Unit A23 4.3 Rates of reaction Flashcards
4.3.7 - carry out experiments to determine the rate of a reaction using a variety of methods to determine the concentration of reactants and/or products
Using a colorimeter
• This is used if the reactant or product is coloured
• Prepare a range of samples of different concentrations of the required solutions
• Calibrate the colorimeter using the appropriate filter and a blank solution
• Record the absorbance of each solution of known concentration
• Draw a calibration curve of absorbance against concentration
• Place the reaction mixture in the colorimeter and record the absorbance at different times
• Use the calibration curve to convert absorbance to concentration and plot a graph of concentration against time. A gradient of a tangent at any concentration on the graph gives the rate at that concentration.
4.3.7 - carry out experiments to determine the rate of a reaction using a variety of methods to determine the concentration of reactants and/or products
Titrating
• The reaction is started by mixing the solutions and a sample is removed by pipette at various times
• Quench the sample to stop the reaction by rapid cooling/adding a large volume of water/adding a chemical to remove a reactant which is not being monitored
• Titrate the sample to find the concentration of the reactant or product
• Plot a graph of concentration against time; the gradient of a tangent at any concentration on the graph gives the rate at that concentration.
For example, to determine the rate of an esterification reaction the samples are titrated with standard alkali. To determine the rate of a reaction involving iodine, the remaining unreacted iodine is titrated with standard sodium thiosulfate solution until straw/yellow, then starch indicator is added and the titration continued until the starch changes from blue-black to colourless.
4.3.7 - carry out experiments to determine the rate of a reaction using a variety of methods to determine the concentration of reactants and/or products
Measuring pH over time
• This is used if H+ or OH− ions are a reactant or a product
• Start the reaction
• Record the pH at different times using a pH meter
• Calculate the hydrogen ion concentration using pH = -log10[H+]
• A graph of [H+] against time can be drawn and again gradients of tangents at various [H+] can be taken, which equal the rate at each point.
4.3.7 - carry out experiments to determine the rate of a reaction using a variety of methods to determine the concentration of reactants and/or products
Measuring the volume of a gaseous product over time
- This can be used if the product is a gas
- Attach a gas syringe to a sealed reaction vessel, or collect the gas over water in an inverted measuring cylinder/burette
• Start the reaction
• Record the volume of gas produced at different time intervals
• A graph of volume of gas against time can be plotted.
4.3.7 - carry out experiments to determine the rate of a reaction using a variety of methods to determine the concentration of reactants and/or products
Measuring the mass of a gaseous product over time
- This can be used if the product is a gas
- A flask containing one reactant is placed in a flask on a balance and the mass recorded
• Add the second reactant and start the clock
• Record the mass at different time intervals until the reaction is finished
• A graph of mass against time is plotted
