Energy changes

• What is conserved in chemical reactions?

  Energy is conserved in chemical reactions.
• What happens to the energy in the universe after a chemical reaction?

  The amount of energy remains the same as before the reaction.
• What does it mean if a reaction transfers energy to the surroundings?

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

  An exothermic reaction transfers energy to the surroundings.
• What is the effect of an exothermic reaction on the temperature of the surroundings?

  The temperature of the surroundings increases.
• Which reactions are examples of exothermic reactions?

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

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

  An endothermic reaction takes in energy from the surroundings.
• What happens to the temperature of the surroundings during an endothermic reaction?

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

  Thermal decompositions 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.
• How can students distinguish between exothermic and endothermic reactions?

  • Based on the temperature change of the surroundings.
• What should students evaluate regarding exothermic and endothermic reactions?

  • Uses and applications given appropriate information.
• What is the limitation regarding the measurement of temperature change in reactions?

  Calculation of energy changes or ΔH is not required.
• What is the purpose of the required practical activity regarding temperature changes?

  To investigate the variables that affect temperature changes in reacting solutions.
• What are some examples of reactions to investigate for temperature changes?

  • Acid plus metals
  • Acid plus carbonates
  • Neutralisations
  • Displacement of metals
• What is the activation energy in a chemical reaction?

  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 students use reaction profiles?

  To identify reactions as exothermic or endothermic.
• What does the activation energy explain?

  It is the energy needed for a reaction to occur.
• What happens to energy during a chemical reaction?

  Energy must be supplied to break bonds in the reactants.
• What occurs when bonds in the products are formed?

  Energy is released.
• How is the overall energy change of a reaction calculated?

  By finding the difference between the energy needed to break bonds and the energy released when bonds are formed.
• In an exothermic reaction, what is true about the energy changes?

  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 changes?

  The energy needed to break existing bonds is greater than the energy released from forming new bonds.
• What do cells contain that produces electricity?

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

  The type of electrode and electrolyte.
• How can a simple cell be made?

  By connecting two different metals in contact with an electrolyte.
• What is a battery composed of?

  Two or more cells connected together in series.
• What happens in non-rechargeable cells and batteries?

  The chemical reactions stop when one of the reactants has been used up.
• What type of batteries are alkaline batteries?

  Non-rechargeable.
• What is the key feature of rechargeable cells and batteries?

  The chemical reactions are reversed when an external electrical current is supplied.
• What are the uses of hydrogen fuel cells?

  • Supplied by an external source of fuel (e.g., hydrogen) and oxygen or air.
  • 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.
• Why are hydrogen fuel cells considered an alternative to rechargeable cells and batteries?

  They offer a potential alternative due to their efficiency and sustainability.
• What should students be able to do regarding hydrogen fuel cells?

  • Evaluate the use of hydrogen fuel cells in comparison with rechargeable cells and batteries.
  • Write the half equations for the electrode reactions in the hydrogen fuel cell (HT only).