Alcanes

Question 1

What is general formula for alkanes? How about for alkyl R grps and saturated alkane rings?

Answer 1

alkane: CnH2n+2

alkyl R grp: CnH2n+1

alkane ring: CnH2n

Question 2

Give possible explanation why alkanes hv low mp, bp

Answer 2

• alkanes hv simple molecular structure
• low amt energy needed to overcome weak id-id attract n
  => low mp,bp

Question 3

Why is there diff in bp in branched chain alkanes and corresponding straight chain isomers (of same molecular formula)?

Answer 3

• extent of surface area of contact: greatest for straight chain and smallest for most branched chain
• extent of distort n of e- cloud: highest for straight chain, lowest for most branched chain
• extent of intermolecular id-id attract n: greatest for straight chain, smallest for most branched chain

=> energy required to overcome imf of attract n: greatest for straight chain, least for most branched chain

Question 4

Explain solubility of alkanes in non-polar and polar solvents

Answer 4

• Soluble in non-polar eg benzene
  bcos energy released fr form of id-id attract n btw alkane molecules & solvent molecules enough to overcome id-id attract n btw alkane molecules & that btw solvent molecules
• Insoluble in polar eg water
  bcos energy released fr form of id-id btw alkane molecules & solvent molecules inadequate to overcome id-id attract n btw alkane molecules & pd-pd/H bond (for solvent eg water) btw solvent molecules

Question 5

Why are alkanes non-polar?

Answer 5

C & H atoms hv almost similar electronegativity

Question 6

Why r alkanes generally unreactive?

Answer 6

1. saturated & non-polar
2. no region of low or high e- density, so unreactive towards polar reagents
3. Strong C-C or C-H bonds (require large amt energy to break)

Question 7

What 2 main reactions do alkanes undergo?

Answer 7

1. Combustion
2. Free radical substitution (FRS)

Question 8

What is the general eqn for complete combustion of alkane? So what are general pdt?

What is a possible eqn of incomplete combustion of methane? So what r the pdt?

Answer 8

CxHy + (x+0.25y)O2 –> xCO2 + 0.5yH2O
(CO2 & H2O r pdt)

possible eqn:
CH4 + 1.5 O2 –> CO + 2H2O
(CO2, H2O AND CO r pdt)

Question 9

What are free radicals?

Answer 9

highly reactive species (atom or grp of atoms) w an unpaired e- formed by homolytic fission

Question 10

Define homolytic fission

Answer 10

breaking of covalent bond btw 2 atoms so each atom retain oni 1 of shared e-, causing radical form n

Question 11

Define heterolytic fission

Answer 11

breaking of covalent bond btw 2 atoms so 1 atom retain both shared e-, causing form n of ions

Question 12

What does a full and half arrow represent?

Answer 12

full arrow: transfer of 2e- (pair of e-)

half arrow: transfer of one e-

Question 13

Describe FRS mechanism using rxn of propane w limited Br to form bromo-1-propane

Answer 13

Name: Free radical sub n

Initiation:
homolytic fission (2 half arrow) of Br-Br under UV light –> 2Br ·

Propagation: (radical + molecule -> another radical)
1) H-CH2CH2CH3 + Br · –> ·CH2CH2CH3 + HBr
2) ·CH2CH2CH3 + Br2 –> Br-CH2CH2CH3 + Br ·

Termination: (3 eqn! A+A, B+B & A+B)
2Br · –> Br2
·CH2CH2CH3 + Br · –> CH3CH2CH2Br
·CH2CH2CH3 + ·CH2CH2CH3 –> CH3CH2CH2CH2CH2CH3
(in other situations where C radical is in the middle, u write formula sth like this:
CH3CHCH3
|
CH3CHCH3
)

Question 14

Assuming equal probability of H being substituted, how to determine ratio of mono sub pdt formed?

Answer 14

• Draw out structure of alkane for easy visualisation
• determine H atoms that gives specific pdt and group them
  eg sub for Ha gives 1-chloropropane, sub for Hb gives 2-chloropropane
• take no of Ha & Hb atoms and put them into a ratio to get the final ratio

Question 15

If eg Cl is limited vs excess, how does it affect product of FRS?

Answer 15

• Limited: major pdt is monosub
• Excess: disub, trisub, tetrasub oso obtained

Question 16

Name the main pollutants that motorcar engines emit. How do they form and what are their harmful effects?

Answer 16

i. CO
- fr incomplete fuel combust n
- cause CO poisoning

ii. Unburnt hydrocarbons
- fr incomplete fuel combust n
- bcome photochemical smog in strong sunlight, cause lung damage

iii. Lead compounds
- eg (TO REMEMBER) Lead bromide vapour
- fr add n of tetraethyllead (IV) to petrol
- lead poisoning, accumulat n cause brain damage

iv. Oxides of nitrogen
- eg NO, NO2; referred as NOx
- form acid. rain, corrode buildings, destroy crop, marine life & cause respiratory problem in human

v. SO2
- form acid rain, corrode buildings, destroy crops, marine life

Question 17

What does catalytic converter do? What condition does it work?

Answer 17

• speeds up conversion of harmful pollutants eg CO, NOx, unburnt hydrocarbons into harmless pdt eg H2O, CO2, N2, in motor vehicle exhaust system
• car must b run on unleaded petrol (bcos lead poisons/deactivates catalyst)

Question 18

Give 3 rxn occurring in a catalytic converter. Name catalyst for each

Answer 18

1. 2NO + 2CO –> 2CO2 + N2 (Rh as catalyst)
2. CxHy + (x+0.25y)O2 –> xCO2 + 0.5yH2O (Pd/Pt as catalyst)
3. 2CO + O2 –> 2CO2 (Pd/Pt as catalyst)

Question 19

Name greenhouse gases. What are the consequences of enhanced greenhouse effect?

Answer 19

• H2O, CO2, CH4, N2O, O3, CFCs
  Consequences:
• more drought, flooding
• less snow, ice
• more extreme weather incidences
• rising sea lvl
  stronger gh effect warm ocean, partially melt glaciers, other ice, increasing sea lvl. Ocean water oso expand if warmed, contribute further to rise in sea lvl

Question 20

What is the problem with petroleum use? How to deal with it?

Answer 20

• chemical feedstock (raw material to produce many sorts of pdt eg plastic, fuel), natural source of hydrocarbon
  BUT
• non-renewable resource bcos natural petroleum deposits r finite; once exhausted, no way to restore them
  THUS
• must use alternative, renewable sources to preserve resources as well as minimise harmful pollut n (save environ)
  eg solar, wind pwr, bio-fuels
• must oso recycle

Question 21

How is recycling important?

Answer 21

preserves Earth’s natural resources (eg petroleum), protect environ by reducing:
- pollut n caused by dispose waste, plastic incinerat n
eg harmful chemicals, ghg gases
- need for raw materials; rainforests can b preserved
eg deforestation, cause habitat destruct n, global warming
- amt energy needed in making pdt
eg recycle aluminium less energy need than extract fr ore

Question 22

Explain stability of radical

Answer 22

Case 1: if there is adjacent benzene ring:
- lone (unpaired) electron is delocalised into benzene ring, stabilising radical

Case 2: number of electron donating group (not adjacent bond to benzene):
- the more electron donating alkyl group, the more stable (to better disperse +ve charge of radical)