C5- 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 is one that transfers energy to the surroundings.
• 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 is one that takes in energy from the surroundings.
• What are some examples of endothermic reactions?

  Thermal decomposition and the reaction of citric acid and sodium hydrogencarbonate.
• What is a common application 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 when bonds are broken and formed?

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

  By subtracting the sum of energy released to form bonds from the sum of energy taken in to break bonds.
• What indicates an endothermic reaction in terms of energy changes?

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

  Energy released when bonds are formed is greater than energy taken in to break bonds.
• 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 is 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 allows rechargeable cells and batteries to be recharged?

  The chemical reactions are reversed when an external electrical current is supplied.
• What is a fuel cell supplied with?

  A fuel cell is supplied by an external source of fuel and oxygen or air.
• What is the overall reaction in a hydrogen fuel cell?

  The oxidation of hydrogen to produce water.
• What are the advantages and disadvantages of hydrogen fuel cells compared to rechargeable cells and batteries?

  Advantages:

  • Can be used constantly if fuel is supplied
  • Produce only water as a byproduct

  
  

  Disadvantages:

  • Hydrogen needs to be stored at high pressure
  • Harder to transport
  • Eventually stop working
• What are the half-cell equations for a hydrogen fuel cell at the anode and cathode?

  At the anode: $H_2(g) \rightarrow 2e^- +$$2H^+$$(aq)$; At the cathode: $4H^+$$(aq) +$$O_2(g) +$$4e^- \rightarrow 2H_2O(g)$
• What is the difference between non-rechargeable and rechargeable batteries?

  • Non-rechargeable: Chemical reactions stop when reactants are used up.
  • Rechargeable: Chemical reactions can be reversed with an external current.