Alkanes

Question 1

what is a hydrocarbon?

Answer 1

compounds that only contain hydrogen and carbon

Question 2

What does saturated mean?

Answer 2

compound which contains only single bonds between carbon atoms

Question 3

What is the general formula for alkanes?

Answer 3

Cn H2n+2

Question 4

What is a functional group?

Answer 4

an atom or group of atoms that gives a compound its characteristic and chemical reactivity

Question 5

What is a homologus series?

Answer 5

a series of compounds that have the same functional group and the same general formula and in which the successive members of the series differ by an extra CH2 unit

Question 6

What shape are the hydrogens around each carbon in an alkane?

Answer 6

tetrahedral - bond angle 109.5’

Question 7

What are isomers?

Answer 7

different compounds that have the same molecular formula

Question 8

What are structural isomers?

Answer 8

compounds that have the same molecular formula but different structural formula

Question 9

What is a branch in the alkane called and what is its general formula?

Answer 9

akyl group - CnH2n+1
eg: methyl, ethyl

Question 10

What does it mean if the alkane is substituted?

Answer 10

1 or more of the hydrogens is replaced by a different atom or group of atoms

Question 11

What is a cycloalkane’s general formula?

Answer 11

CnH2n

Question 12

Why do alkanes have relatively low boiling points?

Answer 12

C-H bonds are non-polar so they can only form london forces which are the weakest type of intermolecular forces

Question 13

What increases an alkanes boiling point?

Answer 13

increasing the number of carbon atoms

Question 14

Why does an alkanes boiling point increase with more carbon atoms?

Answer 14

the size of the atom increases = the surface contact area with other molecules increases
therefore:
more london forces to break and more energy is needed to break them

Question 15

Why do straight chain alkanes have a higher boiling point than branched isomers?

Answer 15

the straight chain isomer has a larger surface area for contact with other molecules - can form more london forces - more energy needed

Question 16

What is the density of alkanes like?

Answer 16

all solid and liquid alkanes are less dense than water so they will float on water

Question 17

Why are alkanes insoluble?

Answer 17

non-polar so cannot form hydrogen bonds

Question 18

Is combustion of alkanes endothermic or exothermic?

Answer 18

exothermic

Question 19

What is the expression for the number of products and reactants in the combustion of alkanes?

Answer 19

CnH2n+2 + 3n+1/2 O2 —–> nCO2 + n+1 H2O

Question 20

What are the products of incomplete combustion of alkanes?

Answer 20

CO or C (soot)

Question 21

What does alkanes reacting with halogens produce?

Answer 21

haloalkanes

Question 22

What type of reaction is alkanes reacting with halogens?

Answer 22

substitution

Question 23

What are the conditions for alkanes reacting with halogens?

Answer 23

room temp under exposure to UV light, does not occur in the dark

Question 24

What are the observations for alkanes reacting with bromine?

Answer 24

bromine is decolourised - orange –> colourless
steamy, acidic fumes formed = HBr

Question 25

What does aliphatic mean?

Answer 25

C atoms are joined to each other in unbranched or branched chains or non-aromatic rings

Question 26

What does alicyclic mean?

Answer 26

C atoms are joined to each other in a ring structure with or without branches

Question 27

What does aromatic mean?

Answer 27

some or all of the C atoms are found in a bezene ring

Question 28

What is a reaction mechanism?

Answer 28

a description of the key events on the pathway that leads reactants to come together and react to produce the products

Question 29

What is a radical?

Answer 29

an atom or group of atoms which has an unpaired electron, they are often highly reactive

Question 30

What happens during the initiation of alkanes with halogens?

Answer 30

starts the reaction, brought about bu UV light which provides the energy needed to break a halogen molecule apart to produce its radical
the bond breaking is by homolytic fission

Question 31

What is homolytic fission?

Answer 31

a process in which a covalent bond breaks so that one of the electrons that one of the electrons that made up the bond stays with one of the atoms and the other electron stays with the other atom

Question 32

What happens during the propagation stage in the reaction of alkanes with halogens?

Answer 32

x2 reactions:
1. a highly reactive halogen collides with a methane molecule and extracts a H atom from it, forming a molecule of acid and a methyl radical - CH3-
2. the reactive methyl radical collides with a halogen molecule and extracts a halogen molecule from it, forming a molecule of halogenmethane and a halogen radical
the two reactions can repeat themselves = a chain reaction

Question 33

What is the termination stage for the reaction with alkanes and halogens?

Answer 33

if two radicals collide with each other, they combine to form a molecule which now has no unpaired electrons this stops the reaction

Question 34

What is heterolytic fissison?

Answer 34

when a covalent bond breaks, one of the bonded atoms takes both of electrons from the bond