2. Alkanes

Question 1

what is a hydrocarbon?

Answer 1

a compound with only H and C atoms

Question 2

general formula of alkane [without additional functional groups]

Answer 2

C₂H₂ₙ₊₂

Question 3

are alkanes saturated or unsaturated?

Answer 3

saturated

Question 4

alkanes cannot have cis-trans isomerism. T/F?

Answer 4

false. cyclohexanes CAN have cis trans isomerism due to ring structure restricting free rotation of bonds in the ring.

Question 5

physical property of alkane: are they polar or non-polar? why?

Answer 5

non-polar. electronegativity between C and H negligible-> no dipole -> non-polar

Question 6

physical property of alkane: boiling points increase/decrease with increasing number of carbons. why?

Answer 6

increase.
1. as no. of C atoms increase, no. of electrons in alkane molecule increase
2. size of electron cloud in molecule increases
3. ease of polarisation of electron cloud increases
4. strength of id-id attractive forces increases -> require more heat energy to overcome

Question 7

Boiling points of isomeric alkanes ____ with increasing degree of branching.

Answer 7

Decrease.

Question 8

Explain why boiling point of isomeric alkanes decrease with increasing degree of branching.

Answer 8

1. With branching, molecule becomes more spherical/compact.
2. Surface area available for intermolecular interactions decreases.
3. This reduce extent of contact between neighbouring molecules.
4. Weaker id-id attractive forces -> less heat energy required to overcome

Question 9

Describe the melting point trend of alkanes

Answer 9

Increases as C atoms increases, but not a straight line, will be zigzag.

Question 10

Describe the zigzag trend as observed for melting point of alkanes.

Answer 10

Alkanes with even number of carbon atoms are packed more closely than those w odd number of C atoms. Hence attractive forces between individual molecules are greater and melting point are higher.

Question 11

solubility of alkanes

Answer 11

alkanes are non-polar->soluble in non-polar solvents but insoluble in polar solvents eg. water
solubility standard explanation
“soluble in organic solvents”
the energy released by the id-id between alkane & organic solvent molecules are sufficient to overcome the id-id between alkane & id-id between organic solvent molecules
“insoluble in water”
the energy released by the id-id between water and alkane molecules are insufficient to overcome the hydrogen bonding between water molecules & id-id between alkane molecules

Question 12

trend of density of alkanes + compared to water’s density [2]

Answer 12

1. increases with increasing molecular size, but plateaus at 0.8g/cm-3
2. so all alkanes are less dense than water

Question 13

how can alkanes be formed?

Answer 13

reduction of alkenes (catalytic hydrogenation of alkenes)

Question 14

state reagents and conditions for reduction of alkenes [2]

Answer 14

1. (H2 + Ni + heat) OR
2. (H2 + Pt/Pd)

when using Nickel, need heat bc it is the least reactive catalyst

Question 15

other than alkenes, what other kinds of compounds can be reduced? [2]

Answer 15

compounds that are unsaturated [aka double/triple bond]

eg. -CN, benzene, -C=O

Question 16

are alkanes reactive? why or why not? [3]

Answer 16

alkanes are generally unreactive
1. non-polar, hence cannot attract charged species eg. H+, OH-
2. are saturated hydrocarbons, no double/triple bonds -> no regions of high electron density-> dont attract electrophilic reagents. also no e- deficient sites to attract nucleophiles
3. C-C & C-H bonds are strong and generally do not break under normal conditions

Question 17

state the effect of adding NaOH to alkanes

Answer 17

no effect, whether hot or cold

Question 18

state the effect of adding HCl to alkanes

Answer 18

no effect, whether hot or cold

Question 19

state the effect of adding conc H2SO4 to alkanes

recall: what is conc H2SO4 used for?

Answer 19

no effect

conc H2SO4 used to dehydrate alcohol to form alkenes

Question 20

state the effect of adding acidified KMnO4 to alkanes

recall: what is KMnO4 used for?

Answer 20

no decolourisation of purple KMnO4

KMnO4 used for oxidation

Question 21

state the effect of reacting air with alkanes

recall: what is air used for?

Answer 21

no effect when cold, burns when heated

air usually for combustion [high heat] of hydrocarbon

Question 22

state the effect of adding bromine to alkanes

recall: what kinds of reactions

Answer 22

no effect in the dark. bromine decolourised in presence of sunlight (UV)

free radical substitution

Question 23

state the 3 steps of FRS

Answer 23

1. initiation: homolytic fission of Cl-Cl bond, forming Cl radicals in UV/sunlight
2. propagation: Cl radical extracts H atom from CH4 to form HCl & CH3 methyl radical ; CH3 radical abstracts Cl atom from Cl2 molecule to form CH3Cl & Cl radical ; Steps 1 & 2 repeated to form a chain reaction
3. termination: all the radicals form bonds w each other to form diff compounds

Question 24

How is CO formed? environmental consequence of CO

Answer 24

Incomplete combustion of fuel. Effect: Combines w Hb in blood and reduce blood capacity to transport oxygen

Question 25

environmental consequence of unburnt hydrocarbons

Answer 25

form photochemical smog in presence of sunlight-> irritates respiratory tract and results in chest pain and breathing problems

Question 26

How is nitrogen oxides formed? environmental consequence of their formation.

Answer 26

combination of nitrogen and oxygen in air at high temp in internal combustion engines Effect: contribute to formation of acid rain -> - makes farmlands unsuitable for cultivation - corrode architecture - negatively affect marine life [due to increased acidity of water] - contributor of photochemical smog

Question 27

solution to reducing pollutants eg CO, NO2, unburnt hydrocarbons]

Answer 27

fit cars with catalytic converter, which converts these pollutants to CO2,N2 and water vapour.

Question 28

stability of radicals depends on?

Answer 28

number of alkyl groups bonded to C atom with unpaired electrons

Question 29

state whether there is bond formation or bond breaking in the following steps of FRS: **Initiation** | Recall what happens in initiation

Answer 29

bond breaking

Question 30

state whether there is bond formation or bond breaking in the following steps of FRS: **Propagation**

Answer 30

both formation and breaking

Question 31

state whether there is bond formation or bond breaking in the following steps of FRS: **Termination**

Answer 31

formation. recall: all the radicals produced in initation and propagation bond with each other

Question 32

FRS can happen in the dark/room temp. T/F?

Answer 32

FALSE. need UV or light [aka energy to BREAK BONDS in initiation]

Question 32

FRS can happen in the dark/room temp. T/F?

Answer 32

FALSE. need UV or light [aka energy to BREAK BONDS in initiation]

Question 33

why is the synthesis of halogenoalkane from alkane through free radical substitution not preferred?

Answer 33

1. difficult to control the extent of substitution 2. formation of by-products 3. difficult to control position of substitution