topic 5-energy changes

• What is the conservation of energy principle?

  Energy is conserved in chemical reactions
• What does the conservation of energy principle state about the universe's energy?

  The energy remains the same before and after reactions
• What is an exothermic reaction?

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

  The temperature of surroundings increases
• What is an endothermic reaction?

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

  The temperature of surroundings decreases
• What is activation energy?

  Minimum energy needed for particles to react
• What is a reaction profile?

  A graph showing energies of reactants and products
• What does a reaction profile illustrate about activation energy?

  It shows the activation energy of the reaction
• Describe the energy changes in exothermic and endothermic reactions in terms of bond energies.

  • Energy is supplied to break bonds
  • Energy is released when bonds are formed
  • Exothermic: energy released > energy needed
  • Endothermic: energy needed > energy released
• What is the equation to find enthalpy change in terms of bond energies?

  Energy of reaction = bonds broken - bonds made
• What is a cell?

  A device producing electricity from chemical reaction
• What is a battery?

  A combination of two or more cells in series
• What determines the voltage obtained from a cell?

  Metals used as electrodes and electrolyte concentration
• What are the advantages and disadvantages of using cells and batteries?

  Advantages:

  • Cheap
  • Some are rechargeable
  • Convenient energy source

  
  

  Disadvantages:

  • Harmful chemicals
• Describe rechargeable and non-rechargeable cells.

  • Rechargeable: reactions reversed with external current
  • Non-rechargeable: reactants used up, cannot be recharged
• What is a fuel cell?

  A device generating electricity from fuel and oxygen
• What is the overall reaction in a hydrogen fuel cell?

  2 H2 + O2 → 2 H2O
• What are the half equations in a hydrogen fuel cell?

  2 H2 → 4 H+ + 4 e− and O2 + 4 H+ + 4 e− → 2 H2O
• What are the advantages and disadvantages of hydrogen fuel cells?

  Advantages:

  • No pollutants
  • No recharging needed

  
  

  Disadvantages:

  • Flammable
  • Difficult to store H2
  • Fossil fuel production
  • Toxic chemicals
  • Expensive H2 production by electrolysis