Alkanes And Alkenes Flashcards
Why are alkane called SATURATED HYDROCARBONS?
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.
How do the physical properties of alkane change going down a series?
- Their MELTING and BOILING points INCREASE.
- They become MORE VISCOUS
- They become LESS FLAMMABLE
Why do the melting and boiling points of alkane increase going down the series?
Their molecular sizes increase, thus the forces of attraction between the molecules also increase.
Why do alkanes become less flammable going down the series?
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
What are the chemical properties of Alkanes?
Alkanes are GENERALLY UNREACTIVE
Alkanes can COMBUST
Alkanes undergo SUBSTITUTION REACTIONS.
What is the general equation for the combustion of alkane?
Alkane+oxygen—->carbon dioxide+water vapour
What are substitution reactions?
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.
What are alkenes?
Alkebes are hydrocarbons that contain one or more C=C DOUBLE BONDS
Why are alkenes UNSATURATED HYDROCARBONS?
Any molecules that contain C=C double bonds are unsaturated.
What is alkene combustion?
Same with alkanes.
They burn with a much sootier flame.
What are addition reactions and what are the types?
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)
What is hydrogenation and why is it useful?
At 200°C and with Ni catalyst, alkenes react with H² to form an alkane.
Used in manufacture of margarine.
What is bromination?
Br²(aq) is reddish brown.
If an alkene is added to Br² (aq) it decolourises immediately and a colourless liquid is formed.
How is bromination useful to us?
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)
What is hydration?
Alkenes react with steam to produce alcohols. Conditions required are 300°C, 60atm and H³PO⁴ catalyst
What is cracking?
Cracking is the breakinng down of long chain hydrocarbons into smaller molecules.
What is an example of cracking?
Hexane (C6H14) —-> Butane (C4H10) + Ethene (C2H4)
How is industrial cracking carried out?
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
What is the importance of cracking?
- Cracking is used to produce short-chain alkenes.
- Cracking is used to produce hydrogen.
- Cracking is used to produce petrol.
Why is producing petrol useful?
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.
What are some of the similarities between alkenes and alkanes?
S:
Both are hydrocarbons.
Both are flammable and produce Carbon dioxide and water on combustion.
What are polyunsaturated fats?
Polyunsaturated fats or oils contain more than one C=C double bond.
How is margarine produced?
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.
