Topic 5 - Energy Changes

• What is the conversation of energy principle?

  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? Give examples.

  A reaction where energy is transferred to the surroundings so that the surroundings‘ temperature increases. E.g: combustion, oxidation reactions and neutralisation (acid+alkali) reactions. Negative sign of energy change.
• What is an endothermic reaction? Give examples.

  A reaction where energy is taken in from the surroundings so the surroundings’ temperature decreases. E.g: thermal decomposition, reaction of citrine acid and sodium hydrogencarbonate. Positive sign of energy change.
• What is activation energy?

  Activation energy is the minimum amount of energy required for a chemical reaction to occur.
• What is a reaction profile?

  A graphical representation which Shows the relative energies of reactants and products, 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 supposed to break bonds and energy is released when bonds are made;

  Exothermic - energy released from forming bonds is greater then 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 broken bonds - 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.
• 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 infectious and concentration of an electrolyte.
• State the advantages/disadvantages of using cells and batteries.

  Advantage: more or less cheap, some are rechargeable, a convenient source of electrical energy.

  Disadvantage: 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 fuels and oxygen to oxidise the fuel to generate electricty.
• What is the overall reaction in a hydrogen fuel cell? What are the half equations?

  Overall reaction: 2H2 + O2 -> 2H2O

  Half equations:

  1. Cathode: 2H2 -> 4H+ + 4e-

  2. Anode: O2 + 4H+ + 4e- -> 2H2O
• What are the advantages and disadvantages of hydrogen fuel cells?

  Advantages: No pollutants, no recharging. Disadvantages: Flammable, fossil fuel production, expensive, limited infrastructure.