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First order reaction: A first order reaction is a chemical reaction where the rate of reaction is directly proportional to the concentration of the reactants. In other words, if we double the concentration of the reactants, the rate of the reaction will also double. There can be one or two reactants in first order reaction, as in the case of the decomposition reaction.
In NCERT Class 12 Chemistry, we learn first order reaction in chapter Chemical Kinetics. Students can expect questions related to first order reaction for board exams as well as competitive exams like JEE Main, NEET, etc. This article explains definition, equation for first order reaction, and solved examples.
- Order of First order Reaction
- Rate of a first-order reaction
- Examples of first-order reaction
- FAQs on First Order reaction
Order of First order Reaction
The order of first order reaction is equal to 1.
Rate of a first-order reaction
The rate of a first-order reaction can be described using the following equation:
rate = k[A]m
where:
- rate is the rate of the reaction
- k is the rate constant of the reaction
- [A] is the concentration of the reactant, A
- m is the order of the reaction (in this case, m = 1, since the reaction is first order)
The rate constant (k) is a constant value that depends on the specific reaction and is related to the frequency of the successful collisions between the reactants.
First-order reactions are typically characterized by a single inflection point, and the reaction rate increases as the reactant concentration increases.
Examples of first-order reaction
The reaction of hydrogen gas with oxygen gas to form water:
2H2 + O2 → 2H2O
This reaction is a first-order reaction, as the rate of the reaction increases with the concentration of hydrogen gas.
The decomposition of hydrogen peroxide:
2H2O2 → 2H2O + O2
This reaction is also a first-order reaction, as the rate of the reaction increases with the concentration of hydrogen peroxide
FAQs on First Order reaction
Q: For a first order reaction, show that time required for 99% completion is twice the time required for the completion of 90% of reaction.
A:
Let, initial concentration be [R]°
Concentration at 90% completion be ((100-90)/100)×[R]°
∴Concentration at 90% be 0.1[R]°
Concentration at 99% completion be ((100-99)/100)× [R]°∴Concentration at 99% be 0.01[R]°
we know time, t= 2.303/K log R0 / R
Time taken for 90% completion is
T90 = 2.303 / K log R0 / 0.1 R0
T90 = 2.303 / K log 1 / 0.1
T90 = 2.303 / K log 10 / 1
T90 = 2.303 / K
Time taken for 99% completion is
T99 = 2.303 / K log R0 / 0.01 R0
T99 = 2.303 / K log 1 / 0.01
T99 = 2.303 / K log 100 / 1
T99 = 2 X 2.303 / K
T99 = 2 T90
Hence, the time taken to complete 9% of the first order reaction is twice the time required for the completion of 90% of the reaction.
Chemistry Atoms and Molecules Exam
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