Tag: chemistry

Questions Related to chemistry

For the second order reaction, concentration $(x)$ of the product at time $t$ starting with initial concentration $[A] _0$ is:

  1. $\dfrac{kt[A _0]^2}{1 + kt[A _0]}$

  2. $\dfrac{k + [A _0]^2}{1 + kt}$

  3. $\dfrac{1 + kt[A _0]^2}{k + [A _0]^2}$

  4. none of these


Correct Option: A
Explanation:

A reaction said to be a second order when the overall order is $2$. The rate of second order reaction may be proportional to one concentration squared.

$R=K[A]^2$
For rate proportional to single concentration squared, the time dependance of concentration is given by
$\cfrac{1}{[A]}=\cfrac{1}{A _0}+Kt
Therefore, concentration of product after time $t=\cfrac{kt[A_0]^2}{1+kt[A_0]}$.

Units of rate constant of a first order reaction is :

  1. $mole.lit^{-1}$

  2. $lit. mole$

  3. $mole. sec^{-1}$

  4. $sec^{-1}$


Correct Option: D
Explanation:

A $\rightarrow$ product

For first order reaction, rate is dependent on single reactant A for example, rate = k[A]
$k=\frac{rate}{[A]}$

$=\frac{mole}{liter}sec\times \frac{liter}{mole}$
$=sec^{-1}$

A gaseous reaction, $A _{2}\left ( g \right )\rightarrow B\left ( g \right )+\frac{1}{2}\left ( g \right )$ 
Show increase in pressure from 40 mm to 120 mm in 5 minutes. the rate of disappearance of$A _{2}$ is ?

  1. 4 mm $min ^{-1}$

  2. 8mm $min^{-1}$

  3. 16 mm $min^{-1}$

  4. 2 mm $min^{-1}$


Correct Option: B

Consider the reaction $2A+B$ $\rightarrow$products,when the concentration of a alone was doubled, the half-life of the  reaction did not change.When the concentration of B alone was double,the rate was not altered.The unit of rate constant for this reaction is

  1. $S^{-1}$

  2. $L\ mol^{-1}\ s^{-1}$

  3. $mol\ L^{-1}\ s^{-1}$

  4. $mol^{-2}\ L^{5}\ S^{-1}$


Correct Option: B

For the reaction $A\rightarrow C+D$, the initial concentration of $A$ is $1000 M$. After $10^{2} sec$ concentration of $A$ is $100\ M$. The rate constant of the reaction has the numerical value of $9.0$. What is the unit of the reaction rate constant? 

  1. $M^{-1}s^{-1}$

  2. $Ms^{-1}$

  3. $s^{-1}$

  4. $M^{-1.5}s^{-1}$


Correct Option: B

The second order rate constant is usually expressed as :

  1. $Mol\ L\ s^{-1}$

  2. $Mol^{-1}\, L^{-1}\, s^{-1}$

  3. $Mol\, L^{-1}\, s^{-1}$

  4. $Mol^{-1}\, L\, s^{-1}$


Correct Option: D
Explanation:

As we know,
for second order reaction:
$r = k[A]^2$
so unit of rate constant is $Mol^{-1}\, L\, s^{-1}$.

The unit of rate constant obeying the rate expression, $r=k{ \left[ A \right]  }{ \left[ B \right]  }^{ { 2 }/{ 3 } }$ is:

  1. ${ mol }^{ { -2 }/{ 3 } }\ { litre }^{ { 2 }/{ 3 } }\ { time }^{ -1 }$

  2. ${ mol }^{ { 2 }/{ 3 } }\ { litre }^{ { -2 }/{ 3 } }\ { time }^{ -1 }$

  3. ${ mol }^{ { -5 }/{ 3 } }\ { litre }^{ { 5 }/{ 3 } }\ { time }^{ -1 }$

  4. none of the above


Correct Option: D
Explanation:

The unit of rate constant is ${ mol }^{ { -2 }/{ 3 } }\ { litre }^{ { 2 }/{ 3 } }\ { time }^{ 2/3 }$ and it does not match with any of the A, B, C options. So option D is correct

For the second order reaction, if the concentration of reactant changes from $0.08M$ to $0.04M$ in 10 minutes. Calculate the time at which concentration of reactant becomes $0.01M$.

  1. $20min$

  2. $30min$

  3. $50min$

  4. $70min$


Correct Option: D
Explanation:

For a second order reaction we have 

$ \cfrac { 1 }{ { [A] } _{ t } } -\cfrac { 1 }{ [A] _{ o } } =kt$
 In $10$ minutes,  concentration changes from $0.08M $ to $0.04M$
$ \left( \cfrac { 1 }{ 0.04 } -\cfrac { 1 }{ 0.08 }  \right) \cfrac { 1 }{ 10 } =k{ \quad min }^{ -1 }\ 1.25{ min }^{ -1 }=k$ 
Substituting value of $k$, we get it become $ \left( \cfrac { 1 }{ 0.01 } -\cfrac { 1 }{ 0.08 }  \right) \cfrac { 1 }{ 1.25 } =t\ 70min=t$
 Hence answer is $[D]$

For a second order reaction rate at a particular time is $x$. Ifthe initial concentration is trapled, the rate will becomes?

  1. $3x$

  2. $9x^{2}$

  3. $9x$

  4. $27x$


Correct Option: C
Explanation:

Its a second Order reaction.


The rate at a particular time is x. 

$x = k{[A]}^{2}$

If the initial concentration is tripled, the rate becomes

$rate = k[{3[A]}^{2}]$ = $9x$

The given reaction 
$2FeCl _{3}+SnCl _{2}\rightarrow 2FeCl _{2}+SnCl _{4}$
Is an example of:

  1. Third order reaction

  2. First order reaction

  3. Second order reaction

  4. None of these


Correct Option: A
Explanation:

For the third order reaction, the general reaction can be written as,

$A+B+C\rightarrow Products\ r={ K } _{ 3 }\left[ A \right] ^{ 2 }\left[ B \right] ^{ 1 }$
$r=\left[ Fe{ Cl } _{ 2 } \right] ^{ 2 }\left[ Sn{ Cl } _{ 4 } \right] ^{ 1 }$
the order of products are third order.