• Rate of a reaction can be defined as the change of concentration (either decrease of
concentration of reactants or increase of concentration in products) within a unit time.

• Factors affecting rate of reactions
– Temperature
– Concentration (Pressure)
– Physical nature (Surface area of reactants)
– Catalysts (Heterogeneous/Homogeneous)

Experiment I : Effect of concentration on the rate of a reaction.

• Equal volumes of water (10.0 cm³) are added to test tubes along with two pieces of magnesium ribbon of 2 cm long. Apparatus are arranged as in the diagram keeping all the factors identical except the concentration of an acid by adding 2 drops of a hydrochloric acid solution to one test tube and 4 drops of the same solution to the other.

• Gas bubbles evolve faster in the test tube containing acid in higher concentration.

• It can be concluded that concentration affects the rate of a reaction.

Experiment II : Effect of temperature on the rate of a reaction.

• Apparatus are set as shown in the diagram keeping all the factors identical except
temperature.

• It can be seen that the solution kept at higher temperature decolourizes faster.
• Therefore, temperature affects the rate of a reaction.

Experiment III : Effect of the physical nature (surface area) of reactants on the rate of
a reaction.

• To two test tubes separately add a lump of CaCO₃ and CaCO₃ powder of equal mass.
• To each test tube, add equal volumes of a HCl solution of the same concentration (e.g.0.01 mol dm^-3)
• Keep both tubes in a water bath.
• Observe the difference in the rates of evolution of gas.
• Emphasize the need to immerse in a water bath.
• It can be observed that the tube containing CaCO₃ powder effervesces faster.
• Thus, it can be concluded that the physical nature of reactants affects the rate of a reaction.

Experiment IV : Effect of catalysts on the rate of a reaction

• Add 10.0 cm³ samples of ’20 volume’ H₂O₂ to two boiling tubes. (’20 volume’ means that unit volume of H₂O₂ solution gives twenty volumes of O₂ at STP)
• Add 5.0 cm^-3 of water to one tube and 5.0 cm³ of 0.1 mol dm^-3 NaOH to the other.
• It is seen that gas bubbles evolve faster in the tube with NaOH.
• This leads to the conclusion that NaOH increases the rate of decomposition of H₂O₂.

(a) Homogeneous catalysts : If the catalyst and the reactants are in the same phase, they are called homogeneous catalysts.

(b) Heterogeneous catalysts : If the catalyst and the reactants are in different phases, they are heterogeneous catalysts.