Some reactions are not represented in the reaction equation; such as,- Absorbing light energy- Colliding to the walls of the container

Many reactions proceed through a sequence of steps to arrive at the products

Each step is called an elementary reaction and occurs through collision of atoms, ions and molecules.

The slowest step in a reaction mechanism is called the Rate-determining Step

In multistep reactions, increasing the concentration of reactants at any steps other than the rate-determining step will not increase the rate of reaction

A chemical species that form and then are consumed in the reaction is called Reaction Intermediates

mX + nY -----> products of reaction or intermediates

then r α [X]m [Y]n

r = k[X]m [Y]n

When writing the reaction mechanism, there are three rules:- each step must be elementary, with no more than three

reactants- The rate-determining step must be consistent with the rate

equation- The elementary steps must all add up to be the overall

equation

Elementary reactions (Molecularity) Unimolecular; Decomposition

(ex. N2O5(g) NO2(g) + NO3(g))rate = k[N2O5]

Bimolecular; Collision of 2 atoms, ions or molecules(ex. NO(g) + O3(g) NO2(g) + O2(g))rate = k[No][O3]

Termolecular; Simultaneous collision of 3 molecules(ex. 2I(g) + Ar(g) I2(g) + Ar(g) )rate = k[I][I][Ar] = k[I]2[Ar]

Reaction mechanism is a detailed sequence of elementary reactions, with their rate, that are combined to yield the overall reaction.

Consider the mechanism:1) Cl2(g) 2Cl(g) 2) Cl(g) + CHCl3(g) HCl(g) + CCl3 (g) 3) CCl3 (g) + Cl (g) CCl4(g)

Some of the possible questions are…

a) What's the molecularity of each step?1) Unimolecular2) Bimolecular3) Bimolecular

b) Write the overall equation for the reactionCl2(g) + CHCl3(g) HCl(g) + CCl4(g)

c) Identify the reaction intermediate(s)Cl(g) & CCl3

To explain the large effect of temperature and catalysis, the Arrhenius Equation is used to account for their effects

k = Ae-Ea/RT

Ea is the activation energy (J)A is constant related to the geometry

of moleculeR is the gas constant (8.314 J/(mol•K))T is the temperature (K)

the answer k is the rate constant for the rate law equation r = k[A]n[B]m

By taking ln (natural log) to each side,ln k = -Ea/RT + ln A

Arrhenius equation is written for the rate constant determined at each temperature giving, ln k1 = -Ea/RT1 + ln A

ln k2 = -Ea/RT2 + ln A Subtracting second equation from the first,

ln(k1/k2) = -Ea/R(1/T1 – 1/T2) The rate and the rate constant are directly

proportional to each other as long as the concentrations are held constant.

Examples At 200K the rate constant for a reaction is

3.5 X 10-3s-1 , and at 250K the rate constant is 4.0X 10-3s-1 . What is the activation energy?Ea = 1.11KJ/mol

What is the rate of reaction at 450oC if the reaction rate is 6.75 X 10-6mol/(L*s) at 25oC? The activation energy was previously determined to be 35.5kJ/mol.The rate of reaction at 450oC is 3.05 X 10-2 mol/(L*s)

To test the answer for reasonableness, two principles must be remembered:1) The larger rate constant (or rate) will always be associated with the higher temperature2) The activation energy always has a positive sign.

Reactions that contain more than 3 molecules colliding occurs in multi-steps

Each elementary step should only have 3 or less reactants

r = k[A]n[B]m k = Ae-Ea/RT

Temperature and activation energy affects the value of k exponentially