Topic 5 - Energy Changes

• What happens to energy during chemical reactions?

  Energy is conserved.
• What does it mean when a reaction transfers energy to the surroundings?

  The product molecules must have less energy than the reactants, by the amount transferred.
• What is an exothermic reaction?

  An exothermic reaction transfers energy to the surroundings, increasing their temperature.
• What are some examples of exothermic reactions?

  Combustion, many oxidation reactions, and neutralisation.
• What are everyday examples of exothermic reactions?

  Self-heating cans and hand warmers.
• What is an endothermic reaction?

  An endothermic reaction takes in energy from the surroundings, decreasing their temperature.
• What are examples of endothermic reactions?

  Thermal decomposition and the reaction of citric acid and sodium hydrogencarbonate.
• What are some applications of endothermic reactions?

  Some sports injury packs are based on endothermic reactions.
• What is activation energy?

  Activation energy is the minimum amount of energy that particles must have to react.
• What do reaction profiles show in chemical reactions?

  • Relative energies of reactants and products
  • Activation energy
  • Overall energy change of a reaction
• How can you tell if a reaction is exothermic or endothermic from a reaction profile?

  You can tell by looking at whether the reactants or products have more energy.
• What happens to energy during a chemical reaction?

  Energy must be supplied to break bonds in the reactants and released when bonds in the products are formed.
• How is the overall energy change of a reaction calculated?

  Sum of energy taken in to break bonds - sum of energy released to form bonds.
• What indicates an endothermic reaction in terms of energy?

  Energy taken in to break bonds is greater than energy released when formed.
• What indicates an exothermic reaction in terms of energy?

  Energy taken in to break bonds is less than energy released when formed.
• What do cells contain that produces electricity?

  Cells contain chemicals that react to produce electricity.
• What factors affect the voltage produced by a cell?

  The type of electrode and electrolyte.
• What is a simple cell made of?

  A simple cell is made by connecting two different metals in contact with an electrolyte.
• What is a battery?

  A battery consists of two or more cells connected together in series to provide a greater voltage.
• What happens in non-rechargeable cells and batteries?

  Chemical reactions stop when one of the reactants has been used up.
• What is an example of a non-rechargeable battery?

  Alkaline batteries are non-rechargeable.
• What characterizes rechargeable cells and batteries?

  They can be recharged because the chemical reactions are reversed when an external electrical current is supplied.
• What is a fuel cell?

  A fuel cell is supplied by an external source of fuel and oxygen or air, where the fuel is oxidised electrochemically to produce a potential difference.
• What is the overall reaction in a hydrogen fuel cell?

  The oxidation of hydrogen to produce water.
• What is a potential advantage of hydrogen fuel cells over rechargeable cells and batteries?

  Hydrogen fuel cells can be used constantly as long as fuel is supplied.
• What is a disadvantage of hydrogen as a fuel?

  Hydrogen is a gas that needs to be stored at high pressure, making it harder to transport.
• What is produced when hydrogen is burnt in a fuel cell?

  Water is produced when hydrogen is burnt.
• What are the half-cell equations for a hydrogen fuel cell at the anode?

  At the anode: $2H_2(g) \rightarrow 4H^+$$(aq) +$$4e^-$
• What are the half-cell equations for a hydrogen fuel cell at the cathode?

  At the cathode: $4H^+$$(aq) +$$O_2(g) +$$4e^- \rightarrow 2H_2O(g)$
• What are the differences between hydrogen fuel cells and rechargeable cells and batteries?

  • Hydrogen fuel cells can be used constantly as long as fuel is supplied.
  • Rechargeable cells can be recharged by reversing the reaction.
  • Hydrogen fuel cells produce only water when burnt.
  • Rechargeable cells may produce waste products.