Topic 5: 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? Give examples

  A reaction where energy is transferred to the surroundings so that the surroundings temperature increases - combustion, oxidation reactions, and neutralistaion (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 - thermal decomposition, reaction of citric acid and sodium hydrogen carbonate. Positive sign of energy change
• What is activation energy?

  Minimum amount of energy that particles need to react
• What is a reaction profile?

  Reaction profile is a graph that shows the relative energies of reactants and product, as well as the activation energy of the reaction.
• What occurs in a chemical reaction in terms of bond energies? Describe exothermic and endothermic reactions in terms of bonding 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?

  ΔH = Σ (bond energies of bonds broken) - Σ (bond energies of bonds formed)
• 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 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 are the advantages and disadvantages of hydrogen fuel cells?

  Advantages: no pollutants, no recharging

  Disadvantages: flammable, difficult to store, fossil fuels production, toxic chemicals, expensive production of h2 by electrolysis
• Does breaking bonds release energy or require energy? 

  Require energy 
• Is breaking bonds exothermic or endothermic?

  Endothermic
• Bond energy values are always:

  Positive 
• Is forming a bond an exothermic or endothermic process?

  Exothermic
• What type of reaction produces a negative energy change?

  An endothermic reaction