Alkanes

Question 1

What is the general formula for alkanes?

Answer 1

CnH2n+2

Question 2

Describe the boiling point and melting point of alkanes.

Answer 2

• Alkanes have a relatively low boiling and melting point as boiling or melting involves overcoming the weak instantaneous dipole - induced dipole interaction between molecules
• Melting and boiling points increase with increasing Mr due to the LARGER ELECTRON CLOUD SIZE which results in GREATER POLARIZATION OF THE ELECTRON CLOUD. This leads to STRONGER AND MORE EXTENSIVE INSTANTANEOUS DIPOLE - INDUCED DIPOLE ATTRACTIONS BETWEEN MOLECULES. Thus, more energy is needed to overcome these intermolecular forces of attractions during melting and boiling.
• Boiling point of a branched isomer is lower than that of its straight chain isomer. The greater the extent of branching, the lower the boiling point. INCREASED BRANCHING gives the molecule a MORE SPHERICAL SHAPE. This REDUCEDTHE SURFACE AREA OF CONTACT BETWEEN MOLECULES, leading to LESS EXTENSIVE INSTANTANEOUS DIPOLE - INDUCED DIPOLE INTERACTIONS between molecules. Thus, less energy is required to overcome these intermolecular forces of attraction.

Question 3

Density of alkanes

Answer 3

Density of alkanes INCREASES with increasing Mr as more extensive id-id interactions hold the molecules MORE CLOSELY together

Question 4

Solubility of alkanes

Answer 4

Alkanes being non-polar are SOLUBLES IN NON-POLAR SOLVENTS and are INSOLUBLE IN POLAR SOLVENTS.

Question 5

State the reagent and conditions as well as describe the mechanism of free radical substitution of chlorine

Answer 5

Question 6

Explain the relative amount of mono-subsituted isomeric products in a F.R.S. Reaction.

Answer 6

• Free radical subsitution takes place randomly and there is a lack of control on the creation.
• Assuming all H-atoms are of the same reactivity and substitution is random, ratio of isomeric products formed based on the number of unique H atoms
• However, not all H atoms are of the same reactivity. H atoms may have different reactivity since radicals of different stability are formed when H atoms in different structural environments are abstracted during the reaction

Question 7

Explain the stability of radicals

Answer 7

• The tertiary radical is the most stable as the TERTIARY RADICAL HAS THE GREATEST NUMBER OF ELECTRON-DONATING ALKYL GROUPS which will STABILISE THE RADICAL TO A GREATEST EXTENT.
• The more stable the radical, the faster it is formed. This increases the proportion of the isomer formed from the more stable radical during the reaction

Question 8

What is the reagent and conditions for reduction (Hydrogenation) of alkenes

Answer 8

C_nH_2n + H₂ —> C_nH_2n+2

H₂ (g) Pt or Pd catalyst at room temperature and pressure
OR
H₂ with Ni catalyst at elevated temperature and pressure

Question 9

What are the equations for the nitrates found in exhaust fumes

Answer 9

• O₂ + N₂ —> 2NO
• 2NO + O₂ —> 2NO₂

Question 10

What are the equations for the conversions of SO₂ to SO₃

Answer 10

SO₂ + NO₂ —> SO₃ + NO

NO + 1/2 O₂ —> NO₂

Question 11

What are the equations of the reactions in a catalytic converter?

Answer 11

• 2NO + 2CO —> N₂ + 2CO₂
• 2CO + O₂ —> 2CO₂
• C_xH_y + (x+y/4)O₂ —> xCO₂ + y/2H₂O

Question 12

What is the catalyst used in catalytic converters?

Answer 12

Ceramic honeycomb coated with precious metals such as PLATINUM, PAALLADIUM, and RHODIUM