Kinetics II

Rate of reaction

Rate of reaction

Change in conc of product, or a reactant, divided by the time taken

Rate constant K,

Constant of proportionality in the rate expression. It connects the rate of reaction with the conc of reaction. It’s value increases with increasing temp and decreases with decreasing temp. It decreases with increasing activation energy

Order of a reaction

Is the power to which the conc of the given reactant is raised in rate equation

Overall order

Is sun of the powers to which the reactant conc terms are raised in the rate equation eg for the reaction

A+B - Products

Rate = K [A]^x[B]^y

Where K = rate constant; and the reaction is x order wrt A, y order wrt, B and x+y order overall

Rate determining step

Slowest step of a reaction mechanism which determines overall reaction rate

Reagents that appear in rate equation = rds

Reactant not in rate equation is in faster step

Transition state

High energy state through which the reactants pass after collision in order to become products. Occurs at the top of the Ea barrier for a reaction step. They are unstable species such as carbocations or carbanions.

Half-life

Time taken for the conc of a reactant to reach half that of its original value.

Activation energy

Minimum amount of energy that the colliding molecules must have in order for that collision to result in a reaction

Initial rate of reaction

Rate of reaction at the time when reactants are mixed. It’s value can be estimated by measuring the conc of a reactant at the start and after a short time period

How to measure rate

Rate = change in conc reactants/change in time Units = moldm^-3 s-1

Conc of reactants will decreases with time

Rate may also be calculated by measuring the change in conc of products over time: this will increase over time

Rate of reaction can also be calculated by measuring change in conc of products over time - this will increases over time

Rate of reaction at any point can be calculated from the gradient of a conc time graph

Method for measuring rate of reaction: reactions involving gases

• Continuous method

• Measure rate of gas given off

• Done by either using a measuring cylinder (less accurate) or gas syringe (more accurate) and measuring the volume of gas given off at set time intervals

An example of a reaction involving gases

Calcium carbonate (chips or powder) and acid why do we not use H2SO4

Bc CaSO4 in insoluble - coats chip stops reaction

CaCO3 + HCl - CO2 - dissolves in H2O initial vol wrong

Method for measuring rates of reaction - measuring change in mass

• Method is continuous

• Done when reacting mix is placed in conical flask on a balance + change in mass measured over set time intervaks

• If gas is produced mass will decrease

• Care must be taken that splashes of liquid are not lost during the course of the reaction, so a loose swab of cotton wool is placed in mouth of the flask to prevent splashes being lost whilst the gas can still escape

E.g. Same as above for CO2 not H2 as mass is too low

Reactions which involve a colour change - use a colorimeter

• Continuous method

• Colorimeter - measure changes in intensity of colour during the course of a reaction e.g. result of appearance or disappearance of iodine or bromine.

E.g.

• ethandioate ions reduce purple manganate (VII) ions to colourless Mn2+ ions in acidic solution

• Brown yellow iodine to colourless I-

• Orange Br2 to colourless Br-

• Orange Cr2O72- to blue/green Cr3+.

Adv - more subtle than human eye can observe and so v fast to measure exact times unknown

The Iodine Clock method

Initial rates method - used for any reaction that produced iodine

• Small amount of sodium thiosulphate is added to reaction mix also contains starch indicator

• 2 colourless gases are mixed no visible reaction initially the thiosulphate ions react with the iodine as soon as it is formed turning it back to I- ons

• (Very quick reaction no effect on overall rate)

• All thiosulphate is used up, free I2 is formed which reacts with starch indicator - blue-black colour

• Time, t is measured from mixing until first formation of blue colour . Rate of reaction is proportional to 1/t.

• Exp is repeated with diff conditions e.g. diff concs (dilution of water) each time using same amount of thiosulphate - means sudden colour change always happens when same amount of iodine is formed

Titrimetric methods

Can be used for initial rates or continuous method used when change in conc of acid present

• Portions of a reaction mix may be removed at intervals during course of reaction, these portions are quenched either by putting into large amount of water or adding to a reagent which effectively stops the reaction

• Conc of 1 product or reactant is then determined by titration

Example:

Reaction between Propanone and iodine with an acid catalyst:

CH3COCH3 (aq) + I2 (aq) = CH2ICOCH3 (aq) + H+ (aq) + I- (aq) - zero order wrt iodine

• Sodium hydrogencarbonate is used to quench reaction as it reacts with acid catalyst. Conc of iodine can then be determined by titration with standard thiosulphate solution (using starch indicator added near the end point)

What is quenching?

Stop a reaction where it is

Conductivity measurements

• Changes in numbers of charged particles occurs during the course of the reaction, changes in conductivity may be used to follow reaction rate

• E.g. H2O2 (aq) + 2H+ (aq) + 2I- (aq) - 2H2O (l) + I2 (aq)

• Use a pH meter

• Continuous method for measuring rate of reaction

What is the initial rates method?

• Finds the rate immediately after the start of the reaction

• Sep exps are carried out where diff initial concs of 1 reagent are used, while others remain constant

• Uses a pan experimental method = initial rate -may draw a conc-time graph find grad

• Clock reactions

What is the continuous method?

Monitoring method is used to generate data to enable conc-time or vol-time graphs to be plotted

Then can deduce order of reaction

First order reactions

Directly proportional to conc of the reactant

rate = k[A]

Units: s-1

If [A] doubles, rate doubles

When plot a curve of conc against time for first orde reaction can be seen curve exponential

All first order reactions have constant half lives

Means time taken for conc of a reactant to fall by half is constant in any first ride reaction

Second order reactions

rate = k[A]²

If [A] doubles rate quadruples 2²

Units of k (second order) = mol^-1 dm³ s^-1

Appears to look sim to first order but is not constant

Zero order reactions

Rate = k

This is independent of [A] ([A] = 1)

Units of k = moldm³ s^-1

Examples A+B = C

Rate is dependent on conc of A only and not B at all.

Then Rate of reaction = k[A]

Reaction is 1st order with respect to A

Rate is zero order with respect to B (effectively [B]^0)

• Overall order of reaction is 1

Example 2 D+E = F

It is found that rate of the reaction depends directly on the conc of both D and E

Rate = k[D][E]

Order of the reaction to D and E 1st order

Overall the reaction is therefore second order

Example 3: G+H = I

• The reaction is found to depend directly upon the conc of G (first order with respect to G) and the square of the conc of H (second order with respect to H) and third order overall, giving the rate equation:

• Rate = k[G][H]²

How to work out the units

The initial rates method

Remember to list what experiments you are looking at

The primary halogenoalkane reaction

Can be shown by exp to be first order with respect to the hydroxide ion and 1st order wrt halogenoalkane I.e. rate k[CH3CH2CH2CH2Br] [OH-]

• a collision between these two leads to the formation of a transition stats in the reaction SN2

The tertiary halogenoalkane reaction

• Can be shown by exp to be first order wrt the halogenoalkane, and zero order wrt the hydroxide ion I.e. rate = k[(CH3)3CBr

• The SN1 reaction involves the initial reaction breaking of the C-Br bond to form a tertiary carbocation intermediate in the slow rds - only one molecule involved

The reaction of iodine with propanone

• Another example which shows that it is not possible to deduce the rate equation from the balanced equation for the reaction

I2 (aq) + CH3COCH3 (aq) = CH2ICOCH3 (aq) + H+ (aq) + I- (aq)

• Exps show that the reaction is first order with respect to propanone and first order with respect to H+ ions but zero order with respect to iodine. The iodine conc does not affect the rate of reaction. This shows that iodine is not involved in the rds of the reaction mechanism

• I.e. rate = k[CH3COCH3][H+]

• In the presence of H+ ions propanone molecules react to form an intermediate species with a double bond and an OH group and ‘enol’

• Intermediate formed than reacts with iodine