KPK Class 10 Chemistry Notes Chapter #12 (Hydrocarbons) Contains solved exercises, reviews Acid, Base and Salts Chapter #12.
Table of Contents
Chapter #12 Chemistry Class 10 Notes Short question
Q.1) How would you test that alkenes undergo an addition reaction:
Answer:Â
Alkenes are unsaturated hydrocarbons. Due to the unsaturated nature of alkenes, they easily undergo addition reactions and in this way, they are converted into saturated compounds. Bromine water is used to test for the addition reaction of alkenes. When bromine is added to an alkene in an inert solvent like carbon tetrachloride (CCl4), the double bond is converted into a single bond. It decolorizes the reddish-brown colour of bromine and produces 1,2-dibromoethane (colourless liquid).
Q.2) Which one is more reactive between alkane and alkene? Explain. *
Answer:Â
Alkenes are more reactive than alkanes. Alkenes have one pi-bond between two carbon atoms along with an sp2-sp2 hybridized orbital bonding. The pi-bond can be easily broken to free the valence shell electrons for combining with other atoms hence they are more reactive. Alkanes have no pi bonds between carbon atoms. They only have an sp3-sp3 hybridized orbital bonding. Alkanes to react, first, the sigma bonds with hydrogen have to be broken and only then can other atoms combine. this needs more energy. So alkanes are least reactive than alkenes. Â
Read more: KPK Class 10 Chemistry Notes Chapter 11 (Organic Chemistry)
Q.3) Justify alkenes and alkynes as unsaturated hydrocarbons.
Answer:
Alkenes and alkynes are said to be unsaturated because of the presence of double and triple bond respectively. Alkene and alkynes both give Baeyer’s test. The purple colour of an acidified aqueous solution of potassium permanganate decolourises when it is treated with alkenes and alkynes. This test is used for the detection of unsaturation. Also, they undergo addition reaction which justifies that alkenes and alkynes as unsaturated hydrocarbons.
Q.4) Why alkanes are inert in nature?
Answer:Â
Alkanes are called saturated hydrocarbons. All the four bonds of carbon in alkanes are fully utilized and no more hydrogen or other atom can attach to it. Therefore, they are least reactive compounds due to the presence of all single covalent bonds. For this reason, alkanes are called paraffins. “Paraffin” is based on the Latin words parum + affinis = “little affinity”.
Few examples of alkanes showing all single covalent bonds are as follows:
Q.5) What happens when alkyl halides is reduced?
Answer:Â
Reduction means addition of nascent hydrogen. In fact, it is a replacement of a halogen atom with a hydrogen atom. This reaction takes place in the presence of Zn metal and HCl.
Q.6) Can you predict the product if KMnO4 solution reacts with alkene?
Answer:Â
Alkenes reacts with an acidified aqueous solution of potassium permanganate (KMnO4). They decolourise its purple colour and form ethylene glycol (1,2-ethanediol), manganese dioxide (MnO2) and potassium hydroxide (KOH).
Read more: KPK G10 Chemistry Notes Chapter-10 (Acids, Bases, Salts)
Q.7) Why the colour of Bromine water discharges on addition to ethene?
Answer:
The colour of bromine water discharges in addition to ethene because of the presence of double bond. If there is no double bond present the reddish-brown colour will remain. Thus decolourizing bromine is the chemical test for the presence of a double bond.
Q.8) Compare the reactivity of alkane and alkene?
Answer:
| Alkane | Alkene |
|---|---|
| Alkanes are least reactive compounds being saturated hydrocarbons. However, they give reactions at high temperatures. | Alkenes are reactive compounds because the electrons of the double bond are easily available for reaction. These compounds have the tendency to react readily by adding other atoms. |
| Alkanes undergo a substitution reaction | Alkenes undergo addition reaction |
| Alkanes give no reaction with bromine | Alkenes when reacts with bromine decolorize reddish-brown bromine |
| Alkanes give no reaction with acidified potassium manganate (VII) solution. | Alkenes decolourise acidified potassium manganate (VII) solution |
Read more: KPK G10 Chemistry Notes Chapter9 (Chemical Equilibrium)
Q.9) Why addition reaction takes place in ethene and ethyne but not in ethane.
Answer:Â
Addition reactions take place in ethene and ethyne because they are unsaturated compounds and have the capacity to add up atoms. In these reactions, double and triple bonds are broken in order to accommodate additional atom or group of atoms. On the other hand, ethane is a saturated compound i.e. all the four bonds of carbon are fully utilized and no more hydrogen or other atom can attach to it. For example:
When bromine water is added to ethene, the Br2Â adds on across the double bond of ethene to produce dibromoethane.
Ethyne undergoes addition reactions in two steps because two molecules of bromine will add across the triple bond.
Q.10) Write equation for the preparation of ethene from ethyl alcohol and ethyl chloride?
Answer:Â
Preparation of ethene from ethyl alcohol:
Preparation of ethene from ethyl chloride:
