GCSE C2 Chapter 4

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Rate of reactions

  • The rate of a reaction measures the speed of a reaction
  • Rate of reaction = amount of product fromed / time
  • Rate of reaction = amount of reactant used / time
  • An average rate of reaction can be found by measuring time taken for a specific amount of product to be formed or for a specific amount of reactant to be used up
  • Graphs can be used to measure the amount of product formed or amount of reactant used on the Y axis aginst the time taken on the X axis
  • To find the rate of reaction at a given time is the gradient of the slope
  • The steeper the gradient the faster the rate of reaction
  • The Y axis can represent the amount of product formed, the amount reactant released, change in colour, change in pH, change in concentration or pressure or change in temperature
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Collision theory

  • Collision theory states that for a reaction to occur the particles must collide with enough energy to change into new substances. 
  • The minimum energy required is the activation energy for the specific reaction
  • Factors that affect the rate of reaction are:
    • Surface area
    • Temperature
    • Concentration / pressure
    • Using a catalyst
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The effect of surface area

  • The larger the surface area of solids in a reaction the more collisions in a given time
  • Braking large pieces of solid into small pieces exposes new surfaces for collisions meaning more collisions will occur in a given time increasing the rate of reaction
  • So powder reacts faster than large lumps of a substance
  • The finer the powder the faster the rate of reaction
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The effect of temperature

  • Increasing the temperature causes an increase in rate of reaction:
    • Increasing the temperature increases the energy of particles 
    • This means they move around much quicker and with more energy
    • More frequent collisions
    • More collisions with the activation energy (more successful collisions)
    • Faster rate of reaction
  • In most reactions an increase of 10C will double rate of reaction and a decrease in 10C will halve rate of reacion
  • This is why we refrigerate foods because it decreases the rate of reaction so the growth of bacteria is greatly reduced. Keeps food fresh for longer 
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The effect of concentration and pressure

  • Increasing the concentration or pressure will increase the rate of reaction:
    • Particles in fluids move around randomly because of the energy they have
    • If the concentration or pressure is increased there are more particles in the same volume
    • Therefore there are more frequent collisions
    • Rate of rection increases
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The effect of catalysts

  • Catalysts increase the rate of reactions by lowering the activation energy
  • Therefore more collisions result in a reaction
  • They are not used up in the reaction
  • Only work for a specific reaction
  • Catalysts that are solids are usually in foms that have large surface areas maximising rate of reaction
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Catalysts in action

  • Catalysts can be expensive to produce however they can be economical because they can be used over and over again
  • They are used in many industrial processes to speed up reactions
  • This helps to reduce costs and energy, preservelimited resources and reduce pollution
  • Cataylsts are often made from transition metals and their compounds which can be toxic and can cause harm if they get into the environment
  • Finding new and better catalysts is a major source of research
  • Nanoparticles would provide excellent cataylsts because of their huge surface area
  • Biological catalysts such as enzymes are also being researched as these can bring aout reactions at normal temperatures and pressures reducing energy requirements and reducing the impact on the environment
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Exothermic and endothermic reactions

  • When chemical reactions take place, energy is required to break the bonds between the compounds and energy is released when new bonds are formed
  • Exothermic reactions release energy into the surroundings causing temperature increase:
    • Oxidation
    • Combustion
    • Neutralisation
  • Endothermic reactions take energy in from the surroundings or require a constant supply of energy:
    • Thermal decomposition
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Energy and reversible reactions

  • In reversible reactions the forward and reverse reactions involve equal and opposite energy transfers
  • If a reaction is exothermic in one direction it is endothermic in the other
  • The amount of energy released in the exothermic reaction is equal to the amount of energy taken in by the endothermic reaction
  • Blue crystals       =    white crystals
  • CuSO4.5H2O     =          CuSO4        +  5H2O
  •    Hydrated  Anohydrous
  • Copper sulphate =  Copper sulphate + water
  • When water is added to anhydrous copper sulphate the reaction is exothermic
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Using energy transfers from reactions

Exothermic reactions can be used to heat things e.g in hand warmers:

Reaction can be reversible if made using the crystallisation of a salt:

  • Once hand warmer has been used
  • Place in boiling water and the salt will re-dissolve into the solution
  • Hand warmer becomes warm again
  • Can be used many times

Reaction may also be irreversible:

  • Reactants are used up and cannot be used again
  • These use reactions - oxidation of iron or reaction of calcium oxide with water

Endothermic reactions can be used to cool things e.g sports injury packs

Some contain ammonium nitrate and water:

  • The two substances are kept seperate
  • Upon usage the substances are allowed to mix (by bursting bag seperating them)
  • The reaction which occurs is endothermic taking in energy from surroundings
  • This reaction is reversible but not in cold pack because they cannot be seperated
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