Chem - Energy Changes

• What is involved when a chemical reaction takes place?

  Energy is involved.
• What happens to energy during chemical reactions?

  Energy is transferred when chemical bonds are broken and when new bonds are made.
• What are exothermic reactions?

  Reactions that involve the transfer of energy from the reacting chemicals to the surroundings.
• What is observed during a practical investigation of an exothermic reaction?

  An increase in temperature as the reaction takes place.
• Give examples of exothermic reactions.

  Combustion, respiration, and neutralisation reactions.
• What are everyday examples of exothermic reactions?

  Hand warmers and self-heating cans.
• What are endothermic reactions?

  Reactions that involve the transfer of energy from the surroundings to the reacting chemicals.
• What is observed during a practical investigation of an endothermic reaction?

  A decrease in temperature as the reaction takes place.
• Give an example of an endothermic reaction.

  Thermal decomposition of calcium carbonate.
• How does eating sherbet demonstrate an endothermic reaction?

  It produces a cooling effect when it dissolves in saliva.
• What is another everyday example of an endothermic reaction?

  Instant ice packs used to treat sporting injuries.
• What do energy level diagrams show in chemical reactions?

  • Difference in energy between reactants and products
  • In exothermic reactions, reactants are at a higher energy level than products
  • In endothermic reactions, reactants are at a lower energy level than products
• What happens to the temperature of the surroundings in an exothermic reaction?

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

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

  The minimum amount of energy required for a chemical reaction to take place.
• How do catalysts affect chemical reactions?

  Catalysts increase the rate of a reaction by lowering the activation energy.
• What is the energy change (ΔH) in an endothermic reaction?

  Positive.
• What is the energy change (ΔH) in an exothermic reaction?

  Negative.
• How are bond energies measured?

  In kJ/mol.
• How do you calculate the change in energy for a chemical reaction using bond energies?

  1. Write the symbol equation for the reaction.
  2. Identify bonds breaking and bonds made.
  3. Calculate total bond energy of bonds broken.
  4. Calculate total bond energy of bonds made.
  5. Use the formula: ΔH = sum (bonds broken) - sum (bonds made).
• What is the change in energy for the reaction: 2H₂O₂ → 2H₂O + O₂?

  ΔH = -206 kJ/mol.
• Why is the reaction 2H₂O₂ → 2H₂O + O₂ considered exothermic?

  Because ΔH is negative.
• What is the aim of the required practical in investigating temperature changes in reacting solutions?

  • To investigate variables affecting temperature changes
  • Examples include acid plus metal, acid plus carbonates, neutralisations, and displacement of metals
• What equipment is needed for the required practical on temperature changes?

  • Polystyrene cup
  • Thermometer
  • Measuring cylinder
  • 250cm³ glass beaker
  • Top pan balance
• What is the method for conducting the required practical on temperature changes?

  1. Gather equipment.
  2. Place polystyrene cup inside beaker.
  3. Measure 30cm³ of acid and pour into cup.
  4. Record temperature of acid.
  5. Measure solid or metal and add to acid.
  6. Record temperature changes over time.
• Calculating ΔH using bond energies


  
  
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  Identify bonds breaking and bonds made, calculate total bond energy of bonds broken and bonds made, and use the formula: ΔH = sum (bonds broken) - sum (bonds made)
• Bond dissociation energy


  
  
• 
  

  The energy required to break a bond, usually expressed in kJ/mol.
• Endothermic reaction


  
  
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  A reaction that absorbs energy, indicated by a positive ΔH (ΔH > 0).
• Exothermic reaction