Energy changes

• What is conserved in chemical reactions?

  Energy is conserved in chemical reactions.
• What happens to the energy of product molecules in an endothermic reaction?

  The product molecules have less energy than the reactants.
• What occurs in an exothermic reaction regarding energy transfer?

  The product molecules have more energy than the reactants.
• What are examples of exothermic reactions?

  • Combustion (burning of fuels)
  • Oxidation reactions (e.g., sodium and water)
  • Neutralisation
• What are everyday uses of exothermic reactions?

  • Self-heating cans (reaction between chemicals in mixed bases)
  • Hand warmers (iron in air with a salt solution)
• What happens to the temperature of surroundings during exothermic reactions?

  The temperature of the surroundings increases.
• What happens to the temperature of surroundings during endothermic reactions?

  The temperature of the surroundings decreases.
• What are examples of endothermic reactions?

  • Reaction of citric acid and sodium hydrogencarbonate
  • Thermal decomposition (e.g., heating calcium carbonate)
• What are everyday uses of endothermic reactions?

  • Sports injury packs (chemical reaction cools the pack)
• What is activation energy?

  Activation energy is the minimum amount of energy particles must have to collide and react.
• What do reaction profiles or energy level diagrams show?

  • Relative energies of reactants and products
  • Activation energy
  • Overall energy change of a reaction
• What happens during a chemical reaction regarding energy?

  Energy is taken in to break bonds in the reactants and released to form new bonds in the products.
• How is the overall energy change of a reaction calculated?

  It is the difference between the energy needed to break bonds in the reactants and the energy released when bonds in the products are formed.
• In an exothermic reaction, what is true about the energy released and needed?

  The energy released from forming new bonds is greater than the energy needed to break existing bonds.
• In an endothermic reaction, what is true about the energy needed and released?

  The energy needed to break existing bonds is greater than the energy released from forming new bonds.
• How can you measure the amount of energy released by a chemical reaction?

  1. Measure the initial temperature of the reagents.
  2. Mix them in a polystyrene cup.
  3. Measure the final temperature of the solution.
• What types of reactions can the energy measurement method be used for?

  • Neutralisation reactions
  • Reactions between metals and acids
  • Reactions between carbonates and acids
• What is the method to test the effect of acid concentration on energy released in a neutralisation reaction?

  1. Prepare HCl and NaOH in separate beakers.
  2. Equalize their temperatures in a water bath.
  3. Mix them in a polystyrene cup.
  4. Record the temperature every thirty seconds.
  5. Repeat with different concentrations of HCl.
• What is a major problem with energy measurements in reactions?

  The amount of energy lost to the surroundings.
• How can you reduce energy loss during measurements?

  By insulating the polystyrene cup with cotton wool and using a lid to reduce evaporation.
• What happens to the temperature after the acid is neutralised?

  The temperature starts to decrease because no more energy can be released.
• What do cells contain that produces electricity?

  Cells contain chemicals that react to produce electricity.
• How can simple electrochemical cells be made?

  By connecting two different metal electrodes in contact with an electrolyte.
• What is the role of the electrolyte in a cell?

  The electrolyte contains ions that can move and react with the electrodes.
• What happens to the more reactive metal in a cell?

  The more reactive metal loses electrons and becomes the negative electrode (anode).
• What happens to the less reactive metal in a cell?

  The less reactive metal gains electrons and becomes the positive electrode (cathode).
• What factors affect the voltage of a cell?

  • Type of electrode
  • Type of electrolyte
  • Temperature of the electrolyte
  • Concentration of the electrolyte
• How does the difference in reactivity between two metals affect voltage?

  The greater the difference in reactivity, the higher the voltage produced.
• What type of reactions do non-rechargeable cells use?

  Non-rechargeable cells use irreversible reactions.
• What is an advantage of non-rechargeable cells?

  They are cheap.
• What is a disadvantage of non-rechargeable cells?

  They contain harmful and toxic chemicals that can damage the environment.
• What type of reactions do rechargeable cells use?

  Rechargeable cells use reversible reactions.
• What is an advantage of rechargeable cells?

  They can be used many times.
• What is a disadvantage of rechargeable cells?

  They take time to recharge and are more expensive than non-rechargeable cells.
• What do fuel cells use to produce electricity?

  Fuel cells use an external source of fuel and oxygen.
• What is produced when hydrogen and oxygen combine in a fuel cell?

  Water and energy are produced.
• What happens at the anode in a hydrogen fuel cell?

  Hydrogen loses electrons and is oxidised.
• What happens at the cathode in a hydrogen fuel cell?

  Oxygen gains electrons and is reduced.
• What are the advantages of fuel cells?

  • Easy to maintain (no moving parts)
  • Small size and lightweight
  • Renewable with a continuous supply of fuel and oxygen
  • Water is the only product
  • Very efficient with 100% atom economy
• What are the disadvantages of fuel cells?

  • Hydrogen is difficult to store
  • Expensive to manufacture
  • Hydrogen is flammable and explosive
  • Fossil fuels may be used to make hydrogen