Energy changes

• The amount of energy in the universe at the end of a chemical reaction is....

  the same as before the reaction took place
• What is an exothermic reaction?

  One that transfers energy to the surroundings so the temperature of the surroundings increases (EXIT)
• What are examples of exothermic reactions?

  • combustion
  • oxidation
  • neutralisation
• What are some everyday uses of exothermic reactions?

  • self-heating cans
  • hand warmers
• What is an endothermic reaction?

  one that takes in energy from the surroundings so the temperature of the surroundings decreases
• What are some examples of endothermic reactions?

  • thermal decomposition
  • photosynthesis
• What are some everyday uses of endothermic reactions?

  Sports injury packs
• Energy much be supplied to...

  break bonds
• In exothermic reactions, the energy released from forming new bonds is...

  greater than the energy needed to break exiting bonds
• In endothermic reactions, the energy needed to break bonds is...

  greater then the energy released from forming new bonds
• In non-rechargeable cells and batteries the chemical reaction stops because...

  all the reactants have been used up. Alkaline batteries are non-rechargeable
• Rechargeable cells and batteries can be recharged because...

  The chemical reactions are reversed when an external electrical current is supplied
• What is the half equation at the negative anode in a fuel cell?

  $H_2\rightarrow2H^+$$+$$2e^-$
• What is the half equation at the positive cathode in a fuel cell?

  $O_2+$$4H^+$$+$$4e^-\rightarrow2H_2O$
• What happens at the anode in a fuel cell?

  Oxidation of hydrogen so loss of electrons
• What happens at the cathode in a fuel cell?

  Reduction of oxygen so gain of electrons
• What are the pros of hydrogen fuel cells?

  • only require hydrogen and oxygen
  • don't produce any carbon dioxide
  • simple to produce
  • less polluting
  • last longer
• What are the cons of hydrogen fuel cells?

  • Hydrogen is a gas, so it requires more space to be stored
  • can be explosive when mixed with air
  • Requires a lot of energy with is mainly from fossil fuels
  • expensive to produce
• What is the overall reaction in a hydrogen fuel cell?

  oxidation of hydrogen to produce water