Monday 26 September 2011

Chemistry note: Kinetics I, the measurement of rate

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Chemical Kinetics
Rate of reaction = change in concentration of reactant or product / time
For instantaneous rate we say rate = d[X]/dt.
Considering reactant consumed = product formed, for a general equation aA+bBcC+dD, the rate = -(1/a)d[A]/dt = -(1/b)d[B]/dt = (1/c)d[C]/dt = (1/d)d[D]/dt. Note that the negative sign for product is to ensure the rate is positive and can be compared with the rate of formation of the product. In a concentration-time graph, rate of reaction can be obtained from the graph.
Measurement of rate of reaction
1)       Measuring volume of gaseous product, e.g. Mg + 2HCl MgCl2 + H2
In this case only H2 is a gas involved in the reaction. Put the reactant in a conical flask and connect the flask to a gas syringe and start the stop watch at the same time. We can record the gas produced in the reaction in different time interval. Converting volume to number of moles the concentration can be obtained. Note that we can also use data-logger to measure the gas pressure change in a closed environment and find the gas produced by PV=nRT like placing HCl and Mg ribbon separately in a suction flask connected to data-logger.
2)       Measuring change in mass of reaction mixture left, e.g. CaCO3 + 2HCl CaCl2 + H2O + CO2
Put the reactant together into a conical flask with a piece of cotton wool at the top of the flask. The flask is put under an electronic balance and the stop watch starts as the reaction starts. Only gaseous product left the flask so by measuring change in mass we can know amount of product formed in different time intervals.
3)       Measuring transmittance, e.g. S2O82- + 2I- 2SO42- + I2
When light passing through a coloured solution, proportion of light absorbed is given by A = abc where a is a constant, b is the length of path that light undergoes in the solution and c is the concentration. In a cuvette b is fixed so the absorbance is proportional to the concentration solely.  In a colorimeter, a light source gives out light passing through a monochromatic filter (to give the light that wavelength fits the solution the most) and strike through the sample and into the light detector. Then absorbance is than can be measured. Since concentration proportional to absorbance, relative concentration-time can be deduced. However we need a calibration curve to convert absorbance to concentration. It can be obtained by measuring the absorbance for that coloured species of known concentration lonely.
4)       Measuring light transmittance, e.g. S2O32- + H2SO4 SO42- + SO2 + H2O +S
Place the beaker (8 well reaction strip) under a piece of white paper with a cross. Place the reaction mixture in the beaker and start the reaction. Record the time when the cross can’t be seen. It follows that a small, fixed amount of sulphur is produced. Since the reaction only goes a small proportion from completion, we say rate is proportional to 1/(time taken to form small, fixed amount of sulphur).
5)       Iodine clock reaction: 2H2O2 + 3I- + 2H+ I3- + H2O
(Note that triiodide I3- is equivalent to I2 + I- as they are in equilibrium) Consider the reaction   I3- + 2S2O32- 3I- + S4O62- which is extremely fast, and consume all triiodide produced. When starch solution exist in reaction mixture, blue-black colour is only shown after the small fixed amount of thiosulphate is consumed. Then rate is proportional to 1/t.
6)       Following acidity, e.g. neutralization
Data-logger can be put in the reaction mixture to measure its acidity which implies the concentration of H+.
Without the existence of data-logger, we have to use titrimetric analysis in which we take out a small proportion of reaction mixture and find concentration of species and different time interval. In order to ensure that the reaction stops after the mixture is taken out, quenching is used by the following method:
-          Ice bath on the reaction mixture (T↓→Rate)
-          Add a large amount of water to dilute the mixture (Concentration↓→Rate)
-          Eliminating catalyst/reactants
Besides acid-alkali titration, we can titrate thiosulphate as well. Put a small amount of starch solution into the thiosulphate solution and titrate against I2. By estimating I2 used we can know the concentration of thiosulphate ion.

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