March 23, 2012

Chemical Kinetics - II Three Marks

1. Define half life period.
The time required to reduce the concentration of a reactant to one half of its initial value.
2. Derive an equation for the half-life period of a first order reaction. Or Show that half-life period of a first order reaction is independent of the initial concentration of the reactant
For first order reaction,
Thus half life period of a first order reaction is independent of the initial concentration of the reactant and also, inversely proportional to the rate constant of the reaction.
3. Give any three examples for opposing reactions.
4. Show that for a first order reaction time required for 99% completion is twice the time required for 90% completion of the reaction.
5. The initial rate of a first order reaction is 5.2 x 10-6 mol lit-1 s-1 at 298 K, when the initial concentration of the reactant is 2.6 x 10-3 mol lit-1. Calculate the first order rate constant of the reaction at the same temperature.
Rate of a first order reaction = 5.2 x 10-6 mol lit-1 s-1
Concentration of the reactant [A] = 2.6 x 10-3 mol lit-1
First order rate constant k1 = ?
Rate of reaction = k1 [A]1.0
k1 = Rate of reaction / [A]
k1 = 5.2 x 10-6 mol lit-1 s-1 / 2.6 x 10-3 mol lit-1
     = 2 x 10-3 s-1
6. What are simple and complex reactions?
A simple reaction takes place in a single step. Simple reactions are also known as elementary reactions. One step reactions are elementary reactions.
Reactions which do not take place in a single step but take place in a sequence of a number of elementary steps are called as complex reactions.
7. What is pseudo first order reaction? Give an example.
In a second order reaction, when one of the reactants concentration is in excess (10 to 100 times) of the other reactant, then the reaction follows a first order kinetics and such a reaction is called pseudo-first order reaction.
Example: The acid catalysed hydrolysis of an ester
/ Hydrolysis of an Ester in presence of mineral acid           
             CH3COOCH3 + H2O CH3COOH + CH3OH
8. Write Arrhenius equation and explain the terms.
             k = A e– Ea / RT  
  k = Rate constant,
Ea = Activation energy,
A = Frequency factor,
R = Gas constant,
T = Temperature in Kelvin.
9. Define order of a reaction.
The sum of the powers of the exponential powers to which each concentration term is raised in the experimentally determined rate law of a chemical reaction.
10. The half-life period of a first order reaction is 20 mins. Calculate the rate constant.
Half-life period of a first order reaction t½ = 20 mins
Rate constant k = ?
t½ = 0 693 / k
k = 0 693 / t½
k = 0 693 / 20 mins
    = 0.03465 mins–1
    = 3.47 x 102 mins1


k = 0.693 / 20 x 60 sec

k = 0.0005775 sec–1

k = 5.78 x 10–4 sec–1
11. The rate constant for a first order reaction is 1.54 x 10–3sec–1. Calculate its half-life period.
12. What are consecutive reactions? Give an example. Or Write a (brief) note on consecutive reactions.
The reactions in which the reactant forms an intermediate and the intermediate forms the product in one or many subsequent reactions are called as consecutive or sequential reactions.
In such reactions the product is not formed directly from the reactant. Various steps in the consecutive reaction are shown as below :
    k1          k2 
A = reactant; B = intermediate; C = product.
Saponification of a diester in presence of an alkali:
                                          k1                                              k2
R'OOC– (CH2)n– COOR R'OOC– (CH2)n– COOH HOOC – (CH2)n – COOH
13. What are opposing reactions? Give an example.
In opposing reactions the products formed react back simultaneously to form the reactants. These reactions are also called as reversible reactions.
Dissociation of hydrogen iodide in gas phase:
14. What are parallel reactions? Give an example.
In these group of reactions, one or more reactants react simultaneously in two or more pathways to give two or more products. The parallel reactions are also called as side reactions
Bromination of bromobenzene:
15. Write a note on 'activation energy’.
1. The additional energy required by the molecules to attain the threshold energy in addition to the energy of colliding molecules.
2. Activation energy = Threshold energy Energy of colliding molecules.
3. An energy barrier that must be crossed by the reactant molecules before getting converted to actual product molecules.
4. The energy required to form the activated state or the intermediate, which is necessary to form the products.
5. Ea is a characteristic value of a reaction.
6. The rate, rate constant, and their temperature dependence are determined by the value of Ea.
7. Higher the value of Ea, slower is the rate of the reaction.
8. Plot of log k against 1/T gives a negative slope straight line. From the slope of the straight line, activation energy Ea can be calculated.
16. Write any three characteristics of simple reaction
1. A simple reaction takes place in a single step
2. Overall order values are small. Total and pseudo order values lie between 0, 1, 2 and 3. / Simple reactions possess low values of order like n = 0, 1, 2,
3. No side reactions
4. Products are formed directly
5. Experimental rate constant values agree with the calculated values.
6. Theories of reaction rates apply well on simple reactions
 17. What is threshold energy?
Reactant molecules come into contact through collisions for transformation into product molecules. Since, not all collisions are successful in producing the product molecules, all colliding molecules must possess
certain minimum energy called as the threshold energy which is needed to make the collisions effective and successful.
 Threshold energy = Activation energy + Energy of colliding molecules Or
Activation energy = Threshold energy Energy of colliding molecules
18. Give three examples for first order reaction.
1. All radioactive transformations follow first order kinetics.
92U238 90Th234 + 2He4
2. Decomposition of sulphuryl chloride in the gas phase
SO2Cl2(g) → SO2(g) + Cl2(g)
3. Inversion of sucrose in acidic aqueous medium
C12H22O11 + H2O → C6H12O6 + C6H12O6
                                                                              Glucose       Fructose
4. Decomposition of nitrogen pentoxide in CCl4 medium
N2O5 → 2NO2 + ½ O2
5. Decomposition of H2O2 in aqueous solution
H2O2 → H2O + ½O2

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