chemistry

• When analysing the rate of a chemical reaction, there are several ways to measure it
• Change in pH


  Useful if the reaction changes pH from reactants to products. A pH meter can measure pH at regular intervals to calculate H+ concentration
• Mass loss


  Useful for reactions that produce a gas. The mass loss can be measured on a balance as the gas escapes
• Volume of gas produced

  Useful for containing and measuring the volume of gas produced over time using a gas syringe
• Color change


  A colorimeter can measure the absorbance of a colored sample to monitor the progress of a reaction
• Using a colorimeter


  1. Prepare a calibration curve of known concentrations

  2. Measure the absorbance of an unknown sample

  3. Use the calibration curve to determine the unknown concentration
• Rate of reaction


  How fast the reaction is progressing
• Measuring rate of reaction

  1. Take regular measurements

  2. Plot the results on a graph

  3. Draw a line of best fit

  4. Calculate the gradient of the line to determine the rate
• Gradient


  The change in Y (dependent variable) over the change in X (independent variable)
• Initial rate


  The rate of reaction at the very start, determined by the gradient of the tangent line at time zero
• The initial rate is useful for determining the order of a reaction and the rate equation
• Initial rate


  The rate right at the start of the reaction
• Calculating initial rate


  1. Take the gradient of the tangent at zero minutes

  2. Change in Y over change in X
• There is a little bit of tolerance when calculating the exact initial rate figure
• Clock reaction


  A reaction used to simplify the initial rate method
• Assumptions for clock reactions

  • Temperature of reaction must remain constant
  • Concentration of reactants does not change significantly during reaction time
  • Reaction has not proceeded too far when endpoint is seen
• Endpoint


  The point at which a colour change or other indicator is observed in a clock reaction
• The rate of a clock reaction is a good estimate of the initial rate of the reaction
• Iodine clock reaction


  Uses sodium thiosulfate and starch to monitor the production of iodine
• Iodine clock reaction


  1. Add sodium thiosulfate and starch to excess hydrogen peroxide

  2. Sodium thiosulfate reacts immediately with iodine produced

  3. When thiosulfate runs out, iodine reacts with starch to produce a blue-black colour
• Rate equation


  Equation that links the rate of reaction with the concentrations of reactants
• Order of reaction


  The power to which the concentration of a reactant is raised in the rate equation
• Orders of reaction


  • Zero order (concentration has no effect on rate)
  • First order (proportional change in rate with concentration)
  • Second order (squared proportional change in rate with concentration)
• Orders of reaction can only be determined by experiment, not deduced theoretically
• Rate constant (K)


  A number that allows the rate equation to balance, varies with temperature
• As temperature increases


  The rate constant (K) increases, increasing the rate of reaction
• The larger the value of the rate constant (K), the faster the rate of reaction
• Rate


  The speed at which a reaction occurs
• Calculating rate

  1. Write rate expression

  2. Substitute values

  3. Calculate rate
• Rate constant (K)


  A measure of the speed of a reaction, calculated using the rate equation
• Calculating rate constant


  1. Write rate expression

  2. Rearrange to make K the subject

  3. Substitute values

  4. Calculate K and determine units
• Rate-concentration graphs


  • Zero order: flat line
  • First order: straight diagonal line
  • Second order: curved upwards
• Half-life


  The time it takes for half of the reactant to be used up
• Calculating rate constant from half-life

  1. Measure half-life

  2. Use equation: K = ln(2) / t1/2
• Initial rate


  The rate of reaction at the very start, before any products have been formed
• Determining rate equation from initial rates

  1. Repeat experiment changing one reactant concentration at a time

  2. Record concentrations and initial rates

  3. Determine order with respect to each reactant

  4. Write rate equation
• The rate equation must be determined experimentally, it cannot be deduced from the balanced equation alone
• Determining the rate equation and initial rate for experiment 4

  1. Compare experiments where [NO] changes and [Cl2] is constant to determine order w.r.t. NO

  2. Compare experiments where [Cl2] changes and [NO] is constant to determine order w.r.t. Cl2

  3. Use the rate equation and the value of k calculated from another experiment to determine the initial rate for experiment 4
• The order with respect to NO is first-order
• The order with respect to Cl2 is also first-order