Mod 7.4 - Alcohols

Question 1

What is an alcohol?

Answer 1

It is a carbon chain containing one or more -OH groups (hydroxyl groups)

Question 2

How do you name alcohols

Answer 2

Alcohols have the ending -ol

Question 3

How are alcohols classified?

Answer 3

primary Secondary and Tertiary

Question 4

What are primary alcohols?

Answer 4

The C-OH is attached to ONE other carbon atom

Question 5

What are secondary alcohols?

Answer 5

The C-OH is attached to TWO other carbon atoms

Question 6

What are tertiary alcohols?

Answer 6

The C-OH is attached to THREE other carbon atoms

Question 7

What is the general formula for alkanols?

Answer 7

C(n) H(n+1) OH

Question 8

What is the trend in primary alcohols in terms of boiling point?

Answer 8

As molecular weight increases, boiling point also increases.

This is due to the intermolecular
force of dispersion forces. The more atoms a molecule has, the greater its potential for dispersion forces, and therefore an increase in boiling point. This can form dispersion forces as a form of IMF between molecules –> weak bond

On top of that, it initially has high BP due to the presence of intermolecular hydrogen bonding

Essentially remember that the strength of alcohols is influenced by the fact that it has hydrogen bonds (IMF) and dispersion forces (weakest force)’

Intramolecular bonding is bonding that occurs between atoms, such as covalent bonds, and are thus stronger than intermolecular forces

Question 9

What are dispersion forces?

Answer 9

Because the electrons move around a lot, sometimes they may move in a way that creates a temporary dipole moment. The more electrons an atom has, the more easily this can happen, because the electrons are held more loosely, far from the nucleus.

They are the weakest IMF and contribute the most to an increasing trend as molecular weight increases

Question 10

How do you calculate enthalpy and molar heat of combustion?

Answer 10

q = quantity of heat = mcΔT

mass = mass of water (g)
C = specific heat capacity
ΔT = change in temp

ΔH = -q/n = enthalpy of combustion in Jmol^-1 or kJmol^-1)

Meanwhile, for molar heat of combustion it is just q/n (no negative since it is a measure of total heat)

Question 11

What is the difference between molar heat of combustion and enthalpy of combustion?

Answer 11

Molar heat of combustion - a measure of total heat released / mol, thus it is always positive

Enthalpy of combustion refers to the change in heat, so it should be negative under combustion

Question 12

What are the different reactions that occur with alcohols?

Answer 12

Combustion
Dehydration
Substitution with HX
Oxidation

Question 13

What is required to react alcohol with oxygen?

Answer 13

Heat

Question 14

What is the formula for combustion of alcohols?

Answer 14

Alcohol + oxygen –> carbon dioxide gas + water vapour

Question 15

What is required for a dehydration of alcohol reaction?

Answer 15

Concentrated H2So4 and Heat

Question 17

What is the formula for the dehydration of alcohols? WHat are the 3 outcomes?

Answer 17

Alcohol –> (Conc H2SO4 and heat) alkene + water vapour)

Primary alcohol - double bond will lie on the terminal alkene

Secondary alcohol - Double bond can lie to the left or to do the right . There can be more than one product

Tertiary alcohol - becomes a non terminal alkene. There can be more than one product

Question 18

What is a dehydration reaction?

Answer 18

If alcohols are dehydrated, they are returned to their alkene form. Alkanols are easily dehydrated by heating it with concentrated sulfuric or phosphoric acid which also acts as a catalyst:

The resultant alkene is dependent on whether the alkanol is a primary, secondary or tertiary
alcohol as the double bond changes position.

Question 19

What is a substitution reaction?

Answer 19

When a hydrogen halide reacts with an alcohol, a substitution reaction occurs forming water. Essentially, a hydroxyl group is replaced by a halogen

Question 20

What is the substitution reactio equation?

Answer 20

HX + alcohol –> H2O + haloalkane

Question 21

What is the Lucas Test?

Answer 21

It is a test which can be used to distinguish between primary, secondary and tertiary alcohols

Question 22

How do you determine different alcohols from the Lucas test?

Answer 22

Primary alcohol - Produce a solution that remains colourless unless it is subjected to heat. Solution forms an oily layer when heated

Secondary alcohol - turn the solution turbid and form an oily layer in 3-5 mins depending on solubility

Tertiary alcohol - Turns turbid and forms an oily layer immediately

Question 23

What reagents are required for oxidation reactions involving alcohols?

Answer 23

Acidified KMnO3 or K2CrO7

Question 25

What are the different products/equations for oxidation reactions involving alcohols?

Answer 25

Primary Alcohol (heated in acidified KMnO4 or K2CrO7) –> aldehyde –> alkanoic acids (as a final product)

Secondary Alcohol (heated in acidified KMnO4 or K2CrO7) –> ketone

Tertiary alcohol - no reaction

Question 26

Why are the BP of alcohols higher than in alkanes?

Answer 26

Because of the hydrogen bonding in alcohols as compared to the weaker dispersion forces in alkanes.

As chain length increases within the homologous series, the BP also increases. this is due to increasing strength of dispersion forces due to growing alkyl chain

Question 27

What is fermentation?

Answer 27

Fermentation refers to a biochemical reaction in which sugars can be converted into ethanol and carbon dioxide under the action of an enzyme produced by microbes (e.g. yeast or bacteria).

Enzymes in the mixture first convert any starch or sucrose in the mixture into glucose and/or fructose, then other enzymes convert glucose or fructose into ethanol and CO2

Question 28

What is the formula for producing alcohols by fermentation?

Answer 28

glucose (yeast as a catalyst)–> ethanol + CO2

C6H12O6 (aq) + yeast as a catalyst –> 2C2H4OH + 2CO2

Question 29

How are high concentrations of ethanol achieved

Answer 29

Fractional distillation. However, due to strong hydrogen bonding between ethanol abd water, water cannot be completely removed by these methods

Question 30

What is the formula for glucose?

Answer 30

C6H12O6

Question 32

What is reliability?

Answer 32

What the experiment repeated under similar circumstances?
How could reliability be improved? (Through increasing the
number of trials, removing outliers in data, taking averages,
reduction of relative error (or spread) in the results as much as
possible and removal of random errors such as parallax error
and resetting a measuring device.

Question 33

What is validity?

Answer 33

How well did the experiment measure what was intended? An experiment is valid if it measured the effect of the independent
variables on the dependent variable. The controlled variables must be kept constant. It must be a fair test, which is defined
as an experiment where there is one independent variable. All systematic errors (error within experiment) need to be
eliminated and random errors must be reduced.

Question 34

What is accuracy?

Answer 34

How accurate was the experimental results? An accurate experiment gives exact results that are close to the ideal (data book) value as possible. In order to improve accuracy, all systematic and random errors must be eliminated where possible. All variables must be kept constant apart from those that are being investigated.

Question 35

Assess the solubility of alcohols in water

Answer 35

Hydrogen bonding explains the high water solubility of the first four members of the alkanol homologous series. The idea is that ‘like dissolves like’, and the stronger the intermolecular forces between solute molecule and solvent molecule, the greater the solubility of the solute in the solvent.

Thus, because water is a polar solvent it will form hydrogen bonds with an alcohols’ tail end –> ability to be soluble initially

As the chain length increases, however, they
become increasingly insoluble. While the
hydroxyl group of an alcohol can form
hydrogen bonds with water molecules, the
alkyl group cannot. Only dispersion forces
occur between the alkyl chain and water
molecules, opposing the tendency for the
molecule to dissolve.

For smaller alcohols, the energy released when these new hydrogen bonds are made compensates for that needed to break the original ones (bond breaking > bond making).

Meanwhile, in larger alcohols, The -OH end of the alcohol molecules can form new hydrogen bonds with water molecules, but the hydrocarbon
“tail” does not form hydrogen bonds. Hence, a lot of the
original hydrogen bonds being broken are not replaced by
new ones. In place of those original hydrogen bonds are
dispersion forces between the water and the hydrocarbon
“tails”. These attractions are much weaker. Even allowing
for the increase in disorder, the process becomes less
feasible.

Question 36

Explain the increased move towards ethanol as a fuel source? Assess ethanol as a carbon neutral fuel source

Answer 36

It is touted as ‘greenhouse netural’ since the net carbon dioxide released appears to be 0. When we consider the equations for photosynthesis, fermentation and combustion:

The amount of CO2 produced in fermentation and combustion is the same amount consumered in photosynthesis

Despite these equations however, ethanol is not actually greenhouse netrual since many other energy inputs have been ignored such as the energy needed for fertiliser production, crop cultivation and the distillation of ethanol after fermentation

Question 37

What are the advantages of using ethanol as a biofuel

Answer 37

Unlike petroleum, ethanol is a renewable resource.

Ethanol burns more cleanly in air than petroleum, producing less carbon (soot)
and carbon monoxide.

The use of ethanol as opposed to petroleum could reduce carbon dioxide emissions,
provided that a renewable energy resource was used to produce crops required to
obtain ethanol and to distil fermented ethanol.

Question 38

What are the disadvantages of using ethanol as a biofuel

Answer 38

Ethanol has a lower heat of combustion (per mole, per unit of volume, and per unit of
mass) than petroleum.

Large amounts of arable land are required to produce the crops required to obtain
ethanol, leading to problems such as soil erosion, deforestation, fertiliser run-off and
salinity. This will also reduce amounts of land to grow crops –> decreased food production

Major environmental problems would arise out of the disposal of waste fermentation
liquors.

Typical current engines would require modification to use high concentrations of
ethanol.

The energy used by trucks to transport crops to to biofuel manufacturing plants and the ethanol product to relevant stores to sell to consumers are currently derived from the burning of fossil fuels.

Due to limited amount of land on Earth, the landfills are already burdened with the large amount of rubbish. There will be problems in the future in storing large volume of fermentation waste if a switch from fossil fuels to completely renewable biofuel production via fermentation in the future.

Question 39

WHat are the advantages of using fossil fuels?

Answer 39

The use of fossil fuels in vehicles (mainly octane) saves time used in refueling as octane has a higher energy output per gram (and thus per mole) compared to ethanol, thus can travel a longer distance before the need for re-fuelling. The values are 46 kJ/g and 20.6 kJ/g respectively.

Plus all the disadvantages of biofuels

Question 40

What are the disadvantages of using fossil fuels?

Answer 40

As the formation of fossil fuels takes millions of years, they are considered non-renewable. Therefore, it is not sustainable in the long term to meet the growing world population’s demand for energy usage.

Furthermore, as quantity of fossil fuel decreases, the price of fossil fuels will rise which will become less affordable to consumers.

Fossil fuels used in cars (majority octane) burn less cleanly than biofuels such as ethanol as it requires more oxygen for one mole of octane to undergo complete combustion. This would mean that toxic carbon monoxide will be produced as a product which can harmful to living organisms such as humans.