27: Alkanes and Alkenes

Question 1

What is the meaning of the old name of alkanes?

Answer 1

Paraffin, was derived from Latin, meaning “little affinity”, showing that alkanes have limited reactivity.

Question 2

What is the chemical equation of combustion of a hydrocarbon with formula C_xH_y?

Answer 2

C_xH_y + (x+y/4)O₂ -> xCO₂ + y/2 H₂O

Question 3

What is the chemical equation of complete combustion of an alkane with n carbon atoms?

Answer 3

C_nH_2n+2 + (3/2 n+1/2) O₂ -> nCO₂ + (n+1)H₂O

Question 4

What is a substitution reaction?

Answer 4

A substitution reaction is a reaction in which an atom or a group of atoms in a molecule is replaced by another atom of group of atoms.

Question 5

What is the chemical equation of the reaction of chlorine gas and methane gas under sunlight?

Answer 5

CH₄ (g) + Cl₂ (g) —UV—> CH₃Cl (g) + HCl (g)

Question 6

What are the stages that take place in a substation reaction?

Answer 6

Initiation, propagation, termination

Question 7

What is the definition of free radical?

Answer 7

A free radical is an atom with an unpaired electron in its outermost electron shell. It is highly reactive.

Question 8

What are the criteria for a single non-metal atom to be a free radical?

Answer 8

It is not a noble gas, meaning it cannot achieve an octet structure on its own, and thus must have some unpaired electrons

Question 9

Why is ultraviolet radiation or heat needed to begin the initiation stage of substitution reactions?

Answer 9

Ultraviolet radiation or heat provides the energy required to break the covalent bond and form a free radical.

Question 10

What is the chemical equation of the initiation of the substitution reaction between methane and chlorine gas?

Answer 10

Cl—Cl —UV—> Cl`+ Cl`

Question 11

What are the chemical equations of the chain propagation of the reaction between methane and chlorine under sunlight?

Answer 11

1. Chlorine free radical reacts with methane to give methyl free radicals and hydrogen chloride: CH₄ + Cl` -> CH<sub>3</sub>` + HCl
2. Methyl free radical reacts with chlorine to give chloromethane and chlorine free radical: CH₃` + Cl<sub>2</sub> -> CH<sub>3</sub>Cl + Cl`

Question 12

Why is the substitution reaction between alkanes and halogens a type of chain reaction?

Answer 12

One initial event causes a large number of subsequent reactions, with the relative chemical species (free radical) being regenerated in each cycle of reactions.

Question 13

What are the chemical equations of the chain termination of the reaction between methane and chlorine under sunlight?

Answer 13

1. Two chlorine free radicals collide to make chlorine molecules: 2Cl` -> Cl₂
2. Two methyl free radicals collide to make ethane molecules: 2CH₃` -> C₂H₆
3. Chlorine free radicals and methyl free radicals collide to make chloromethane molecules: Cl` + CH<sub>3</sub>` -> CH₃Cl

Question 14

How many different substitution reactions can take place when methane and chlorine gas are mixed in sunlight?

Answer 14

4, since there are 4 hydrogen atoms on methane, and they can be replaced by chlorine atoms one by one.

Question 15

How can pure chloromethane be collected from the reaction of methane and chlorine under sunlight?

Answer 15

Add an excess of methane to reduce the likelihood of further substitution. There will still be other chloromethanes formed and they have to be separated by fractional distillation.

Question 16

What is the definition of cracking?

Answer 16

Cracking is the breaking down of large hydrocarbon molecules with heat or in the presence of a catalyst to produce smaller hydrocarbon molecules.

Question 17

What is the difference between thermal cracking and catalytic cracking?

Answer 17

Thermal cracking is carried out at a higher temperature than catalytic cracking. The process is more expensive due to the higher temperature, but the composition of the product mixture is different.

Question 18

What are the products of catalytic cracking?

Answer 18

Short-chain alkanes and alkenes.

Question 19

What are the advantages of catalytic cracking?

Answer 19

It makes the best use of petroleum by breaking long-chain alkanes into shorter-chain alkanes, which are better fuels.
It also produces small alkene molecules, which are feedstocks for making other compounds like synthetic polymers and a variety of products.

Question 20

What is the catalyst used when cracking octane in a laboratory?

Answer 20

Broken unglazed porcelain

Question 21

Why must the piece of unglazed porcelain be broken to be used as a catalyst when cracking in a laboratory?

Answer 21

Broken unglazed porcelain has a larger surface area comped with a large piece of porcelain and the reaction will be faster.

Question 22

What are the expected results when cracking octane in the school laboratory?

Answer 22

A gaseous product is collected in the inverted test tube. It is colourless, has a pungent smell, burns readily, and is more reactive than octane, which is a thick, colourless liquid difficult to burn with no smell.

Question 23

What is the expected result if cracking of octane has failed in a school laboratory?

Answer 23

There is a layer of oil on top of the water instead of gas, indicating that the boiling point of the product is still higher than room temperature, and it is likely that no reaction has happened.

Question 24

What is the chemical equation of the complete combustion of alkenes?

Answer 24

C_nH_2n + 3/2 n O₂ -> nCO₂ + nH₂O

Question 25

What are the differences between the flames when burning alkanes and alkenes of the same chain length and why?

Answer 25

The flame given from burning alkenes is sootier because the alkene molecules have a higher carbon to hydrogen ratio and thus a higher chance to undergo incomplete combustion and give carbon soot when burning.

Question 26

What is the definition of addition reaction?

Answer 26

An addition reaction is a chemical reaction in which two or more molecules react to form a single, larger molecule.

Question 27

What is the product of reaction between propene and bromine dissolved in 1,1,1-trichloroethane?

Answer 27

1,2-dibromopropane

Question 28

What is the product of reaction between propene and bromine water?

Answer 28

2-bromopropan-1-ol

Question 29

What is the expected result when propene is mixed with bromine dissolved in 1,1,1-trichloroethane?

Answer 29

The orange colour of bromine dissolved in the solvent disappears quickly, even in darkness.

Question 30

What is the product of reaction between propene and cold acidified dilute potassium permanganate solution?

Answer 30

propane-1,2-diol

Question 31

Why must cold and dilute acidified potassium permanganate solution be used in a reaction with alkenes?

Answer 31

This reduces the oxidising power of the acidified potassium permanganate solution, making it a mild oxidising agent.

Question 32

What is the expected result of reaction between propene and cold acidified dilute potassium permanganate solution?

Answer 32

The purple colour of the potassium permanganate solution disappears quickly.

Question 33

What are the two reactions that test for unsaturation?

Answer 33

1. The orange solution of bromine dissolved in 1,1,1-trichloethane becomes colourless quickly when shaken with an unsaturated hydrocarbon.
2. The purple solution of cold acidified diluted potassium permanganate solution becomes colourless quickly when shaken with an unsaturated hydrocarbon.