Topic 5 : Energy Changes

• What is an endothermic reaction?

  It takes in energy from surroundings
• What happens to the temperature of surroundings during an endothermic reaction?

  The temperature of surroundings decreases
• Name two examples of endothermic reactions.

  Thermal decompositions and citric acid reaction
• What does the conservation of energy state in chemical reactions?

  Energy amount remains the same before and after
• What must be true if a reaction transfers energy to the surroundings?

  Products have less energy than reactants
• What is an exothermic reaction?

  It transfers energy to the surroundings
• What happens to the temperature of surroundings during an exothermic reaction?

  The temperature of surroundings increases
• Give two examples of exothermic reactions.

  Combustion and neutralisation reactions
• What are everyday uses of exothermic reactions?

  Self-heating cans and hand warmers
• What type of reactions are some sports injury packs based on?

  Endothermic reactions
• What is activation energy?

  Minimum energy needed for particles to react
• What is the general method for measuring energy change in a reaction?

  1. Use a polystyrene cup for insulation
  2. Measure solution volumes
  3. Clamp thermometer in place
  4. Measure initial temperatures
  5. Transfer reagents to cup
  6. Stir mixture
  7. Measure final temperature
• What are the steps for measuring energy change during neutralisation?

  1. Use a polystyrene cup for insulation
  2. Measure hydrochloric acid volume
  3. Clamp thermometer in place
  4. Measure initial temperature of acid
  5. Add sodium hydroxide and stir
  6. Measure temperature after each addition
• What happens to energy during a chemical reaction?

  Energy is supplied to break bonds and released when formed
• How is the overall energy change of a reaction calculated?

  Energy change = bonds broken - bonds made
• What is the bond energy of H-H?

  436 kJ/mol
• What is the bond energy of O=O?

  498 kJ/mol
• What is the bond energy of O-H?

  464 kJ/mol
• Calculate the energy change for the reaction: 2 H-H + O=O → 2 H-O-H.

  -486 kJ/mol
• What does a reaction profile show?

  • Relative energies of reactants and products
  • Activation energy
  • Overall energy change of a reaction
• In an exothermic reaction, how do the energy levels of products compare to reactants?

  Products are lower in energy than reactants
• In an endothermic reaction, how do the energy levels of products compare to reactants?

  Products are higher in energy than reactants
• What happens to chemical reactions in non-rechargeable batteries?

  They stop when a reactant is used up
• What type of batteries are alkaline batteries?

  Non-rechargeable
• What is a fuel cell supplied by?

  An external source of fuel and oxygen
• What happens in a hydrogen fuel cell?

  Hydrogen is oxidised to produce water
• What is the overall reaction in a hydrogen fuel cell?

  2H2 + O2 → 2H2O
• What is produced at the negative electrode of a hydrogen fuel cell?

  Water and electrons
• What is produced at the positive electrode of a hydrogen fuel cell?

  Hydroxide ions
• What are the advantages and disadvantages of hydrogen fuel cells?

  Advantages:

  • No pollutants produced
  • Do not need recharging
  • Hydrogen can be renewable

  
  

  Disadvantages:

  • Highly flammable/explosive
  • Difficult to store
  • Not many filling stations