The Rate and Extent of Chemical Changes

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  • The Rate and Extent of Chemical Change
    • Rates of Reaction
      • Factors affecting Rates of reaction
        • temperature
          • when the temperature increases the particles move faster = more frequent collisions and more energy
        • concentration of  solution or pressure of gas
          • if the solution is more concentrated then there are more particles in the same volume
          • if the pressure of the gas increases it means that the same number of particles occupies a smaller space
          • collisions = more frequent
        • surface area
          • means that for the same volume of the solid the particles will have more area to work on = more frequent collisions
          • if one of the reactants is a solid then breaking up into smaller pieces will increase the surface area to volume ratio
        • presence of a catalyst
          • a catalyst is a substance that speeds up a reaction without being used up
          • different catalysts are needed for different reactions- they all work by decreasing the activation energy needed for the reaction
            • they do this by providing an alternate reaction pathway with a lower activation energy
            • activational energy = the minimum amount of energy that particles need to react
          • enzymes are biological catalysts - catalyse reactions in living things
        • more collisions increases the rate of reaction
      • the rates of a chemical reaction is how fast the reaction is
      • examples
        • rusting of iron = slow
        • burning = fast
        • metal magnesium reacting with acid = moderate speed
      • graph*
        • find the speed of the reaction
        • steeper line = faster rate
          • over time the line becomes less steep as the reactants are used up
      • the collision theory
        • the rate of the reaction depends on the collision frequency of the reacting particles
        • the rate of reaction depends on the energy transferred during a collision
      • rate of reaction = amount of reactant used or amount of product formed / time
      • ways of measuring the rate of reaction
        • precipitation and colour change
          • you can observe a mark and measure how long it takes for it disappear
            • the faster the reaction the faster the mark disappears
          • if the reactants are coloured and the products are colourless you can time how long it takes for the solution to lose or gain its colour
        • change in mass
          • measuring the speed of a reaction that  produces a gas can be carried out using a mass balance
          • as the gas is released is mass disappearing is measured on the balance
          • the quicker the reading on the balance drops the faster the reaction
          • most accurate method
        • the volume of gas given off
          • involves the use of a syringe to measure
          • the more gas given off during a given time interval , the faster the reaction
    • Reversible Reactions
      • equilibrium
        • after a while the forward reaction will be going at exactly the same rate as the backward one - the system is at equilibrium
        • at equilibrium, both reactions are still happening but theres no effect - the concentrations of reactants and products have reached a balance and wont change
        • equilibrium is only reached if the reversible reaction takes place in a closed system
          • closed system = none of the reactants or products can escape and nothing can get in
        • doesn't mean the amounts of reactants and products are equal
        • position of the equilibrium
          • depends on temperature, pressure and concentration
          • lies to the right = concentration of products is greater than the reactants
          • lies to the left = concentration of reactants is greater than the products
      • A + B ¬ C +D
      • as the reactants react, their concentrations fall - the forward reaction will slow down
      • as more and more products are made their concentrations will rise - the backward reaction will speed up
      • can be both endothermic and exothermic
        • if the reaction is endothermic in one direction it will be exothermic in the other
    • Le Chatelier's Principle
      • the idea that if you change the conditions of a reversible reaction at equilibrium the system will try to counteract that change
      • can be used to predict the effect of any changes you make to a reactive system
      • changes to the...
        • temperature
          • decrease temperature = the equilibrium will move in the exothermic direction to produce more heat
            • means you'll get more products for the  exothermic reaction and fewer for the endothermic reaction
          • increase temperature = the equilibrium will move in the endothermic direction to try and decrease it
            • you'll have more products for the endothermic reaction and fewer for the exothermic reaction
        • pressure
          • increase pressure = the equilibrium moves in the direction where there are fewer molecules of gas
          • decrease pressure = equilibrium moves in the direction where there are more molecules of gas
        • concention
          • if you change the concentration of the reactants or the products the system will no longer be at equilibrium
          • increase concentration of reactants = the system tries to decrease it by making more products
          • decrease concentration of products = the system tries to increase it again by reducing the amount of reactants

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