[3 Mark Questions]

Question 13 Marks

Write the structural formulae of all the isomers of hexane.

Answer

$\text{CH}_3-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{CH}_3\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{Hexane}$
$\ \ \ \ \ \ \ \ \ \ \ \ \text{CH}_3\\\ \ \ \ \ \ \ \ \ \ \ \ \ |\\\text{CH}_3-\text{CH}-\text{CH}_2-\text{CH}_2-\text{CH}_3\\\ \ \ \ \ \ \ \ 2-\text{Methylpentane}$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{CH}_3\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ |\\\text{CH}_3-\text{CH}_2-\text{CH}-\text{CH}_2-\text{CH}_3\\\ \ \ \ \ \ \ \ \ \ \ 3-\text{Methylpentane}$
$\ \ \ \ \ \ \ \ \ \ \ \ \text{CH}_3\ \ \ \ \text{CH}_3\\\ \ \ \ \ \ \ \ \ \ \ \ \ |\ \ \ \ \ \ \ \ \ \ |\\\text{CH}_3-\text{CH}-\text{CH}-\text{CH}_3\\\ \ \ \ \ \ 2,3\text{ dimethylbutane}$
$\ \ \ \ \ \ \ \ \ \ \ \ \text{CH}_3\\\ \ \ \ \ \ \ \ \ \ \ \ \ |\\\text{CH}_3-\text{C}-\text{CH}_2-\text{CH}_3\\\ \ \ \ \ \ \ \ \ \ \ \ \ |\\\ \ \ \ \ \ \ \ \ \ \ \ \text{CH}_3\\2,2\text{ dimethylbutane}$

Question 23 Marks

How is ethene prepared from ethanol? Give the reaction involved in it.

Answer

From ethanol: This method involves the slow oxidation of a dilute solution of ethanol (10-15 per cent) by oxygen present in air in the presence of an enzyme acetobactor.$\text{CH}_3\text{CH}_2\text{OH}+\text{O}_2\xrightarrow{\ \ \text{Acetobactor}\ \ }\text{CH}_3\text{COOH}+\text{H}_2\text{O}\\\ \ \ \ \ \text{Ethanol}\ \ \ \ \ (\text{Air})\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{Ethanoic acid}\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (\text{as vinegar})$
The acid obtained is in the form of dilute solution called vinegar.
We have also studied under ethanol that it gets oxidized to ethanoic acid in the presence of dilute solution of alkaline $KMnO _4$ or acidified $K _2 Cr _2 O _7$.

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Question 33 Marks

Intake of small quantity of methanol can be lethal. Comment.

Answer

When methanol is entered in our body, it is metabolized in the liver and form formaldehyde which further converted to formate. Formate is essential for survival because it acts as building block for many biomolecules but high levels of it can cause toxicity.
Formaldehyde also reacts with living cells and can cause coagulation of protoplasm. Excess of methanol in our body also affects the optic nerve and can cause blindness.

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Question 43 Marks

Ethene is formed when ethanol at 443K is heated with excess of concentrated sulphuric acid. What is the role of sulphuric acid in this reaction? Write the balanced chemical equation of this reaction.

Answer

Conversion of ethanol to ethene is a dehydration reaction which occurs in the presence of excess of concentrated sulphuric acid. Here concentrated sulphuric acid acts as dehydrating agent and remove water molecule from ethanol to form ethene.$\text{CH}_3\text{CH}_2\text{OH}\xrightarrow[\ \ \ \text{Dehydration}\ \ \ ]{\ \ \text{Conc. H}_2\text{SO}_4, 443\text{K}\ \ }\text{CH}_2=\text{CH}_2+\text{H}_2\text{O}$
Hence concentrated sulphuric acid acts as a dehydrating agent as it removes water molecule.

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Question 53 Marks

Unsaturated hydrocarbons contain multiple bonds between the two C-atoms and show addition reactions. Give the test to distinguish ethane from ethene.

Answer

Distinction between ethane and ethene can be done with the help of bromine water test. Whereas ethene dicolourises the yellow colour of bromine water, ethane does not.

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Question 63 Marks

Draw the electron dot structure of ethyne and also draw its structural formula.

Answer

Ethyne is a hydrocarbon with molecular formula $C_2H_2$. Here two carbon atoms are bonded together by a triple covalent bond and one hydrogen atom is bonded with each carbon atom through a single covalent bond. Hence the electron dot structure of ethyne molecule can be representes as:

Structural formula of ethyne is: H ─ C ≡ C ─ H.

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Science [3 Mark Questions]

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