Topic 2: The chemistry of chemical compounds

The Alkanes – a family of hydrocarbons 2A 

Physical properties

1) Oily      2) insoluble     3) less dense than H2O, mainly gasses        4) Insoluble – doesn’t mix with H2O

 

States:

Gas: 4 carbon atoms               Liquid: 4 – 17 carbon atoms               Solid: 17+ carbon atoms

Combustion

 

Hydrocarbons + burning -> Carbon dioxide + water

When there is a lack of oxygen:          Hydrocarbons + burning -> Carbon monoxide + soot

 

Alkanes don’t react with aqueous reagents e.g. acids or alkali’s.

This is because they are saturated – their C – H and C – C bonds are difficult to break = highly un-reactive.

 

The formula CnH (2n+2), where n = no of carbon atoms, can be used to determine the formula for each alkane. E.g. To find propane: C3H (2×3 +2) = C3H8

The Alkenes

The alkenes are unsaturated and are more reactive b/c of the C =C double bond.

The double bond allows them to melt and form into different things e.g. plastic

The Alcohols – OH group 2B          

 

Alcohols have similar properties b/c they have the OH at the end.

The formula CnH (2n+1) OH, where n = no of carbon atoms, can be used to determine the formula for each alcohol.

 

Alcohol (contains hydrocarbons) + burning -> Carbon dioxide + water

 

Properties

  • Good solvent – Can dissolve in compounds that water can’t
  • Soluble in water
  • Liquid at room temp. – evaporate + volatile (give of fumes).
  • Intermolecular forces are stronger with the–OH functional group; other molecules can join to the alcohol easily. Alkanes are gasses so their intermolecular forces are less strong.
  • Flammable – catches a clean flame.
  • b/c it is pure, can be used as fuel
  • High boiling point

An alcohol can become a water molecule or an alkane when you replace the – H atom.

Ethanol molecules have more tendencies to stick to each other than Alkanes but fewer tendencies to stick to each other compared to water.

 

B/c the – OH group allows other molecules to cling together; alcohol and water can mix together.

  • But if hydrocarbon length is too long the oiliness of the alcohol will dominate and it will be too difficult to mix.

Reaction with sodium

 

Only the hydrogen atom in the – OH is involved in the reaction. The hydrogen atoms linked to the carbon

(C –H) are inert or un-reactive.

Na + H2O à NaOH + H2 (+ squeaky pop)              

Na + C2H5OH à NaO + H2

 

An ionic bond is formed between the positive sodium and negative oxygen.

E.g.  2C2H5OH + Na à 2C2H5ONa + H2

The production of ethanol 2B

 

Uses: fuel, solvent, feedstock.

Principles of green chemistry must be applied to the process + modifications must be made if necessary.

 

Fermentation

 

PRO’s of fermentation

 

  • Renewable feedstock – e.g. waste plant material – maize, sugar cane.
  • Un-fermented parts used as animal food
  • 21st century = further developments à more parts CAN be fermented
  • Agricultural waste / sludge can be fermented

 

CON’s of fermentation

 

  • Lots of land needed
  • May need the space for human foods
  • Some parts can’t be fermented

 

Reaction

 

Cellulose polymers from feedstock are heated with acid to break it down into simple sugars.

 

Glucose -> Ethanol + Carbon dioxide

Optimum conditions

 

Opt. Temp: 30 degrees

  • Too high -> denatured enzymes
  • Too low -> Rate of Reaction too slow b/c enzymes working slow

Limited conc. of ethanol is produced (14/15%) b/c if the ethanol production is any higher, the yeast gets killed and the fermentation gets stopped.

Distillation

 

Distillation – separating chemicals based on boiling points

This is used to obtain higher conc. of ethanol.

       

Is fermentation sustainable?

Analyse the data and think about:

  • Raw materials e.g. renewable feedstock
  • Atom economy à waste = low AE
  • Waste à released? Recycled?
  • Energy costs e.g. for opt. temp.
  • Environment e.g. GHG
  • Health + safety
  • Profit
  • Benefits / risks

Energy balance

Energy balance – Energy output needs to be greater than energy input

  • Higher EB = greener process.
  • Different feedstock release different EB

Biotechnology

GM of microorganisms + biomass

PRO’s

  • Bacteria breaks down wide range of sugars into ethanol
  • Fungi breaks down biomass into glucose -> ethanol
  • Yeast converts glucose into ethanol
  • Yeats withstands high concs. of ethanol -> more profit
  • Biomass used = less waste

 

PRO’s of GM

  • Waste biomass can’t be fermented normally b/c contains cellulose; cellulose can’t be converted by yeast, into ethanol.
  • M E.Coli converts cellulose into ethanol
  • Yeast only breaks down glucose NOT other types of sugars available in plant feedstock’s
  • GM E.Coli converts all types of plant sugars into ethanol

 

Equation: All sugars -> ethanol + carbon dioxide

Opt temp: 35 degrees             Opt pH: pH6

 

Chemical synthesis

 

Fermentation is too slow for making ethanol on a large scale.

Using ethane to produce high quality ethanol is quicker on an industrial scale = profitable

 

Crude oil/natural gas (non-renewable) contains ethane, which is cracked to form ethane.

 

Natural gas -> ethane -> [cracked] -> ethene + hydrogen released

Crude oil -> Naphtha ->[cracked] -> ethene + hydrogen released

 

Ethane -> Ethene + hydrogen

Ethene + steam [+ phosphoric acid catalyst]-> ethanol

 

Catalyst: Phosphoric acid                   Temperature: 300 degrees Celsius                 Pressure: 60-70atm.

 

Atom economy: 100% (all atoms are used up)          Energy costs: High temp + pressure required

Yield: 95% (b/c by-products are produced)                Environment: oil spills

Un-reacted molecules are recycled.                           Health + safety: High temp + pressure be controlled

 

Purifying

Difficult to remove water and obtain 100% ethanol. Zeolites are used as a dehydrating agent, which absorbs small water molecules and leave ethanol remaining.

Carboxylic acids – organic acids – COOH group 2D

They belong to – COOH group so all have similar properties.

Name ends in “anoic”.

General formula: CnH (2n+1) COOH

 

They have strong smells b/c of the breakdown of fats.

  • More carbon atom s = stronger smells

 

Oxidation of ethanol produces ethanoic acid e.g. acetic acid.

Ethanol + Oxygen -> Ethanoic acid + Water

Acidity

 

Strong acids: hydrogen ionises completely when dissolved in water.

Weak acids: Carboxylic acids are weak b/c they aren’t very reactive (only the – OH part is reactive)

  • Not all the molecules ionise the hydrogen are released as ions into the solution.

 

The O in – OH has a negative charge whilst the H has a positive charge. This makes them both reactive.

 

Ethanoic acid + water -> Ethanoate (salt) + hydrogen

Esters – COO group 2E

Properties

  • Fruity smell b/c organic
  • Low molecular mass + B.P = high volatility (molecules evaporate quickly ) -> good for solvents
  • Highly flammable
  • Some can hydrolyse back to original reactants
  • Acetate is a colourless liquid at room temp.

 

Uses

  • Perfumes
  • Plasticisers
  • Solvents
  • Flood flavourings

 

Extracting natural esters is too expensive. Instead you can make synthetic “nature identical” esters.

 

Alcohol + Carboxylic acid [+sulphuric ester catalyst]-> Ester

Esters 2E

Ethanol + Ethanoic acid [+ Conc. Sulfuric acid (catalyst)] àEthyl Ethanoate

 

Making an ester

  • Refluxing
  • Ethanol + Ethanoic acid + conc. Sulfuric acid is in the flask
  • Heating speeds up the reaction, although gently so that the ethanol doesn’t evaporate/ catch on fire
  • The flask is fitted with a condenser
  • The condenser catches the ethanol vapour and recycles it back into the flask
  • This enables the chemicals to react

 

  • Distillation (separating ester from waste chemicals)

 

  • Reacted mixture is heated below a fractionating column (FC)
  • As mixture boils, vapour goes up FC
  • When at top of the FC reaches boiling point of ethyl ethanoate, it become a liquid
  • The liquid flows through condenser and is collected in a container
  • The liquid is impure ethyl ethanoate

 

  • Purification
  • The distillate ( liquid collected) is poured into a tap funnel

 

  • First part
  • The mixture is shaken with the aqueous reagent, sodium carbonate (Na2SO3),  to remove acidic impurities
  • The mixture separates into two layers b/c the ethyl ethanoate doesn’t mix with the water in the sodium carbonate solution
  • The top layer is ethyl ethanoate. The lower layer is sodium carbonate solution.
  • The lower layer can be tapped off

 

  • Second part
  • The remaining upper layer is shaken with conc. calcium chloride (CaCl2).
  • The CaCl2 removes any remaining ethanol
  • The lower layer is tapped off again
  • Only ethyl ethanoate and some water in the solution remains

 

  • Drying

 

  • Water still remains in the solution
  • Anhydrous granules of calcium chloride (a drying agent) can absorb the water.

 

  • Filtration

 

  • The granules/ lumps of calcium chloride can be filtered
  • Only pure ethyl ethanoate remains

 

Fats & Oils 2F

 

Fats + oils have more than one ester link.

Fats/oils + oxygen à lots of energy (stored) [can be used later].

 

Fatty acids are carboxylic acids with long hydrocarbons chains.

 

Fatty acids [carboxylic acids] + glycerol [alcohol] -> fats /oils

 

Un/saturated fats

 

Animal fats: saturated hydrocarbon chains which mainly contain single C – C bonds

  • The molecule has as much hydrogen as it can take.
  • The saturate molecules are straight and in a regular shape -> solid at room temp.
  • It tastes quite bitter

Vegetable oils: unsaturated hydrocarbon chains with lots of C = C double bonds.

  • The double bond means that there are carbon atoms that don’t form four bonds with other atoms and thus they are highly reactive à
  • The double bond means that the molecule isn’t straight and it in a more irregular shape. It is difficult to pack it into an ordered pattern à liquid at room temp.