energy changes

• Exothermic reaction


  A reaction that transfers energy from the reacting molecules to the surroundings, causing the temperature of the surroundings to increase
• Endothermic reaction


  A reaction that takes in energy from the surroundings, causing the temperature of the surroundings to decrease
• Energy profile diagram for an exothermic reaction

  1. Reactants have more energy than products

  2. Energy is released to the surroundings
• Energy profile diagram for an endothermic reaction

  1. Products have more energy than reactants

  2. Energy is taken in from the surroundings
• Activation energy


  The minimum amount of energy that particles must have in order to react
• Representing activation energy on an energy profile diagram

  Activation energy is the energy from the reactants to the peak of the curve
• Reactions can only occur when the particles collide with each other and they have to do that with sufficient energy
• Uses of exothermic reactions

  • Hand-warmers
  • Self-heating cans (e.g. for food or drink)
• Exothermic reaction


  A reaction that releases energy in the form of heat
• Endothermic reaction


  A reaction that absorbs energy in the form of heat
• Investigating temperature changes in an exothermic reaction

  1. Measure 30 cm³ of dilute hydrochloric acid

  2. Transfer acid to polystyrene cup

  3. Measure initial temperature of acid

  4. Measure 5 cm³ of sodium hydroxide solution

  5. Add sodium hydroxide solution to acid

  6. Fit lid with thermometer and stir

  7. Record maximum temperature reached

  8. Repeat experiment with increasing volumes of sodium hydroxide
• Independent variable


  • Volume of sodium hydroxide solution
• Dependent variable


  • Maximum temperature reached
• Control variables


  • Volume of hydrochloric acid
  • Concentrations of hydrochloric acid and sodium hydroxide
• Increasing volume of sodium hydroxide

  Increases maximum temperature reached
• Reaching maximum volume of sodium hydroxide

  Maximum temperature reached starts to decrease
• This is a required practical
• Cell


  Two different metals placed in an electrolyte can produce electricity
• Producing electricity in a cell

  1. Chemical reaction takes place on the surface of the two metals

  2. Generates an electric current
• Cell


  • Can only produce electricity for a certain period of time
  • Requires metals with different reactivity to produce electricity
• Reactivity series of metals

  • Reactive metals (e.g. magnesium, aluminium)
  • Less reactive metals (e.g. lead, copper)
• Difference in reactivity between metals

  Greater the difference, greater the potential difference produced by the cell
• Battery


  Contains two or more cells connected in series, producing a greater voltage
• Types of batteries


  • Alkaline (non-rechargeable)
  • Rechargeable
• Alkaline (non-rechargeable) batteries


  Reactants run out, no way to reverse the reactions
• Rechargeable batteries


  Chemical reactions can be reversed by applying an electrical current
• Fuel cell


  A device that generates electricity through a chemical reaction, typically using hydrogen and oxygen
• Hydrogen fuel cell operation

  1. Hydrogen molecules split into hydrogen ions and electrons at the negative electrode

  2. Oxygen molecules combine with the electrons and hydrogen ions at the positive electrode to form water
• Advantages of hydrogen fuel cells compared to rechargeable batteries

  • Produce electricity as long as hydrogen is provided
  • Do not become less efficient over time
  • Can produce drinking water as a byproduct
• Disadvantages of hydrogen fuel cells compared to rechargeable batteries

  • Hydrogen is an explosive gas and difficult to store safely
  • Produce a relatively low voltage
• Rechargeable batteries can catch fire if not manufactured correctly
• Rechargeable batteries can produce a greater potential difference than hydrogen fuel cells