ENERGY CHANGE

• what is an exothermic reaction?


  reactions which transfer energy to their surroundings, usually by heating
• what is an endothermic reaction?


  reactions which take in energy from their surroundings
• what happens during an exothermic reaction?

  bonds are formed and energy is transferred
• what happens during an endothermic reaction?


  bonds are broken and energy is absorbed
• what are some examples of exothermic reactions?


  combustion, neutralization reactions, oxidation reactions
• what are some examples of endothermic reactions?

  photosynthesis, thermal decomposition
• what is activation energy?

  the energy needed to start a reaction
• what is the reaction profile for exothermic reactions?

  
• what is the reaction profile for endothermic reactions?

  
• what is an electrochemical cell?

  a basic system made up of two different electrodes in contact with an electrolyte
• how does the difference in reactivity affect the voltage?

  the bigger the difference in reactivity, the bigger the voltage of the cell
• what is a fuel cell?


  an electrical cell that is supplied with a fuel and oxygen and uses energy from the reaction to efficiently produce electrical energy
• what occurs inside a fuel cell?

  the fuel enters the cell and becomes oxidized and sets up a voltage
• what is produced from a hydrogen-oxygen fuel cell?

  water and energy
• which electrode does oxygen go to?


  cathode
• which electrode does hydrogen go to?

  anode
• what is the reduction equation in a hydrogen-oxygen fuel cell?


  O₂ + 4H⁺ + 4e⁻ -> 2H₂O
• what is the oxidation equation in a hydrogen-oxygen fuel cell?

  H₂ -> 2H⁺ + 2e⁻
• what is the overall reaction in a fuel cell?

  2H₂ + O₂ -> 2H₂O
• what are the advantages of h-o fuel cells?

  no pollutants, less expensive than batteries, stores more energy than batteries
• what are the disadvantages of h-o fuel cells?

  gas takes up a lot of space, explosive, made from hydrocarbons/electricity
• 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
• Exothermic reaction


  A reaction where energy is transferred to the surroundings so that the surroundings temperature increases
• Exothermic reactions


  • Combustion
  • Oxidation reactions
  • Neutralisation (acid + alkali) reactions
• Endothermic reaction


  A reaction where energy is taken in from the surroundings so the surroundings temperature decreases
• Endothermic reactions


  • Thermal decomposition
  • Reaction of citric acid and sodium hydrogencarbonate
• Activation energy

  Minimum amount of energy that particles need to react
• Reaction profile


  A graph which shows the relative energies of reactants and product, as well as activation energy of the reaction
• In a chemical reaction, energy is supplied to break bonds and energy is released when bonds are formed
• Exothermic reaction


  Energy released from forming bonds is greater than that needed to break the bonds
• Endothermic reaction


  Energy needed to break bonds is greater than energy released making them
• Equation to find enthalpy change in terms of bond energies

  Energy of reaction = sum of bonds broken – sum of bonds made
• Cell


  Composed of two electrodes dipped in an electrolyte solution. It produces electricity from a chemical reaction.
• Battery


  Consists of two or more cells connected in series
• Voltage obtained from a cell


  Determined by the identities of metals used as electrodes and the identity and concentration of an electrolyte
• Advantages and disadvantages of using cells and batteries

  • Advantages: cheap, some are rechargeable, a convenient source of electrical energy
  • Disadvantages: harmful chemicals
• Rechargeable cells

  Chemical reactions are reversed when an external current is supplied
• Non-rechargeable cells


  Reactants are used up, cannot be recharged
• Fuel cell


  Supplied by fuel and oxygen to oxidise the fuel to generate electricity
• Overall reaction in a hydrogen fuel cell

  1. Cathode: 2 H2 → 4 H+ + 4 e−

  2. Anode: O2 + 4 H+ + 4 e−→ 2 H2O

  3. Overall: 2 H2 + O2 → 2 H2O