Sunday, February 9, 2014

CBSE Board guess Questions class 10 Chapter 04 Carbon and its Compounds

An organic acid ‘X’ is a liquid which often freezes during winter time in cold countries, has the molecular formula, C2H4O2. On warming   it with ethanol in the presence of a few drops of concentrated  sulphuric acid, a compound ‘Y’ with a sweet smell is formed
 (i) Identify ‘X’ and ‘Y’. (ii) Write a chemical equation for the reaction involved.

(i) The organic acid X is ethanoic acid (CH3COOH) and The compound Y is ethyl Ethanoate
CH3 – COOH + CH3 – CH2OH ---------->  (CH3  –  C – O – CH2 – CH3 )



Write name of the following –
i. Alkaline earth metal belonging to the third period
ii. The alkali metal atom having largest atomic radius
iii. The halogen atom belonging to fourth period
iv. The element having lowest ionization energy
v. The element having second lowest electronegativity

   i.  Magnesium
  ii.  Francium
 iii.  Bromine
 iv.  Cesium 375.6 kJ per mole 
  v.  Francium 380 kJ per mole

Organic compound 'x' of molecular formula C2H4O2 gives brisk effervescence with sodium bi carbonate . give name and molecular formula of x with balanced equation

The compound X is ethanoic acid, commonly known as acetic acid. On reaction with sodium bicarbonate, it forms sodium acetate, water and carbon dioxide gas. Thus the brisk effervescence observed is due to carbon dioxide gas. Its molecular formula is C2H4O2 .
Following is the chemical equation for the reaction

What is Saponification Reaction?

Esters react in the presence of an acid or a base to give back alcohol and carboxylic acid. This reaction is used in the manufacture of soaps and is known as saponification. Soap, is nothing but the sodium salt of the carboxylate.
CH3COOC2H5 ---(NaOH)--> CH3COOH + C2H5OH
Ester  (ethyl ethanoate) --Base (sodium hydroxide)--> Carboxylic Acid (ethanoic acid) + Alcohol (ethanol)
CH3COOH + NaOH -----> CH3COONa + H2O
Carboxylic Acid (Ethanoic Acid) + Base (Sodium Hydroxide) -----> Soap (Sodium Ethanoate / acetate) + Water 

Soaps are not considered as effective cleansing agent. Why?

Soaps are not considered as effective cleansing agent because it is glyceride salt of soft metal like sodium which works in neutral and alkaline medium but in acidic medium get neutralized to form scum or ppt.

How does melting and boiling points of hydrocarbon change with the increase in molecular mass ?

With increase in molar mass, the melting point and boiling points of hydrocarbons increases generally.
It is because, with increase in molar mass, the molecule becomes larger and so the van der waal's forces and other binding forces between those molecules become stronger, which holds the particles very firmly together in solid or liquid phase and so it requires more energy to break these bonds and thus the melting and boiling points of the hydrocarbons increases.

Write down the relevant chemical equation involved in decolourisation.

Ethene is an unsaturated has one double bond.When Bromine water(brown in colour) is added to ethene it reacts with ethene to form a new compound 1,2 dibromoethane which is colourless. The colour of bromine water is gone now. Generally bromine water is decolourise by all the compounds that contain double or triple bond. Ethane is a saturated compound with no multiple bond hence does not decolourise Br water
CH2 = CH2     + Br2                 ----------> Br-CH2 - CH2- Br   

A compound X has molecular formula C3H4 one mole of X reacts with 2 moles of hydrogen to yield a compound Y deduce the structure of X and Y.

The molecular formula suggests that the molecule is an alkyne, as the general formula for alkyne is CnH2n-2.  So, X is propyne.  Addition of two moles of hydrogen will form the corresponding alkane, propane (C3H6).  So, X is propyne and Y is propane.

HC º C - CH3 + 2 H2 --------------> CH3  -  C H2- CH3  

What is dehydration reaction? Give one example.

The removal of water molecule from a compound is known as dehydration. Example: Dehydration of alcohols gives alkenes.
R-CH2-CHOH-R ------------->  R-CH=CH-R + H2O

What is hydrolysis?

Hydrolysis means addition of water to a molecule resulting in breaking of bonds that are present in the molecule.
In scientific language;-
Hydrolysis is a type of decomposition reaction where one reactant is water.
The general formula of a hydrolysis reaction is:

Organic hydrolysis reactions involve the reaction of water and a ester. This reaction follows the general formula:

RCO-OR' + H20 RCO-OH + R'-OH
Dissolving a salt of a weak acid or base in water is an example of a hydrolysis reaction.

Why doesn’t soap form micelles in ethanol as they form in water?

A molecule of soap is made up of two parts- a hydrophilic  polar end and a hydrophobic hydrocarbon part.In a soap solution, the hydrocarbon portion of several soap molecules huddle together  whereas the polar ends project outward into the polar solvent, water. These spherical clusters of hundreds of molecules are called micelle.
In organic solvent like ethanol, soap is completely soluble, hence no micelle formation

Three elements X,Y and Z belong to 17TH group but to 2nd 3th and 4th period respectively. Number of valance electrons in X is 7Find the number of valance electrons in X and Z.
Since X, Y and Z all three belong to same group that is 17th therefore they all have same number of valence electrons which is equal to 7. Having different period will not affect on the number of valence electrons provided the group number remain same.
The X, Y and Z are Fluorine, chlorine and bromine belonging to 2nd, 3rd and 4th period respectively.  
What is the use of oxyacetylene flame?

The oxyacetylene flame is a gas torch that burns a mixture of acetylene and oxygen to produce a high-temperature flame (3,000°C or 5,400°F) that can be used for welding, cutting steel, and other metal working.

What is observed on adding 5% solution of alkaline potassium permanganate solution drop by drop to some warm ethanol taken in testube.
Write the name of the compound formed during chemical reaction.

When a 5% solution of alkaline potassium permanganate solution is added drop  by drop to some warm ethanol taken in a test tube, magenta colour of alkaline  potassium permanganate disappears and ethanol has been changed to carboxylic acid
. The reaction is as follows.
CH3   - CH2OH        --------------- > CH3COOH

How would you distinguish experimentally between an alcohol and a carboxylic acid on the basis of a chemical property?        

To distinguish experimentally between an alcohol and a carboxylic acid on the basis of a chemical property, we take these samples in two separate test tubes. Add sodium hydrogen carbonate in both the test tubes. The test tube, from which the effectiveness evolves, contain carboxylic acid. The effervescence is due to the evolution of carbon dioxide gas. However, from  test tube containing alcohol there is to effervescence, because no carbon  dioxide gas is produced there.

Why are vegetable oils healthy as compared to vegetable ghee ? how are vegetable oils converted into vegetable ghee name the process.

The fatty acid with a single double bond is called mono unsaturated fatty acid (e.g. oleic acid), and if it has multiple double bonds, it’s polyunsaturated (e.g. linoleic acid). By virtue of their tightly packed structure, the saturated fatty acids increase the levels of bad cholesterol (LDL) and clog the arteries. On the other hand, the unsaturated fatty acids increase the levels of good cholesterol (HDL) by taking the LDL to the liver to be broken down and removed from the body.
Vegetable oil can be converted into ghee by the process of hydrogenation.
The process of conversion of oil into ghee is known hydrogenation. This process is called so because hydrogen is added and it takes place at 4500c and with nickel or palladium as a catalyst.
CH2=CH+H2  ----Ni---------> CH3-CH3

When acetic acid reacts with X, a salt is formed which on reaction with soda lime gives a gas Y. Identify X and Y

CH3COOH + NaHCO3 --------------> CH3COONa + H2O +CO2
CH3COONa +NaOH     ---------- ->  CH4 + Na 2 CO3

“Alkenes form a homologous series" Explain. 

Homologous series is a group or family of compounds that have the same general formula but vary by a single parameter.
Alkenes have general formula, CnH2n and the varying parameter is the length of carbon chain.  C2H4, C3H6, C4H8  are members with different number of carbon atoms.  

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J Sunil said...

Q 1. Justify the nature of physical and chemical properties of the compound of homologous series -2
CBSE 2013-14

A Homologous Series is a group of organic chemical compounds, usually listed in order of increasing size, that have a similar structure (and hence also similar properties) and whose structures differ only by the number of CH2 units in the main carbon chain.

1. Chemical Properties

Organic compounds that are part of the same homologous series generally have similar chemical properties as each other, due to the presence of the same functional group in the molecules of all compounds in the series.

2. Physical Properties

The physical property of any particular member of a homologous series depends on its size, or its position within the homologous series. The physical properties, such as solubility, melting point, boiling point, specific gravity etc. show a gradual change with the increase in their relative molecular masses.

Q 2. Apart from organic being where else we find carbon? Mention the form in which it is available there and also its percentage

Apart from organic being we find carbon in the earth’s crust about 0.02% in the form of minerals like carbonates, hydrogen carbonates, coal and petroleum CBSE 2013-14

Q3. How are carboxylic acids from mineral acid on the basis of ionization point of view?

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