Rate of Chemical Reactions

Reaction Rate

- rate of change of amount or concentration of a particular reactant or product

- rate = change in amount or concentration / time taken

Following Reactions

(1) Rate of disappearance of reactant

(2) Rate of appearance of product

(3) Conductance of product

(4) Volume of gas formed with a gas syringe

(5) Light intensity with Colorimeter

(6) Titration method: A small sample of known volume of reacting mixture is withdrawn with a pipette.

     Reaction is quenched by cooling, dilution or adding suitable reagent. The sample is titrated with

     suitable indicator.

Factors affecting reaction rates

(1) Concentration and Pressure: The higher the concentration is, the more number of collision between

      reactants and hence higher rates.

(2) Surface area of reactants: Larger surface area increases area of contact between reactants and hence

      higher rates.

(3) Temperature: When increasing the temperature by 10*C, the rate of reaction is doubled. Higher

      temperature causes a greater number of collision between reactants. More molecules have enough

      energy to react.

(4) Catalyst: The presence of catalyst lower the activation energy of reaction and so higher rates.

Simple Collision Theory

- For a reaction to occur, molecules must collide in a correct orientation AND with enough energy to react.

Activation Energy

- It is the minimum energy required to overcome the energy barrier of a reaction

Kinetic Stability vs Thermodynamic Stability

- Activation energy --> kinetic stability

   Enthalpy change --> thermodynamic stability

- Enthalpy change very negative means that the products are thermodynamically stable with respect to the

  elements.

- Very large activation energy means slow rate of reaction and hence kinetically stable.

Using Activation energy to explain the effect of temperature on reaction rate

- Maxwell-Boltzmann distribution curve: graphical sketch

- Most probable kinetic energy increases as temperature increases.

- The curve becomes flat as temperature increases, thus wider distribution of molecular speeds.

- Area beneath each curve proportional to total number of molecules.

- Increase in temperature results an increase in the proportion of molecules having the minimum

  Activation energy to react.

(98/4)

Catalysts and their effect on reaction rates (2000 essay question)

- Action: The presence of catalyst reduces the activation energy by providing an alternative path for

  reaction. Therefore, greater proportion of molecules have enough energy to react and hence faster

  reaction. Graphical sketch illustration. (96/9(c)(iii))

- Properties of catalyst:

  (1) can be regenerated

  (2) chemically unchanged (though may be physically changed)

  (3) effective in small amount

  (4) specific

  (5) may be promoted or poisoned

  (6) percentage yield remain unchanged

- Homogeneous and heterogeneous Catalysis (94/10(b))

- Applications of Catalysis

   (1) In catalytic converter: Platinum catalyst. Unleaded petrol must be used because catalyst is easily

         poisoned by lead.

   (2) In industry: Contact Process (V2O5), Haber Process (Fe), Manufacture of margarine from vegetable

        oil (Ni).

   (3) In enzymes: Enzymes are biological catalyst (proteins). e.g. hydrolysis of starch to sugar.

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