Define the Following Terms: Half-life Period of Reaction (T1/2). - Chemistry


Define the following terms:

Half-life period of reaction (t1/2).



 Half-life period of a reaction
Half-life period of a reaction is the time taken for half of the reaction to be completed. In other words, the time in which the concentration of a reactant is reduced to half of its original value is called half-life period of the reaction.
For  zero-order reaction,
For first-order reaction,
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2013-2014 (March) Delhi Set 2

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Define "zero order reaction".

In a first order reaction x → y, 40% of the given sample of compound remains unreacted in 45 minutes. Calculate rate constant of the reaction.

A → B is a first order reaction with rate 6.6 × 10-5m-s-1. When [A] is 0.6m, rate constant of the reaction is

  • 1.1 × 10-5s-1
  • 1.1 × 10-4s-1
  • 9 × 10-5s-1
  • 9 × 10-4s-1

A reaction is second order in A and first order in B.

(i) Write the differential rate equation.

(ii) How is the rate affected on increasing the concentration of A three times?

(iii) How is the rate affected when the concentrations of both A and B are doubled?


What is pseudo first order reaction? Give one· example of it.

Write molecularity of the following reaction:


For a reaction: 

Rate = k

(i) Write the order and molecularity of this reaction.

(ii) Write the unit of k.

Write two factors that affect the rate of reaction.

For a reaction A + B ⟶ P, the rate is given by

Rate = k [A] [B]2

How is the rate of reaction affected if the concentration of B is doubled?

For a reaction A + B ⟶ P, the rate is given by

Rate = k [A] [B]2

What is the overall order of reaction if A is present in large excess?

For a reaction : 

(i) Write the order and molecularity of this reaction.

(ii) Write the unit of k.

For the hydrolysis of methyl acetate in aqueous solution, the following results were obtained :

t/s 0 30 60
[CH3COOCH3] / mol L–1 0.60 0.30 0.15

(i) Show that it follows pseudo first order reaction, as the concentration of water remains constant.

(ii) Calculate the average rate of reaction between the time interval 30 to 60 seconds.

(Given log 2 = 0.3010, log 4 = 0.6021)

For a chemical reaction R → P, the variation in the concentration (R) vs. time (t) plot is given as:

(i) Predict the order of the reaction.

(ii) What is the slope of the curve ?

(iii) Write the unit of rate constant for this reaction.

The following data were obtained during the first order thermal decomposition of SO2Cl2 at a constant volume :

SO2Cl2 (g) → SO2 (g) + Cl2 (g)

Experiment Time/s–1 Total pressure/atm
1 0 0.4
2 100 0.7

Calculate the rate constant.

(Given : log 4 = 0.6021, log 2 = 0.3010)

For a reaction, A + B → Product; the rate law is given by, `r = k[A]^(1/2)[B]^2` . What is the order of the reaction?

The conversion of molecules X to Y follows second order kinetics. If concentration of X is increased to three times how will it affect the rate of formation of Y?

A first order reaction has a rate constant 1.15 10−3 s−1. How long will 5 g of this reactant take to reduce to 3 g?

A reaction is second order with respect to a reactant. How is the rate of reaction affected if the concentration of the reactant is (i) doubled (ii) reduced to half?

For a reaction R ---> P, half-life (t1/2) is observed to be independent of the initial concentration of reactants. What is the order of reaction?

How does calcination differ from roasting?

Write resonating structures of ozone.

The decomposition of N2O5(g) at 320K according to the following equation follows first order reaction:


The initial concentration of N2O5(g) is 1.24 x 10-2 mol. L-1 and after 60 minutes 0.20x10-2 molL-1. Calculate the rate constant of the reaction at 320K.

Which of the following statements is not correct about order of a reaction.

In any unimolecular reaction:

(i) only one reacting species is involved in the rate determining step.

(ii) the order and the molecularity of slowest step are equal to one.

(iii) the molecularity of the reaction is one and order is zero.

(iv) both molecularity and order of the reaction are one.

Assertion: Rate constants determined from Arrhenius equation are fairly accurate for simple as well as complex molecules.

Reason: Reactant molecules undergo chemical change irrespective of their orientation during collision.

Use Molecular Orbital theory to determine the bond order in each of species, [He2j+ and [He2]2+?

A catalyst in a reaction changes which of the following?

The role of a catalyst is to change

In the presence of a catalyst, the heat evolved or absorbed during the reaction.

For a reaction A + B → products, the rate law is given by: r = `K[A]^(1/2)`. What is the order of reaction?

For a reaction R → p the concentration of reactant change from 0.03 m to 0.02 m in minute, calculate the average rate of the reaction using the unit of second.

If the 0.05 molar solution of m+ is replaced by a 0.0025 molar m+ solution, then the magnitude of the cell potential would be

For reaction 2A + B → BC + D which of the following does not Express the reaction rates

The half-life period of a. substance in a certain enzyme catalysed reaction is 138 s. The time required for the concentration of the substance to fall from 1.28 mol–1 to 0.04 mg L–1 is

At concentration of 0.1 and 0.2 mol L–1 the rates of deem position of a compound were found to be 0.18 and 0.72 mol L–1 m–1. What is the order of the reaction?

The number of molecules of the reactants taking part in a single step of the reaction is indicative of ______.

Read the following passage and answer the question that follows:

The rate of reaction is concerned with decrease in the concentration of reactants or increase in the concentration of products per unit of time. It can be expressed as instantaneous rate at a particular instant of time and average rate over a large interval of time. A number of factors such as temperature, concentration of reactants, catalyst affect the rate of reaction. Mathematical representation of rate of a reaction is given by rate law:

Rate = k[A]x [B]y

x and y indicate how sensitive the rate is to change in concentration of A and B. Sum of x + y gives the overall order of a reaction.

When a sequence of elementary reactions gives us the products, the reaction is called complex reaction. Molecularity and order of an elementary reaction are same. Zero-order reactions are relatively uncommon but they occur under special conditions. All natural and artificial radioactive decay of unstable nuclei takes place by first-order kinetics.

  1. What is the effect of temperature on the rate constant of a reason?
  2. For a reaction \[\ce{A + B -> Product}\], the rate given by, rate k[A]2 [B]1/2. What is the order of the reaction?
  3. How order and molecularity are different for complex reactions?
  4. A first-order reaction has a rate constant 2 × 10–3 s–1. How long will 6 g of this reactant take to reduce?
    The half-life for radioactive decay of 14C is 6930 years. An archaeological artefact containing wood had only 75% of the 14C found in a living tree. Find the age of the sample.
    [log 4 = 0.6021 log 3 = 0.4771 log 2 = 0.3010 log 10 = 1]

On heating compound (A) gives a gas (B) which is constituent of air. The gas when treated with H2 in the presence of catalyst gives another gas (C) which is basic in nature, (A) should not be ______.

The following data was obtained for chemical reaction given below at 975 K.

\[\ce{2NO(g) + 2H2(g) -> N2(g) + 2H2O(g)}\]

  [NO] [H2] Rate
  Mol L-1 Mol L-1 Mol L-1 s-1
(1) 8 × 10-5 8 × 10-5 7 × 10-9
(2) 24 × 10-5 8 × 10-5 2.1 × 10-8
(3) 24 × 10-5 32 × 10-5 8.4 × 10-8

The order of the reaction with respect to NO is ______. (Integer answer)

A drop of solution (volume 0.05 ml) contains 3.0 × 10-6 mole of H+. If the rate constant of disappearance of H+ is 1.0 × 107 mole l-1s-1. It would take for H+ in drop to disappear in ______ × 10-9s.

For a chemical reaction starting with some initial concentration of reactant At as a function of time (t) is given by the equation,

`1/("A"_"t"^4) = 2 + 1.5 xx 10^-3` t

The rate of disappearance of [A] is ____ × 10-2 M/sec when [A] = 2 M.

[Given: [At] in M and t in sec.]
[Express your answer in terms of 10-2 M /s]
[Round off your answer if required]

A flask contains a mixture of compounds A and B. Both compounds decompose by first-order kinetics. The half-lives for A and B are 300 s and 180 s, respectively. If the concentrations of A and B are equal initially, the time required for the concentration of A to be four times that of B (in s) is ______. (Use ln 2 = 0.693)


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