What is a homologous series?
A homologous series is a group of similar compunds.
They have the same general formula - all alkanes, for example, have the formula CnH2n+2 which dictates the number of carbon and hydrogen atoms in the compound.
They also have the same functional group, meaning that all atoms in the same functional group are bonded in a very similar arrangement. Alcohols, for example, all have the functional group R-O-H, meaning the carbon chain will always have an oxygen and hydrogen molecule on the end of its structure.
Compounds in the same homologous series have a gradual variation in physical properties.
Lastly, all compounds in the same homologous series have similar chemical properties, and will react in similar ways when part of a reaction.
Alkanes and Alkenes
Alkanes and alkenes are both individual homologous series. Alkanes and alkenes contain carbon and hydrogen atoms only.
C1 Knowledge: Alkanes do not contain any carbon-carbon double bonds, so do not decolourise bromine water.
Alkanes all have the same general formula CnH2n+2. For example, methane has 1 carbon atoms so it will have 4 hydrogen atoms, and becomes CH4.
C1 Knowledge: Alkenes contain one or more carbon-carbon double bonds, so can decolourise bromine water.
The general formula of alkenes is CnH2n; there are twice as many hydrogen atoms than carbon atoms.
The alcohols are another homologous series.
The functional group of an alcohol is R-O-H, 'R' signifying the carbon chain.
The general formula of an alchohol is CnH2n+1OH. The naming system (meth-, eth-) remains the same as it was for the alkane and alkene series, but the final '-e' is replaced with an '-o'.
Methanol - CH3OH Ethanol - C2H5OH Propanal - C3H7OH
Carboxylic acids have the same general formaula CnH2n+1COOH.
The carboxylic acids all end with the ending '-anoic acid'
Esters are formed when alchohol reacts with a carboxylic acid. They have the functional group -COO-.
Esters have distinct fruity smells, so are used in flavourings and perfumes in food and cosmetics. Polyesters can makes plastic products, and fibres for clothing such as fleece.
Producing ethanol by fermentation
Ethanol can be made by fermentation. Fermentation is the process of using yeast to convert carbohydrates (sugars) into alcohol: Glucose = Ethanol + Carbon Dioxide. The carbohydate can come from any source, but sugar cane and sugar beet are often used.
1) Firstly, the sugar is dissolved in water to form a sugar solution.
2) Yeast is added to the sugar solution. Yeast is a biological catalyst and helps the reaction to speed up.
3) The vessel is sealed. This ensures anaerobic respiration takes place. Otherwise, oxygen would turn the ethanol into ethanoic acid (vinegar) which tastes nasty.The solution is then heated to 37 degrees, the optimum temperature for any biological catalyst.
4) When the concentration of alcohol reaches 10-20%, the reaction stops as the yeast is killed off by the alcohol.
5) The fermented mixture is distilled to produce more concentrated alcohol using a fractionating column.
Producing ethanol by reacting ethene with steam
Ethene can be reacted with steam to produce ethanol. This is how ethanol is produced industrially.
The raw material ethene is obtained from cracking crude oil; it is the shorter hydrocarbon chain produced.
Ethene reacts with steam to produce ethanol:
Ethene + Water (Steam) = Ethanol
This is a fairly cheap process; ethene is cheap and not much is wasted. It is made is a large processing plant so can be made quickly and continuously, a major advantage over biofuels used in fermentation. Ethene is, however, obtained from fossil fuels, a non-renewable resource, which is the major disadvantage of this process.
Negative social impacts of alcohol
The GCSE chemistry course requires you to recognise the negative affects of alcohol on the human body and health, as well as society.
Negative social impacts of alchohol
- Getting drunk causes lowered inhibitions and impaired judgement. This leads to an increase in violent behaviour and fights, increased levels of crime and a higher risk of physical and sexual assult. Irresponsible sexual behaviour also leads to a greater risk of sexually transmitted diseases, as people cannot make sensible desicions about protection.
- People can become addicted to alcohol. It can lead to family breakdowns, and alcoholics losing their jobs and even homes. Some alcoholics may act criminally to fuel their addiction.
- Drink impaires vision and judgement, causing death and injury from drunk driving
- Costs government money to police and treat alcohol-related incidents
Negative impacts of alcohol on health
Alcohol lowers inhibitons so can help people to relax and socialise. There are, however, many negative affects of alcohol on the body.
Negative impacts of alcohol on health and the body
- Alcoholis a depressant so reduces the activity of the nervous system. Getting drunk leads to increased reaction times, impaired judgement, poor bablance and coordination and unconsciousness.
- Excess alcohol can also cause dehydration, which leads to brain cell damage. This leads to a temporary drop in brain function and even long term memory loss.
- Alcohol is a toxin, so excessive drinking causes severe scarring of the liver, leading to liver scarring and liver cirrhosis. This is becoming more common for young binge-drinkers in their 20s.
Dehydration of ethanol
Ethanol can be dehyrdated to form ethene.
Companies who make plastics and polymers require a lot of ethene.
Ethanol vapour is passed over a hot aluminium oxide catalyst:
Ethanol = Ethene + Water
This is the reverse of the reaction between ethene and steam to make ethanol.
Ethanoic acid is found in vinegar.
If wine is left open, the ethanol is oxidised to form ethanoic acid. This is because the oxidation of ethanol produces ethanoic acid:
ethanol + oxygen = vinegar + water
This reaction is used for the commericial production of vinegar.
Vinegar is used for
- Flavouring food
- Preserving food
Reactions with carboxylic acids
The salts formed from ethanoic acid are all ethanoates:
Reaction with a metal
ethanoic acid + magnesium = hydrogen + magnesium ethanoate
Reaction with a base
ethanoic acid + sodium hydroxide = sodium ethanoate + water
Reaction with a carbonate
ethanoic acid + sodium carbonate = sodium ethanoate + carbon dioxide + water
- Ethanoic acid is a typical acid and this will show on indicators:
- Universal indicator will turn orange / red
- Blue litmus paper will turn red
Uses of esters
Soaps are made from an ester reacting with an alkali.
Fats and oils are a type of ester. They are boiled up in a concentrated alkali solution to make soap.
The oils and fats break down into glycerol and long-chain carboxylic acids, and the carboxylic acids go on to react with an alkali. An acid + alkali = salt + water.
Some soaps are just sodium or potassium salts of long-chain carboxylic acids.
Soap anions have a hydrophobic (water-hating) and hydrophilic (water-loving) end. The hydrophobic tail dislikes water so sticks into the oil or fat droplet, whilst the hydrophilic head dissolves in water. Soap anions let the oil and the water mix, helping lift oily stains out of the fabric.
Hydrogenating vegetable oils
Unsaturated oils, which have at least one carbon-carbon double bond, are less viscous (runny) than saturated oils.
Liquid unsaturated molecules can be changed to solid staurated oils by breaking the double bonds and adding hydrogen.
A nickel catalyst is used to speed up the reaction. This is known as catalytic hydrogenation.
The nickel catalyst is solid and can be filtered out to use again.
As the filtered oil cools down to room temperature, it turns into a solid fat.
Polyunsaturated vegetable oils are hydrogenated to make margarine. Not all of the double bonds are hydrogenated so some of the margarine remains unsaturated. This is, however, still lower in saturates than butter, which is healthier.