Calculating energy changes - Higher

• What is the difference between exothermic and endothermic reactions?

  Exothermic reactions transfer energy to the surroundings, while endothermic reactions take in energy.
• What happens to the temperature of the surroundings during an exothermic reaction?

  The temperature of the surroundings increases.
• What happens to the temperature of the surroundings during an endothermic reaction?

  The temperature of the surroundings decreases.
• What is bond energy?

  • The amount of energy needed to break one mole of a particular covalent bond.
  • Different bonds have different bond energies.
• How is energy measured in the context of bond energy?

  Energy is measured in joules (J).
• What is a mole in chemistry?

  A mole is the amount of substance that contains the same number of particles as there are atoms in 12 g of carbon-12.
• What is Avogadro's constant?

  Avogadro's constant is approximately \(6.0 \times 10^{23}\) number of particles.
• How do you calculate the energy change for a reaction?

  1. Add together the bond energies for all the bonds in the reactants (energy in).
  2. Add together the bond energies for all the bonds in the products (energy out).
  3. Energy change = energy in - energy out.
• What is the energy change for the reaction of hydrogen and chlorine to form hydrogen chloride gas?

  The energy change is \(-185 \text{ kJ mol}^{-1}\), indicating an exothermic reaction.
• What is the energy change for the decomposition of hydrogen bromide?

  The energy change is \(+103 \text{ kJ mol}^{-1}\), indicating an endothermic reaction.
• How do you calculate the energy change for the reaction of hydrogen and oxygen to form water?

  The energy change is \(-486 \text{ kJ mol}^{-1}\), indicating an exothermic reaction.
• What does a negative energy change indicate about a reaction?

  • A negative energy change indicates that the reaction is exothermic.
  • Energy is released to the surroundings.
• What does a positive energy change indicate about a reaction?

  • A positive energy change indicates that the reaction is endothermic.
  • Energy is absorbed from the surroundings.