# Important 5 Marks Questions for CBSE 12th Chemistry Exam

Question 1:

(a) Why do transition elements show variable oxidation states ?

1. Name the elements showing maximum number of oxidation states among the first series of transition metals from Sc (Z = 21) to Zn (Z=30)
2. Name the element which shows only +3 oxidation state

(b) What is lanthanide contraction ? Name an important alloy which contains some of the lanthanoid metals

Question 2:

(a) Given reasons for the following :

1. Mn3+ is a good oxidizing agent
2. $E^{o}_{M^{2+}/M}$  values are not regular for first row transition metals(3d series)
3. Although ‘F’ is more electronegative than ‘O’, the highest Mn fluoride is MnF4 whereas the highest oxide is Mn2O7,

(b) Complete the following equation :

1. 2 CrO42− + 2H+ →
2. KMnO4 $\overset{heat}{\rightarrow}$

Question 3:

(a) Define the following terms :

1. Ideal solution
2. Azeotrope
3. Osmotic pressure

(b) A solution of glucose (C6H12O6) in water is labelled as 10% by weight. What would be the molality of the solution ? (Molar mass of glucose = 180 g mol–1)

Question 4:
(a) How will you convert the following :

1. Propanone to Propan-2-ol
2. Ethanal to 2-hydroxy propanoic acid
3. Toluene to benzoic acid

(b) Give simple chemical test to distinguish between :

1. Pentan-2-one and Pentan-3-one
2. Ethanal and Propanal

Question 5:

(b) Give simple chemical tests to distinguish between the following pairs of compounds :

1. Butanal and Butan-2-one
2. Benzoic acid and Phenol

Question 6:

(a) What type of deviation is shown by a mixture of ethanol and acetone ? Give reason.

(b) A solution of glucose (molar mass = 180 g mol–1) in water is labeled as 10% (by mass). What would be the molality and molarity of the solution ? (Density of solution = 1.2 g mL–1)

Question 7:

(a) Define the following terms :

1. Molarity
2. Molal elevation constant (Kb)

(b) A solution containing 15 g urea (molar mass = 60 g mol–1) per litre of solution in water has the same osmotic pressure (isotonic) as a solution of glucose (molar mass = 190 g mol–1) in water. Calculate the mass of glucose present in one litre of its solution.

Question 8:

a) Account for the following :

1. Transition metals show variable oxidation states.
2. Zn, Cd and Hg are soft metals.
3. Eº value for the Mn3+/Mn2+ couple is highly positive (+1.57 V) as compared to Cr3+/Cr2+.

b) Write one similarity and one difference between the chemistry of lanthanoid and actinoid elements.

Question 9:

(a) Account for the following :

1. Cl–CH2COOH is a stronger acid than CH3COOH.
2. Carboxylic acids do not give reactions of carbonyl group.

(b) Write the chemical equations to illustrate the following name reactions :

1. Rosenmund reduction
2. Cannizzaro’s reaction

(c) Out of CH3CH2–CO–CH2–CH3 and CH3CH2–CH2–CO–CH3, which give iodoform test ?

Question 10:

(i) With reference to structural variability and chemical reactivity, write the difference between lanthanoids and action ions.

(ii) Name of member of the lanthanoid series which is well known to exhibit +4 oxidation state.

(iii) Complete the following equations : MnO4- + 8H+ + 5e– →

(iv) Out of Mn3+ and Cr3+, which is more paramagnetic and why ?
(atomic nos. : Mn = 25, r = 24)

Question 11:

(a) Write the product(s) in the following reactions :

(b) Give simple chemical tests to distinguish between the following pairs of compounds :

1. Butanal and Butan-2-one
2. Benzoic acid and Phenol

Question 12:

(a) Complete the following equations:

1. Cr2O7 2– + 2OH– →
2. MnO4– + 4H+ + 3e– →

(b) Account for the following :

1. Zn is not considered as a transition element.
2. Transition metals form a larger number of complexes.
3. The Eº value for the Mn3+ / Mn2+ couple is much more positive than that for Cr3+/Cr2+ couple.

Question 13:

(a) Write the structures of the main products when aniline reacts with the following reagents :

1. Br2 water
2. HCl
3. (CH3CO)2O / pyridine

(b) Arrange the following in the increasing order of their boiling point :

C2H5NH2, C2H5OH, (CH3)3N

(c) Give a simple chemical test to distinguish between the following pair of compounds : (CH3)2-NH and (CH3)3N

Question 14:

(a) Which poisonous gas is evolved when white phosphorus is heated with conc. NaOH solution? Write the chemical equation.

(b) Write the formula of first noble gas compound prepared by N. Bartlett. What inspired N. Bartlett to prepare this compound?

(c) Fluorine is a stronger oxidizing agent than chlorine. Why?

(d) Write one use of chlorine gas.

(e) Complete the following equation : CaF2 + H2SO4 →

Question 15:

(a) Account for the following :

1. Bond angle in NH4+ is greater than that in NH3.
2. Reducing character decreases from SO2 to TeO2.
3. HClO4 is a stronger acid than HClO.

(b) Draw the structures of the following :

1. H2S2O8
2. XeOF4

Question 16:

(a) For a reaction A + B → P the rate is given by Rate = k [A]2 [B]

1. How is the rate of reaction affected if the concentration of A is doubled?
2. What is the overall order of reaction if B is present in large excess?

(b) A first order reaction takes 23.1 minutes for 50% completion. Calculate the time required for 75% completion of this reaction. (Given : log 2 = 0.301, log 3 = 0.4771, log 4 = 0.6021)

Question 17:

(a) Write the chemical reaction involved in Etard reaction.

(b) Arrange the following in the increasing order of their reactivity towards nucleophilic addition reaction : CH3–CHO C6H5COCH3, HCHO

(c) Why pKa of Cl–CH2–COOH is lower than the pKa of CH3COOH ?

(d) Write the product in the following reaction. CH3CH2CH=CH–CH2CN $\rightarrow[2.H_2O]{1.{(i-B)}_2AlH}$

(e) A and B are two functional isomers of compound C3H6O. On heating with NaOH and I2, isomer A forms yellow precipitate of iodoform whereas isomer B does not form any precipitate. Write the formulae of A and B

Question 18:

For the hydrolysis of methyl acetate in aqueous solution, the following results were obtained :

 t/s 0 30 60 [CH3COOCH3]/mol L-1 0.6 0.3 0.15

(i) Show that is follows pseudo first order reaction, as the concentration of water remains constant.

(ii) Calculate the average rate of reaction between the time interval 30 to 60 seconds.

Question 19:

The elements of 3d transition series are given as : Sc Ti V Cr Mn Fe Co Ni Cu Zn Answer the following :

(i) Write the element which is not regarded as a transition element. Give reason.

(ii) Which element has the highest m.p ?

(iii) Write the element which can show an oxidation state of +1.

(iv) Which element is a strong oxidizing agent in +3 oxidation state and why ?

Question 20:

(a) Account for the following :

1. Mn shows the highest oxidation state of +7 with oxygen but with fluorine it shows the highest oxidation state of +4
2. Zirconium and Hafnium exhibit similar properties.
3. Transition metals act as catalysts.

(b) Complete the following equations :

1. 2MnO2 + 4KOH + O2 $\overset{\Delta }{\rightarrow}$
2. Cr2O22- + 14H+ + 6I– →

Question 21:

(a) The conductivity of 0.001 mol L-1 solution of CH3COOH is 3.905 × 10–5 S cm-1. Calculate its molar conductivity and degree of dissociation(α).
Given λº(H+ ) = 349.65 cm2 mol–1 & λº(CH3COO– ) = 40.95 cm2 mol-1

(b) Define electrochemical cell. What happens if external potential applied becomes greater than Eºcell of electrochemical cell ?

Question 22:

(a) Calculate Eºcell for the following reaction at 298 K:

2Cr(s) + 3Fe2+ (0.01 M) → 2Cr3+ (0.01 M) + 3Fe(s)

Given : Ecell = 0.261 V

(b) Using the Eº values of A and B, predict which one is better for coating the surface of iron [Eº(Fe2+/Fe) = –0.44 V] to prevent corrosion and why ?
Given : Eº(A2+/A) = –2.37 V : Eº(B2+/B) = –0.14 V

Question 23:

(a) Write the reactions involved in the following :

(i) Etard reaction

(ii) Stephen reduction

(b) How will you convert the following in not more than two steps :

(i) Benzoic acid to Benzaldehyde

(ii) Acetophenone to Benzoic acid

(iii) Ethanoic acid to 2-Hydroxyethanoic acid

Question 24:

(a) 30 g of urea (M = 60 g mol-1) is dissolved in 846 g of water. Calculate the vapour pressure of water for this solution if vapour pressure of pure water at 298 K is 23.8 mm Hg.

(b) Write two differences between ideal solutions and non-ideal solutions.

Question 25:

(a) Writhe the products formed when CH3CHO reacts with the following reagents :

(i) HCN

(ii) H2N – OH

(iii) CH3CHO in the presence of dilute NaOH

(b) Give simple chemical tests to distinguish between the following Pairs of compounds.

(i) Benzoic acid and Phenol

(ii) Propanal and Propanone.

Question 26:

(a) A 10% solution (by mass) of sucrose in water has a freezing point of 269.15 K. Calculate the freezing point of 10% glucose in water if the freezing point of pure water is 273.15 K. Given : (Molar mass of sucrose = 342 g mol-1) (Molar mass of glucose = 180 g mol-1)

(b) Define the following terms :

(i) Molality (m)

(ii) Abnormal molar mass

Question 27:

(a) Following are the transition metal ions of 3d series :

Ti4+, V2+, Mn3+, Cr3+

(Atomic numbers : Ti = 22, V = 23, Mn = 25, Cr = 24)

(i) Which ion is most stable in an aqueous solution and why ?

(ii) Which ion is a strong oxidising agent and why ?

(iii) Which ion is colourless and why ?

(b) Complete the following equations :

(i)  2 MnO4- + 16H+ + 5S2- →

(ii) KMnO4 $\overset{heat}{\rightarrow}$

Question 28:

a) Write the reactions involved in the following :

1. Etard reaction
2. Stephen reduction

b)How will you convert the following in not more than two steps :

1. Benzoic acid to Benzaldehyde
2. Acetophenone to Benzoic acid
3. Ethanoic acid to 2-Hydroxyethanoic acid

Question 29:

(a) 30 g of urea (M = 60 g mol-1) is dissolved in 846 g of water. Calculate the vapour pressure of water for this solution if vapour pressure of pure water at 298 K is 23·8 mm Hg.

(b) Write two differences between ideal solutions and non-ideal solutions.

Question 30:

(a) Write the products of the following reactions :

(b) Which acid of each pair shown here would you expect to be stronger ?

Question 31:

(a) State Raoult’s law for a solution containing volatile components. How does Raoult’s law become a special case of Henry’s law ?

(b) 1.00 g of a non-electrolyte solute dissolved in 50 g of benzene lowered the freezing point of benzene by 0.40 K. Find the molar mass of the solute. (Kf for benzene = 5.12 K kg mol–1)

Question 32:

a) A 10% solution (by mass) of sucrose in water has a freezing point of 269·15 K. Calculate the freezing point of 10% glucose in water if the freezing point of pure water is 273.15 K.

Given :(Molar mass of sucrose = 342 g mol-1)

(Molar mass of glucose = 180 g mol-1)

b) Define the following terms :

1. Molality (m)
2. Abnormal molar mass

Question 33:

1. Following are the transition metal ions of 3d series:
Ti4+, V2+, Mn3+, Cr3+
(Atomic number: Ti = 22, V = 23, Mn = 25, Cr = 24)
i) Which ion is most stable in an aqueous solution and why ?
ii) Which ion is a strong oxidising agent and why?
iii) Which ion is colourless and why?
2. Complete the following equations :
i) 2 MnO4- + 16 H+ + 5 S2- gives out
ii) KMnO4 on heating gives out

Question 34:

Account for the following :

1. (i) Transition metals show variable oxidation states.
(ii) Zn, Cd and Hg are soft metals.
(iii) E value for the Mn3+/Mn2+ couple is highly positive (+ 1·57 V) as compared to Cr3+/Cr2+.
2. Write one similarity and one difference between the chemistry of lanthanoid and actinoid elements.