Kinetics

Reaction rates

Concerned with why some reactions are fast and others are slow.

A reaction rate is quantitatively written as either:Change in concentration of a reactant/time Change in concentration of a product/timeUnits may vary. Some examples are:

Molarity/hour, nmol/sec.

Reaction rate graph showing two curves: reactant and a product

Reaction: NO2 + CO NO + CO2

concentration reactants

products

equilibrium

equilibrium

Factors which influence a reaction rate. Internal factors

More bond breaking and bond making, slower the reaction. Each bond broken or made adds a step to the overall reaction.

The physical state ( s,l,aq,or g) in the reactants: solids can slow down a reaction. Gases are faster. (Slow: s,l, aq,g: fast)

Example: CH4(g) + 2O2(g) 2H2O(g) + CO2(g)

C8H18(l) + 25/2 O2(g) 8CO2(g) + 9H2O(g)

Factors which influence a reaction rate continued

External FactorsTemperature: Increasing a temp. will increase a

rate because: K.E. increases so the velocity of the molecules increases. Therefore, the molecules collide with more force and more often, so there are more successful collisions. A change in temp. has more impact on the rate of a reaction, more than any other factor.

External factors continued

Concentration- increasing the concentration of a reactant will increase the rate of a reaction because there will be more collisions/time. Applies to gases and aq solutions.

Pressure-increasing the pressure will increase the rate of a reaction if there are any gaseous reactants only. The increased pressure increases the collisions/time. Applies to gases.

Surface area- increasing the surface area will increase the rate of a reaction only if there are any liquid or solid reactants. The increase in surface area increases the collisions/time. Applies to solids and liquids.

The effects of concentration and a solid

Reaction rate graph showing the effects of surface area

A quantitative look at a reaction rate. A rate is the Δ concentration/time The change in concentration can be for any reactant

or product. Just pick the easiest species to measure. Example: determining the rate of H2 gas produced by

Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)

Measure the volume of the H2 at regular time intervals .(use a gas law to turn the vol. into a conc. )

Plot the volumes or concentrations on a graph with time as x axis, and volume and concentration as y.

Take the slope of the line early in the reaction: the Δy/ Δx. This equals the change in volume or concentration over time which is a rate.

This rate changes at any point on the graph since the slope is not constant.

The rate obtained through this method is an instantaneous rate-accurate at that point in time.

Since rates change during a reaction the rate law of the reaction is more useful.

Making a rate graph

Ml of H2

time

Volume of H2 collected

Reaction mechanisms

A reaction mechanism is the series of steps that occur in order for a reaction to reach completion.

The mechanism must be determined experimentally.

Restrictions to a reaction mechanism are: Each step in the mechanism usually involves a collision

between 2 molecules or the splitting of one molecule The individual steps must add up to the overall final

reaction. The mechanism must support the rate law.

The individual steps must add up to equal the overall reaction. The reaction:

AB + CD AC + BD AB A + B B + CD C +BD A + C AC

AB + CD AC + BD

Each step usually involves the collision between 2 molecules only example reaction is: HBr + CH3CH2OH CH3CH2Br + H2O

Reaction mechanisms: the slow step (rate determining step) The slow step or rate determining step is the step in

the reaction mechanism which is the slowest. It is the slowest for these possible reasons: It has the highest energy requirement for a successful

collision. (The bond being broken has a high bond enthalpy). This energy requirement is called the Activation energy. The molecule formed during this step is called the activated complex.

There may be a special angle of orientation for a successful collision.

There may be a special angle of orientation.

Catalysts

A catalyst is a substance which can speed up an existing reaction. A catalyst works by:Speeding up the slow step in the reaction

mechanism through: Lowering the activation energy required for a

successful collision. Widening the angle of orientation required for a

successful collision.

Catalysts continued.

Catalysts do not:Cause a reaction. They only speed up

existing ones.Change either the reactants or products, or

the enthalpy for a reaction.Get used up. They are recycled and used

over again.

Catalysts continued

Some examples of catalysts are:EnzymesVitamins (actually they are coenzymes)Catalytic converters in carsTransition metals such as Platinum, Osmium,

Manganese.

Diagrams to know:

Maxwell Boltzman diagram: temperature effects and activation energy (Ea)

energy

MolesOf Mole-cules

1T1

EaT2 is a higher temp.

Diagrams to know continued

Potential energy diagrams: shows the energy changes that occur during the progress of a reaction.

Progress of the reaction

PotentialEnergykJ/mole

RDS/activated complex

Ea H

This reaction is Endothermic. The Products have moreEnergy than the Reactants. Enthalpy isPositive.

(products)(reactants)

Potential energy diagram of an exothermic reaction with a catalyst

Potential energy diagrams continued On PE diagrams, you must know how to:

determine if the reaction is endothermic or exothermic.

determine the enthalpy for the reactiondetermine the Ea determine the location of the R.D.S. and the

activated complex.Determine the line for a catalyst.Estimate whether a reaction will be slow or fast.

Rate LawsIB optional material

General information:A rate law is constant for a reaction unless the

temperature is changed.A rate law uses only reactants.A rate law is determined using the initial

concentrations of reactants.A rate law helps explain a reaction mechanism.

If we know the rate law we have a better understanding on how to control the reaction.

Rate laws continued.

The general form of a rate law is:Given the reaction: 2A + B A2BThe rate law expression will look like:

rate = k[A]n[B]m

k is a rate constantn and m are rate ordersBoth of the above are determined experimentally.Coefficients are not used in the expression

Rate Laws and graphs

2nd order

1st order

Zero order

Intial rate

Initial concentration

Summary of rate orders

Zero order [x]0 altering the concentration of the reactant has no effect on the rate.

First order [x]1 doubling the concentration of the reactant will double the rate, cutting the concentration in half will cut the rate in half etc.

Second order [x]2 doubling the concentration will quadruple the rate. Cutting in1/2 the concentration will cut the rate to ¼ etc.

Overall rate order: add the rate orders together.

Half-life of first-order reactants

A half-life is the time it takes for the concentration of a reactant (1st order) to decrease to half of its original concentration. In the 2nd half-life interval, the concentration will be cut in half again to 1/4th its original concentration. So the change in concentration is continually being cut in half with each successive half-life.

Determining the half-life.

First method: make a rate graph using concentration over time. From the graph determine where the concentration is cut in half.This time interval is the half-life. This half-life is constant. t1/2= 2t1/2=3t1/2=4t1/2 For a zero order: each half-life is half the time of the preceding

half-life. ex. T1/2=1min. 2t1/2 = 0.5min., 3t1/2= 0.25min. For 2nd order: each half-life is double the preceding half-life.

ex. T1/2= 1min. 2t1/2= 2min., 3t1/2= 4min.,

Second method: use t1/2 = ln2 / k ln 2 = 0.693, k= the rate constant for the reaction.

Graph showing the half-life of a 1st order reactant.

Conc.Of areactant

Time

Reaction mechanisms: Molecularityoptional IB materialThere are two basic types of steps or processes in a mechanism:

1. unimolecular: A reactant breaks up into two products. If the species is in the r.d.s. then the reacting species is first order.2.bimolecular: two species collide to form the product(s). It they are in the slow step then each reacting species is first order, and 2nd order over all.

There is initially a product made which quickly breaks down into either reactants or products. This temporary product is called the activated complex and has the maximum energy requirement, ie, activation energy, for a successful collision.

Whether a reaction step is unimolecular or bimolecular is called the step’s molecularity.

Molecularity continued

These molecularity concepts along with the rate law are used to determine the rate determining step for a reaction.

Any reactant in the rate determining step or in the step preceding the r.d.s. determines the overall rate of the reaction. So it is in the rate law

Knowing the rate law expression for the reaction can help to predict the process or molecularity in the r.d.s.

Determining a rate expression based upon the mechanism IB text pg 234, 235The chart refers to the reaction: 2A + B C + D

I. A+B X+C Slow RDS

A+X D Fast

II. A+B X Fast

A+X C+D Slow RDS

III. A+A A2 Fast

A2+B C+D Slow RDS

IV. A+A A2 Slow RDS

A2+B C+D Fast

V. BX Slow RDS

X+A Y+C Fast

Y+A D Fast

Possible mechanisms Rate expression

Rate k [A] [B]

Rate k [A]2[B]

Rate k [A]2[B]

Rate k [A]2

Rate k [B]