9c Flashcards
What are alcohols?
The alcohols are a group of compounds that make up a homologous series. They all contain an -OH functional group (an oxygen atom covalently bonded to a hydrogen atom).
The general formula of an alcohol is CnH2n+1OH. So, an alcohol with two carbons has the formula C2H5OH. The basic naming system for alcohols is the same as for alkanes - but you replace the final ‘-e’ with ‘-ol’. So, instead of methane you have methanol.
Structures and formulae of the first four alcohols in the homologous series
Using alcohols as fuel
When alcohols burn (combust) they release energy. This energy could power cars, heat homes or generate electricity, so alcohols can be used as fuels in place of hydrocarbons.
Investigating alcohols as fuels
The amount of energy given out when an alcohol combusts determines how good a fuel it is. The more energy released the better the fuel it is. You can investigate the energy released by the combustion of alcohols by using them to heat a known mass of water and measuring the temperature rise.
Investigating alcohols as fuels - method
- Put some alcohol into a spirit burner and measure the mass of the burner and fuel using a mass balance.
- Measure 100 cm³ distilled water into a copper calorimeter.
- Insulate the calorimeter by using a draught excluder, then cover with an insulating lid after placing a thermometer inside. This helps to make sure that minimal energy is lost to the surroundings.
- Take the initial temperature of the water then put the burner under the calorimeter and light the wick.
- Stir the water throughout using the thermometer. When the heat from the burner has made the temperature of the water rise by 20 °C, blow out the spirit burner.
- Immediately reweigh the spirit burner and subtract the burner and fuel’s original mass- this gives you the mass of fuel that has been used.
- Repeat the experiment using other alcohols.
Investigating alcohols as fuels - Variables to keep the same.
Each time you repeat the experiment you need to keep all the variables the same except the type of alcohol. Variables you need to keep constant include:
* mass/volume of water,
* height of the container above the wick,
* length of the wick/height of the flame,
* number of moles of alcohol,
* equipment used e.g. thermometer and calorimeter.
Investigating alcohols as fuels - How to use your results
You can use your results to compare the masses of alcohol needed to increase the temperature of the water and hence their usefulness as fuels.
The lower the mass of fuel needed to raise the temperature of the water by a given amount, the better the alcohol is as a fuel, as it’s released more energy per gram.
These results show that pentanol is the best fuel. Fewer grams of pentanol were required to raise the temperature of the water by 20 °C. They also show that the longer the carbon chain of the alcohol, the more energy there is per gram of fuel.
Fermentation
Fermentation is the process of using yeast to produce alcohols from carbohydrates. During fermentation, sugars, such as glucose, are converted to ethanol and carbon dioxide.
The carbohydrates can come from any source, but the sugar cane and sugar beet plants are often used. The yeast cells contain an enzyme. Enzymes are naturally occurring catalysts they speed up reactions.
Producing ethanol
Here’s how you would make a solution of ethanol by fermentation:
1. Mix yeast and a solution of a carbohydrate (e.g. glucose) in a clean container. Seal the container and leave it in a warm place.
2. Keep the mixture between 30 °C and 40 °C - fermentation happens fastest between these temperatures. At lower temperatures, the reaction slows down. If it’s too hot the enzyme in the yeast denatures (is destroyed) and the reaction would stop.
3. It’s important to keep the mixture in anaerobic conditions - this means no oxygen. Oxygen would convert the ethanol to ethanoic acid (which is what you get in vinegar).
4. When the concentration of alcohol reaches about 10 to 20%, the fermentation reaction stops, because the yeast gets killed off by the alcohol.
5. The yeast will fall to the bottom of the container and you can collect the ethanol solution from the top.
Products of fermentation
Fermentation produces a dilute concentration of ethanol. The fermented mixture can then be distilled (see next page) to produce more concentrated alcohol. Different types of alcoholic drink contain different percentages of ethanol. The typical ethanol concentration of beer is about 4% whilst some spirits (e.g whisky or vodka) have a concentration of 40%.
Fractional distillation of ethanol
To make a concentration of ethanol above 20%, ethanol must be concentrated by fractional distillation of the fermentation mixture. The equipment for fractional distillation can be set up as shown.
Fractional distillation separates mixtures by heating them. Ethanol has a lower boiling point than water. This means that when the fermentation mixture is heated, ethanol evaporates and the vapour rises up the fractionating column, while the water stays as a liquid. A Liebig condenser is used to condense the ethanol vapour by cooling it. The concentrated ethanol can then be collected in a separate flask
What are carboxylic acids?
The carboxylic acids are a group of compounds that all contain a -COOH functional group. The -COOH group consists of a carbon atom that has formed a double bond with a lone oxygen atom and a single bond with an oxygen atom in an -OH group
Examples of carboxylic acids
The names of carboxylic acids end in ‘-anoic acid’ (and start with the normal ‘meth-‘, ‘eth-‘, ‘prop-‘ and ‘but-‘). You need to know the formulae and structure of the first four carboxylic acids in the homologous series. These are methanoic acid, ethanoic acid, propanoic acid and butanoic acid
Chemical formulae of carboxylic acids
When you’re giving the chemical formula of a carboxylic acid, it’s crucial that you show the -COOH group clearly.
To identify whether a compound is a carboxylic acid or not, just look out for a -COOH group or ‘anoic acid’ at the end of its name.
Formation of carboxylic acids
Carboxylic acids are made by oxidising alcohols using an oxidising agent. When a substance is oxidised, it gains oxygen.
