Flashcards in Kinetics 2 Deck (47)
What is order of reaction?
The sum of the powers to which the concentrations of reactants are raised in the experimentally determined rate equation
What is the partial order of one reactant?
It is the power to which the concentration of that reactant raised in the rate equation
What is rate constant, k?
The constant of proportionality that connects the rate equation with the concentration of the reactants
What would the rate equation be for a reaction:
A + 2B + 3C -> products
Where it is first order with respect to A and B, and second order with respect to C?
Rate = k[A][B][C]^2
What does the rate against concentration graph look like for a first order reaction?
A straight line graph passing through the origin, as rate is directly proportional to concentration
What units would k have in a first order reaction with rate equation rate = k[A]?
What happens to the rate of a first order reaction when the concentration doubles?
Rate also doubles
What does the rate against concentration graph look like for a second order reaction/second order with respect to a particular reagent?
A curve, double concentration gives quadruple the rate
How does changing concentration of a reagent that is second order affect rate?
It will square the number that the concentration has been multiplied by
e.g. 2x concentration = 4x rate
3x concentration = 9x rate
4x concentration = 16x rate
n x concentration = n^2 x rate
In the second order reaction:
H2 + I2 -> 2HI
Where it is first order with respect to hydrogen and iodine, how does concentration affect rate?
If [H2] doubles, but [I2] is constant, rate doubles
If [H2] trebles and [I2] trebles, rate is 9x faster
if [H2] increases n fold and [I2] increases m fold, rate increases by nm
What is the rate equation for H2 + I2 = 2HI, which is second order overall, but first order with respect to each reagent?
Rate = k[H2][I2]
What are the units for k in a second order reaction?
mol^-1 dm^3 s^-1
In the third order reaction:
2NO + O2 -> 2NO2
Where it is second order with respect to [NO] and first order with respect to [O2], how does concentration affect rate?
If [NO] doubles and [O2] is constant, rate becomes 4x faster
If [NO] trebles and [O2] doubles, rate becomes 8x faster
If [NO] halves and [O2] doubles, rate becomes half as fast
If [NO] increases n fold and [O2] increases m fold, rate increases me n^2m times
What are the units for k in a third order reaction?
mol^-2 dm^6 s^-1
What does the rate against concentration graph look like for a zero order reaction?
A straight horizontal line. Changing concentration has no effect on rate
What are the experimental methods for measuring rate?
Quenching and titration
Collecting gas evolved
Change in mass
Outline the method for measuring rate of reaction using quenching and titrating
Measure out samples of the reactants with known concentrations
Mix them together, start a clock and stir the mixture thoroughly
At regular time intervals, withdraw samples using a pipette and quench the reaction. This can be done by adding the solution to ice-cold water or to a solution that reacts with one of the reactants or a catalyst to prevent further reaction from taking place
The quenched solution is then titrated against a suitable standard solution. The titre is proportional to the concentration of reactant or product being titrated
Outline the process of using pH measurements to measure rate of reaction
If H3O^+ ions are produced or removed in a reaction, the pH will change and this can be measured (pH = -log[H3O^+])
Therefore if a pH meter is placed in a solution, the change of pH with time will mirror the change of concentration of H3O^+ ions with time. If concentrations of the initial reactants are known then it is possible to find their concentrations at any time from the pH
Outline the process of using colorimetry to measure rate of reaction
If a reactant or product is coloured then a colorimeter is able to measure the concentration of the coloured species. The amount of light of a particular frequency that is absorbed depends on the concentration of the coloured substance.
See how the concentration of the coloured substance varies with time by following the variation of absorbance to find the order of reaction
Outline the process of collecting gases evolved to measure rate of reaction
If a gas is produced in a reaction, the volume produced can be measured at regular intervals by attaching a gas syringe to the reaction vessel. Since volume of gas is proportional to the moles of gas, it can be used to measure the concentration of the product and then how it varies with time, therefore you can work out order of the reaction
Outline the process of using change in mass to measure rate of reaction
The loss of mass can be measured as gas escapes from a reaction vessel. The reactants are mixed and placed on a top-pan balance and the mass is measured and recorded at regular time intervals.
Suggest a limitation of using change in mass to work out the rate of reaction
The changes in mass are very small, particularly with light gases, so a very sensitive balance must be used
Outline the process of using electrical conductivity to measure rate of reaction
The number and type of ions in a solution will affect how it conducts electricity. As some reactions in solution have ionic balance changes, the change in conductivity that results from the changing balance of ions will reflect the change in concentration of those ions.
Outline the process of using polarimeter measurements to measure rate of reaction
If a reactant is optically active and the product has a different optical activity or is a racemic mixture, the reaction can be followed by the amount it will rotate the plane of plane polarised light.
The reaction mixture is placed in a cell and the angle of rotation is measured at regular time intervals, and the angle of rotation is proportional to the concentration of the optically active substance
What methods are used to maintain the concentration of reactants we do not want to measure?
The large excess method
The initial rate method
Outline the large excess method to keep the concentration of other reagents constant
By using a large excess of all reagents except the one we are measuring, we are able to assume the change in concentration of these reagents stays constant in the reaction, so the concentration time graph shows the effect of the only reagent that is not in excess
Outline the initial rate method to keep concentration of other reagents constant
At the beginning of the reaction, the concentrations of all reactants are known. By measuring the initial rate of reaction for different concentrations of one reagent while all other concentrations are kept constant, the order of reaction with respect to this reagent can be found. This is tedious unless a clock reaction is used
Do the substances in the slow step or the fast step in a mechanism appear in the rate equation?
In what circumstances may a substance appear in a rate equation but not in the balanced equation?
When it is a catalyst. It can be a part of the rate determining step, but will not appear as a reactant as it remains unchanged in the experiment