Tag: chemistry

Questions Related to chemistry

The rate of reaction, $A+B\longrightarrow $ Products, is given by the equation, $r=k\left[ A \right] \left[ B \right] $. If $B$ is taken in large excess, the order of reaction would be:

pseudo first order reaction order of reactions chemical kinetics electrochemistry and chemical kinetics chemistry

  1. $2$

  2. $1$

  3. $0$

  4. Unpredictable

Show answer
Correct Option: B
Explanation:

If $B$ is in excess then $A$ is limiting reagent thus

$ \vartheta =k[A]$
$ \therefore$ order$=1.$

In pseudo-unimolecular reactions:

pseudo first order reaction order of reactions chemical kinetics electrochemistry and chemical kinetics chemistry

  1. one of the reactants is present in large excess

  2. both the reactants have same concentration

  3. both the reactants are present in low concentration

  4. one of the reactants is less reactive

Show answer
Correct Option: A
Explanation:

The reactions having molecularity but order of reaction is one are called pseudo unimolecular reactions. In pseudo unimolecular reaction one recatent is always in excess amount. Due to its excess amount there is no significant changes in its comcetatration that's why rate of reaction only depend on second recatent.

Ex. Hydrolysis of ester in water. In this water is in excess.
So answer will be A

Which of the following are pseudo unimolecular reactions?

pseudo first order reaction order of reactions chemical kinetics electrochemistry and chemical kinetics chemistry

  1. $C{ H } _{ 3 }COO{ C } _{ 2 }{ H } _{ 5 }+HOH\xrightarrow [ ]{ \quad { H }^{ + }\quad } C{ H } _{ 3 }COOH+{ C } _{ 2 }{ H } _{ 5 }OH$

  2. $C{ H } _{ 3 }COO{ C } _{ 2 }{ H } _{ 5 }+HOH\xrightarrow [ ]{ \quad O{ H }^{ - }\quad } C{ H } _{ 3 }COOH+{ C } _{ 2 }{ H } _{ 5 }OH$

  3. $\begin{matrix} { C } _{ 12 }{ H } _{ 22 }{ O } _{ 11 } \ \end{matrix}\begin{matrix} +HOH \ \end{matrix}\begin{matrix} \xrightarrow [ ]{ \quad { H }^{ + }\quad } \ \end{matrix}\begin{matrix} { C } _{ 6 }{ H } _{ 12 }{ O } _{ 6 } \ glucose \end{matrix}\begin{matrix} + \ \end{matrix}\begin{matrix} { C } _{ 6 }{ H } _{ 12 }{ O } _{ 6 } \ fructose \end{matrix}$

  4. $S{ O } _{ 2 }{ Cl } _{ 2 }\left( g \right) \longrightarrow S{ O } _{ 2 }\left( g \right) +{ Cl } _{ 2 }\left( g \right) $

Show answer
Correct Option: A,C
Explanation:

In a chemical reaction if two or more reactants are involved but the rate of reaction depends only upon the concentration of one of the reactant and independent of other reactants

$C{ H } _{ 3 }COO{ C } _{ 2 }{ H } _{ 5 }+HOH\xrightarrow [ ]{ \quad { H }^{ + }\quad } C{ H } _{ 3 }COOH+{ C } _{ 2 }{ H } _{ 5 }OH$

$\begin{matrix} { C } _{ 12 }{ H } _{ 22 }{ O } _{ 11 } \ \end{matrix}\begin{matrix} +HOH \ \end{matrix}\begin{matrix} \xrightarrow [ ]{ \quad { H }^{ + }\quad } \ \end{matrix}\begin{matrix} { C } _{ 6 }{ H } _{ 12 }{ O } _{ 6 } \ glucose \end{matrix}\begin{matrix} + \ \end{matrix}\begin{matrix} { C } _{ 6 }{ H } _{ 12 }{ O } _{ 6 } \ fructose \end{matrix}$ 
is unimolecular reaction 

The rate of a first order reaction is $0.04$ mole ${ \ell  }^{ -1 }{ S }^{ -1 }$ at $10$ seconds and $0.03$ mol ${ \ell  }^{ -1 }{ S }^{ -1 }$ at $20$ seconds after initiation of the reaction. The half-life period of the reaction is:

pseudo first order reaction order of reactions chemical kinetics electrochemistry and chemical kinetics chemistry

  1. $48.1$ s

  2. $24.1$ s

  3. $34.1$ s

  4. $44.1$ s

Show answer
Correct Option: A
Explanation:
$\vartheta =kA\rightarrow $first order

$\vartheta =kA _{0}e^{-kt}$

$\vartheta =\vartheta _{0}e^{-kt}\rightarrow $first order wrt $\vartheta$
so,

$\vartheta _{10}=0.04=\vartheta _{0}e^{-10k}\rightarrow (1)$

$\vartheta _{30}=0.03=\vartheta _{0}e^{-30k}\rightarrow (2)$

now using (1)/(2)
$+l _{n}(1.33)=20k$

$k=0.0144$

now,$t _{\frac{1}{2}}=\frac{0.693}{k}$

$=48.125$

$\cong 50$ seconds

In a pseudo first order acid catalysed hydrolysis of ester in water the following results were obtained:
t/s             0      30       60        90
[ester]/M    0.55 0.31   0.17       0.085
Which of the following is/are correct for the given reaction?

pseudo first order reaction order of reactions chemical kinetics electrochemistry and chemical kinetics chemistry

  1. The average rate of reaction between time interval 30 to 60 seconds is $4.67\times 10^{-3} mol L^{-1} s^{-1}$

  2. Order of reaction is 2.

  3. Pseudo first order rate constant for the acid catalysed hydrolysis of ester is $1.92\times 10^{-2}s^{-1}$

  4. All are correct.

Show answer
Correct Option: A,C
Explanation:

$rate _{avg}=0.14/30=4.67\times 10^{-3}$
$\therefore k=1.92\times 10^{-2}$

An optically active compound A upon acid catalysed hydrolysis yield two optically active compound B and C by pseudo first order kinetics. The observed rotation of the mixture after 20 min was $ 20^{\circ}$ while after completion of the reaction it was $-20 ^{\circ} .$ If optical rotation per mole of A,B &C are 60$ ^{\circ} ,40$ $^{\circ}$& $-80 ^{\circ} . $Calculate half life of the reaction.

pseudo first order reaction order of reactions chemical kinetics electrochemistry and chemical kinetics chemistry

  1. 20 min

  2. 24 min

  3. 28 min

  4. 32 min

Show answer
Correct Option: A
Explanation:

As after completion of the reaction rotation is $-20 ^{\circ} $
So according to first order reaction
$k = 2.303log(r _0 - r _{\infty })/(r _t - r _{\infty })/20 = 2.303log(80/40)/20$
$t _{1/2} = 0.693/k = 20 min$

Which of the following is/are examples of pseudo unimolecular reactions ?

pseudo first order reaction order of reactions chemical kinetics electrochemistry and chemical kinetics chemistry

  1. $\displaystyle CH _3CO _2C _2H _5\, +\, H _2O\, \overset{H^{\oplus}}{\rightarrow}\, CH _3CO _2H\, +\, C _2H _5OH$

  2. $\displaystyle C _{12}H _{22}O _{11}\, +\, H _2O\, \overset{H^{\oplus}}{\rightarrow}\, \begin{matrix} & C _6H _{12}O _6\ & (Glucose)\end{matrix}\, =\, \begin{matrix} & C _6H _{12}O _6\ & (Fructose)\end{matrix}$

  3. $CH _3COCl\, +\, H _2O\, \rightarrow\, CH _3CO _2H\, +\, HCl$

  4. $\displaystyle CH _3CO _2C _2H _5\, +\, H _2O\, \overset{OH^{\oplus}}{\rightarrow}\, CH _3CO _2H\, +\, C _2H _5OH$

Show answer
Correct Option: A,B,C
Explanation:

The reactions of the options A, B and C represents pseudo unimolecular reactions as water is present in large excess. Hence, the concentration of water remains practically unchanged and does not affect the rate of the reaction.  

Hydrolysis of ester in acidic medium is pseudo unimolecular and in alkaline medium is bimolecular. Inversion of cane sugar and the hydrolysis of acetyl chloride is also pseudo unimolecular.

Consider the reaction represented by the equation represented by the equation
$CH _3Cl(g)\, +\, H _2O(g)\, \rightarrow\, CH _3OH(g)\, +\, HCl(g)$
These kinetic data were obtained for the given reaction concentrations

Initial conc (M) Initial conc (M) Initial rate of disappearance of
$[CH _3Cl]$ $[H _2O]$ $CH _3Cl\, (M\, s^{-1}$
0.2 0.2 1
0.4 0.2 2
0.4 0.4 8

If $H _2O$ is taken in large excess, the order of the reaction will be:

pseudo first order reaction order of reactions chemical kinetics electrochemistry and chemical kinetics chemistry

  1. 1

  2. 2

  3. 3

  4. None of these

Show answer
Correct Option: A
Explanation:

Comparing the rate law, the order w.r.t. $H _2O\, =\, 2$.
w.r.t. $[CH _3Cl]\, =\, 1$
$\therefore \, r\, =\, [CH _3Cl]\, [H _2O]^2$
Overall order = 1 + 2 = 3
If $H _2O$ is in excess, the order of the reaction will be 1.

Select the  correct statement about $Mg^{2+}$   ions : 

chemistry the s-block elements (alkali and alkaline earth metals) biological importance of magnesium and calcium biological importance of elements biological importance of s block elements

  1. These ions are concentrated more in intracelluar then in extracellular fluids in animal bodies

  2. Activation of phosphate transfer enzyme takes place in their presencde

  3. They are involved in carbohydrate metablism

  4. All are correct statement

Show answer
Correct Option: C

The ion helpful for controlling heart beating and muscle contraction is:

chemistry the s-block elements (alkali and alkaline earth metals) biological importance of magnesium and calcium biological importance of elements biological importance of s block elements biological importance of sodium and potassium

  1. $Mg^{2+}$

  2. $Na^{+}$

  3. $K^{+}$

  4. $Ca^{2+}$

Show answer
Correct Option: D
Explanation:

${ Ca }^{ 2+ }$ is the ion helpful for controlling heart beat and muscle contraction.