MODULE 7: ORGANIC CHEMISTRY Flashcards
Homologous Series
A series of compounds which have the same functional group, general formula, and similar physical and chemical properties.
What type of compounds are hydrocarbons?
Covalent Molecular Compounds
Are hydrocarbons polar or non-polar?
- Non-polar
- the sum of dipoles (difference in EN between C & H) is zero due to the symmetry of hydrocarbons
- Hence there is no net / overall dipole, therefore non-polar.
Which intermolecular force occurs between hydrocarbon molecules?
Dispersion forces, as they are non-polar.
Dispersion force: electrostatic attraction between temporary fluctuating dipoles
What affects the strength of the dispersian force in hydrocarbon molecules?
Size of molecule
* the strength increases with an increasing number of electrons which is proportional to molar mass.
Boiling and Melting points
Boiling and Melting points are the measures of thermal energy recquired to break intermolecular forces.
Relationship between the strength of IMF and the BP / MP of a substance
Stronger IMF = higher BP / MP
(as more thermal energy is recquired to break stronger IMF)
What does the packing of molecules refer to?
Packing: the way molecules fit together
What does packing depend on?
Molecular shape
* molecules that are small or unbranched tend to be able to pack closer together.
IMF and packing relationship
The closer molecules pack, the stronger the IMF between them.
* strength of IMF depends on the distance between molecules.
* stronger IMF = decreasing distance between molecules (closer packing)
Explain why BP & MP increases with increasing chain length
- As chain length increases, molar mass increases
- hence, the strength of dispersion forces will also increase.*
- More heat energy is recquired to break the stronger intermolecular forces and cause boiling / melting.
*molecules that are larger (more molar mass) tend to have more electrons that are generally held further away from the nucleus = stronger dispersian force.
How does packing explain unusually high BP / MP points?
- Smaller or unbranched molecules pack more efficiently.
- Hence, the strength of intermolecular forces will increase.
- more heat energy is recquired to break the stronger intermolecular forces and cause boiling / melting.
What does the solubility of a substance depend on?
The strength of:
* solute - solute cohesive forces
* solute - solvent cohesive forces
* solute - solvent adhesive forces
if a substance is soluble, the adhesive forces are strong enough to break the cohesive forces to cause them to mix.
cohesive forces: the force of attraction between molecules of the same substance.
adhesive forces: force of attraction between different substances.
Density
a measure of its mass per unit volume, expressed in gmL-1 or gcm-3
d = m/v
Explain why the density of alkanes increase with increasing molar mass.
- As MM increases, the strength of dispersion forces also increases.
- This results in molecules being more attracted to each other, causing them to be closer together.
- As a result, there will be more molecules per volume, meaning a greater mass per volume i.e higher density.
What would be observed when alkanes are mixed with water?
- Two immisible substances would be observed.
- Alkanes would float on top as they are less dense than water.
Volatility
the ability of a liquid or a solid to form a vapour (gas)
What is the relationship between volatility and BP.
higher volatility = lower BP
substances with a low BP will boil quicker, hence turn into vapour quick
Relationship between a hydrocarbons MM and volatility
a higher MM = lower volatility
- larger MM (and BP) means stromger dispersion forces
- hence, more energy is recquired to break the dispersion forces
- therefore, it will be more difficult to vaporise
viscosity
a subatnce’s resistance to fluid flow
liquids with a relatively high resisatnce to flow have high viscosity
What does viscosity depend on?
- strength of IMF
- size of molecule
- temperature
- greater strength = greater v
- greater size = greater v
- greater temperature = lower viscosity
Why do hydrocarbons have a low viscosity?
- Hydrocarbons have weak dispersion forces
- little energy is needed to break them
- thus, molecules are able to easily separate and flow
Explain greater IMF strength = greater viscosity
Greater IMF strength recquires more energy to break the dispersion forces. Hence, it is harder for the molecules to separate and flow.
Explain greater size = greater viscosity
- Larger moelcules have stronger IMF, meaning more energy is needed to break the dispersion forces, therefore more viscous.
- Larger molecules also tend to become tangled up, which prevents easy flow.
Explain greater temperature = lower viscosity
- At higher temperatures, molecules will have mroe kinetic energy.
- Thus, the molecules will move around more, which increases the space between molecules.
- This causes weaker IMF and hence, viscosity decreases.
Enthalpy of combustion
the amount of heat energy released when one mole of a substance burns completely in oxygen.
Catalyst for dehydration
H+/H2So4
concentrated sulfuric or phosphoric acid
Products of dehydration
alkene + water
Condition for dehydration
heat
Products of substitution
haloalkane + water
Catalyst for substitution
ZnCl2 (acidic salt)
Catalyst for oxidation
Potassium dichromate = Cr2O7(2-)
Potassium permanganate = Mno4-
Product for primary alcohol oxidation
aldehyde -> carboxylic acid
Product for secondary alcohol oxidation
ketone
How to stop the oxidation reaction at aldehyde
the oxidising agent can be slowly added to a heated solution of the alcohol and the aldehyde can be quickly distilled off, through the process of distillation.
This is as aldehydes have a lower BP than alcohols and carboxylic acids, as alcohols and carboxylic acids have hydrogen bonding = higher BP. Therefore, the aldehyde can be distilled off.
Fermentation
the conversion of carbbohydrates into simple alcohols by the action of enzymes in microorganisms, such as yeast or bacteria.
Fermentation of glucose
glucose (aq) -> ethanol (aq) + carbon dioxide (g)
Conditions for fermentation
- catalyst: yeast / bacteria
- warm temperature (30-40c)
- anaroabic environment >
- aqueous solution of sugar
> otherwise yeast will respire and alcohol will oxidise
Ways of producing alcohol
fermentation and substitution of haloalkanes
Production of alcohol by substitution
Alcohol can be produced by heating the haloalkane w/ sodium (NaOH) or potassium hydroxide (KOH). A hydroxide ion replaces the halogen atom to give an alcohol and a halide salt.
Biofuels
Biofuels are derived from biomass, which is material that has been derived from ** living organisms**. This could either be animal or plant derived. They are renewable.
Fossil Fuel
A fossil fuel is a fuel which has been formed through natural processes such as anaerobic decomposition of dead organisms. The fossil fuel is a non-renewable energy source since the rate of its consumption is much faster than its reformation.
Examples of biofuels
ethanol and biodiesel
Alcohol contains a highly electronegative oxygen atom, which results in polar bonds.
yes.
Why do odd chained alkanes have a lower MP than even chained alkanes?
Even numberd carbon compounds pacl togrther more efficiently.
In general, are alcohols soluble in water?
Alcohols are soluble in water, as they are able to form dipole-dipole and hydrogen bonds with water. This hydroxyl group enhances the alcohols solubility in water.
Relationship between the solubiltiy of alcohols and increasing chain length
An increased chain length = less soluble in water
This is as, as the chain length increases, the more non-polar the alcohol molecule becomes, hence, the solubility will decrease.
Alcohols have a higher MP / BP than hydrocarbons
yes.
Saturated
single bond
s for single
Unsaturated
double / triple bond
Why is the hydroxyl group in alcohol polar?
As oxygen is much more electronegative than hydrogen, making the molecule polar
