Chemical Kinetics Book Abstract
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Published: February 17, 2008
Chemical Kinetic
“It is the branch of chemistry which deals with the study of rate of reactions and factors affecting the rate.”
Rate of reaction
Substance that are react in a chemical reaction are called Reactant and those that are produced are called Product.
During the course of a reaction, the molar concentration of reactants gradually decreases, whereas that of product increases. Consider a general chemical reaction.
A(Reactant) ® B(Product)
Rate of reaction is defined as,
“The quantity of a reactant consumed or the quantity of a product formed per unit time.”
The concentration of a reactant or a product is usually expressed in tem of mole/dm3. The time taken for the change is expressed in seconds. The rate so expressed in mole/dm3/sec.
The rate of reaction is not uniform throughout it decreases with the passage of time as the reactant are gradually consumed during chemical reaction.
Velocity of Reaction
It is defined as:
“Rate of reaction at a specific time.”
If in a small interval of time ‘dt’ amount of substance changed is ‘dx’ and during this interval, the rate of reaction remain constant then:
Velocity of reaction may also be termed as instantaneous rate of reaction. Velocity of reaction is the slope of the tangent. Consider the reaction between Mg and HCl.
Mg + 2HCl ® MgCl2 + H2
Velocity of reaction at 48s = Slope of Tangent
= = 0.83c.c/s
Rate Constant
Consider the reaction:
Reactant ® Product
According to the law of mass action, the rate of a chemical reaction is directly proportional to the active mass or molar concentration of the reactant.
Rate of reaction µ
But we know that:
Þ Þ Where K is the rate constant specified. The rate constant is called specific rate constant. If the concentration of reactant is 1 mole/dm3 at given temperature then:
= V
Þ K = V /
Since, R = 1mole/dm3, therefore,
K = V
So rate constant is the ratio between the rate of reaction and concentration of reactant.
Energy of Activation
“The minimum amount of energy required in addition to the average energy, just to convert the reactants into the products is called the Activation Energy.”
It is represented by EA.
According to the collision theory of reaction rate, the reaction takes place when molecules of reacting substances collide together.
It has been found that only those collisions are effective in which the colliding molecules possess a minimum amount of energy called ‘Threshold Energy’. Thus,
Activation Energy = Threshold Energy – Average Internal Energy
Every reaction has its own energy of activation. A reaction with low activation energy takes place at low temperature and a reaction with higher activation energy will take place at high temperature
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