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6.1 DETERMINING THE RATE OF A CHEMICAL RXN

What is the rate of a chemical rxn?

The speed at which reactants are used up and products are formed

EXPERIMENTS TO MEASURE RATE OF RXN

Consider this rxn:

CaCO3(s) + 2HCl(aq) CaCl2(aq) + CO2(g) + H2O

We can measure the rate of rxn in 2 ways1.Measure rate that CO2 is produced

2.Measure the rate at which the mass decreases

MEASURING CO2 PRODUCTION

HCl(aq)

CaCO3(s)

Delivery Tube

Water

Measuring Cylinder

CO2(g)

MEASUREMENT OF THE RATE MASS DECREASES

Mass decreases as CO2 is given off

Measure mass decrease every 10 sec

Plot the data

HCl(aq)

CaCO3(s)

Balance

Cotton Wool

RATE OF RXN DEFINED

Change in concentration of reactants or products per unit of time

(Time could be 1 sec, 1 min, etc)Average rate= (change in

concentration)/(time)Unit are dm-3 s-1, dm-3 min-1, etcIf a given amount of reactant is used up,

the same amount will be produced

6.2 COLLISION THEORY

What is the product of particles colliding?Reactions

For collisions to result, there are two conditions that must be fulfilled

1. Collision must involve more than a certain minimum amount of energy

2. Molecules must collide with the correct orientations

COLLISION MUST INVOLVE A CERTAIN MIN. AMOUNT OF ENERGY

To react, particles must collide with sufficient energy

The min. energy must result in the activation energy (Ea)

Ea is the minimum amount of energy that colliding particles must posses for a collision to result in a rxn

If particles collide and do not overcome Ea, they will just bounce off each other

Collisions resulting in a reaction are called successful or effective collisions

Not all rxns that overcome Ea will result in rxn

MOLECULES COLLIDE WITH CORRECT ORIENTATION

If molecules do not collide with correct orientation they will not react

Main factors affecting rate of rxn

1. Concentration of reactants

2. Pressure for reactions involving gases

3. Surface area of solid reactants

4. Temperature

5. Catalysis

EFFECTS OF CONC.

Does higher reactant concentrations, how will it effect the rate?

With more particles in a given volume, there will be more collisions

Increases the chances of successful collisions

EFFECTS OF PRESSURE

How will increased pressure effect rate?

Similar to concentration increase

Increases collision frequency

Only reactions involving gases are significantly affected by changing pressure

EFFECT OF SURFACE AREA OF SOLID REACTANTS

Reactions generally only occur on the surface of solids

How can surface area of a solid be increase?

Finely divide the solid to open up more areas for

reactions

Allows for more potential collision opportunities

RELATIONSHIP BETWEEN TEMP AND THE ENERGY OF PARTICLES IN A GAS

How does temp effect the movement of particles?

Ideal gas: kinetic energy of the particles in a gas is

proportional to its temp in K

If temp is doubled, average energy is generally

doubled

Does not depend on identity of gas, however lighter

molecules will be traveling faster than heavier

ones

EFFECTS OF TEMP ON RATE

How does temp effect rate?

The rate will generally double for every 10 K increase in

temp

Why?

Collision frequency increases due to particles moving faster

The also collide harder

Increases the chance for collision

MAXWELL-BOLTZMANN

Distribution of molecular kinetic energies at a particular temp

Only a few particles with high energy and only a few low energy

Most particles have average energy

CATALYSIS

Catalyst A substance that increases the rate of a

chemical rxn without itself being used up in the rxn

Often written above the yield arrow in the equation

Also allow the rxn to proceed by an alternative

pathway of lower activation energy

May be homogeneous (same state as reactants) or

heterogeneous

HL2

6.3 THE RATE EXPRESSION

Rate equation/ expression

Consider A B

Rate is directly proportional to [A]

Rate= k[A]

k is the rate constant

The rate expression is experimentally determined equation relating to the rate of rxn to the concentration of substances in the rxn mixture

RATE EXPRESSION

General equation

xA + yB C + D

Rate= k[A]m[B]n

Rate constant a constant of proportionality relating the concentrations in the experimentally determined rate expression to the rate of the rxn

Only constant at a particular temp

ORDER OF RXN

In respect to a particular reactant is the power

of the reactant’s concentration in the

experimentally determined rate equation

m and n are the orders of the reactants

Overall order is m+ n

Rate expression can be determined

experimentally

EXPERIMENTAL DETERMINATION OF RATE EXPRESSION

Consider, A + B CThe initial rate can be taken because

we know the initial concentrations of A and B

An experiment with a fixed amount of B and varied concentrations of A is performed

Then do the same with fixed A and varied B

Use data to determine orders of A and B

DETERMINING ORDER OF REACTION AND RATE EXPRESSION FROM EXPERIMENTAL DATA

Given the reaction 2A B

Experiment

[A]/ mol dm-3

Rate/ mol dm-3 s-1

1 1.0 0.60

2 2.0 1.2

3 5.0 3.0

CONT

We want to determine:1.The order with respect to A2.The rate expression3.The value of the rate constant (w/

units)4.The rate of reaction when [A]=

1.3 mol dm-3

ZERO-ORDER RXNS

Rate independent of concentrationRate equation is: rate=kUnits of k are conc. time-1

Could be mol dm-3 s-1

mol dm-3 h-1

etc.

FIRST –ORDER RXNS

Rate is directly proportional to the concentration

Half- life is first-order

Half-life is related to rate constant:

rate constant= 0.693/ half-life

Rate equation: rate=k[A]

Units of k are time-1

SECOND-ORDER RNS

Rate of rxn is proportional to concentration squared

Rate expression: rate=k[A]2

UNITS OF RATE CONSTANT

Overall order

Units of k Example of units

0 Concentration time-1

mol dm-3 s-1

1 time-1 s-1

2 Concentration-1

time-1

mol-1 dm3 s-1

3 Concentration-2

time-1

mol-2 dm6 s-1

6.4 THE ARRHENIUS EQUATION

Shows the variation of the rate constant with temperature

As temp. increases, the rate constant increases exponentially

Equation: k=Ae-Ea/RT

A pre-exponential factor, A-factor or frequency factor

Relates to frequency and orientation of collision

A constant that varies slightly with temperature

e-Ea/RT the fraction of collision where E> Ea (Energy is greater than activation energy)

Not all reactions where E> Ea will result in collision

THE OTHER ARRHENIOUS EQUATION

May also be written as lnk= (-Ea/R) x (1/T) + lnA

This form is used to solve for activation energy, if these procedures are followed

1. Conduct a series of temperature-varied experiments

2. Calculate rate constant for each temp.

3. Plot a graph of lnk (y-axis) vs. 1/T (x-axis)

Temp in K

Slope = -Ea/R (R= gas constant)

EFFECTS OF A CATALYST ON RATE CONSTANT

If the rate equation is: rate= k[A][B]

Catalyst increases rate constant

6.5 MECHANISMS OF REACTIONS

Consider the reaction, 2NO2(g) +F2(g) 2NO2F(g)

If this rxn were to occur in a single step, all 3 molecules would have to collide in correct orientation at the same time

We could assume that if the concentration was increased the chances of proper collision would increase

This makes the rate dependent upon reactant concentrations

Rate equation will be: rate= [NO2]2[F2]

The 2 superscript comes from the coefficient in the balanced equation

CON’TThe rate derived from experimentation was

found to be

rate= [NO2][F2]

This suggests that the rxn does not occur in a single step

Thus, this reaction (as many more) must occur in multiple steps

The chances of molecules colliding in perfect orientation simultaneously is quite low

SUGGESTEDMECHANISMS OF THIS RXN

NO2 + F2 NO2F +F Step 1

NO2 + F NO2F Step 2

2NO2 + F2 2NO2F Overall Equation

Step 1:rate= k1[NO2][F2]

Step 2:rate= k2[NO2][F]

CON’T

Step 1 is the same as the overall equation, thus must be the step determining the rate for the overall rxn

Called the rate-determining step (the slow step)

Step 2 is the fast step and does not influence the overall rate of rxn to a great extent

thus, the concentrations of the step species are not included in the rate equation

GENERIC EXAMPLE

A + 2B C

B + B Q Step 1 rate- determining step

Q + A C Step 2 fast

Mechanism

B + B Q

Q + A C

2B + A C

Thus, rate= k[B]2 since it is the only species remaining from the first step

RATE DETERMINING STEP AS SECOND STEP

B + B Q Step 1 fast

Q + A C Step 2 rate-determining step

Process of determining overall rate is basically the same as if the first step as the slow step

Rate= k[Q][A]

Because Q is produced by 2 B molecules in the first step, we can replace [Q] with [B]2

Rate= k[B]2[A]

ANOTHER MECHANISM

A + B S step 1 fast

S + B C step 2 rate-determining step

Reactants involved up to and including the rate determining step are included in rate equation

Rate= k[B]2[A]

REACTION INVOLVING A CATALYST

CH3COCH3(aq) + I2(aq) CH3COCH2I(aq) + HI(aq)

The rxn is acid (H+) catalysed

Experimental rate expression is

Rate= k[CH3COCH3][H+]

Does not include I2, so it is only involved after the rate-determining step

CON’T

Proposed mechanism:

CH3COCH3 + H+ X rate-determining step

X + I2 CH3COCH2I + HI + H+ fast

Catalyst is involved in the rate- determining step but is regenerated in second step and does not appear in the overall chemical equation

X is an intermediate

H+ cancels out

SN1 VS SN2 MECHANISMS

(CH3)3CBr + OH- (CH3)3COH + Br –

This is a nucleophilic substitution

Experimental rate expression: rate= k[(CH3)3CBr]

Since OH- is not included, it is in the fast step

Suggested mechanism:

(CH3)3CBr (CH3)3C+ + Br – rate- determining step

(CH3)3C+ + OH- (CH3)3COH fast

CON’T

This is considered SN1

S= substitution

N= nucelophilic

1= molecularity of rate- determining step

Molecularity # of ‘molecules’ that react in a particular step (usually rate-determining)

SUMMARY OF MECHANISM RULES

1. Mechanism must agree with overall stoichiometric equation

2. Maximum of 2 particles can react in any one step

3. All species in rate equ. Must appear in mechanism in or before the rate-determining step

4. The power of a particular reactant’s concentration in the rate equ. Indicates the # of times it appears in the mechanism up to and including the rate determining step