Sunday, October 21, 2007

Study Guide Ch.10. CHEMICAL KINETICS

Jee Syllabus

Chemical kinetics:
Rates of chemical reactions;
Order of reactions;
Rate constant;
First order reactions;
Temperature dependence of rate constant (Arrhenius equation).
--------------------------
Main topics in TMH book

CHEMICAL KINETICS
-------------------------
The material given in the TMH book is very brief. So I have to elaborate the contents. This chapter should have been given before the chapter on Chemical Equilibrium.

The topic "Chemical kinetics" consists of reaction rate and reaction mechanism.

Reaction rate is the speed with which a reaction takes place. This shows the rate or speed at which the reactants are consumed and products are formed.

Reaction mechanism is the path by which a reaction takes place.

Rate of reaction

The rate of reaction is a quantity that tells how the concentration of reactants or product changes with time.

So this can be expressed as Δ contentration/Δ time. That is change in concenation divided by time taken for the change.

Molar concentration i.e., moles per liter (M), is used in these equations.

The brackets, [ ] are always used to to indicate molar concentrations.

Rate of reactions can be expressed as d[A]/dt. [A] represents molar concentration of a reactant or a product.

Rate law

The rate for a reaction is a mathematical expression that relates the rate of reaction to the concentrations of the reactants.

For the reaction aA + bB → products

The rate law is expressed as, rate of reaction is proportional to [A]^x[B]^y.
x and y are determined experimentally. These values can be whole or fractional numbers or zero.

Reaction Mechanisms

Many reactions go in two or more steps. When this is the case, there is usually one slow step, and the rate of this step determines the rate of whole reaction.. The slow step is called the rate determining step.

Law of Mass Action

In 1867, Cato Guldberg, and Peter Waage, proposed this law. According to this law, for the rate determining step in a reaction, the rate of reaction is proportional to the product of the concentrations of the reactants, each raised to the power of its coefficient in the balanced equation.

For the reaction aA + bB → cC (when it is a rate determining step)

Rate of reaction is proportional to [A]^a[B]^b

The above proportionality can be written as an equation, by putting in a proportionality constant k.

Rate = k *[A]^a[B]^b

K is called the specific rate constant (TMH book used the term “rate constant”)

But, remember that for the overall rate law (rate law for total reaction), must be determined experimentally. The law of mass action only applies to the rate determining step. TMH book uses the term elementary equation for the equation for which law of mass action applies.

Example:

For the rate determining step

A + 2B → C

Rate = k *[A][B]^2

Order of Reaction

From the rate law for a reaction order of reaction can be determined.

For a particular species or reactant, the order is equal to the exponent for that species in the rate law.

For example for Rate = k *[A][B]^2
for B the order of reaction is 2. For A it is 1.

The overall order of reaction is equal to the sum of all the individual orders of reactants.

From the orders of reactions, we can determine how the rate of a reaction will change as the concentration of each of the reactants is changed.

For the above reaction, if the concentration of B is double, the rate will become 4 times.
For gases, as molar concentrations are proportional of partial pressures, they can be used instead of concentrations in the rate equations.

Collision Theory of Reaction Rates

Collision theory can be used to explain what is happening in reactions. The collisions between particles of reactants results in reaction. The collisions will be effective when colliding particles have sufficient energy. The energy that is necessary for effective collisions is called the activation energy. Activation energy is energy barrier that must be overcome in order for a reaction to take place.

Factors that Control Reactions Rates

Concentration of reactants, temperature, and catalysts determine the rates of reaction for a given set of reactants. The nature of reactants determines the activation energy required.

Concentration

As concentration increases, more reactant molecules collide with other reactant molecules in a given period of time, and more product molecules are formed.

Temperature

As temperature increases, the average kinetic energy increases. So there are more molecules with activation energy and hence reaction rate increases.

As a general approximation, the rate roughly doubles for each 10°C rise in temperature.

Catalyst

A catalyst is a substance that alters the speed of a reaction without being consumed. While a catalyst is normally used for speeding up the reaction, there are also catalysts that slow down a reaction. They are called negative catalysts or inhibitors.

A catalyst alters the speed of the reaction by changing the activation energy. It speeds up the reaction by decreasing the activation energy. A negative catalyst increases the activation energy.

Determining Reaction Mechanisms

When alternative paths are possible for a reaction, the various paths are identified. The rate laws for each possible paths are written down. Experimentally, the rate law for the reaction is determined. The mechanism that gives this rate law (experimentally determined law) is taken as the reaction mechanism.

How to Observe Change in rates of Reactions

If a reaction has H^+ ions as one of the products, change in pH can be used to determine the rate of reaction.

If the product is a gas, the measurement of gas given off in various periods of time
can be used to determine the rate.

If the products contain a species with distinctive colour, the progress can be followed by measuring the absorbency of light passing through the solution, in a colorimeter or other type of spectrophotometer.
--------------
web sites
Factors affecting the Speed-Rates of Chemical Reactions
http://www.docbrown.info/page03/3_31rates.htm

----------------------
JEE Question 2007 paper II

For the process H-2O(l)(1 bar, 373 K) --> H-2O(g)(1 bar, 373 K), the correct set of thermodynamic parameters is

(A) ∆G=0,∆S= + ve
(B)∆G=0,∆S= -ve
(C)∆G=+ve,∆S=0
(D)∆G=-ve, ∆S= + ve

Solution: A

This question belongs to the chapter Energetics. The answer is A because, because at 100 degree C, the steam and water mixture is at equilibrium. Hence ΔG = 0(G = H - TS), and ΔS is positive.

-----------------------

See some questions for practice on this chapter in the page

http://iit-jee-chemistry-ps.blogspot.com/2007/10/iit-jee-chemistry-questions-ch10.html