Topic 16: Kinetics II
1. understand the terms:
Rate of reaction: the change in amount of reactants or products per unit time
Rate equation: tells you how the rate of a reaction is affected by the concentrations of the
reactants
Order with respect to a substance in a rate equation: tells you how the substance’s
concentration affects the rate
Overall order of reaction: the sum of the orders of all the reactants
Rate constant: k is the rate constant – the bigger it is, the faster the reaction
Half-life: the time it takes for half the reactant to be used up
Rate-determining step: the slowest step in a multi-step reaction. The overall rate is decided by
the rate-determining step.
Activation energy: the minimum amount of kinetic energy particles need to react
Heterogenous catalyst: the catalyst is in a different physical state from the reactant
Homogenous catalyst: the catalyst is in the same state as the reactant
2. be able to determine and use rate equations of the form:
rate = k[A]m[B]n, where m and n are 0, 1 or 2
Rate Equations
For the general equation A + B C + D, the rate equation is:
k – rate constant
Rate = k[A]m[B]n m/n – orders of reaction with respect to A and B
Overall order = m + n
If something is 2nd order, if the concentration multiplies by x, the rate multiplies by x 2.
Example: In the reaction, CH3COCH3(aq) + I2(aq) ----H+----> CH3COCH2I(aq) + H+(aq) + I-(aq), the reaction
is first order with respect to propanone and H+ and zero order with respect to iodine. Write the
rate equation.
Even though H+ is a catalyst it can still appear in the rate equation.
Rate = k[CH3COCH3(aq)][H+(aq)]
Spectator ions are normally not included in the rate equation.
3. be able to select and justify a suitable experimental technique to obtain
rate data for a given reaction, including:
How to find rate of reaction
Titration
Take small samples of a reaction at regular intervals and titrate them using a standard solution.
Rate can be found by measuring change in concentration of the products or reactant over time.
Colorimetry
A colorimeter measures absorbance (the amount of light absorbed by a
solution). More concentrated the colour = higher the absorbance.
E.g. in reaction between propanone and iodine, brown colour fades so
absorbance will decrease. First plot a calibration curve of known
concentrations against absorbance. Then take small samples of your
solution at regular intervals and read the absorbance. Use the calibration
curve to find the corresponding concentration.
Mass change
If a gas is given off, the system will lose mass. You can measure this at regular intervals with a
balance and use mole calculations to work out how many moles of the gas you have lost, and
therefore how many moles of reactants are left.
1. understand the terms:
Rate of reaction: the change in amount of reactants or products per unit time
Rate equation: tells you how the rate of a reaction is affected by the concentrations of the
reactants
Order with respect to a substance in a rate equation: tells you how the substance’s
concentration affects the rate
Overall order of reaction: the sum of the orders of all the reactants
Rate constant: k is the rate constant – the bigger it is, the faster the reaction
Half-life: the time it takes for half the reactant to be used up
Rate-determining step: the slowest step in a multi-step reaction. The overall rate is decided by
the rate-determining step.
Activation energy: the minimum amount of kinetic energy particles need to react
Heterogenous catalyst: the catalyst is in a different physical state from the reactant
Homogenous catalyst: the catalyst is in the same state as the reactant
2. be able to determine and use rate equations of the form:
rate = k[A]m[B]n, where m and n are 0, 1 or 2
Rate Equations
For the general equation A + B C + D, the rate equation is:
k – rate constant
Rate = k[A]m[B]n m/n – orders of reaction with respect to A and B
Overall order = m + n
If something is 2nd order, if the concentration multiplies by x, the rate multiplies by x 2.
Example: In the reaction, CH3COCH3(aq) + I2(aq) ----H+----> CH3COCH2I(aq) + H+(aq) + I-(aq), the reaction
is first order with respect to propanone and H+ and zero order with respect to iodine. Write the
rate equation.
Even though H+ is a catalyst it can still appear in the rate equation.
Rate = k[CH3COCH3(aq)][H+(aq)]
Spectator ions are normally not included in the rate equation.
3. be able to select and justify a suitable experimental technique to obtain
rate data for a given reaction, including:
How to find rate of reaction
Titration
Take small samples of a reaction at regular intervals and titrate them using a standard solution.
Rate can be found by measuring change in concentration of the products or reactant over time.
Colorimetry
A colorimeter measures absorbance (the amount of light absorbed by a
solution). More concentrated the colour = higher the absorbance.
E.g. in reaction between propanone and iodine, brown colour fades so
absorbance will decrease. First plot a calibration curve of known
concentrations against absorbance. Then take small samples of your
solution at regular intervals and read the absorbance. Use the calibration
curve to find the corresponding concentration.
Mass change
If a gas is given off, the system will lose mass. You can measure this at regular intervals with a
balance and use mole calculations to work out how many moles of the gas you have lost, and
therefore how many moles of reactants are left.
