Rates and Kinetics

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  • Created on: 25-04-14 00:08
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Kinetics ­ Answers
1. The rate of a chemical reaction is the change in concentration of a reactant or product with time. Its units are mol
dm-3 s-1. The average rate of reaction is defined as rate = concentration
2. The rate equation is an equation that links the reaction rate with concentrations or pressures of reactants and
constant parameters (normally rate coefficients and partial reaction orders). It can only be found experimentally and
not by just looking at the equation.
3. The order of a reaction is the sum of the powers to which the concentrations of the reactants are raised in the
experimentally determined rate equation. The partial order of one reactant is the power to which the concentration
of that reactant is raised in the rate equation.
4. The rate constant is a proportionality constant that relates the concentration of reactants to the reaction rate. The
only thing affecting the rate constant is temperature.
1st Order ­ k=s-1
2nd order ­ k= mol-1 dm3 s-1
3rd order ­ k= mol-2 dm6 s-1
5. Half-life is the time taken for the concentration of a reactant to halve
6. The rate-determining step is the slowest step in a multi-step reaction. The overall rate of a multi-step process is
governed by the slowest step.
7. Activation energy is the fraction of molecules that have sufficient energy to react on collision, providing that the
orientation of collision is correct. Measured in J/mol or kJ/mol.
8. Heterogeneous catalysts are in a different phase from the reactants, whereas homogeneous catalysts are in the
same phase as the reactants (usually the gas phase or in solution).
9. Titration can be used when an acid, alkali or iodine is a reactant or product. Acids can be titrated with a standard
alkali, alkalis with a standard acid and iodine with a standard solution of sodium thiosulfate. The reaction can be
followed using this technique:
1) Measure out samples of the reactants with known concentration
2) Mix them together, start a clock and stir the mixture thoroughly
3) At regular time intervals, withdraw samples using a pipette and quench the reaction
This can be achieved by adding the solution from the pipette to ice-cold water or to a solution that reacts
with on of the reactants to prevent further reaction taking place
4) The quenched solution is then titrated against a suitable standard solution
10. If a reactant or product is coloured, the concentration of the coloured species can be measured using a colorimeter.
The amount of light of a particular frequency that is absorbed depends on the concentration of the coloured
substance. The reactants are mixed and a clock started. The light absorbed is measured at set time intervals.
11. Infrared spectroscopy can be used in a similar way to colorimetry. The spectrometer is set at a particular frequency
and the amount of infrared radiation absorbed at that frequency is measured at regular time intervals.
12. If a reactant is optically active and the product either has a different optical activity or a racemic mixture, the reaction
can be followed by measuring the extent to which the plane of polarisation of plane-polarised light is rotated.
1) The reaction mixture is placed in a cell in the polarimeter
2) The angle of rotation is measured at regular time intervals
3) The angle of rotation is proportional to the concentration of the optically active substance
13. If the reaction produces a gas, the volume of gas produced can be measured at regular time intervals. The volume of
gas is proportional to the moles of gas and can, therefore, be used to measure the concentration of the product.
14. An approximate method to measure rate of reaction is to add the two reactants and time how long it takes for the
reaction to stop. The assumption is made that the rate if proportional to 1/time. However, this is only valid if the
concentration of the acid in the reaction has fallen by less than 10-15% and if the temperature did not change by
more than 1 degree.
15. In a `clock' reaction, the reactants are mixed and the time taken to produce a fixed amount of product is measured.
The experiment is then repeated several times using different starting concentrations. The average rate for each
experiment is proportional to 1/t.

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A] [B] Rate
1 0.25 0.25 4
2 0.25 0.50 8
3 0.5 0.…read more

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Zero Order: Horizontal line
First Order: Straight line sloping upwards
Second Order: Curved line sloping upwards against [ ] or straight line sloping upwards against [ ]2
20. The value of the rate constant depends on:
The orientation factor of the molecules
The activation energy for the reaction
The temperature
21.…read more


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