Topic 5 - C5

• What is an exothermic reaction?

  transfers energy into surrounding (EXO - exit)
• What are some examples of exothermal reactions?


  ~ combustion

  ~ neutralisation reactions

  ~ oxidation reactions
• What are uses of exothermic reactions?


  ~ hand warmers

  ~ self heating cans (for hot drinks)
• What is an endothermic reaction?


  takes energy in from surroundings (END - end of reaction has more)
• What are some examples of endothermic reactions?

  ~ reaction between citric acid and sodium hydrocarbonate

  ~ thermal decomposition
• What are uses of endothermic reactions?

  sports injury packs
• What happens in a chemical reaction?


  ~ old bonds broken

  ~ new bonds formed
• How do you break existing bonds?

  energy must be supplied - endothermic process
• How do you make new bonds?

  energy is released - exothermic process
• What is an electrochemical cell?


  basic system made of two different electrodes in contact with an electrolyte
• How does an electrochemical cell produce electricity?

  1. two electrodes must be able to conduct electricity (mainly metals)
  2. electrolyte is a liquid that contains ions which react with electrodes
  3. chemical reactions between electrodes and the electrolyte set up a charge difference
  4. if top of electrodes are connected by wire the charge is able to flow and produce electricity
• How do you increase voltage?


  • type of electrodes (react to electrolyte differently)

  • the bigger the difference in reactivity if electrodes

  • electrolyte used (different ions in solution react differently to metal electrodes)
• How is a battery formed?


  by connecting two or more cells together in series
• How do non-rechargeable batteries work?


  • chemical reactions at electrodes are irreversible

  • ions in electrolyte and metals on electrodes get used up and turned into products

  • once one reactant is used up reactions can't happen and electricity isn't produced
• How do rechargeable batteries work?


  reactions can be reversed by connecting it to an external electric current
• What is a fuel cell?


  is an electrical cell thats supplied with fuel and oxygen (or air) and uses energy from reaction between them to produce electrical energy efficiently
• How does a fuel cell work?


  • fuel enters the cell becoming oxidised and sets up potential difference within cell

  • hydrogen- oxygen fuel cells react (H2O and O2) to produce clean water and release energy
• How does a hydrogen-oxygen fuel cell work?

  • electrolyte often an acid (phosphoric) and electrodes often porous carbon with a catalyst

  • hydrogen goes into anode compartment and oxygen into cathode

  • at anode hydrogen loses electrons (oxidation) producing H+ ions

  • H+ ions in electrolyte move to cathode

  • ar cathode oxygen gains electrons (reduction) producing water

  • electrons flow through an external circuit from anode to cathode this is the electric current
• Advantages of hydrogen-oxygen fuel cells instead of batteries :

  • don't produce as many pollutants
  • batteries are polluting to dispose due to high toxic metal compounds
  • batteries are rechargeable but limited times
  • batteries store less fuel then fuel cellsp
  • fuel cells are less expensive to make
• Disadvantages of using hydrogen-oxygen fuel cells instead of betteries?

  • hydrogen is gas so takes up more space to store
  • hydrogen is explosive when mixed with air so hard to store safely
  • hydrogen fuel is made from hydrocarbons (fossil fuel - crude oil) or electrolysis which uses energy (normally from fossil fuels)