Effect of Temperature over rate of a Chemical Reaction

Factors affecting Reaction rate

The rate of a chemical reaction depends on the rate of encounter between the molecules of the reactants which in turn depends on the following things.

(1)     Effect of temperature on reaction rate : The rate of chemical reaction generally increases on increasing the temperature.

(2)     Nature of reactants : (i) Reactions involving polar and ionic substances including the proton transfer reactions are usually very fast. On the other hand, the reaction in which bonds is rearranged, or electrons transferred are slow.

(ii)    Oxidation-reduction reactions, which involve transfer of electrons, are also slow as compared to the ionic substance.

(iii)   Substitution reactions are relatively much slower.

(3)     pH of the medium : The rate of a reaction taking place in aqueous solution often depends upon the  ion concentration. Some reactions become fast on increasing the H+ ion concentration while some become slow.

(4)     Concentration of reactants : The rate of a chemical reaction is directly proportional to the concentration of the reactants means rate of reaction decreases with decrease in concentration.

(5)     Surface area of reactant : Larger the surface area of reactant, the probability of collisions on the surface of the reactant particles by the surrounding  molecules increases and thus rate of reaction increases.

(6)     Presence of catalyst : The function of a catalyst is to lower down  the activation energy. The greater the decrease in the activation energy caused by the catalyst, higher will be the reaction rate. In the presence of a catalyst, the reaction follows a path of lower activation energy. Under this condition, a large number of reacting molecules are able to cross over the energy barrier and thus the rate of reaction increases.  Fig. shows how the activation energy is lowered in presence of a catalyst.

(7)       Effect of sunlight :  There are many chemical reactions whose rate are influenced by radiations particularly by ultraviolet and visible light. Such reactions are called photochemical reactions. For example, Photosynthesis, Photography, Blue printing, Photochemical synthesis of compounds etc.

H2 + Cl2 \underrightarrow { \quad Sunlight\quad (hv)\quad } 2HI : The radiant energy initiates the chemical reaction by supplying the necessary activation energy required for the reaction.

 

Rate law, Law of mass action and Rate constant

(1)     Rate law : The actual relationship between the concentration of reacting species and the reaction rate is determined experimentally and is given by the expression called rate law.

For any hypothetical reaction, aA + bB → cC + dD

Rate law expression may be, rate = k[A]a[B]b

Where a and b are constant numbers or the powers of the concentrations of the reactants  and  respectively on which the rate of reaction depends.

(i)    Rate of chemical reaction is directly proportional to the concentration of the reactants.

(ii)     The rate law represents the experimentally observed rate of reaction, which depends upon the slowest step of the reaction.

(iii)    Rate law cannot be deduced from the relationship for a given equation. It can be found by experiment only.

(iv)    It may not depend upon the concentration of species which do not appear in the equation for the over all reaction.

(2)       Law of mass action : (Guldberg and Wage 1864) This law relates rate of reaction with active mass or molar concentration of reactants. According to this law, “At a given temperature, the rate of a reaction at a particular instant is proportional to the product of the reactants at that instant raised to powers which are numerically equal to the numbers of their respective molecules in the stoichiometric equation describing the reactions.”

Active mass = Molar concentration of the substance

=  \frac { Number\quad of\quad gram\quad moles\quad of\quad the\quad substance }{ Volume\quad in\quad litres } =\frac { W/m }{ V } =\frac { n }{ V }

Where W = mass of the substance, m is the molecular mass in grams, ‘n’ is the number of g moles and V is volume in litre.

Consider the following general reaction,

m1A1 + m2A2 + m3A3 → Products

Rate of reaction  [A1]m1[A]m2[A3]m3

(3)     Rate constant : Consider a simple reaction, A → B. If C4 is the molar concentration of active mass of A at a particular instant, then,  \frac { dx }{ dt } ∝ CA  or   \frac { dx }{ dt } = kCA ; Where k is a proportionality constant, called velocity constant or rate constant or specific reaction rate constant.

At a fixed temperature, if CA = 1, then Rate =  \frac { dx }{ dt } = k

“Rate of a reaction at unit concentration of reactants is called rate constant.”

(i)      The value of rate constant depends on, Nature of reactant, Temperature and Catalyst

(It is independent of concentration of the reactants)

(ii)     Unit of rate constant :  Unit of rate constant = \left[ \frac { litre }{ mol } \right] ^{ 1-n } × sec-1  or   \left[ \frac { mol }{ litre } \right] ^{ 1-n } × sec-1

Where n = order of reaction

Difference between Rate law and Law of mass action
Rate law Law of mass action
It is an experimentally observed law. It is a theoretical law.
It depends on the concentration terms on which the rate of reaction actually depends It is based upon the stoichiometry of the equation
Example for the reaction, aA + bB → Products Example for the reaction, aA + bB → Products
Rate = k[A]m[B]n Rate = k[A]a[B]b

 

Difference between Rate of reaction and Rate constant

Rate of reaction Rate constant
It is the speed with which reactants are converted into products. It is proportionality constant.
It is measured as the rate of decrease of the concentration of reactants or the rate of increase of concentration of products with time. It is equal to rate of reaction when the concentration of each of the reactants is unity.
It depends upon the initial concentration of the reactants. It is independent of the initial concentration of the reactants. It has a constant value at fixed temperature.