C5 - 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 it takes place.
• What type of reaction transfers energy to the surroundings, increasing the surrounding's temperature?

  Exothermic reaction
• What is the sign of the energy change in an exothermic reaction?

  Negative
• What type of reaction takes in energy from the surroundings, decreasing the surrounding's temperature?

  Endothermic reaction
• What is the sign of the energy change in an endothermic reaction?

  Positive
• What is activation energy?

  Minimum energy particles need to react
• What type of graph is a reaction profile?

  A graph which shows the relative energies of reactants and products
• What does a reaction profile show in addition to the relative energies of reactants and products?

  Activation energy of the reaction
• In terms of bond energies, what generally occurs in a chemical reaction?

  Energy is supplied to break bonds and energy is released when bonds are made
• For an exothermic reaction, how does the energy released from forming bonds compare to the energy needed to break them?

  Energy released is greater than energy needed
• For an endothermic reaction, how does the energy needed to break bonds compare to the energy released from making them?

  Energy needed is greater than energy released
• What is the equation to calculate the energy of a reaction in terms of bond energies?

  Energy of reaction = sum of bonds broken – sum of bonds made
• What are the components of a cell?

  Two electrodes dipped in an electrolyte solution
• How does a cell produce electricity?

  From a chemical reaction
• What does a battery consist of?

  Two or more cells connected in series
• What factors determine the voltage obtained from a cell?

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

  Advantages:

  • More or less cheap
  • Some are rechargeable
  • Convenient source of electrical energy

  Disadvantages:

  • Harmful chemicals
• In rechargeable cells, what happens when an external current is supplied?

  Chemical reactions are reversed
• What happens to the reactants in non-rechargeable cells?

  Reactants are used up, cannot be recharged
• What are fuel cells supplied by?

  Fuel and oxygen
• What process do fuel cells use to generate electricity?

  Oxidation of the fuel
• What is the cathode half equation in a hydrogen fuel cell?

  $2 H_2 → 4 H^+$$+$$4 e^-$
• What is the anode half equation in a hydrogen fuel cell?

  $O_2 +$$4 H^+$$+$$4 e^- → 2 H_2O$
• What is the overall reaction in a hydrogen fuel cell?

  $2 H_2 +$$O_2 → 2 H_2O$
• What are the advantages and disadvantages of hydrogen fuel cells?

  Advantages:

  • No pollutants
  • No recharging

  Disadvantages:

  • Flammable
  • $H_2$ difficult to store
  • Fossil fuel production
  • Toxic chemicals
  • Expensive production of $H_2$ by electrolysis