Hydrocarbons Unit 12 Chapter 12 Punjab board Class 10 Chemsitry Notes Short questions, log questions, and mcqs Pdf Download.
Differentiate between saturated and unsaturated hydrocarbons.
| Saturated hydrocarbons | Unsaturated hydrocarbons |
| Compounds in which all the four bonds of carbon are fully utilized and no more hydrogen or other atoms can attach to it are called saturated hydrocarbons. | Compounds in which all the bonds of carbon are not fully utilized and more hydrogen or other atoms can be attached to them are called unsaturated hydrocarbons. |
| Carbon atoms are bonded together with single bonds. | Carbon atoms are bonded together with double or even triple bonds. |
| These compounds are less reactive. | These compounds are more reactive. |
| Example Ethane: H3C – CH3 | Example Ethene: CH2 = CH2 Ethyne: HC≡CH |
Why are the alkanes called ‘paraffin’?
“Paraffin” is based on the Latin words parum + affinis = “little affinity”. Alkanes are called paraffin as they are the least reactive compounds due to the presence of all single covalent bonds.
What do you know about the hydrogenation of alkenes?
Hydrogenation is the addition of hydrogen molecules across carbon-carbon multiple bonds. Nickel catalyst is used to carry out this reaction at 200 – 300oC. However, the reaction can also take place in the presence of platinum or palladium at room temperature.
This reaction is extensively used to convert vegetable oil into margarine on a commercial scale.
How are alkyl halides reduced?
Reduction means the addition of hydrogen atoms. Reduction of alkyl halides means replacement of a halogen atom by a hydrogen atom. An alkyl halide is treated with Zn in the presence of aqueous acid HCl. Zn reacts with HCl and a nascent hydrogen atom is liberated. The nascent hydrogen reduces the alkyl halide.
Why alkanes are used as fuel?
Alkanes burn in the presence of excess oxygen to produce carbon dioxide gas and water, as well as energy in the form of heat and light. It is a highly exothermic reaction. This makes alkanes useful as fuels.
CH4 + 2O2 → CO2 + 2H2O + heat
How can you prepare ethene from alcohol and ethyl bromide?
Preparation of ethene from:
a) alcohol
Ethene is produced by dehydration (removal of water) of ethanol. A mixture of ethanol and concentrated sulphuric acid is heated at 180oC. First, ethyl hydrogen sulphate is produced which is heated to decompose into ethene.
b) ethyl bromide
Ethyl bromide is heated with alcoholic potassium hydroxide to produce ethene. In this reaction, hydrogen and bromine are removed from ethyl bromide.
Identify propane from propene with a chemical test.
Propane and propane can be distinguished by adding bromine water to both. Bromine water in propene will become colourless whereas in propane it will remain red-brown.
This is because the Br2Â will add on across the double bond of propane to produce dibromopropane. The Br2Â is removed from the water which becomes clear.
Propane is already saturated, so no reaction takes place.
Q.9) Why are the alkenes called ‘olefins’?
Answer:
Olefin is derived from a Latin word which means oil forming. Alkenes are called olefins because their lower members react with halogens to form oily products.
Q.10) Why alkane can’t be oxidized with KMnO4 solution?
Answer:
Alkanes are very stable as they are saturated compounds. It has strong covalent bonding which cannot be broken by alkaline KMnO4solution. Therefore, alkanes can’t be oxidized with KMnO4 solution.
Q.11) What are the addition reactions? Explain with an example.
Answer:
Addition reactions are the chemical reactions in which an atom or group of atoms is added to a molecule. Addition reactions are common in alkenes and alkynes as they are unsaturated compounds and have the capacity to add up atoms. In this reaction, double or triple bonds are broken in order to accommodate additional atoms or groups of atoms. For example, the addition of hydrogen in ethene is an additional reaction.
Q.12) Justify that alkanes give substitution reactions.
Answer:
Substitution reactions are chemical reactions in which an atom or group of atoms in a molecule is replaced by another atom or group of atoms. Alkanes are saturated compounds; therefore, they give only substitution reactions. For example halogenation of alkanes is a substitution reaction. It is a replacement of one or more hydrogen atoms by a halogen as shown below:
CH4 + Cl2 → CH3 – Cl + HCl
Q.13) Both, alkenes and alkynes are unsaturated hydrocarbons. State the one most significant difference between them.
| Alkenes | Alkynes |
| Alkenes are hydrocarbons with one or more carbon-carbon double bonds. The C=C is their functional group. | Alkynes are hydrocarbons with one or more carbon-carbon triple bonds. The C≡C is their functional group. |
Q.15) Why hydrocarbons are soluble in organic solvents?
Answer:
Hydrocarbons follow the general rule of solubility i.e. like dissolve like. Hydrocarbons are non-polar and organic solvents are also non-polar in nature. So, hydrocarbons are soluble in organic solvents.
Q.16) Give the physical properties of alkanes.
Answer:
Physical properties of alkanes
i) The first four alkanes are gases at room temperature. The alkanes consisting of C5Â to C10Â are liquids and higher members are solids.
ii) Alkanes are insoluble in water but dissolve in organic solvents.
iii) As their molecular sizes increase, alkanes become more viscous and less flammable.
iv) The melting and boiling points of alkanes increase gradually as the molecular sizes increases.
Q.17) How can you identify ethane from ethene?
Answer:
Ethane can be identified from ethene by adding bromine water to both. Bromine water in ethene will become colorless whereas in ethane it will remain red-brown.
This is because the Br2Â will add on across the double bond of ethene to produce dibromoethane. The Br2Â is removed from the water which becomes clear.
Ethane is already saturated, so no reaction takes place. Therefore, the liquid decolourized is identified as ethene, and the liquid that remained unchanged is identified as ethane.
Q.19) State one important use of each.
i) Ethene                      Â
ii) Acetylene
iii) Chloroform                  Â
iv) Carbon tetrachloride
Answer:
Uses of:
i) Ethene
Ethene is used to accelerate the ripening of fruits and is most commonly used on bananas and citrus fruits.
ii) Acetylene
Acetylene is used in the preparation of other chemicals, such as aldehydes, alcohols and acids.
iii) Chloroform
Chloroform is used for giving anaesthesia.
iv) Carbon tetrachloride
Carbon tetrachloride is used as a solvent for oils and fats.
