Topic 16: Further Kinetics

Question 1

How do you find reaction rate from a graph?

Answer 1

RR is measured by recording how quickly chemical curves change. As this varies throughout the reaction we always consider the initial rate, which is the gradient at 0.

Question 2

What is the rate eqn?

Answer 2

The rate eqn tells us the relationship between the concs of the reagents and the reaction rate.

eg Rate= k[A]^x[B]^y k= rate eqn. x and y= order of the reaction with respect to A and B

Question 3

What are the types of orders?

Answer 3

First order: if you double A the rate is doubled. The value of X=1. Rate is directly proportional to [A]^1

Second order: if you double A the rate is squared, increasing by a factor of 4. X=2. Rate is directly proportional to [A]^2

Zero order= Changing the concentration of A has no effect on the overall rate. Rate is directly proportional to [A]^0

Question 4

State the definition for the order of a reaction

Answer 4

Is the power to which the concentration of a given reagent is raised in the rate equation.

Question 5

What is the overall order of a reaction?

Answer 5

The overall order of reaction equals the sum of the orders, x+y. eg overall order for [A][B]^2 = 3

Question 6

Describe k, the rate constant.

Answer 6

k is large for fast reactions and small for slow reactions.

Increase in temperature is accompanied by an increase in the value of k. k is only a temp dependent constant (like Kc!)

Rate doubles every 10 degrees bc temp affects both collision Hz and the no of particles with correct AE.

Question 7

Deduce the units for k in the following rate equations:

Answer 7

Question 8

Do catalysts appear in the rate equation?

Answer 8

Don’t appear in chemical equations but may appear in the rate of expression or rate eqn.

Question 9

Describe RATE CONCENTRATION graphs for 0, 1st and 2nd orders

Answer 9

r= rate. To find an order>1, plot 1/time against the concentration SQUARED. If the line is straight, then it is second order

Question 10

Describe CONCENTRATION- TIME graphs for 0, 1st and 2nd orders

Answer 10

Question 11

How can we find the orders from the INITIAL rate method?

Answer 11

A series of experiments are carried out at constant temp, changing only the concs of the reagents.

By comparing initial concs and rates of pairs of experiments the order can be found

Question 12

What is the half life of a reaction?

Answer 12

The time taken for the concentration of the reagent to become half of its initial value.

Question 13

How can we find the orders using half lives in RATE CONC graphs?

Answer 13

To find the half life, find half of the highest concentration. Dot along the graph until you hit the curve. Mark down the time taken, eg 10mins. Find half of the concentration again, and do the same again.

If taking several consecutive half lives on a graph give you the same amount of time, the order of the reagent is 1

Question 14

What is the rate determining step?

Answer 14

Many reactions consist of separate steps. The overall rate of a multi step process is governed by the slowest step. This is the rate determining step.

The rate eqn shows ALL involved species up to and including the RDS.

Therefore the RDS is the step with ALL the species that are in the rate eqn!!

Question 15

The RDS for a 3 step reaction is this:

H2O2 + I- ——> IO- +H20

What is the rate eqn for the reaction?

Answer 15

Every reactant in the RDS is in the rate eqn.

So the answer is: Rate = k[H202][I-]

Question 16

The RDS for a 3 step reaction is this: AB + AB —-> A2B2

What is the rate eqn for the reaction?

Answer 16

Every reactant in the RDS is in the rate eqn.

So the answer is: Rate = k[A]^2[B]^2

The reason it is squared is bc there are 2 A’s and 2 B’s in the RDS.

Question 17

X + Y +2Z ——> XYZ2

The rate eqn for the following reaction: Rate= k[X][Y]

Suggest the mechanisms for the reaction. Give the fast steps and the Rate determining step.

Answer 17

X + Y —–> XY SLOW (RDS)

XY + Z —–> XYZ FAST

XYZ + Z —–> XYZ2 FAST

Question 18

Describe the reaction between iodine and propanone

Answer 18

The reaction between iodine and propanone: CH2COCH3(aq) + I2(aq) —> CH3COCH2I(aq) + H+ +I-

The reaction investigates the change in iodine conc as the reaction occurs.

By varying the starting conc of iodine, it is possible to find the order with respect to iodine.

The reaction is acid catalysed. A colorimeter measures the rate at which the iodine is being decolourised.

Question 19

Describe clock reactions

Answer 19

All clock reactions have a 2nd reaction which delays a noticeable change, eg colour change. In this reaction all the iodine made in the main reaction then quickly reacts w the thiosulfate and disappears.

A clock reaction is a more convenient way of obtaining initial rr by taking a single measurement. Time is measured from the start of an experiment for a visual change to be observed.

Si no hay significant change in rate during this time, assume average rr sera el mismo que initial rate. The initial rate is then directly proportional to 1/time.

Question 20

Describe the hydrolysis of Halogenoalkanes

Answer 20

In the hydrolysis of halogenoalkanes, the hydroxide ions acts as a nucleophile and replaces the halogen in the Halogenoalkanes.

The reaction is therefore a nucleophilic substitution.

Question 21

what is the diff entre hydrolysis of primary and tertiary Halogenoalkanes?

Answer 21

In hydrolysis of a tertiary haloalkane, the reaction is 1st order with respect to the haloalkane, but zero order w respect to the OH- ion.

In hydrolysis of a primary haloalkane, the reaction is 1st order with respect to the Haloalkane, and 1st order with respect to the hydroxide ion.

Question 22

What does the Arhenius eqn show? State the Arhenius eqn

Answer 22

The Arhenius eqn shows how changes in temp and AE affect the value of k.

k = A x e^-(Ea / RT)

We can use the arrhenius eqn to find the AE from the rate constant or vice versa:

lnk = lnA - Ea/RT

ln is a button on the calculator. R=gas constant, 8.31. T=temp in kelvin

Question 23

How can we use the arrhenius eqn to find the AE from the rate constant or vice versa?

Answer 23

In exams, you may be given results of T and K. You MUST convert these into lnk and 1/T before drawing the graph. PLS remember that lnk is plotted on the y axis, 1/T is plotted on the x axis.

Once you’ve plotted the graph, find the gradient (change in y/ change in x). If 1/T is given as 1/T x 10^3, then DIVIDE 1/T BY 1000 before calculating the gradient.

From there, find Ea from the equation: gradient= -Ea / R

Ea is given in J mol-1. Divide by 1000 to get the answer in KJ mol-1.

Question 24

Why are the values given as 1/T x 10^3?

Answer 24

To make the graph easier to plot.

If 1/T is given as 1/T x 10^3, then DIVIDE 1/T BY 1000 before calculating the gradient.

Question 25

Describe the experiment between phenol and bromine. What is your objective?

Answer 25

C6H5OH + 3Br2 → C6H2Br3OH + 3HBr

When all the phenol has reacted, the Br continuously produced in the first reaction will react w methyl red indicator and bleach its colour.

From the start of each expt, record time taken for the red colour to disappear. The initial rate can be determined from this time. Measure the time at several diff temps using hot/cold water baths.

Then use the Arrhenius equation to determine the activation energy, Ea, for the reaction.

Question 26

Describe SN1 and SN2 reactions

Answer 26

SN1 and SN2 are types of nucleophilic substitution reactions. The SN1 mechanism occurs in 2 steps, SN2 occurs in one step.

SN1 reactions only have 1 molecule in the RDS, SN2 have 2 molecules in the RDS. Primary haloalkanes react via SN2, secondary can react via both and tertiary reacts via SN1.

SN1 reactions occur when there is little space around the carbon atom, SN2 occurs when there is lots of space around the C atom.

Question 27

What does the rate of SN1 reactions depend on?

Answer 27

The rate of SN1 reactions depends on the stability of the carbocation intermediate.

Therefore, tertiary haloalkanes react the fastest in SN1 reactions since they’re most stable.

Question 28

What does the rate of SN2 reactions depend on?

Answer 28

The rate of SN2 reactions depends on how easily the nucleophile can access the carbon attached to the leaving group.

Therefore, primary haloalkanes react fastest in SN2 reactions since there are smaller groups surrounding the carbon attached to the leaving group.

Question 29

Using bromoethane and aqueous potassium hydroxide as an example, draw the SN2 mechanism.

Rate = k[CH3CH2Br] [OH-]

Answer 29

Question 30

Using (CH3)3CBr and aqueous potassium hydroxide as an example, draw the mechanism.

Rate = k[(CH3)3CBr]

Answer 30

This is SN1 as there is only 1 molecule in the RDS.