Kinetics II

Question 1

How could you measure the rate of a reaction experimentally?

Answer 1

To measure the rate of a reaction, you need to measure the amount of products or reactants over time.
- for reactions which produce a gas, you can measure the volume of gas produced using a gas syringe over time
- also for gas-producing reactions, you can measure a loss in mass of the reactants (as the gas has mass, but is given off)
- for reactions which change colour, you can track the colour change using a colorimeter and plot a calibration curve, then use this to determine concentrations over time
- if the reaction results in a change in pH, you can track this using a pH meter
- you can use repeated titrations of samples of a running experiment to work out the concentrations over time
- if the number of ions in the reaction mixture changes, this will change the electrical conductivity, which can be tested using an ammeter

Question 2

How can you work out the rate of reaction from a concentration-time graph?

Answer 2

Draw a tangent at a particular point and do change in y/change in x to get the gradient, which is the rate

Question 3

What does it mean if a reaction is zeroeth order with respect to a particular reactant?

Answer 3

This means that the concentration of that reactant has no effect on rate, so if you double the concentration of that reactant, the rate will be unchanged.

Question 4

What does it mean if a reaction is first order with respect to a particular reactant?

Answer 4

This means that the reactant’s concentration is proportional to the rate, so doubling the concentration doubles the rate, tripling the concentration triples the rate etc.

Question 5

What does it mean if a reaction is second order with respect to a particular reactant?

Answer 5

This means that the rate of the reaction is proportional to the concentration of the reactant^2, so doubling the concentration x4 the rate, tripling the concentration x9 the rate etc.

Question 6

How do you work out the order of a particular reactant?

Answer 6

This can only be determined experimentally. The experiment must be complete at different concentrations of the reactants, and the rate measured. If enough trials are run, you will see what orders the reactants are.

Question 7

How do you work out the overall order of a reaction?

Answer 7

The overall order is given by the sum of all of the orders of all the reactants.

Question 8

What are the shapes of the concentration graphs for each order?

Answer 8

• zeroeth - straight line with negative gradient
• first - curve with decreasing negative gradient
• second - curve with steeper decreasing negative gradient

Question 9

What are the shapes of the rate-concentration graphs for each order?

Answer 9

• zeroeth - straight line with zero gradient (horizontal)
• first - straight line with positive gradient going through the origin (direct proportion)
• second - quadratic curve going through the origin (y=x^2)

Question 10

What is the half-life of a reaction?

Answer 10

The half-life is the time taken for half the reactant to be used up. The half-life of a first order reaction is constant.

Question 11

What happens to the half-life of a zero order reaction over time?

Answer 11

Over time, the concentration of the reactants decreases. As this happens, the half-life also decreases.

Question 12

What happens to the half-life of a second order reaction over time?

Answer 12

Over time, the concentration of the reactants decreases. As this happens, the half-life increases (each half life is twice as long as the one before it)

Question 13

How do you write a rate equation?

Answer 13

For the reaction A + B —> C + D, if the reaction is first order wrt A and second order wrt B, the rate equation is rate=k[A][B]^2, where k is the rate constant

Question 14

What is the initial rates method for working out orders of reaction?

Answer 14

Carry out experiments with different initial concentrations of one reactant. Use a continuous monitoring technique to get data for the concentration of that reactant over time. Plot a concentration-time graph and find the gradient at t=0 using a tangent. Compare the initial rates against the initial concentrations to work out the order wrt that reactant.

Question 15

What is a clock reaction?

Answer 15

A clock reaction is when you measure how the time taken for a set amount of product to form varies as you vary the concentration of a reactant. There will be a sudden increase in the concentration of a certain product as the limiting reactant is used up. There’s an easily observable end point, like a colour change, to tell you when this happens. The quicker the end point is reached, the faster the initial rate of reaction.

Question 16

What assumptions are made using the clock reaction method?

Answer 16

• the concentration of each reactant doesn’t change significantly over the time period of the clock reaction
• the temperature stays constant
• when the end point is seen, the reaction has not progressed too far

Question 17

Describe the iodine clock reaction

Answer 17

H2O2 + 2I- + 2H+ —> 2H2O + I2
- sodium thiosulfate solution and starch (indicator) are added to an excess of hydrogen peroxide and iodide ions in acidic solution
- the sodium thiosulfate reacts instantaneously with any iodine produced, so the starch doesn’t turn black at this point
- the second reaction is 2S2O3 2- + I2 —> 2I- + S4O6 2-
- once all the thiosulfate is used up, the iodine produced stays in the solution and the starch turns black
- varying the iodide or hydrogen peroxide concentration, while keeping other concentrations constant, will give different times for the colour change to occur
- the time and initial concentration of reactants allows you to calculate the initial rate at that concentration, allowing you to work out the order

Question 18

Describe the bromine clock reaction

Answer 18

• BrO3- + 5Br- + 6H+ —> 3Br2 + 3H2O
• C6H5OH + 3Br2 —> C6H2Br3OH + 3HBr
• the bromide ions and bromate ions react together, producing bromine
• the bromine reacts with the phenol, but once the phenol is used up, the bromine bleaches the indicator (usually methyl red or orange)
• repeat with different initial concentrations of bromide or bromate to work out the orders

Question 19

What is the rate equation for the iodine clock reaction?

Answer 19

rate = k[I-][S2O8 2-]

Question 20

What is the rate equation for the bromine clock reaction?

Answer 20

Rate = k[BrO3-][Br-][H+]^2

Question 21

Describe the iodine-propanone reaction

Answer 21

• CH3COCH3 + I2 —> CH3COCH2I + H+ + I-
• take samples of the reaction mixture at fixed intervals (e.g. every 5 minutes). Add sodium hydrogencarbonate to neutralise the acid (acidic conditions are required)
• this stops the reaction
• titrate the sample against sodium thiosulfate and starch to work out the concentration of iodine present at each time
• plot a graph and use this to calculate the initial rate
• compare initial rates with initial concentrations to work out the orders

Question 22

What is the rate equation for the iodine propanone reaction and what does this tell you about the mechanism?

Answer 22

rate = k[CH3COCH3][H+]
- this tells you that the reaction is first order with respect to propanone and H+, but zeroeth order wrt iodine
- therefore propanone and H+ are involved in the rate-determining step, but iodine is not
- this allows a mechanism to be worked out
- the propanone is protonated first (this is the RDS)
- the H+ is regenerated next, so acts as a catalyst, and a C=C bond is formed
- the iodine then reacts later

Question 23

How do you calculate the rate constant from the rate equation?

Answer 23

Rearrange the rate equation to get k=rate/[reactant concentration]^order
Use experimental results to get a value for k, the larger the value of k, the faster the rate. The units of the rate constant are given by doing mol dm-3 s-1 / (mol dm-3)^overall order = units

Question 24

What is a rate-determining step?

Answer 24

The rate determining step is the slowest step in a multi-step reaction, which determines the overall rate of the reaction.

Question 25

What does the order wrt a reactant tell you about that reactant’s involvement in the RDS?

Answer 25

The order tells you the number of molecules of that reactant which are involved in or before the rate-determining step. This can help you work out an overall reaction mechanism.

Question 26

How does the rate equation for an SN1 reaction vary compared to for an SN2 reaction?

Answer 26

An SN1 reaction has only one reactant involved in the rate-determining step, the halogenoalkane, so the rate equation is rate=k[RX] where RX is a haloalkane
For SN2, there are 2 species involved in the rate-determining step, the halogenoalkane and the nucleophile, so the rate equation is rate=k[RX][Nu]
The rate equations are first order wrt each reactant as only one molecule of each is involved in the RDS.

Question 27

What is the Arrhenius equation?

Answer 27

k= A e^-Ea/RT
Or in logarithmic form
ln k = -Ea/R (1/T) + ln A
k = rate constant
Ea = activation energy
R = molar gas constant
T = temperature (in K)
A = Arrhenius constant

Question 28

How can you work out the activation energy of a reaction using the Arrhenius equation?

Answer 28

• complete a reaction with a clear end point (e.g. bromine clock) at different temperatures
• measure the time taken for the colour to change and the concentration of each reactant to work out the rate constant
• plot ln k against 1/T, ln A is the y-intercept, -Ea/R is the gradient
• find the gradient of the graph and times by -R to get Ea