Energy Changes

• what is the conservation of energy principle?

  • energy is conserved in chemical reactions
  • amount of energy in the universe at the end of a chemical reaction is the same as before the reaction takes places
• what is an exothermic reaction? give examples

  • reaction where energy is transferred to the surroundings so that the surrounding temperature increases
  • combustion, oxidation reactions, neutralisation
  • negative sign of energy change
• what is an endothermic reaction? give examples

  • reaction where energy is taken in from the surroundings so that the surrounding temperature decreases
  • thermal decomposition, reaction of citric acid and sodium hydrogencarbonate
• what is activation energy?

  minimum amount of energy that particles need to react
• what is a reaction profile?

  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
  • exo: energy released from forming bonds is greater than that needed to break the bonds endo: 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
• what is a cell?

  • composed of 2 electrodes dipped in an electrolyte solution
  • produces electricity from a chemical reaction
• what is a battery?

  2 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
• advantages and disadvantages of using cells and batteries

  • quite cheap, some are rechargeable, a convenient source of electrical energy
  • harmful chemicals
• describe rechargeable and non-rechargeable cells

  • r - chemical reactions are reversed when an external current is supplied
  • n - reactants are used up, can’t be recharged
• what is a fuel cell?

  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: 2H2 > 4H+ + 4e-
  • anode: O2 + 4H+ + 4e- > 2H2O
  • overall: > 2H2 + O2 > 2H2O
• advantages and disadvantages of hydrogen fuel cells

  • a: no pollutants, no recharging
  • d: flammable, H2 difficult to store, fossil fuel production, toxic chemicals, expensive production of H2 by electrolysis