energy changes

• what is an endothermic reaction?

  takes in heat from surroundings - shown by a fall in temperature
• what is an exothermic reaction?

  transfers energy to surroundings - shown by a rise in temperature
• what are 2 endothermic reaction examples?

  • thermal decompositions
  • citric acid + sodium hydrogen carbonate
• what are 3 exothermic reaction examples?

  • combustion
  • neutralisation
  • most oxidation reactions
• what is a use of an endothermic reaction?

  sports cooling packs
• what is a use if exothermic reactions?

  hand warmers
• what is activation energy?

  minimum amount of energy that reactants need to react
• what type of reaction is bond breaking?

  endothermic - energy used to break bonds is greater than energy released by forming new bonds
• what type of reaction is bond forming?

  exothermic - energy released by forming bonds is greater than energy used to break existing bonds
• how do you calculate the overall energy change?

  total energy needed to break bonds - total energy released by forming new bonds
• what is a battery?

  two or more cells connected in series
• What do electrochemical cells use to produce electricity?


  Chemical reactions
• What device measures the voltage of a cell?

  Voltmeter
• What is the function of a wire in an electrochemical cell?

  Allows the charge to flow
• What creates a charge difference between the electrodes in a cell?

  Reactions between electrolyte and electrodes
• How do the type of electrodes and electrolyte affect a cell?

  They affect the size of the potential difference
• How are the voltages of cells in a battery combined?

  They are added together
• How can a rechargeable cell be recharged?

  By connecting it to an external current
• What is a hydrogen fuel cell used for?

  To produce electrical current
• What does electrical current represent?

  The flow of electrons through the external circuit
• What are the differences between non-rechargeable and rechargeable cells?

  Non-rechargeable cells:

  • Irreversible reaction
  • Cannot be recharged

  
  

  Rechargeable cells:

  • Reversible reaction
  • Can be recharged by external current
• what is the equation for an anode reaction / oxidation?

  h2 -> 2h+ + 2e-
• what is the equation for a cathode reaction / reduction?

  o2 + 4h+ + 4e- -> 2h2o
• what are the advantages of hydrogen fuel cells compared to batteries?

  • less pollution
  • dont need recharging as often
  • cheaper to make
• what are the disadvantages of hydrogen fuel cells compared with batteries?

  • storage of h2 gas takes up more space
  • h2 production can emit greenhouse gases
  • h2 is explosive - hard to store safely