Chemistry 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
• What does the conservation of energy state in chemical reactions?

  Energy amount remains the same before and after
• What happens to product molecules in an exothermic reaction?

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

  It transfers energy to the surroundings
• What are everyday uses of exothermic reactions?

  Self-heating cans and hand warmers
• What are some applications of endothermic reactions?

  Sports injury packs
• What is activation energy?

  The 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 and stir
  6. 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
  5. Add sodium hydroxide and stir
  6. Repeat until 40cm³ of NaOH added
• What happens to temperature during an exothermic reaction?

  Temperature rises as energy is released
• What energy changes occur during a chemical reaction?

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

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

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

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

  498 kJ/mol
• How do you calculate the energy change for the reaction 2 H-H + O=O → 2 H-O-H?

  Energy change = (2x436 + 498) - (4x464)
• What is 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 the reaction
• What happens in non-rechargeable cells and batteries?

  Chemical reactions stop when reactants are used up
• What type of batteries are alkaline batteries?

  Non-rechargeable
• What is a fuel cell?

  A device that produces electricity from fuel and oxygen
• What factors affect the voltage produced by a cell?

  Type of electrode and electrolyte
• What is a simple cell made of?

  Two different metals and an electrolyte
• How does recharging work in rechargeable cells and batteries?

  The chemical reactions are reversed by electrical current
• What is the relationship between reactivity and voltage in cells?

  Greater reactivity difference leads to larger voltage
• What is the overall reaction in a hydrogen fuel cell?

  Hydrogen + oxygen → water
• What is the equation for the overall reaction in a hydrogen fuel cell?

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

  Hydrogen reacts with hydroxide ions to produce water
• What happens at the positive electrode of a hydrogen fuel cell?

  Oxygen reacts with water to produce hydroxide ions
• What are the advantages of hydrogen fuel cells?

  • No pollutants produced, only water
  • Quicker to refuel than recharge
  • Can travel further before refueling
  • No toxic chemicals at end of life
  • Hydrogen can be renewable
• What are the disadvantages of hydrogen fuel cells?

  • Hydrogen is highly flammable/explosive
  • Difficult to store and takes up space
  • Not renewable if produced from fossil fuels
  • Limited hydrogen filling stations