Energy changes

• What is the conservation of energy principle?

  - Energy is conserved in chemical reactions.

  
  

  - The amount of energy in the universe at the end of a chemical reaction is the same as before the reaction takes place.
• What is an exothermic reaction?

  A reaction where energy is transferred to the surroundings so that the surrounding temperatures increases.
• Examples of exothermic reactions

  - Combustion

  - Neutralisation

  - Many oxidation reactions
• Everyday uses of exothermic reactions

  - Hand warmers,

  - self heating cans
• What is an endothermic reaction?

  A reaction where energy is transferred from the surroundings so the surrounding temperature decreases
• Examples of endothermic reactions

  - Thermal decomposition

  
  

  - the reaction of citric acid and sodium hydrogencarbonate
• Everyday uses of endothermic reactions

  Sports injury packs
• What is activation energy?

  the minimum amount of energy needed that particles need to react
• What is a reaction profile?

  a graph which shows the relative energies of reactants and product, as well as activation energy of the reaction.
• What occurs in a chemical reaction in terms of bond energies? Describe exothermic and endothermic reactions in terms of bond breaking/forming.

  • Energy is supplied to break bonds and energy is released when bonds are made

  
  

  • exothermic - energy released from forming bonds is greater than that needed to break the bonds;

  
  

  • endothermic - energy needed to break bonds is greater than energy released making them
• What is the equation to find enthalpy change in terms of bond energies?

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

  A cell is composed of two electrodes dipped in an electrolyte solution. It produces electricity from a chemical reaction.
• How can a simple cell be made?

  By connecting two different metals in contact with an electrolyte
• What is a battery?

  A battery consists of two or more cells connected in series.
• What determines the voltage obtained from a cell?

  Identities of metals used as electrodes and the identity and concentration of an electrolyte.
• State the advantages and disadvantages of using cells and batteries.

  • (+) more or less cheap, some are rechargeable, a convenient source of electrical energy

  
  

  • (-) harmful chemicals
• Describe rechargeable and non-rechargeable cells

  • Rechargeable - chemical reactions are reversed when an external current is supplied

  
  

  • Non-rechargeable - reactants are used up, cannot be recharged
• What is a fuel cell?

  - Fuel cells are supplied by fuel and oxygen to oxidise the fuel to generate electricity.
• What is the overall reaction in a hydrogen fuel cell? What are the half equations?

  Cathode: 2 H2 → 4 H+ + 4 e−

  
  

  Anode: O2 + 4 H+ + 4 e−→ 2 H2O

  
  

  Overall: → 2 H2 + O2 → 2 H2O
• What are the advantages and disadvantages of hydrogen fuel cells?

  • Advantages:

  
  

  - no pollutants,

  - no recharging

  
  

  • Disadvantages:

  
  

  - flammable,

  - H2 difficult to store,

  - fossil fuel production,

  - toxic chemicals,

  - expensive production of H2 by electrolysis