Factors Affecting the Rate of Reaction

   Factors Affecting the Rate of Reaction

       1.) Temperature

·         Reaction rate increase with an increase with an increase in temperature this happens because the kinetic energies of the reactants increase with a rise in temperature.

·         The faster the reactant particles move, the greater the chance that they will collide and react with each other.

·         As general rule, a 10°C rise in temperature increases the reaction twice or thrice.

Example: We refrigerate perishable food such as milk. The bacterial reactions that lead to the spoiling of milk proceed much more rapidly at room temperature that they do at the lower temperature of a refrigerator.

       2.) Concentration of Reactants

·         An increase in Concentration means increase in the molecules or particles per unit volume and thus a decrease in space between the reacting particles. With less distance to travel inside the vessel, the more frequent collision, the faster the reaction.

  

Example: Steel wool burns with difficulty in air, which contains 20% O2, but burst into a brilliant white flame in pure oxygen.

                  In 1967, Ground Test of Apollo 1, the spacecraft exploded because there was a leak in its fuel container. Its fuel (concentrated O₂) is very combustible.

       3.) Presence of Catalyst

·        Catalyst – a substance which controls the rate of reaction without itself undergoing a permanent chemical change. Catalysts are of two types.

·         Positive Catalyst - increases the rate of reaction by lowering the energy of activation. Thus in the presence of a positive catalyst, the greater fraction of the total molecule will posses lower energy of activation and collided successfully in a short period of time, there by increasing the rate of reaction. A positive catalyst functions by providing an alternate path to the reaction or by the formation of a transition (intermediate) compound having low energy of activation. The activation energy of this path is lower. As a result rate of reaction is increased.

·         Negative Catalysts or Inhibitors –  retards the rate of reaction. Negative catalyst do not lower the energy of activation rather they are combined with reactant molecule thus decreasing the number of colliding reactant molecules. This decreases the effective collisions, hence rate of reaction. A negative catalyst does not lower the energy of activation rather it combines with the reactant molecules, thus decreasing the number of colliding reactant molecules. This decreases the effective collisions, hence rate of reaction.

Example: The physiology of most living species depends crucially on enzymes, protein molecules that acts as catalysts, which increase the rate of selected biochemical reactions.

  4.) Nature of Reactants

·         Ionic Compound – reacts easily

·         Covalent Compound – reacts slowly

·         Polar-Polar – mixes or dissolves easily

·         Nonpolar-Nonpolar – mixes or dissolves easily

·         Polar-Nonpolar – mixes or dissolves slowly

·         Homogenous reactants reacts faster than heterogeneous reactants

Example: Reactions occurring between  oppositely charged ions are generally rapid at room temperature. In reactions involving covalent substances where there are many covalent bonds to be broken, the reaction usually takes place slowly at room temperature. the H-H and O-O bonds in the H2 and O2 molecules must be broken and then two bonds between H and O must be formed to produce the H-O-H molecule.

        5.) Surface Area

·         The greater the surface area, the faster the rate of reaction.

·         The smaller the size of particles, the larger the surface area exposed.

Example: With the same volume, between the small ice cubes and a block of ice, the ice cubes will melt faster because it has greater surface area exposed.