Download App

Chemical Kinetics — Chemistry STD 12 Science — Question

Gujarat BoardEnglish MediumSTD 12 ScienceChemistryChemical Kinetics4 Marks
Question
For a first order reaction, A → Products, $\text{k}=\frac{2.303}{\text{t}}\log\frac{\text{a}}{\text{a}-\text{x}},$ where a is the initial concentration of A and (a - x) is the concentration of A after time t. k is rate constant. Its value is constant at constant temperature for a reaction. The time in which half of the reactant is consumed is called half-life period. Half-life period of a first order reaction is constant. Its value is independent of initial concentration or any other external conditions.In these questions (Q. No. i-iv), a statement of assertion followed by a statement ofreason is given. Choose the correct answer out of the following choices.
  1. Assertion and reason both are correct statements and reason is correct explanation for assertion.
  2. Assertion and reason both are correct statements but reason is not correct explanation for assertion.
  3. Assertion is correct statement but reason is wrong statement.
  4. Assertion is wrong statement but reason is correct statement.
  1. Assertion: Rate of reaction doubles when concentration of reactant is doubled if it is a first order reaction.
Reason: Rate constant also doubles.
  1. Assertion: For the first order reaction, half-life period is expressed as $\text{t}_\frac{1}{2}=\frac{2.303}{\text{k}}\log2.$
Reason: The half-life time of a first order reaction is not always constant and it depends upon the initial concentration of reactants.
  1. Reason: The half-life time of a first order reaction is not always constant and it depends upon the initial concentration of reactants.
Reason: Acid only acts as a catalyst whereas alkali acts as one of the reactants.
  1. Assertion: For a first order reaction, the concentration of the reactant decreases exponentially with time.
Reason: Rate of reaction at any time depends upon the concentration of the reactant at that time.
  1. Assertion: Half-life period for a first order reaction is independent of initial concentration of the reactant.
Reason: For a first order reaction, $\text{t}_\frac{1}{2}=\frac{0.693}{\text{k}},$ where k is rate constant.
✓

Answer

  1. (c) Assertion is correct statement but reason is wrong statement.
Explanation:
For first order reaction, Rate, = $k[A_1]$ According to question,
$[A_2] = [2A_1]$
$\therefore Rate2 = k[2A_1]$
$\Rightarrow Rate_2 = 2 Rate_1$
For a given reaction, rate constant is constant and independent of the concentration of reactant.
  1. (c) Assertion is correct statement but reason is wrong statement.
Explanation:
For first order reaction $\text{k}=\frac{2.303}{\text{t}}\log\frac{\text{a}}{\text{a}-\text{x}}$
$\text{k}=\frac{2.303}{\text{t}_\frac{1}{2}}\log\frac{\text{a}}{\text{a}-\frac{\text{a}}{2}}=\frac{2.303}{\text{t}_\frac{1}{2}}\log\frac{\text{a}}{\frac{\text{a}}{2}}=\frac{2.303}{\text{t}_\frac{1}{2}}\log2$
Therefore half-life period $\text{t}_\frac{1}{2}=\frac{2.303}{\text{k}}\log2.$
Thus $\text{t}_\frac{1}{2}$ is independent of initial concentration of reactant for first order reaction.
  1. (a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
Explanation:
$\text{CH}_3\text{COOC}_2\text{H}_5+\text{H}_2\text{O}\xrightarrow{\ \ \text{H}^+\ \ }\text{CH}_3\text{COOH}+\text{C}_2\text{H}_5\text{OH}$
Rate $\propto[\text{CH}_3\text{COOC}_2\text{H}_5]$
$\text{CH}_3\text{COOC}_2\text{H}_5+\text{NaOH}\rightarrow\text{CH}_3\text{COONa}+\text{C}_2\text{H}_5\text{OH}$
Rate $\propto[\text{CH}_3\text{COOC}_2\text{H}_5][\text{NaOH}]$
  1. (a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
Explanation:
For a first order reaction, $[\text{A}]=[\text{A}]_0\text{e}^{-\text{kt}}$
or $\log[\text{A}]=-\frac{\text{kt}}{2.303}+\log[\text{A}]_0$
  1. (a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
Explanation:
For a first order reaction, $\text{t}_\frac{1}{2}$ is inversely proportional to k, it does not depend on the initial concentration of the reactant.

Need a full question paper?

Generate a complete, print-ready paper with questions like this in minutes — across 16+ boards, with answer keys.

Start Generating Free

Explore more

More "Case study (4 Marks)" from Chemical Kinetics→Chemical Kinetics — all question types→Chemistry STD 12 Science question papers→Gujarat Board STD 12 Science question papers→Gujarat Board question paper generator→

Similar questions

Read the passage given below and answer the following questions:
In haloalkanes, when a nucleophile stronger than the halide ion approaches the positively charged carbon atom of an alkyl halide, the halogen atom along with its bonding electron pair gets displaced and a new bond with the carbon and the nucleophile is formed. These reactions are called nucleophilic substitution reactions.

In these reactions the atom or group of atoms which loses its bond from carbon and takes on an additional pair of electrons is called leaving group. Halide ions are good leaving groups. Some important nucleophilic substitution reactions ofhaloalkanes with common nucleophiles are given in the table below.
 
Reagent
Nucleophile (Nu^-)
Substitution product R-Nu
Class of main product
1.
$NaOH$ or $KOH$ or moist $Ag_2O$
$^-OH$
$ROH$
Alcohol
2.
$H_2O$
$H^2O$
$ROH$
Alcohol
3.
$Nal$
$I^-$
$R – I$
Alkyl iodide
4.
$R'NH_2$
$\text{R'}\ddot{\text{N}}\text{H}_2$
$RNHR'$
Sec. amine
5.
$KCN$
$\overline{\text{C}}\equiv\text{N}:$
$RCN$
Nitrile (cyanide)
6.
$KNO_2$
$O = N – O^-$
$R – O – N = O$
Alkvl nitrite
In these questions (Q. No. i-iv), a statement of assertion followed by a statement ofreason is given. Choose the correct answer out of the following choices.
  1. Assertion and reason both are correct statements and reason is correct explanation for assertion.
  2. Assertion and reason both are correct statements but reason is not correct explanation for assertion.
  3. Assertion is correct statement but reason is wrong statement.
  4. Assertion is wrong statement but reason is correct statement.
  1. Assertion: Alkyl halides are hydrolysed to alcohols by moist silver oxide.
Reason: $RCI$ is hydrolysed to $ROH$ easily but reactions slow down on addition of KI.
  1. Assertion: Alkyl halides fonn alkenes when heated above $300^\circ C.$
Reason: $CH3CH21$ reacts slowly with strong base as compared to $CD_3CH_2I.$
  1. Assertion: RBr reacts with $AgNO_2$ to give nitroalkane.
Reason: Silver nitrite $(AgNO_2)$ is an ionic compound, therefore the negative charge on nitrogen is the attacking site.
  1. Assertion: The nucleophilic substitution of vinyl chloride is difficult than ethyl chloride.
Reason: Vinyl group is electron donating group.
  1. Assertion: $2-$Bromobutane on reaction with sodium ethoxide in ethanol gives 1-butene as the major product.
Reason: $1-$Butene is less stable than $2-$butene.
→
Read the passage given below and answer the following questions:
All the elements of group $16$ have $ns^2np^4$ configuration in their outermost shell. Therefore, the atoms of these elements try to gain or share two electrons to achieve noble gas configuration. Sulphur and other elements of group $16$ are less electronegative than oxygen, so, they cannot accept electrons easily. By sharing of two electrons with other elements, these elements acquire $ns^2np^6$ configuration and exhibit $+2$ oxidation state. Except oxygen, group $16$ elements have vacant d-orbitals in their valence shell to which electrons Call be promoted from P- and s-orbitals of the same shell As a result, they Call show $+4$ and $+6$ oxidation states also.
The following questions are multiple choice questions. Choose the most appropriate answer:
  1. Oxygen shows $+2$ oxidation state in.
  1. $OF_2$​​​​​​​
  2. $H_2O$
  3. $CI_2O$
  4. $H_2O_2$
  1. Like sulphur, oxygen is not able to show $+4$ and $+6$ oxidation states because?
  1. Oxygen is a gas while sulphur is a solid.
  2. Sulphur has high ionisation enthalpy as compared to oxygen.
  3. Oxygen has nod-orbitals in its valence shell.
  4. Oxygen has high electron affmity as compared to sulphur.
  1. Compounds of sulphur with $+4$ oxidation state acts as a/ an.
  1. Oxidising agent.
  2. Reducing agent.
  3. Both oxidising as well as reducing agen.
  4. Cannot be predicted.
  1. Oxidation state of sulphur in $Na_2S_4O_6$​​​​​​​ is:
  1. $\frac{7}{2}$
  2. $\frac{5}{2}$
  3. $\frac{1}{2}$
  4. $\frac{3}{2}$
  1. The oxidation states of sulphur in $S_8, SO_3​​​​​​​$​​​​​​​ and $H_2S$ are respectively.
  1. $0, +6$ and $-2$
  2. $+6, 0$ and $-2$
  3. $-2, 0$ and $+6$
  4. $+2, +6$ and $-2$
→
Read the passage given below and answer the following questions: The order of reactivity towards $S_N1$ reaction depends upon the stability of carbocation in the first step. Greater the stability of the carbocation, greater will be its ease of formation from alkyl halide and hence faster will be the rate of the reaction. As we know, $3^\circ$ carbocation is most stable, therefore, the tert-alkyl that halides will undergo $S_N1$ reaction very fast. For example, it has been observed that the reaction $(CH_3)_3CBr$ with $OH^-$ ion to give 2-methyl-2-propanol is about I million times as fast as the corresponding reaction of the methyl bromide to give methanol. The primary alkyl halides always react predominantly by $S_N2$ mechanism. On the other hand, the tertiary alkyl halides react predominantly by $S_N1$ mechanism. Secondary alkyl halides may react by either mechanism or by both the mechanisms without much preference depending upon the nature of the nucleophile and solvent. In these questions (Q. No. i-tv), a statement of assertion followed by a statement of reason is given. Choose tile correct answer out of tile following choices.
  1. Assertion and reason both are correct statements and reason is correct explanation for assertion.
  2. Assertion and reason both are correct statements but reason is not correct explanation for assertion.
  3. Assertion is correct statement but reason is wrong statement.
  4. Assertion is wrong statement but reason is correct statement.
  1. Assertion: Low concentration of nudeophile favours $S_N1$ mechanism.
Reason: $2^\circ$ alkyl halides are less reactive than $1^\circ$ towards $S_N1$ reactions.
  1. Assertion: Polar solvent slows down $S_N2$ reactions.
Reason: $CH_3-Br$ is less reactive than $CH_3Cl.$
  1. Assertion: Benzyl bromide when kept in acetone- water it produces benzyl alcohol.
Reason: The reaction follows $S_N2$ mechanism.
  1. Assertion: Rate of hydrolysis of methyl chloride to methanol is higher in DMF than in water.
Reason: Hydrolysis of methyl chloride follows second order kinetics.
  1. Assertion: $S_N1$ reaction is carried out in the presence of a polar protic solvent.
Reason: A polar protic solvent increases the stability of carbocation due to solvation.
→
Read the passage given below and answer the following questions:
Haloarenes are less reactive than haloalkanes. The low reactivity of haloarenes can be attributed to:
  • Resonance effect.
  • $sp^2$ hybridisation of C - X bond.
  • Polarity of C - X bond
  • Instability of phenyl cation (formed by self-ionisation of haloarene).
  • Repulsion between the electron rich attacking nucleophiles and electron rich arenes.
Reactivity of haloarenes can be increased or decreased by the presence of certain groups at certain positions for example, nitro ($-NO_{^2}$) group at o/ p positions increases the reactivity of haloarenes towards nucleophilc substitution reactions.
The following questions are multiple choice questions Choose the most appropriate answer:
  1. Aryl halides are less reactive towards nucleophilic substitution reaction as compared to alkyl halides due to
  1. The formation of less stable carbonium ion.
  2. Resonance stabilisation.
  3. Larger carbon-halogen bond.
  4. Inductive effect.
  1. Which of the following aryl halides is the most reactive towards nucleophilic substitution?
  1. Which one of the following will react fastest with aqueous $NaOH$?
  1. Which chloro derivative of benzene among the followings would undergo hydrolysis most readily with aqueous sodium hydroxide to furnish the corresponding hydroxy derivative?
  1. $C_6H_5Cl$
  1. The reactivity of the compounds (i) $MeBr$, (ii) $PhCH_2Br$, (iii) $MeCI$, (iv) $p-MeOC_6H_4Br$ decreases as:
  1. (i) > (ii) > (iii) > (iv)
  2. (iv) > (ii) > (i) > (iii)
  3. (iv) > (iii) > (i) > (ii)
  4. (ii) > (i) > (iii) > (iv)
→

The half-life of a reaction is the time required for the concentration of reactant to decrease by half, i.e.,

$[\text{A}]_\text{t}=\frac{1}{2}[\text{A}]$
For first order reaction,
$\text{t}_\frac{1}{2}=\frac{0.693}{\text{k}}$
this means $\text{t}\frac{1}{2}$ is independent of initial concentration. Figure shows that typical variation of concentration of reactant exhibiting first order kinetics. It may be noted that though the major portion of the first order kinetics may be over in a finite time, but the reaction will never cease as the concentration of reactant will be zero only at infinite time.
The following questions are multiple choice questions. Choose the most appropriate answer:
  1. A first order reaction has a rate constant $k = 3.01 \times 10^{-3} /s.$ How long it will take to decompose half of the reactant?
  1. 2.303s
  2. 23.03s
  3. 230.3s
  4. 2303s
  1. The rate constant for a first order reaction is $7.0 \times 10^{-4} s^{-1}$. If initial concentration ofreactant is 0.080 M, what is the half life of reaction?
  1. 990s
  2. 79.2s
  3. 12375s
  4. $10.10 \times 10^{-4}s$
  1. For the half-life period of a first order reaction, which one of the following statements is generally false?
  1. It is independent of initial concentration.
  2. It is independent of temperature.
  3. It decreases with the introduction of a catalyst.
  4. None of these.
  1. The rate of a first order reaction is 0.04 mol $L^{-1} s^{-1}​​​​​​​$​​​​​​​ at 10 minutes and 0.03 mol $L^{-1} s^{-1}​​​​​​​$ at 20 minutes after initiation. The half-life of the reaction is:
  1. 4.408 min
  2. 44.086 min
  3. 24.086 min
  4. 2.408 min
  1. The plot of $\text{t}_\frac{1}{2}$ vs initial concentration $[A]_0​​​​​​​$​​​​​​​ for a first order reaction is given by:
→
Read the passage given below and answer the following questions:
Noble gases are inert gases with general electronic configuration of $ns^2np^6$. These are monoatomic, colourless, odourless and tasteless gases. The first compound of noble gases was obtained by the reaction of Xe with $PtF_6.$ A large number of compounds of Xe and fluorine have been prepared till now. The structure of these compounds can be explained on the basis of VSEPR theory as well as concept of hybridisation. The compounds of krypton are fewer. Only the diftuoride of krypton $(KrF_2)$ has been studied in detail. Compounds of radon have not isolated but only identified by radio tracer technique. However, no true compounds of helium, neon or argon are yet known.
The following questions are multiple choice questions. Choose the most appropriate answer:
  1. The formula of the compound when $Xe$ and $PtF_6$ are mixed, is:
  1. $XeF_6$
  2. $XeF_4$
  3. $Xe_2PtF_6$
  4. $Xe^+[PtF_6]^-$
  1. The shape and hybridisation of some xenon oxy-fluoride and fluoride compounds are given below. Find the incorrect one.
  1. $XeOF_2 - T-$shape $- sy^3d$
  2. $XeOF_4 -$ square pyramidal $- sp^3d^2$
  3. $XeF_2 -$ linear $- sp^3d$
  4. $XeF_6 -$ square planar $- dsp^2$
  1. Which of the following is not formed by $Xe?$
  1. $XeF_5$
  2. $XeF$
  3. $XeF_3$
  4. All of these.
  1. The number oflone pairs and bond pairs of electrons around $Xe$ in $XeOF_4$ respectively are,
  1. $0$ and $5$
  2. $1$ and $5$
  3. $1$ and $4$
  4. $2$ and $3$
  1. Which of the following compounds has more than one lone pair of electrons around central atom?
  1. $XeO_3$
  2. $XeF_2$
  3. $XeOF_4$
  4. $XeO_2F_2$
→
Read the passage given below and answer the following questions:
Ozone is an unstable, dark blue diamagnetic gas. It absorbs the UV radiation strongly, thus protecting the people on earth from the harmful UV-radiation from the sun. The use of chlorofluorocarbon (CFC) in aerosol and refrigerators and their subsequent escape into the atmosphere, is blamed for making holes in the ozone layer over the Antarctica. Ozone acts as a strong oxidising agent in acidic and alkaline medium. for this property, ozone is used as a germicide and disinfectant for sterilizing water. It is also used in laboratory for the ozonolysis of organic compounds and in industry for the manufacture of potassium permanganate, artificial silk, etc.
The following questions are multiple choice questions. Choose the most appropriate answer:
  1. Which of the following statements is not correct for ozone?
  1. lt oxidises lead sulphide.
  2. It oxidises potassium iodide.
  3. It oxidises mercury.
  4. It cannot act as bleaching agent in dry state.
  1. Ozone gives carbonyl compounds with.
  1. Alkyl chloride
  2. Alkanes
  3. Alkenes followed by decomposition with $\frac{\text{Zn}}{\text{H}_2\text{O}}.$
  4. Alcohols followed by decomposition with $\frac{\text{Zn}}{\text{H}_2\text{O}}.$
  1. Ozone reacts with moist iodine gives.
  1. $HI$
  2. $HIO_3$
  3. $I_2O_5$
  4. $I_2O_4$
  1. Ozone acts as an oxidising agent due to.
  1. Iiberation of nascent oxygen.
  2. Iiberation of nascent oxygen.
  3. Both $(a)$ and $(b).$
  4. Both $(a)$ and $(b).$
  1. The colour of ozone molecule is:
  1. White.
  2. Blue.
  3. Pale green.
  4. Pale yellow.
→
Describe detailed information on classification of carbohydrates.
→
Read the passage given below and answer the following questions:
Due to intermolecular hydrogen bonding, the boiling points of alcohols and phenols are much higher than those of corresponding haloalkanes, haloarenes, aliphatic and aromatic hydrocarbons. Among isomeric alcohols, the boiling points follow the order: primary > secondary > tertiary. Boiling points of ethers are much lower than those of isomeric alcohols. The solubility of alcohols in water decreases as the molecular mass of alcohols increases. Amongst isomeric alcohols solubility increases with branching. The solubility of phenols in water is much lower than that of alcohols. Lower ethers such as dimethyl ether and ethyl methyl ether are soluble in water, but the solubility decreases as the molecular mass increases.
In these questions (Q. No. i-iv), a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices.
  1. Assertion and reason both are correct statements and reason is correct explanation for assertion.
  2. Assertion and reason both are correct statements but reason is not correct explanation for assertion.
  3. Assertion is correct statement but reason is wrong statement.
  4. Assertion is wrong statement but reason is correct statement.
  1. Assertion: Alcohols have higher boiling points than ethers of comparable molecular masses.
Reason: Alcohols and ethers are isomeric in nature.
  1. Assertion: The solubility of phenols in water is much lower than that of alcohols.
Reason: Phenols do not form H-bonds with water.
  1. Assertion: Among n-butane, ethoxyethane, 1-propanol and 2-propanol, the increasing order of boiling points is, 1-butanol < 1-propanol < ethoxyethane < n-butane.
Reason: Boiling point increases with increase in molecular mass.
  1. Assertion: Dimethyl ether and diethylether are soluble in water.
Reason: As the molecular mass increases, solubility of ethers in water decreases.
  1. Assertion: Butan-2-ol has higher boiling point than 2-methylpropan-2-ol.
Reason: Amongst isomeric alcohols, the boiling points decreases with branching.
→
In these questions, a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices.
Assertion: Colloidal particles show Brownian movement.
Reason: Brownian movement arises because of the impact of the molecules of the dispersion medium with the colloidal particles.
→