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Multiple choice chemistry hydrocarbons introduction to alkenes - ethyne acetylene alkynes

A mixture of oxygen and ethyne is burnt for welding. Why a mixture of ethyne and air is not used?

  1. ethene can not burn in air

  2. incomplete combustion takes place

  3. ethene burn in air with explosion

  4. none of these
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

When ethyne is burnt in air incomplete burning takes place and produce sooty flame due to limited supply oxygen present in the air.


 However, if ethyne is burnt in oxygen, it gives a clean flame with high temperature (3000°C ) due to complete combustion of ethyne.

 Hence, This oxy-acetylene flame is used for welding and it is not possible to attain such a high temperature with air. Due this reason a mixture of ethyne and air is not used for welding.

Multiple choice chemistry hydrocarbons introduction to alkenes - ethyne acetylene alkynes

Acetylene may be prepared using Kolbes electrolytic method employing:

  1. Pot. acetate

  2. Pot. succinate

  3. Pot. fumarate

  4. None of these
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Acetylene may be prepared using Kolbes electrolytic method employing potassium fumerate. Acetylene is obtained at anode. The solution is basic due to formation of KOH at cathode.
$\displaystyle  (CHCOONa) _2 \xrightarrow {Electrolysis} C _2H _2+2CO _2+2KOH + H _2$

Multiple choice chemistry hydrocarbons introduction to alkenes - ethyne acetylene alkynes

$C _{2}H _{2}$ on reaction with Na followed by reaction with $C _{2}H _{5}I$ gives 'X'. The molecular formula of 'X' is:

  1. $C _{2}H _{6}$
  2. $C _{4}H _{8}$
  3. $C _{4}H _{6}$
  4. $C _{3}H _{6}$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Reaction: $CH \equiv CH \xrightarrow{Na} Na - C \equiv  CH \xrightarrow{C _2H _5I} C _2H _5 - C \equiv  CH + NaI$


Hence, the correct option is $C$

Multiple choice chemistry hydrocarbons introduction to alkenes - ethyne acetylene alkynes

1, 1, 2, 2-Tetra bromoethane $\xrightarrow {Zn}A$ $\xrightarrow{red hot \ tube}B$,


 Then B is:

  1. Acetylene

  2. Ethylene

  3. Ethane

  4. Benzene
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

$H-\overset{Br}{\overset{|}{\underset{Br}{\underset{|}{C}}}}-\overset{Br}{\overset{|}{\underset{Br}{\underset{|}{C}}}}-H\xrightarrow[\triangle]{2n}H-C\underset{(A)}{\equiv} C-H\xrightarrow{ red hot \ tube} C _6H _6$


Option D is correct.

Multiple choice chemistry hydrocarbons introduction to alkenes - ethyne acetylene alkynes

$H-C\equiv C-H+NaNH _{2}\longrightarrow A\overset{2 \ mole \ CH _3I}{\longrightarrow}B$


Then B is :

  1. 1-Butyne

  2. 2-Butyne

  3. 2-Pentyne

  4. Propyne
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Reaction of process:
$CH\equiv CH+2NaNH _2\rightarrow NaC\equiv CNa\xrightarrow {2 mole CH _3I} CH _3CH\equiv CHCH _3$


Option B is correct.

Multiple choice chemistry hydrocarbons introduction to alkenes - ethyne acetylene alkynes

$X+2Zn\overset{Alcohol}{\rightarrow}H-C\equiv C-H$ 


Here X is :

  1. 1, 1-Dibromoethane

  2. 1, 2-Dibromoethane

  3. Dibromo ethane

  4. 1, 1, 2, 2-Tetrachloroethane
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

This is dehalogenation process:


$CHCl _2-CHCl _2+2Zn\overset{Alcohol}{\longrightarrow}H-C\equiv C-H$

Option D is correct.