endothermic reaction is one that takes in energy from the surroundings so the temperature of the surroundings decreases.
• Endothermic reactions include thermal decompositions and the reaction of citric acid and sodium hydrogencarbonate.
• 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 called the activation energy.
• Reaction profiles can be used to show the relative energies of reactants and products, the activation energy and the overall energy change of a reaction.
• During a chemical reaction:

  • energy must be supplied to break bonds in the reactants
  • energy is released when bonds in the products are formed.
• 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.
• In an exothermic reaction, the energy released from forming new bonds is greater than the energy needed to break existing bonds.
• In an endothermic reaction, the energy needed to break existing bonds is greater than the energy released from forming new bonds.

   
• The energy needed to break bonds and the energy released when bonds are formed can be calculated from bond energies.