1
JAWAHAR NAVODAYA VIDYALAYA, RAVANGLA,SOUTH SIKKIM
Assignment for summer vacations, May & June-2015
CLASS – XII, SUBJECT –CHEMISTRY
Chapter- Surface Chemistry
PREVIOUS YEARS’ CBSE BOARD QUESTIONS
1.How are the colloids classified on the basis of the nature of interaction between dispersed phase and dispersion medium? Describe an important characteristic of each class. Which of these sols need stabilizing agents for preservation?
2. What are detergents? Give their scheme of classification. Why are the detergents
preferred over soaps?
3. Illustrate with examples:
i) Lyophilic and Lyophobic sols.
ii) Multimolecular and Macromolecular colloids.
iii) Homogeneous and Heterogeneous catalysis.
4. Explain the following observations:
i) A beam of light passing through a colloidal solution has a visible path.
ii) Passing an electric current through a colloidal solution removes colloidal sulphate.
iii) Ferric hydroxide sol coagulates on addition of a solution of potassium sulphate.
5. What are the two classes of emulsions? Give one example of each class. State one
activity to test the type of an emulsion.
6.(a) How can a colloidal solution and true solution of the same color be distinguished
from each other?
(b) List four applications of adsorption.
7.Explain the following observations:
(a)Lyophilic colloid is more stable than lyophobic colloid.
(b)Coagulation takes place when sodium chloride solution is added to a colloidal solution of ferric hydroxide.
(c)Sky appears blue in color.
8.Explain the following terms giving a suitable example in each case:
(i) Emulsification
(ii) Homogeneous catalysis.
Or
Define adsorption. Write any two features which distinguish physisorption from
chemisorption.
9. Explain what is observed when:
i) An electrolyte is added to ferric hydroxide sol.
ii) An emulsion is subjected to centrifugation.
iii) Direct current is passed through a colloidal sol.
10. Explain the following terms:
i) Peptization
ii) Dialysis
iii) Hardy- Schulze rule.
11. Explain the following terms giving an example in each case.
i)Emulsification
ii)Coagulation
iii)Electrophoresis.
12. What do you understand by activity and selectivity of catalysts? Describe some
features of catalysis by zeolites.
13. How do size of particles of adsorbent, pressure of gas and prevailing temperature influence the extent of adsorption of a gas on a solid?
14. (a) In which of the following does adsorption take place and why?
i) Silica gel placed in the atmosphere saturated with water.
ii) Anhydrous CaCl2 placed in the atmosphere saturated with water.
(b) How does BF3 act as a catalyst in industrial process?
(c) Give an example of shape-selective catalysis.
- What are micelles? How do they differ from ordinary colloidal particles? Give two
examples of micelles forming substances.
16.Which will be adsorbed more readily on the surface of charcoal and why: NH3 or CO2?
17. How do the size of particles of adsorbent, pressure of gas and prevailing temperature influence the extent of adsorption.
18. What is observed when sodium chloride is added to a colloidal solution of ferric hydroxide?
19. Complete the following:
i)The liquid-liquid colloidal dispersions are called…….
ii)The enthalpy of chemisorption is…… than the enthalpy of physisorption.
iii)The scattering of light by colloidal dispersion is called……
iv)The zig-zag motion of colloidal particles is called……
20. What happens when a freshly precipitated Fe(OH)3 is shaken with little amount of dilute solution of FeCl3?
21.What is the difference between a sol and a gel?
22.State an important use of elecrodialysis in human beings?
Solution:
Human blood is a colloidal solution which consists of sum unwanted impurities which are produced by various bio chemical reactions takes place which also posses some electrolytes, these electrolytes may cause coagulation of blood. So they can be removed before they causes damage to the human blood. This can be done by electro dialysis .
It is used in the purification of blood in the case of Kidney failure.
23. What is the difference in the nature of a dilute soap solution and a concentrated soap
solution?
Solution:
Dilute soap solution behaves like a true solution whereas concentrated soap solution behaves like a colloidal solution.
Chapter- Solutions
Section – A
Important Conceptual Questions
1) A solution of glycol containing 1.82 gm/litre has an osmotic pressure of 51.8 cm of
mercury at 100C. What is the molecular mass of glycol?(1M)
Ans) M2= w R t / P V =1.82 x 0.0821 x 2.83/ (51.8/76) x 1 = 62.04
2) Which salt will have the same value of van’t Hoff factor , as that of K4[Fe (CN)6]?
Ans) Al2(SO4)3(1M)
Hint:
and each dissociates to give 5 ions or i=5
3) A 5% solution of cane sugar (mol. wt. =342) is isotonic with 1% solution of asubstance X. Calculate the molecular mass of X. (2 M)
4) During osmosis, Mention the flow of solvent through a semipermeable membrane.
(1 M)
5) A solution containing 10 g per dm3of urea (molecular mass = 60 g mol–1) is isotonic with a 5% solution of a non-volatile solute. The molecular mass of this non-volatile solute is- (2 M)
6) Vapour pressure of CCl4at 250C is 143 mm Hg. 0.5 g of a non-volatile solute (mol. wt. 65) is dissolved in 100 ml of CCl4. Find the vapour pressure of the solution. (Density of CCl4= 1.58 g/cm3) (2 M)
7) The relationship between osmotic pressure at 273 K when 10g glucose (P1), 10 g urea (P2), and 10g sucrose (P3) are dissolved in 250 ml of water is- (2 M)
8) Two elements A and B form compounds of formula AB2 and AB4. When dissolved in 20.0 g of benzene 1.0 g of AB2 lowers freezing point by 2.30C whereas 1.0 g of AB4 lowers freezing point by 1.30C. The Kf for benzene is 5.4. The atomic masses of A and B will be- (3 M)
9) 250 ml of sodium carbonate solution contains 2.65gm of sodium carbonate. If 10 ml of this solution is diluted to one litre, What is the concentration of resultant solution? (mol. Wt. of sodium carbonate=106)(2 Mark)
10)Write Henry’s law.(1 Mark)
11) What happens when blood cells are placed in pure water?(1 Mark)
12) Two liquids A and B boil at 1200c and 1600c respectively. Which of them has higher vapour pressure at 700 c?(1 Mark)
13) A solution of 1.25gm of a non-electrolyte in 20gm of water freezes at 271.94 K. If Kf= 1.86K/m, then calculate the molar mass of the solute.(2 Mark)
15) A solution of ethanol in water is 1.6 molal. How many grams of ethanol are present in 500g of the solution. (2 Marks )
16) On a hill station pure water boils at 99.820C. The Kb of water is 0.5130C Kg mol–1. Calculate the boiling point of 0.69m solution of urea. (1 Marks )
17) Find the vant Hoff factor for Na3PO4 . (1 Mark )
18) Ethylene glycol solution having molality 0.5 is used as coolant in a car. Calculate the freezing point of solution (given Kf=1.86 K Kg/mole) (2 Marks )
19) An aqueous solution freezes at –0.1860C. Kf = 1.86, Kb = 0.512. Find elevation in boiling point. (2 Marks )
20) Density of 1 M solution of glucose is 1.18g/cm3. Kf for H2O is 1.86 Km–1. Find freezing point of solution. (2 Marks )
21) Vapour pressure of two liquid A and B are 120 and 180mm Hg at a given temperature. If 2 mole of A and 3 mole of B are mixed to form an ideal solution, calculate the vapour pressure of solution at the same temperature. (2Marks )
22) The Osmotic pressure of human blood is 7.65 atm at 370C. For injecting glucose solution it is necessary the glucose solution has same osmotic pressure as of human blood. Find the molarity of glucose solution having same osmotic pressure as of human blood. (2 Marks )
23) A solution contains 25% water, 25% ethanol and 50% acetic acid by mass. Find mole fraction of each of the component. (3 Marks )
24) Conc. H2SO4 has a density 1.9g/ml and is 99% H2SO4 by weight. Find molarity of solution. (3 Marks)
Section-B
CBSE Sample Paper Questions
Q1) Given below is the sketch of a plant for carrying out a process.
(CBSE –Samp -1 – 2M)
(i) Name the process occurring in the above plant.
(ii) To which container does the net flow of solvent take place?
(iii) Name one SPM which can be used in this plant.
(iv) Give one practical use of the plant.
Q2) Heptance and octane form an ideal solution at 373 K, The vapour pressures of the pure liquids at this temperature are 105.2 KPa and 46.8 KPa respectively. If the solution contains 25g of heptance and 28.5g of octane, calculate (i) vapour pressure exerted by heptane(ii) vapour pressure exerted by solution
(iii) mole fraction of octane in the vapour phase.
( CBSE –Samp -1 – 3M)
Q3) Ethylene glycol ( molar mass = 62g/mol) is a common automobile antifreeze. Calculate the freezing point of a solution containing 12.4g of this substance in 100g of water. What it be advisable to keep this substance in the car radiator during summer? ( For water Kf = 1.86K Kg/mol, Kb= 0.512K Kg/mol) ( CBSE –Samp -2 – 2M)
Q4). Give reasons for the following :
(a) At higher altitudes, people suffer from a disease called anoxia. In this disease, they become weak and cannot think clearly.
(b) When mercuric iodide is added to an aqueous solution of KI, the freezing point is raised. ( CBSE –Samp -2 – 2M)
Q5) What is the Van’t Hoff factor for a compound which undergoes tetramerization in an organic solvent? CBSE –Samp -2 – 1M)
Q6) (a) Derive the relationship between relative lowering of vapour pressure and mole fraction of the non volatile solute.
(b) (i) Benzoic acid completely dimerises in benzene. What will be the vapour pressure of a solution containing 61g of benzoic acid per 500g benzene when the vapour pressure of pure benzene at the temperature of experiment is 66.6 torr?
(ii) What would have been the vapour pressure in the absence of dimerisation?
(c) Derive a relationship between mole fraction and vapour pressure of a component of an ideal solution in the liquid phase and vapour phase.
( CBSE –Samp -3 – 5M)
Q7) (a) Which aqueous solution has higher concentration -1 molar or 1 molal solution of the same solule? Give reason.
(b) 0.5g KCl was dissolved in 100g water and the solution originally at 200C, froze at – 0.240C. Calculate the percentage ionization of salt. Kf per 1000g of water = 1.86K.
( CBSE –Samp -3 – 5M)
Q8) State Raoult’s law for solutions of volatile liquid components. Taking a suitable example, explain the meaning of positive deviation from Raoult’s law.
( CBSE –2008– 2M)
Q9) Define the term ‘Osmotic pressure’. Describe how the molecular mass of a substance can be determined on the basis of Osmotic pressure measurement.
( CBSE –2008 – 2M)
Q10) A solution containing 8gm of a substance in 100gm of diethylether boils at 36.860c, whereas pure ether boils at 35.600c. Determine the molecular mass of the solute. ( Kb = 2.02KKgmol-1)
( CBSE –2008 – 3M)
Q14) Explain the following
(i) People taking a lot of salt in their diet suffer from edema.
(ii) Equimolar solutions of glucose and KCl are not isotonic.( CBSE –Samp -4 – 2M)
Q16) Give one use of reverse osmosis.( CBSE –Samp -4 – 1M)
Q17) (i) What would be the value of vant' Hoff factor for dilute solution of K2SO4?
(ii) Which will have higher boiling point—0.1 molar solution of glucose or 0.1 molar solution of NaCl in water?
(iii) Which of the following is not a colligative property—Osmotic pressure or freezing point?
( CBSE –Samp -5 – 3M)
Q21) What is meant by positive and negative deviations from Raoult’s law and how is the sign mixH related to positive and negative deviation from Raoult’s law?
( CBSE –Samp -7 – 3M)
Q23) (a) (i) Define mole fraction.
(ii) Which term affected by temperature molality or molarity .
(b) Two elements A and B form compounds having formula AB2 and AB4. When dissolved in 20 g of benzene(C6H6), 1 g AB2 lowers the freezing point by 2.3 K whereas 1.0 g of AB4 lowers it by 1.3 K. The molar depression constant for benzene is 5.1 K.kg.mol-1. Calculate atomic masses of A and B
( CBSE –Samp -8 – 5M)
REASONING QUESTIONS IN p-BLOCK ELEMENTS
1. Though nitrogen exhibits +5 oxidation state, it does not form pentahalide. Give reason.
Ans: Nitrogen valence electronic configuration ns2np3 due to absence of empty d- orbitals,
it can not extend its valence to 5.
2. PH3 has lower boiling point than NH3. Why?
Ans: Unlike NH3, PH3 molecules are not associated through hydrogen bonding in liquid state. That is why the boiling point of PH3 is lower than NH3.
3. Why are pentahalides more covalent than trihalides ?
Ans: Higher the positive oxidation state of central atom, more will be its
Polarizing power which, in turn, increases the covalent character of bond
formed between the central atom and the other atom.
4. Why is BiH3 the strongest reducing agent amongst all the hydrides of
Group 15 elements ?
Ans: Among hidres of group 15 from NH3 to BiH3 as size of central atom
Increases M—H bond strength decreases. Hence it act as strong reducing
agent.
- Write the reaction of thermal decomposition of sodium azide.
Ans: 2 NaN3 2 Na + 3 N2(g)
6. Why is N2 less reactive at room temperature?
Ans: Due to its high NN bond dissociation energy
7. Why does NH3 act as a Lewis base?
Ans: Due to presence of one lone pair of electron present on Nitrogen
8. Mention the conditions required to maximize the yield of ammonia.
Ans: Optimum Pressure & Optimum Temperature & Removal of ammonia by
cooling.
9. How does ammonia react with a solution of Cu2+?
Ans: Cu2+ + 4 NH3 [Cu(NH3)4]+2 and this is deep blue in color.
10. Why does NO2 dimerise ?
Ans: Due to presence one odd electron on Nitrogen of NO2
11. What is the covalence of nitrogen in N2O5 ?
Ans: Covalency is 4 because N is bonded with 4 bonds
12. In what way can it be proved that PH3 is basic in nature?
Ans: Phosphine is weakly basic and like ammonia, gives phosphonium
compounds with acids e.g., PH3 + HBr PH4Br
Due to lone pair on phosphorus atom, PH3 is acting as a Lewis base in
the above reaction.
13. Bond angle in PH4 + is higher than that in PH3. Why?
Ans: Both are sp3 hybridized. In PH4+ all the four orbital are bonded whereas
in PH3 there is a lone pair of electrons on P, which is responsible for lone
Pair-bond pair repulsion in PH3 reducing the bond angle to less than 109° 28
14. What happens when white phosphorus is heated with concentrated NaOH?
Solution in an inert atmosphere of CO2 ?
15. Why does PCl3 fume in moisture?
Ans: PCl3 react with moisture and gives HCl vapours.
PCl3 + 3 H2O H3PO3 + 3 HCl
16. Are all the five bonds in PCl5 molecule equivalent? Justify your answer.
Ans: PCl5 has a trigonal bipyramidal structure and the three equatorial
P-Cl bonds are equivalent, while the two axial bonds are different and
longer than equatorial bonds.
17. What happens when PCl5 is heated?
Ans: PCl3 hydrolyses in the presence of moisture giving fumes of HCl.
PCl3 + 3 H2O H3PO3 +3HCl
18. Write a balanced equation for the hydrolytic reaction of PCl5 in heavy water.
Ans: PCl5 + D2O POCl3 + 2DCl
19. How do you account for the reducing behavior of H3PO2 on the basis
of its structure ?
Ans: In H3PO2, two H atoms are bonded directly to P atom which imparts
Reducing character to the acid.
20. What is the basicity of H3PO4?
Ans: Three P–OH groups are present in the molecule of H3PO4. Therefore, its
basicity is three.
21. Phosphorous in solid state is ionic, why?
Ans: In the solid state it exists as an ionic solid, [PCl4]+[PCl6]– in which the
cation, [PCl4]+ is tetrahedral and the anion, [PCl6]–octahedral.
22. Elements of Group 16 generally show lower value of first ionization
enthalpy compared to the corresponding periods of group 15. Why?
Ans: Due to extra stable half-filled p orbital electronic configurations of Group 15
elements, larger amount of energy is required to remove electrons compared
to Group 16 elements.
23. H2S is less acidic than H2Te. Why?
Ans: Due to the decrease in bond (E–H) dissociation enthalpy down the group,
acidic character increases.
24. List the important sources of sulphur.
Ans: Traces of sulphur occur as hydrogen sulphide in volcanoes. Organic
materials such as eggs, proteins, garlic, onion, mustard, hair and wool
contain sulphur.
25. Write the order of thermal stability of the hydrides of Group 16 elements.
Ans; H2O>H2S>H2Se>H2Te>H2Po As atomic size increases E—H bond
strength decreases, Hence thermal stability decreases.
26. Why is H2O a liquid and H2S a gas ?
Ans: Because of small size and high electro negativity of oxygen, molecules of
water are highly associated through hydrogen bonding resulting in its
liquid state.
27. Which of the following does not react with oxygen directly?
Zn, Ti, Pt, Fe
Ans: Pt is a noble metal it do not react with any atmospheric gases.
28. Complete the following reactions:
(i) C2H4 + O2 2 CO2 + 2 H2O
(ii) 4Al + 3 O2 l2
Why does O3 act as a powerful oxidizing agent?
Ans: Due to the ease with which it liberates atoms of nascent oxygen (O3 O2 + O),
it acts as a powerful oxidising agent.
30.How is O3 estimated quantitatively?
Ans: When ozone reacts with an excess of potassium iodide solution buffered
with a borate buffer (pH 9.2), iodine is liberated which can be titrated
against a standard solution of sodium thiosulphate. This is a quantitative
method for estimating O3 gas.
31.Which form of sulphur shows paramagnetic behavior ?
Ans: In vapour state sulphur partly exists as S2 molecule which has two unpaired
electrons in the antibonding * orbitals like O2 and, hence, exhibits
paramagnetism.
32. What happens when sulphur dioxide is passed through an aqueous
solution of Fe(III) salt?
Ans: When moist, sulphur dioxide behaves as a reducing agent. For
example, it converts iron(III) ions to iron(II) ions.
33. Comment on the nature of two S–O bonds formed in SO2 molecule. Are
the two S–O bonds in this molecule equal ?
Ans: Both the S–O bonds are covalent and have equal strength due to resonating
Structures.
.
34. How is the presence of SO2 detected ?
Ans: It is a suffocating odor gas, when SO2 gas passed into lime water it gives
Milky white ppt Ca(OH)2 + CO2 CaCO3 + H2O
35. Mention three areas in which H2SO4 plays an important role.
Ans: The bulk of sulphuric acid produced is used in the manufacture of fertilizers
(e.g., ammonium sulphate, super phosphate). Other uses are in:
(a) petroleum refining (b) manufacture of pigments, paints and dyestuff
Intermediates (c) detergent industry (d) metallurgical applications
(e.g., cleansing metals before enameling, electroplating and galvanizing
(e) storage batteries
36. Write the conditions to maximize the yield of H2SO4 by Contact process.
Ans:
The reaction is exothermic, reversible and the forward reaction leads
to a decrease in volume. Therefore, low temperature and high pressure
are the favourable conditions for maximum yield. But the temperature
should not be very low otherwise rate of reaction will become slow.
37. Why is Ka2 < Ka1for H2SO4 in water ?
Ans: H2SO4 is a very strong acid in water largely because of its first ionization
to H3O+ and HSO4–. The ionisation of HSO4– to H3O+ and SO42– is very very
small. That is why Ka1 Ka2 .
38. Halogens have maximum negative electron gain enthalpy in the respective
periods of the periodic table. Why?
Ans: Halogens have the smallest size in their respective periods and therefore
high effective nuclear charge. As a consequence, they readily accept one
electron to acquire noble gas electronic configuration.
39. Although electron gain enthalpy of fluorine is less negative as compared
to chlorine, fluorine is a stronger oxidizing agent than chlorine. Why?
Ans: It is due to (i) low enthalpy of dissociation of F-F bond
(ii) high hydration enthalpy of F–
40. Fluorine exhibits only –1 oxidation state whereas other halogens exhibit + 1,
+ 3, + 5 and + 7 oxidation states also. Explain.
Ans: Fluorine is the most electronegative element and cannot exhibit any
Positive oxidation state. Other halogens have d orbitals and therefore,
can expand their octets and show + 1, + 3, + 5 and + 7 oxidation states also.
41. Considering the parameters such as bond dissociation enthalpy, electron gain
enthalpy and hydration enthalpy, compare the oxidizing power of F2 andCl2.
Ans: Oxidizing power depends on bond dissociation enthalpy, electron gain
enthalpy and hydration enthalpy out of these Fluorine has more hydration
Enthalpy and less bond dissociation energy that can compensate high