Rate Laws and Stoichiometry

8/12/2019 Rate Laws and Stoichiometry

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RATELAWSANDSTOICHIOMETRY

AP DR RUSNAH HJ SAMSUDDIN


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RATE LAWS

A rate law describes the behavior of a reaction.The rate of a reaction is a function of temperature(through the rate constant) and concentration.

RELATIVE RATES OF REACTION

The reaction;

is carried out in a reactor. If at a particular point, therate of disappearance of A is 10 mol/dm3/s, what arethe rates of B and C?


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1. The rate of disappearance of A, -ra, is

or the rate of formation of species A is

The relative rates are


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SPECIES B

The rate of formation of species B is

or The rate of disappearance of B, -rb,is


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SPECIES C

The rate of fomation of C, -rc, is


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POWER LAW MODEL


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Rate Constant, k

k is the specific reaction rate (constant) and is given by theArrhenius Equation:

Where:

E = activation energy (cal/mol)

R = gas constant (cal/mol*K)

T = temperature (K)

A = frequency factor (units of

A, and k, depend on overall

reaction order)


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The larger the activation energy, the

more temperature sensitive k andthus the reaction rate.


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Why is there an Activation Energy?

(1) the molecules need energy to distort or stretch their

bonds in order to break them and to thus form new bonds

(2) as the reacting molecules come close together theymust overcome both steric and electron repulsion forcesin order to react

In our development of collision theory we assumed allmolecules had the same average energy. However, all themolecules dont have the same energy, rather there is

distribution of energies where some molecules have moreenergy than others.The distraction function f(E,T) describes this distributionof the energies of the molecules. The distribution function

is read in conjunction with dE


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f(E, T) dE = fraction of molecules with energies between E and

E + dE

One such distribution of energies is in the following figure.


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By increasing the temperature we increase the kineticenergy of the reactant molecules which can in turn betransferred to internal energy to increase the stretchingand bending of the bonds causing them to be in anactivated state, vulnerable to bond breaking and reaction.

We see that as the temperature is increased we have

greater number of molecules have energies E A orgreater and hence the reaction rate will be greater.

The activation energy can be thought of as a barrier to

the reaction. One way to view the barrier to a reaction isthrough the reaction coordinates. These coordinatesdenote the energy of the system as a function ofprogress along the reaction path. For the reaction


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the reaction coordinate is

We see that for the reaction to occur, the reactants must overcome an energy

barrier or activation energy EA.


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CA -rA Reaction Order Rate Law k

(mol/dm3) (mol/dm3*s) zero -rA= k (mol/dm3*s)

(mol/dm3) (mol/dm3*s) 1st -rA= kCA s-1

(mol/dm3) (mol/dm3*s) 2nd -rA= kCA2 (dm3/mol*s)

You can tell the overall reaction order by the units of k


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The activation energy is a measure of the minimum energy athat the reacting molecules must have in order for thereaction to occur.


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The reaction of AB + C to form A + BC is shown above alongthe reaction coordinate.One way to think of the reaction coordinate is the lineardistance between the AB molecule for a fixed lineardistance between the AC molecule.At the start of the reaction the AB distance is small andthe BC distance is large.

As the reaction proceeds, the AC distance remains fixedbut B moves away for A and closer to C and the energy ofsystem increases.At the top of the barrier molecule B is equal distance fromA and C.But as it crosses the barrier it moved close to C to form theBC molecule and the A molecule alone.


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Rate Laws and Stoichiometry