Exam questions answered.
Q. 1.
Explain why Al and S have diferent charges.
a. Include e- arrangement for both.
b. Relate the charges to the position of atoms in the periodic table.
Answer:- .................Highlight to get the answers.
a.
Al. has the e- configuration as 2, 8. 3.
S . has……………….2, 8, 6.
An atom gets charged either by lending or gaining e-.
Al. lends 3 e- and becomes positively charged as follows.
Al - 3e- = Al 3+ (Positive ion)
S atom gets charged by gaining 2 e-. ( Negative ion)
S + 2e- = S2-.
b.
Position in the periodic table.
As Al has 3 shells it is placed in the third row.
As it has 3 valency e- it fits to the third column.
c.
Compare and contrast the reactions of Ca and Mg with water and dil HCl.
| Reactants. |
Reaction with Ca. |
Reaction with Mg . |
| With cold water, |
Reacts producing bubbles.
Ca +2 H2O = Ca(OH)2 + H2
|
No visible reaction. |
| With hot water. |
Rapid reaction .
Ca +2 H2O = Ca(OH)2 + H2 |
Slow reaction forming H2 bubbles.
Mg +2 H2O = Mg(OH)2 + H2 |
| With dil.HCl. |
Vigorous reaction producing H2 and heat.
Ca +2 HCl = CaCl2 + H2 |
Mg +2 HCl = MgCl2 + H2 |
Comparison :- Both react with water and dil. acids producing hydrogen.
Difference;- Calcium shows a higher rate of reaction than Mg.
Q.2.
a.
Give two Physical properties for S and Pb. (More than two given here)
| Property |
Sulphur |
Lead |
| Melting point |
Low |
High |
| Density, |
Low |
High, |
| Colour |
Yellow |
Greyish or silver. |
| Conductivity. |
Poor conductor of heat and electricity. |
Good conductor of heat and electricity. |
b,
Explain why Zinc blocks are added to the hulls of steel ships.
Answer:-
Steel can easily corrode in the sea environment.
Corrosion of iron or steel is an electrochemical reaction. This is due to the lending of electrons from the Iron atoms. Fe - 2e = Fe 2+ . Ferrous ions go into solution corroding the iron.
These electrochemical cells need and anode, a cathode and an electrolyte.
When zinc is added it becomes the anode and the hull of the ship becomes the cathode. It is the anode that oxidises. Zinc acts as a sacrificial metal as it is higher in the activity series than iron.
Q. 3.
a.
Give the physical properties of Ammonia.
Physical properties generally include
C ..........Colour. ---------------No colour.
O ............Odour. -----Very strong suffocating smell.
W .................Weight (Density) 0.7 kg/m3
S. ....................Solubility in water--Extremely soluble.
Gas NH3 is lighter than air. Normal density of air = 1.2 kg/m3
b.
Explain the reaction of blue copper sulphate crystals with Con. H2SO4 .
CuSO4 .5H2O (s) = CuSO4 (s) + 5H2O
The blue colour of the copper sulphate is due to the crystals. For the formation of crystals a definite proportion of water is necessary. Each CuSO4 molecule takes 5 molecules of water. ( This is known as "water of crystallization".)
Concentrated sulphuric has the property of absorbing water. (Hygroscopic ) When H2SO4 is added to blue crystals they crumble to a white powder which is called "Aanhydrous copper sulphate".
c.
Give the Physical and chemical properties of Chlorine and ozone.
Physical properties.
| Property. |
Air. |
Chlorine. |
Ozone |
| Colour |
Colourless. |
Light yellowish green. |
Colourless. |
| Odour. |
No smell |
suffocating smell. |
Pungent smell. |
| Weight. |
Standard (1).
|
Much heavier than air.(3times.) |
Heavier than air.
(2 times.) |
| Solubility in H2O |
Slightly soluble. |
Dissolves physically and chemically. |
More soluble than oxygen. |
| Melting point |
|
- 101 C |
- 192.5 C |
| Boiling point |
|
-34 C |
-119.5 C |
Chemical properties.
| Property. |
Chlorine. |
Ozone. |
| Molecular mass. |
71 |
48 |
| Reaction with water. |
Cl2 + H2O -> HOCl + H+ + Cl-
HOCl dissociates into H+ and ClO- |
O3 + H2O --> O2 + 2 OH- |
| Toxicity |
Toxic gas in high concentration |
Toxic gas. |
| Preparation |
May be prepared by electrolysis of salt water. |
Forms by oxygen getting exposed to UV rays. |
Resulting water
| |
After Chlorination |
After Ozonization. |
| Smell |
Maintains the smell. |
No smell. |
| Taste |
Slight difference. |
Slight improvement due to O2.. |
| Anticeptic properties, |
Kills bacteria and pathogens. |
Kills bacteria and pathogens. |
| Removing metallic ions. |
Fe ++ + O3 + 5H2O-->
2 Fe(OH)2 O2 + 4H+ |
Fe++ + Cl2 ---> Fe+++ + 2Cl- |
