Topic 5 - Energy changes

• If 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?

  One that transfers energy into the surroundings, so the temperature of the surroundings increases.
• Give 3 examples of exothermic reaction?

  • combustion (burning)
  • Neutralisation
  • Many oxidation reactions
• Give 2 examples of everyday exothermic reactions?

  • Self- heating cans
  • Handwarmers
• What is an endothermic reaction?

  One that takes in energy from the surroundings so the temperature of the surroundings decreases.
• Give 3 examples of endothermic reactions?

  • Thermal decomposition
  • Photosynthesis
  • citric acid and sodium hydrogencarbonate
• give 2 examples of everyday endothermic reactions

  • Some sports injury packs
  • Melting ice
• When can chemical reactions occur?

  When reacting particles collide with each other and with sufficient energy.
• What is the activation energy?

  The minimum amount of energy that particles must have to react
• Why are reaction profiles used?

  to show the relative energies of reactants and products, the activation energy and the overall energy change of a reaction
• This is an exothermic reaction profile

  
• This is an endothermic reaction profile

  
• What is bond energies?

  The amount of energy required to break one mole of a particular covalent bond
• What type of reaction is breaking bonds? and why

  Endothermic, because energy must be supplied to break bonds in the reactants
• What type of reaction is forming bonds? and why

  Exothermic, energy is released when bonds in the products are formed.
• How do you calculate the overall energy change?

  Energy of reactants - Energy of products
• Why is the reaction EXOTHERMIC when the overall energy change is NEGATIVE?

  Because the energy released from forming new bonds is greater than the energy needed to break existing bonds.
• Why is the reaction ENDOTHERMIC when the overall energy change is positive?

  Because  the energy needed to break existing bonds is greater than the energy released from forming new bonds.
• A cell contains chemicals which react to form electricity
• How can a cell be made?

  By connecting two different electrodes( metals) with a wire, and placing them in contact with an electrolyte.
• What is an electrolyte?

  A liquid through which ions can flow (electricity is conducted)
• What is a battery?

  Two or more cells connected together in series to provide a greater voltage.
• Name 3 factors that affect the voltage of a cell?

  • The metals used for the electrodes ( the greater the difference in reactivity of the two metals, the greater the voltage)
  • The type and concentration of the electrolyte used
  • The conditions such as temperature
• What are alkaline batteries?

  • Non-rechargable, because at some point the reactants will run out and no more electricity is produced, and theres no way to reverse these reactions
• What are rechargable batteries?

  Rechargeable because the chemical reaction can be reversed when an external current is applied
• What type of cell is a fuel cell?

  An electrochemical cell (converts chemical energy to electricity)
• Fuel cells are supplied by an external source of fuel (eg hydrogen) and oxygen or air
• When hydrogen gas enters a fuel cell, it  has to loses electrons to become hydrogen ions, what does this mean?

  that is it oxidised to produce a potential difference across the cell
• What is the overall reaction in a hydrogen fuel cell?

  The oxidisation of hydrogen to produce water ( hydrogen + oxygen → water)
• What are the ADVANTAGES to fuel cells?

  • Doesn't produce any pollutants as waste
  • Fuel cells last longer than batteries
  • Hydrogen and oxygen are renewable
• What are the DISADVANTAGES to fuel cells?

  • It requires energy to produce the hydrogen
  • Hydrogen requires a large space to store
  • Hydrogen is highly flammable, so danger to store