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Chemistry M.C.Q (1 Marks)

Chemical Kinetics · CBSE STD 12 Science (CBSE - English Medium). 33 questions.

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M.C.Q (1 Marks)

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33 questions · auto-graded multiple-choice test.

MCQ 11 Mark
Which of the following is not correct reason for the substantially lower rate of reaction than the collision frequency?
  • A
    All the collisions do not attain threshold energy level.
  • ✓
    The activated complex formed is short lived.
  • C
    All the collisions do not have proper orientation.
  • D
    Effective collision are lesser in number than all collisions.
Answer
Correct option: B.
The activated complex formed is short lived.
If there is substantially lower rate of reaction than the collision frequency then the activated complex formed will be long$-$lived and the reaction will proceed at a slower rate.
Because of low energy, the activated complex exists for an extremely long period of time and activated complex will neither reforms to form products.
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MCQ 21 Mark
In general, the rate of a reaction can be increased by all the factors except:
  • A
    Increasing the temperature.
  • B
    Increasing the concentration of reactants.
  • ✓
    Increasing the activation energy.
  • D
    Using a catalyst.
Answer
Correct option: C.
Increasing the activation energy.
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MCQ 31 Mark
If hydrogen and oxygen are mixed and kept in the same vessel at room temperature, the reaction does not take place to form water because:
  • ✓
    Activation energy for the reaction is very high at room temperature.
  • B
    Molecules have no proper orientation to react to form water.
  • C
    The frequency of collisions is not high enough for the reaction to take place.
  • D
    No catalyst is present in the reaction mixture.
Answer
Correct option: A.
Activation energy for the reaction is very high at room temperature.
Activation energy $=$ It is defined as the least possible amount of energy $($minimum$)$ which is required to start a reaction or the amount of energy available in a chemical system for a reaction to take place.

The activation energy for the water formation reaction is very high at room temperature due to this reaction does not take place,
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MCQ 41 Mark
For a first $-$ order reaction, the concentration of reactant:
  • A
    Is independent of time
  • B
    Varies linearly with time
  • ✓
    Varies exponentially with time
  • D
    None
Answer
Correct option: C.
Varies exponentially with time
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MCQ 51 Mark
For a hypothetical reaction$:\ A + B \rightarrow $ Products, the rate law is $r = k[A][B]^0.$ The order of reaction is:
  • A
    $0$
  • ✓
    $1$
  • C
    $2$
  • D
    $3$
Answer
Correct option: B.
$1$
$r = [A][B]^0$
OR $= 1 + o = 1$
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MCQ 61 Mark
The energy of activation is:
  • A
    The energy associated with the activated molecules.
  • ✓
    Threshold energy energy of normal molecules.
  • C
    Threshold energy $+$ energy of normal molecules.
  • D
    Energy of products energy of reactants.
Answer
Correct option: B.
Threshold energy energy of normal molecules.
Activation energy is the energy needed by reactant molecules to gain threshold energy level.
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MCQ 71 Mark
For a given reaction of first order it takes $20$ minutes for the concentration to drop from $1M$ to $0.6M.$ The time required for the concentration to drop from $0.6M$ to $0.36M$ will be:
  • A
    More than $20$ min
  • B
    Less than $20$ min
  • ✓
    Equal to $20$ min
  • D
    Infinity
Answer
Correct option: C.
Equal to $20$ min
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MCQ 81 Mark
What is activation energy?
  • ✓
    It is the minimum amount of energy it takes to start a chemical reaction.
  • B
    It is the intermediate compound in a chemical reaction.
  • C
    It is the energy needed for an exothermic reaction.
  • D
    It is the energy needed for an endothermic reaction.
Answer
Correct option: A.
It is the minimum amount of energy it takes to start a chemical reaction.
Activation Energy$:-$ The minimum energy which must be available to a chemical system with potential reactants to result in a chemical reaction
Activation energy may also be defined as the minimum energy required to start a chemical reaction.
The activation energy of a reaction is usually denoted by Ea and given in units of kilojoules per mole $(kJ$/ mol$)$ or kilocalories per mole $($kcal/ mol$).$
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MCQ 91 Mark
If the activation energy is $65\ kJ$ then how much time faster a reaction proceed at $25^\circ C$ than $0^\circ C?$
  • A
    $12$
  • ✓
    $11$
  • C
    $13$
  • D
    $15$
Answer
Correct option: B.
$11$
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MCQ 101 Mark
Half life for a $I^{st}$ order reaction $A \rightarrow$ product, is $60$ minute. What $\ \%\ $ of $A$ remains after $120$ min?
  • ✓
    $25\%$
  • B
    $50\%$
  • C
    $75\%$
  • D
    $90\%$
Answer
Correct option: A.
$25\%$
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MCQ 111 Mark
The value of rate constant of a pseudo first order reaction $.......$
  • A
    Depends on the concentration of reactants present in small amount.
  • ✓
    Depends on the concentration of reactants present in excess.
  • C
    Is independent of the concentration of reactants.
  • D
    Depends only on temperature.
Answer
Correct option: B.
Depends on the concentration of reactants present in excess.
The value of rate constant of a pseudo first order reaction depends on the concentration of reactants present in excess.
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MCQ 121 Mark
The maximum value of activation energy is equal to:
  • A
    Zero
  • B
    Heat of reaction
  • C
    Threshold energy
  • ✓
    None of these
Answer
Correct option: D.
None of these
Activation energy may be greater than heat of reaction or lesser than threshold energy.
At room temperature, most of the molecules have less than the threshold value.
Hence, if energy is supplied to the reactant molecules in the form of light or heat, they absorb that energy and reach a higher energy level which is equal to or greater than the threshold energy level, is called as activation energy.
Activation energy $(E_a) =$ Threshold energy $-$ Average kinetic energy of molecules
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MCQ 131 Mark
A first order reaction is $50\%$ completed in $1.26 \times 10^{14}s.$ How much time would it take for $100\%$ completion?
  • A
    $1.26 \times 10^{15}s.$
  • B
    $2.52 \times 10^{14}s.$
  • C
    $2.52 \times 10^{28}s.$
  • ✓
    Infinite.
Answer
Correct option: D.
Infinite.
It is impossible to perform $100\%$ of the reaction, whole of the substance never react.
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MCQ 141 Mark
Activation energy of a chemical reaction can be determined by $.......$
  • A
    Determining the rate constant at standard temperature.
  • ✓
    Determining the rate constants at two temperatures.
  • C
    Determining probability of collision.
  • D
    Using catalyst.
Answer
Correct option: B.
Determining the rate constants at two temperatures.
Activation energy of a chemical reaction is related to rate constant of a reaction at two different temperatures i.e., $K_1$ and $K_2$ respectively.
Solution:
$\log\frac{\text{k}_2}{\text{k}_1}=\frac{\text{E}_\text{a}}{2.303}\Big(\frac{1}{\text{T}_1}-\frac{1}{\text{T}_2}\Big)$
Where, $Ea =$ activation energy.
$T_2 =$ higher temprature
$T_1 =$ lower temprature
$K_1 =$ rate constant at temprature $T_1$
$K_2 =$ rate constant at temprature $T_2$
The equation is known as Arrehenius equation.
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MCQ 151 Mark
According to the Arrhenius equation,
  • ✓
    A high activation energy usually implies a fast reaction
  • B
    Rate constant increases with increase in temperature. This is due to a greater number of collisions whose energy exceeds the activation energy
  • C
    Higher the magnitude of activation energy, stronger is the temperature dependence of the rate constant.
  • D
    The pre$-$exponential factor is a measure of the rate at which collisions occure, irrespective of their energy.
Answer
Correct option: A.
A high activation energy usually implies a fast reaction
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MCQ 161 Mark
In which of the following reactions of the following orders the molecularity and order can never be same?
  • ✓
    Zero order
  • B
    First order
  • C
    Second order
  • D
    Third order
Answer
Correct option: A.
Zero order
because molecularity can never be zero.
So, for zero$-$order reaction, molecularity and order can never be same.
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MCQ 171 Mark
Note: In the following question two or more options may be correct.
During decomposition of an activated complex:
  • A
    Energy is always released.
  • B
    Energy is always absorbed.
  • C
    Energy does not change.
  • ✓
    Reactants may be formed.
Answer
Correct option: D.
Reactants may be formed.
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MCQ 181 Mark
For a first order reaction, rate constant is $0.6932hr^{-1},$ then half $-$ life for the reaction is:
  • A
    $0.01hr$
  • ✓
    $1hr$
  • C
    $2hr$
  • D
    $10hr$
Answer
Correct option: B.
$1hr$
$\text{t}_{\frac{1}{2}}=\frac{0.693}{\text{k}}=\frac{0.693}{0.6932\text{hr}^{-1}}=1\text{hr}$
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MCQ 191 Mark
Collision frequency of a gas at $\text{1 atm}$ pressure is $Z.$ Its value at $\text{0.5 atm}$ will be:
  • A
    $0.25Z$
  • B
    $2Z$
  • C
    $0.50Z$
  • ✓
    $Z$
Answer
Correct option: D.
$Z$
$\rightarrow $ As the pressure increases collision frequency increases.
$\rightarrow $ Vice versa pressure is decreased to half, frequency of collision decreases to half.
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MCQ 201 Mark
Which of the following statements is correct?
  • ✓
    The rate of a reaction decreases with passage of time as the concentration of reactants decreases.
  • B
    The rate of a reaction is same at any time during the reaction.
  • C
    The rate of a reaction is independent of temperature change.
  • D
    The rate of a reaction decreases with increase in concentration of reactant$(s).$
Answer
Correct option: A.
The rate of a reaction decreases with passage of time as the concentration of reactants decreases.
The rate of a reaction can be defined as the change in concentration of a reactant of product in unit time. To be more specific, it can be expressed in term of:
  1. The rate of decrease in concentration of any one of the reactants, or
  2. The rate of increase in concentration of any one of the products. Concentration of reactant decreases with the passage of time, we can say that the rate is also decreasing.
$\text{r}=\frac{-\text{dx}}{\text{dt}} (r =$ rate$)$
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MCQ 211 Mark
In a first order reaction, the concentration of product $'x\ '$ at time $'t\ '$ is given by the expression$:\ ($where$,a =$ initial concentration$, k =$ rate constant$, n =$ order$)$
  • ✓
    $\text{x}=\text{a}(1-\text{e}^{_\text{-kt}})$
  • B
    $\text{x}=\frac{1}{(\text{a}-\text{x})}$
  • C
    $\text{x}=\frac{1}{2^{\text{n}-1}}$
  • D
    $\text{x}=\frac{\text{a}}{(\text{a}-\text{x})}$
Answer
Correct option: A.
$\text{x}=\text{a}(1-\text{e}^{_\text{-kt}})$
The integrated rate law for the first order reaction is $[A]=[A]^0 e^{-k t}$.
Substituting the value of $[A]=a-x$ and $[A]_0=a$ in the above expression.
We get$, a-x=a e^{-k t}$.
Rearrange above expression$, x=a\left(1-e^{-k t}\right)$.
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MCQ 221 Mark
Which is correct about zero order reaction?
  • ✓
    Rate of reaction depends on decay constant.
  • B
    Rate of reaction is independent of concentration.
  • C
    Unit of rate constant is concentration.
  • D
    Unit of rate onstant is concentration time.
Answer
Correct option: A.
Rate of reaction depends on decay constant.
For a zero order reaction,
Rate $= K[A]^0$
So, rate of reaction depends on $K$ only. $K$ is called rate constant or decay constant.
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MCQ 231 Mark
The specific rate constant of a first order reaction depends on:
  • A
    Concentration of the reactants
  • B
    Concentration of the products
  • C
    Time
  • ✓
    Temperature
Answer
Correct option: D.
Temperature
$\text{k}=\text{Ae}^{\frac{-\text{E}_\text{a}}{\text{RT}}}$
$\therefore k$ depends on temperature.
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MCQ 241 Mark
The study of reaction kinetics is called $......$
  • A
    Rate of reaction.
  • B
    Mechanism of reaction.
  • C
    Factors which affect the rate of reaction.
  • ✓
    All of the mentioned.
Answer
Correct option: D.
All of the mentioned.
Reaction kinetics is the study of reaction rates, mechanisms, and the factors that influence reaction rates.
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MCQ 251 Mark
Which of the following statement is not correct for the catalyst?
  • A
    It catalyses the forward and backward reaction to the same extent.
  • ✓
    It alters $\Delta\text{G}$ of the reaction.
  • C
    It is a substance that does not change the equilibrium constant of a reaction.
  • D
    It provides an alternate mechanism by reducing activation energy between reactants and products.
Answer
Correct option: B.
It alters $\Delta\text{G}$ of the reaction.
A catalyst does not alter Gibbs energy$, (G)$ of a reaction. It catalyses the spontaneous reactions but does not catalyse non$-$spontaneous reactions.
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MCQ 261 Mark
Which of the following is correct with respect to a first order reaction?
  • A
    A plot of rate and concentration is a positive straight line graph whose slope given the value of rate constant.
  • B
    A plot of $\log [A]$ and time is a straight line graph whose slope is $-\frac{\text{k}}{2.303}.$
  • C
    A plot of $\log​​ \frac{[\text{A}]^0}{[\text{A}]}$and time is a positive straight line graph whose slope is $-\frac{\text{k}}{2.303}.$
  • ✓
    All of the above
Answer
Correct option: D.
All of the above
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MCQ 271 Mark
Effective collisions are those in which molecules must:
  • A
    Have energy equal to or greater than the threshold energy
  • B
    Have proper orientation
  • C
    Acquire the energy of activation
  • ✓
    All of the above
Answer
Correct option: D.
All of the above
These are the characteristics of effective collisions.
Effective collisions are collision between two reactants with the appropriate orientation $\ \&\ $ with sufficient energy to overcome the activation energy barrier.
The number of effective collisions increases exponentially with an increase in temperature.
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MCQ 281 Mark
For a zero$-$order reactio$n:\ 2 \mathrm{NH}_3(\mathrm{\sim g}) \rightarrow \mathrm{N}_2(\mathrm{\sim g})+3 \mathrm{H}_2(\mathrm{\sim g}),$ the rate of reaction is $0.1\ atm/s.$ Initially only $\mathrm{NH}_3(\mathrm{\sim g})$ was present at $3\ atm$ and the reaction is performed at constant volume and temperature. The total pressure of gases after $10s$ from the start of reaction will be:
  • ✓
    $4\ atm$
  • B
    $5\ atm$
  • C
    $3.5\ atm$
  • D
    $4.5\ atm$
Answer
Correct option: A.
$4\ atm$
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MCQ 291 Mark
For a zero $-$ order reaction,
  • A
    The reaction rate is doubled when the initial concentration is doubled.
  • ✓
    The time for half change is half the time taken for completion of the reaction.
  • C
    The time for half change is independent of the initial concentration.
  • D
    The time for completion of the reaction is independent of the initial concentration.
Answer
Correct option: B.
The time for half change is half the time taken for completion of the reaction.
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MCQ 301 Mark
The role of a catalyst is to change $.......$
  • A
    Gibbs energy of reaction.
  • B
    Enthalpy of reaction.
  • ✓
    Activation energy of reaction.
  • D
    Equilibrium constant.
Answer
Correct option: C.
Activation energy of reaction.
Catalyst can change the activation energy of the reaction by changing the path of the reaction.
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MCQ 311 Mark
Collision diameter is least in case of:
  • A
    $H_2$
  • B
    $He$
  • ✓
    $CO_2$​
  • D
    $N_2$​
Answer
Correct option: C.
$CO_2$​
Collision diameter will be least for the low molecular weight.
$\text{d}\alpha\frac{1}{\text{M}}$
In this case, $Co_2$​ has the highest molecular weight.
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MCQ 321 Mark
Reactants molecule must gain a minimum amount of energy called the $........$ before they can transform into product molecules.
  • ✓
    Activation energy
  • B
    Reaction energy
  • C
    Equilibrium energy
  • D
    None of the above
Answer
Correct option: A.
Activation energy
The minimum energy required to start a reaction is called activation energy.
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MCQ 331 Mark
The reactions having very high values of energy of activation are generally $........$
  • ✓
    Very slow
  • B
    Very fast
  • C
    Spontaneous
  • D
    Medium fast.
Answer
Correct option: A.
Very slow
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