Alkanes And Alkenes

Question 1

Why are alkane called SATURATED HYDROCARBONS?

Answer 1

They contain ONLY CARBON CARBON SINGLE COVALENT BONDS.
All outer electrons of each C atom are used in forming single bonds with 4 other atoms (the maximum for C), thus it is saturated.

Question 2

How do the physical properties of alkane change going down a series?

Answer 2

1. Their MELTING and BOILING points INCREASE.
2. They become MORE VISCOUS
3. They become LESS FLAMMABLE

Question 3

Why do the melting and boiling points of alkane increase going down the series?

Answer 3

Their molecular sizes increase, thus the forces of attraction between the molecules also increase.

Question 4

Why do alkanes become less flammable going down the series?

Answer 4

As the sizes of alkanes molecules increase the % of C in alkane molecules also increase. They also produce a smokies flame caused by the incomplete combustion of alkane molecules

Question 5

What are the chemical properties of Alkanes?

Answer 5

Alkanes are GENERALLY UNREACTIVE
Alkanes can COMBUST
Alkanes undergo SUBSTITUTION REACTIONS.

Question 6

What is the general equation for the combustion of alkane?

Answer 6

Alkane+oxygen—->carbon dioxide+water vapour

Question 7

What are substitution reactions?

Answer 7

Alkanes react with halogens (Cl and Br) in the presence of UV light.
A hydrogen atom in an alkane is replaced by a halogen atom, this can be repeated until all H atoms are replaced.

Question 8

What are alkenes?

Answer 8

Alkebes are hydrocarbons that contain one or more C=C DOUBLE BONDS

Question 9

Why are alkenes UNSATURATED HYDROCARBONS?

Answer 9

Any molecules that contain C=C double bonds are unsaturated.

Question 10

What is alkene combustion?

Answer 10

Same with alkanes.

They burn with a much sootier flame.

Question 11

What are addition reactions and what are the types?

Answer 11

C=C bonds in alkenes are reactive, and will undergo addition reactions.
The 4 types are:
Hydrogenation (H²) 
Bromination (Br²)
Hydration (Steam)
Addition Polymerisation (Alkenes)

Question 12

What is hydrogenation and why is it useful?

Answer 12

At 200°C and with Ni catalyst, alkenes react with H² to form an alkane.
Used in manufacture of margarine.

Question 13

What is bromination?

Answer 13

Br²(aq) is reddish brown.

If an alkene is added to Br² (aq) it decolourises immediately and a colourless liquid is formed.

Question 14

How is bromination useful to us?

Answer 14

Serves as the chemical test for presence of unsaturated hydrocarbons.
It is used to distinguish between an alkane and an alkane.
(Alkanes do not decolourise but alkenes do so rapidly)

Question 15

What is hydration?

Answer 15

Alkenes react with steam to produce alcohols. Conditions required are 300°C, 60atm and H³PO⁴ catalyst

Question 16

What is cracking?

Answer 16

Cracking is the breakinng down of long chain hydrocarbons into smaller molecules.

Question 17

What is an example of cracking?

Answer 17

Hexane (C6H14) —-> Butane (C4H10) + Ethene (C2H4)

Question 18

How is industrial cracking carried out?

Answer 18

Is done by passing long chain alkanes over Al2O3 and SiO2 catalyst at 600’C
Long Chain Alkanes —(Catalytic cracking)—-> Short Chain Alkenes + Alkanes/Hydrogen Gas

Question 19

What is the importance of cracking?

Answer 19

1. Cracking is used to produce short-chain alkenes.
2. Cracking is used to produce hydrogen.
3. Cracking is used to produce petrol.

Question 20

Why is producing petrol useful?

Answer 20

Over 1/2 the fractions obtained from petroleum are heavy and less useful.
Long-Chain hydrocarbons can be converted by catalytic cracking into higher demand smaller molecules.

Question 21

What are some of the similarities between alkenes and alkanes?

Answer 21

S:
Both are hydrocarbons.
Both are flammable and produce Carbon dioxide and water on combustion.

Question 22

What are polyunsaturated fats?

Answer 22

Polyunsaturated fats or oils contain more than one C=C double bond.

Question 23

How is margarine produced?

Answer 23

We add hydrogen to vegetable oil. A temperature of 200’C and Ni catalyst are required. The greater the amount of H used, the more solid the margarine.