Factor Affecting Reaction Rate

Factor Affecting Reaction Rate

Objectives

1.    Explain factors affecting reaction rate;

              -Concentration / Pressure   -Temperature

              -Catalyst                                   -Particle size

    2. Explain the effect of temperature on reaction

        rate using Maxwell-Boltzmann distribution

        curve.

    3. Explain the effect of catalyst on activation

       energy based on energy profile diagram for

       exothermic and endothermic reactions.

   4. Relate rate constant to temperature and activation energy using the Arrhenius   

       equation

      

       k = Ae-Ea/RT or ln k = ln A –Ea/RT

   5. Determine k, Ea, T and A using Arrhenius

       equation by calculation and graphical method.

Ln (k1 / k2) = Ea/R (1/T2 – 1/T1)

Factors Affecting Reaction rate

a) Effect of Concentration

o When concentration of reactant increases, frequency

of collision also increases.

o More particles present in the same volume, they are more likely to collide

o The probability of effective collisions increase

frequency of effective collision = collision frequency x fraction of molecules with sufficient energy

o Rate of reaction increases.

   [reactants] increases,the frequency of collision increases

o This observation correlates with the RATE LAW

    Reaction rate = k [ A ]x [ B ]y …

                             (A & B = reactants)

                            (x & y = rate order)

o Based upon this equation,

                       Reaction rate directly proportional to the concentration of reactants

o REMINDER!

    Only in zero order reactions, the rate of reaction is not

    dependant upon the concentration of the reactants.

    (depending on its rate order)

    Reaction rate = k [ A ]0 = k (constant)

b) Effect of Temperature

o At a higher temperature, molecules have higher kinetic energy and move at higher speed

o more collisions will occur in a given time

o Furthermore, the higher the KE, the higher the energy of the collisions

o So more molecules will have energy greater than Ea

o effective collision also will be increased

o thus the reaction rate increases.

Distribution of Kinetic of Molecules(Maxwell-Boltzmann Distributions)

The collision frequency is higher temperature because the fraction of reactant molecules with activation energy is higher.

Maxwell-Boltzmann Distributions

• The figure shows the distribution KE of gaseous molecule at temperature T1 and T2.

• At higher temperature, the fraction of molecules with energy greater than Ea increases.axwel-Boltzmann Distributions

• Thus, rate of reaction increase with increase of

temperature.

Take note:

• Area under the curve ∝ to the total number of molecules

area under the curve is the same, bcoz no. of molecules is the same.

Maxwel-Boltzmann Distributions

• There are a wide range of molecular energy

A few molecules have low and high KE, most have value in the middle.

• The shape of the graph changes with temperature

At higher temperature the peak of the graph moves towards the right to a higher KE value as the curve broadens out.

• At higher temperatures, the fraction of molecules with higher energy increase

Average energy increase at higher temperature, but fraction of molecules with low energy decreases. At higher temperature, fraction of molecules with energy

greater than Ea increase.

c). Effect Of Temperature

ARRHENIUS EQUATION

• the effect of temperature on the rate constant, k:

                                k = A e -Ea⁄RT

Where…

k = rate constant

A = frequency factor

(is a measure of the probability of a favorable collision)

e = natural log exponent

Ea = activation energy for the reaction (kJ/mol)

R = universal gas constant (8.314 J mol-1 K-1)

T = absolute temperature (T in Kelvin)

Arrhenius Equation - Derivation

  k = A e^ -Ea⁄RT

ln k = ln (A.e^ –Ea/RT)

ln k = ln A – Ea/RT

ln k = ln A - Ea / R (1/T)

   y = C + mx

Example

The table below gives the rate constants, k for the reaction between potassium hydroxide and bromoethane at different temperatures.

K(M^-1s^-1)	T(k)
0.63	322
2.50	331
10.0	347
22.6	353

a) Using a graphical method, calculate the activation energy (kJmol-1) for this reaction.

b) What is the overall order of reaction? Explain

c) Calculate the initial rate of reaction at 330 K when the concentrations for both KOH and   CH3CH2Br are 0.1M.

Solution……

1/T	0.0031	0.0030	0.0029	0.0028
ln k	-0.46	0.92	2.30	3.12

a) Slope = Ea/R

              = 12492

Ea = 12492 × 8.314

     = 1.04 ×105 Jmol-1

     = 104 kJmol-1

b) Second order, unit of k = M-1s-1

 c) ln k = -Ea/R(1/T) + ln A

     ln k = -12492 (1/330) + 38.45

         k = 1.81 M^-1s^-1

Rate = k [KOH][CH3CH2Br]

        = 1.81 x 0.1 x 0.1

        = 1.81 x 10-2 Ms-1

Arrhenius Equation – Further Derivation

Rate constant, k varies with T.

The ratio of rate constant at two different T can be calculated.

ln k1 = ln A – Ea / RT1……………………….(1)

ln k2 = ln A – Ea / RT2……………………….(2)

Equation (1) minus (2) gives :

ln ( k1/k2) = Ea / R (1/T2-1/T1)

d). Effect Of Catalyst

• A catalyst is a substance that increases the rate of a chemical reaction without itself being consumed.

• It increases the reaction rate by providing an alternative reaction pathway of which having lower activation energy.

• A catalyst provides a different reaction mechanism.

A catalyst provides an alternative pathway for the reaction to occur (----curve) which has a lower activation energy.

When Ea decreases, k increases, reaction rate increases

e). Effect Of Surface Area

• For reactions that occur on a surface that is for solid, the rate increases as the surface

area is increased.

• A larger surface area increases the contact area between the reactants thus increases

the frequency of collision and the probability of effective collision between the molecules

of the reactants.