Chemical reactions and Energy

• What is an exothermic reaction?

  A reaction that releases energy to the surroundings, causing the temperature to increase.
• What is an endothermic reaction?

  A reaction that absorbs energy from the surroundings, causing the temperature to decrease.
• Give two examples of exothermic reactions.

  Combustion, neutralisation.
• Give two examples of endothermic reactions.

  Thermal decomposition, photosynthesis.
• In an energy profile, how do exothermic reactions appear?

  Reactants start higher than products; the curve goes down, showing energy is released.
• In an energy profile, how do endothermic reactions appear?

  Reactants start lower than products; the curve goes up, showing energy is absorbed.
• What is activation energy?

  The minimum energy required to start a reaction by breaking bonds in reactants.
• What effect does a catalyst have on activation energy?

  It lowers the activation energy, making the reaction occur more easily.
• What happens during bond breaking?

  Energy is absorbed – this is an endothermic process.
• What happens during bond formation?

  Energy is released – this is an exothermic process.
• How do you calculate the overall energy change in a reaction?

  Energy change = Energy to break bonds - Energy released forming bonds
• How can you tell if a reaction is exothermic using bond energies?

  If the result is negative (more energy released than absorbed), it’s exothermic.
• How can you tell if a reaction is endothermic using bond energies?

  If the result is positive (more energy absorbed than released), it’s endothermic.