Rate of reaction

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  • Rates of reaction
    • How to calculate rate of reaction
      • mean rate of reaction= quantity of reactant used / time taken
      • mean rate of reaction= quantity of product formed / time taken
        • The units for rate are usually g/s or g/min.
          • mean rate of reaction= quantity of reactant used / time taken
          • The rate of a chemical reaction can also be measured in Mol s-1.
      • To calculate the rate at the start of a reaction, you need to find the mathematical gradient (steepness) of the gradient. Use this equation:
        • rate of reaction= change in mass o volume of product / change in time
    • Speeding up reactions
      • 1. Catalyst  2. increase temperature  3.  shrink containers  4. increase the surface area  5. increase concentration
      • If the concentration of a reacting solution or the pressure of a reacting gas is increased: the reactant particles become more crowded, the frequency of collisions between reactant particles increases, the rate of reaction increases.
      • If the surface area to volume ratio of a reacting solid is increased: more reactant particles are exposed at surface, the frequency of collisions between reactant particles increases, the rate of reaction increases.
      • If the temperature of the reaction mixture is increased: reactant particles move more quickly, the energy of the particles increases, the frequency of successful collisions between reactant particles increases, the proportion of collisions which are successful increases, the rate of reaction increases.
      • Catalysts
        • A catalyst is a substance that speeds up the rate of a reaction,does not alter the products of the reaction, is not chemically changed or used up at the end of the reaction.
        • Catalysts provide an alternative reaction pathway, that has a lower activation energy. This is so the particles don't have to build up to as much energy.
        • Not all reactions have suitable catalysts. Different substances catalyse different reactions.
    • Collision theory
      • Collision theory states that for a reaction to occur the reactant particles must collide the particles must reach the activation energy
        • Activation energy- minimum amount of energy needed for the reaction
      • 1. Two pairs of particles move towards each other.   2. The pairs collide and reform so that each member of the original pair joins with a member of the other pair, forming two new pairs.   3. The new pairs are now moving away from each other
      • A successful collision is when the reactant particles collide with enough energy (activation energy) to make a reaction.
    • Graphs
      • The rate of reaction can be analysed by plotting a graph of mass or volume of product formed against time.
      • The gradient of the line is equal to the rate of reaction: the steeper the line, the greater the rate of reaction. fast reactions - seen when the line becomes horizontal - finish sooner than slow reactions
    • Required practical
      • Measuring the production of a gas
        • To investigate the effect of changing the concentration on the rate of a reaction.
        • You can tell when the reaction has finished when the line on the graph becomes horizontal or when you get the same result twice.
      • Investigate the rate of reaction by colour change
        • To investigate the effect of changing the temperature on the rate of a reaction.
        • 1. Using a measuring cylinder, add 50 cm3 of dilute sodium thiosulfate solution to a conical flask. 2. Place the conical flask on a piece of paper with a black cross drawn on it. 3. Add 10 cm3 of dilute hydrochloric acid to the conical flask. Immediately swirl the flask to mix its contents, and start a stop clock. 4. When the cross can no longer be seen, record the time on the stop clock.
        • When doing this experiment, the person looking at the black cross should stay the same, as different people will decide that they can no longer see the cross at different at times. this can lead to errors.

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