Topic 16 Kinetics II
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Overall rate of reaction
The change in concentration of a species divided by the time it takes for the change to occur. All reaction rates are positive.
Equation for overall rate of reaction:
Rate= change in concentration of a product/time= -change in concentration of a reactant/time.
Units for rate (most commonly)
mol dm^-3 s^-1
Methods of Measuring Rate of Reaction (6)
• Measuring the volume of a gas evolved. • Measuring the change in mass of a reaction mixture. • Monitoring the change in intensity of colour of a reaction mixture by colourimetry. • Measuring the change in concentration of a reactant or product using titration. • Measuring the change in pH of a solution. • Measuring the change in electrical conductivity of a mixture.
Measuring the volume of a gas evolved
2 possible methods: 1. Collection over water into a measuring cylinder. 2. Collection using a gas syringe.
Measuring the change in mass of a reaction mixture
Used for reactions in which a gas is evolved. The reaction flask & contents are placed on a digital balance, and the decrease in mass is measured as the reaction proceeds.
When is measuring the change in mass of a reaction mixture most suitable for measuring rate of reaction?
When the gas evolved in high-density (e.g., CO2) as the gas given off has a relatively high density, so it’s more precise as measurement uncertainties are less significant.
Measuring the change in concentration of a reactant or product using titration:
Use a pipette to remove aliquots from the reaction mixture at regular intervals. The reaction in the aliquot is then quenched and titrated.
How is the reaction in the aliquot quenched?
The reaction is slowed to almost zero by immersing it in an ice bath, or stopped by adding another species.
Measuring the change in concentration of a reactant or product using titration: iodine + propanone.
This reaction is catalysed by an acid. NaHCO3 is added to the aliquot to stop the reaction by removing the acid catalyst. The remaining iodine is titrated against sodium thiosulfate. CH3COCH3 + I2 –> CH3COCH2I + H+ + I- I2 + 2S2O3^2- –> 2I- + S4O6^2- All species are aqueous.
Any other property that shows a significant change can be measured to determine reaction rate, for example…
Dilatometry/volume of a liquid. Chirality. Refractive index.
Measuring the change in electrical conductivity of a mixture: how?
If the number/type of ions changes, a conductivity meter can be used to measure changes in the conductivity of the solution.
What causes rate constants to change?
Temperature.
Units for the rate constant
Vary. They must be determined in each rate equation. E.g., 0 order is mol dm^-3 s^-1 & 1st order is s^-1.
Reactions involving simultaneous collision of more than 2 particles.
Very rare.
Rate-determining step
The slowest step in the mechanism for a reaction.
Overall order
Sum of all the individual orders.
Order of a reactant species
The power to which the concentration of the species is raised in the rate equation.
Rate equation
An equation expressing the mathematical relationship between the rate of reaction and the concentrations of the reactants.
Which species appear in the rate equation, and so affect the rate of reaction?
All reactant species either in or before the rate-determining step.
Half-life of a reaction
The time taken for the concentration of a reactant to fall to half its initial value.
Instantaneous reaction rate
The gradient of a tangent drawn to the line of the graph of concentration against time. Except for a zero order reaction, the instantaneous rate varies as the reaction proceeds.
What does a concentration-time graph look like for a zero order reaction?
A straight line with a negative gradient.
Determining the rate equation by the continuous method:
Withdraw samples of the reaction mixture at regular time intervals. the reaction in the sample is stopped by quenching. The concentration of a reactant is determined by titration. Draw a concentration-time graph. Determine the half-life for the reaction at different concentrations.