energy changes

• an exothermic reaction is when energy is transferred to the surrounding so theres an increase in temperature
• endothermic reactions take in energy so theres a decrease in temperature
• reaction profile for an exothermic reaction

  A) activation energy

  B) overall energy change

  
• an endothermic reaction

  A) reactants

  B) activation energy

  C) products

  D) activation energy peak

  E) overall energy change

  F) energy

  G) progress of reaction

  
• an electrolyte is a solution that can conduct electricity
• size of potential difference

  • depends on the difference in reactivity between the two metals
  • electrolyte affects p.d
• reactivity series: MAZ ITL C

  1. magnesium
  2. aluminium
  3. zinc
  4. iron
  5. tin
  6. lead
  7. copper
• alkaline batteries are non-rechargable as their reactants run out no more electricity will be produced
• rechargeable batteries can recharge when an electrical current reverses the chemical reaction
• hydrogen fuel cells react hydrogen with oxygen to produce electricity the only waste product is water
• half equation (negative electrode)

  • $4H^+$$\ +$$\ 4e^-\ \rightarrow\ 2H_2$
• overall equation for hygrogen fuel cells:

  
  

  $2H_2\ +$$\ O_2\ \rightarrow\ 2H_2O$
• a battery has two or more cells connected in a series to produce high potential difference
• fuel cells

  half equation(positive electrode)

  $O_2\ +$$\ 4H^{+\ }+$$4e^-\ \rightarrow\ 2H_2O$
• uses of EXOthermic (releases energy) reactions:

  • hand warmers
  • neutralisation
  • combustion
• BEN-MEX

  • breaking bonds is endothermic
  • making bonds in exothermic
  • negative energy change is exothermic
  • positive energy change is endothermic
• activation energy is the minimum amount of energy required so the particles to collide and the reaction to start
• cells

  • only produce electricity for a short amount of time
  • eventually chemicals will run out and the reaction will stop
  • it only produces electricity when two different metals are used with different reactivities
• examples of ENDOthermic ( takes energy in ) reactions are:

  • thermal decomposition
  • cracking alkanes
  • photosynthesis
  • electrolysis
• examples of an exothermic reactions:

  • combustion
  • neutralisation
  • hand warmers
• rechargeable batteries:

  -runs out need to be recharged

  -stores less electricity the more charging cycles they go

  + no dangerous fuels

  + produces more potential difference
• hydrogen fuel cells

  + will produce electricity as long as hydrogen is supplied

  + doesnt get less efficient

  + source of drinking water

  + only waste is water

  • runs on hydrogen which is an explosive gas so its hard to store and transport
  • produces low potential difference