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Exam questions answered.

posted Aug 19, 2019, 12:53 AM by Upali Salpadoru   [ updated Aug 22, 2019, 11:09 PM ]
CHEMISTRY

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:-

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.than Mg

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.
    ............Odour. -----Very strong suffocating smell.
  W   .................Weight  (Density)  0.7 kg/m3
    S.   ....................Solubility in wate--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 .
           CuSO.5H2O (s)   =    CuSO(s)  + 5H2
  The blue colour of the copper sulphate is due to the crystals. For the formation of crystals a definite proportion of water is mecessary. Each CuSO4  molecule takes 5 molecules of water. ( This is known as "water of crystallization".)
  Concentrated sulphuric has the property of absorbing water. (Hydroscopic) 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.(3) Heavier than air.
(2)
 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. Prepared by exposing the gas 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-




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