Chemistry iGCSE Section C

Homogolous Series - same general formula, similar chemical reactions e.g. alkane/ alkene

Saturated - all single bonds

Unsaturated - contain a carbon-carbon double bond

Isomerism - same molecular formula, different displayed formula

ALKANES:

C11H2n+2

ALKENES:

C11H2n

Alkanes:

Complete combustion: Creates CO2

Incomplete combustion: Creates carbon monoxide

With bromine: CH4(g) + Br2(g) --> CH3Br(g) + HBr(g)

Substitution reaction

Alkenes:

Test for carbon double bonds: decolourises bromine.

Addition reactions (alkene and a halogen) break double bonds.

1.Fermentation

• Dissolve sugar or starch in water and add yeast
• Leave the mixture to ferment at 25-40C for several days in the absence of air
• Filter off excess yeast for a dilute soluion of ethanol

When the ethanol content rises above 15%, the yeast is killed. The enzymes in the yeast produce glucose. The glucose is then converted into ethanol.

C6H12O6(aq) --> 2C2H5OH(aq) + 2CO2(g)

2. Direct hydration of ethene

• Ethene+steam is passed over a phosphoric acid catalyst at 300C and 60-70 atm

C2H4(g) + H2O(g) --> C2H5OH(g)

The ethanol is then condensed as a liquid. The ethene used is obtained from crude oil.

Fermentation                                          Hydration

Renewable sources (sugar cane)          Non-renewable sources (crude oil)

Batch process                                        Continuous

Slow                                                        Fast

Dilute ethanol                                          Pure ethanol

Low temps                                              High temp & pressure

REACTIONS OF ETHANOL

Combustion:  C2H5OH(l) + 3O2(g) --> 2CO2(g) + 3H20(l)

Dehydration: passing ethanol vapour over hot aluminium oxide.