Properties of Hydrocarbons and Haloalkanes

Question 1

why is propane a safe and useful fuel for barbecues?

Answer 1

propane has a low boiling point

Question 2

what are physical properties of hydrocarbons determined by?

Answer 2

intermolecular forces, size and shape of molecule

Question 3

when are alkanes colourless gasses?

Answer 3

methane to butane

Question 4

when are alkanes colourless liquids?

Answer 4

pentane to heptadecane

Question 5

what is the relationship of boiling point and chain length of alkanes?

Answer 5

boiling points increase as the size of the alkane molecule increases

Question 6

what are the intermolecular forces present in alkanes?

Answer 6

because alkanes are non-polar, they have weak dispersion forces.

Question 7

why does the overall forces of attraction between molecules increase as chain length increases?

Answer 7

strength of dispersion forces increases because of the increased strength of temporary dipoles within molecules.

Question 8

what happens to boiling point when an alkane chain length increases?

Answer 8

boiling point increases because it is determined by the strength of the intermolecular forces

Question 9

how does molecular shape effect the strength of dispersion forces?

Answer 9

shape affects how well it is able to get close to neighbouring molecules and so if an alkane is branched, molecules are further apart, meaning dispersion forces are weaker, hence a lower boiling point.

Question 10

why do straight chained alkanes have higher boiling points? (draw diagrams)

Answer 10

because they are more ordered and straight, molecules can fit closer together, strengthening the dispersion forces. (reference diagrams on pg 297)

Question 11

boiling points of alkanes vs alkenes and alkynes

Answer 11

alkenes and alkynes have a relatively low boiling point compared to an alkane with the same number of carbon atoms.

Question 12

what happens when a polar functional group is added to a hydrocarbon that is non-polar?

Answer 12

the addition increases the boiling point of the molecule (compared to a similar sized molecule that is non-polar)

Question 13

haloalkanes have what type of bonds?

Answer 13

polar bonds

Question 14

chloromethane is a haloalkane, identify its polar bond and why it is polar. (draw a diagram)

Answer 14

it contains a polar carbon-chlorine bond. the bond is polar because the chlorine atom is more electronegative than the carbon atom which creates a permanent dipole, thus producing dipole-dipole attractions between chloromethane molecules. (reference diagram on pg298)

Question 15

why are haloalkane boiling points generally higher than those of hydrocarbons with a similar number of carbon atoms?

Answer 15

because dipole-dipole attractions ate stronger than dispersion forces.

Question 16

how are melting points of straight-chain hydrocarbons affected when the number of carbon atoms increase?

Answer 16

melting point will increase with increasing carbon atoms

Question 17

what is the exception of melting point trends?

Answer 17

whether or not there is an even or odd number of carbon atoms.

Question 18

what happens to melting point when there is an even number of carbons?

Answer 18

chains with even numbers pack more efficiently, requiring more energy to melt the compound. hence, increasing melting point.

Question 19

what happens to melting point when there is an odd number of carbons?

Answer 19

chains with odd numbers are less efficient at packing, resulting in a lower melting point.

Question 20

sketch a graph of the boiling and melting points for alkanes

Answer 20

boiling point trend steadily increases whereas melting point trend has a step-like curve. (reference graph on pg 298)

Question 21

trend of melting points for alkenes and alkynes

Answer 21

as length of carbon chain increases, melting point increases.

Question 22

at room temp, what state is ethane, propene and butene at?

Answer 22

gas

Question 23

what state are alkenes with 5 to 15 carbons at?

Answer 23

liquids

Question 24

what happens when the triple bond of an alkyne changes position?

Answer 24

melting point is affected greatly as the shape of the molecule changes

Question 25

but-1-yne MP: -122 and but-2-yne MP:-24 why are the melting points so different?

Answer 25

the placement of the triple bond went from carbon 1 to carbon 2.

Question 26

a haloalkane and its corresponding hydrocarbon has a melting point that is ________?

Answer 26

higher

Question 27

why are haloalkane melting points high than corresponding hydrocarbons?

Answer 27

it is because of the stronger dipole-dipole forces between haloalkane molecules, caused by the presence of polar halogen-carbon bonds, hence making the molecule more polar.

Question 28

why are alkanes, alkenes and alkynes insoluble in water? (reference a diagram of water)

Answer 28

water is a polar molecule, held together by strong hydrogen bonds (reference diagram pg 299). the weak dispersion forces between water molecules and hydrocarbon molecules are not strong enough to overcome the strong attraction between water molecules.

Question 29

what happens when hexane is added to water?

Answer 29

it forms a layer on top of the water since hexane is less dense than water dn the two liquids don’t mix.

Question 30

what is it called when liquids don’t mix?

Answer 30

immiscible

Question 31

are haloalkanes soluble in water?

Answer 31

because of the polar halogen-carbon bond they are slightly more soluble, although, solubility is still very low because the influence of the dipole-dipole interaction is small. the influence of the halogen decreases further as the length of the carbo chain increases.

Question 32

what happens if a haloalkane is added to water?

Answer 32

since many haloalkanes are denser than water, they will settle to form a layer on the bottom.

Question 33

why do alkanes mix with other non-polar liquids (eg, containing alkenes or symmetrical haloalkanes)?

Answer 33

the forces of attraction of these liquids are also weak dispersion forces and are similar in strength

Question 34

what is it called when liquids are soluble in each other?

Answer 34

miscible

Question 35

why is the BP and MP of haloalkanes higher than alkanes, alkenes and alkynes of the same length?

Answer 35

haloalkanes contain both dispersion forces and polar bond, which means that between molecules a dipole-dipole interaction occurs.