Prestwick Academy Chemistry department

National 4/5 Unit 2 – Everyday consumer products Summary notesCarbohydrates (National 4)

Plants are a source of carbohydrates and oils which can be used for food or fuel.

Carbohydrates are compounds that contain carbon, hydrogen and oxygen, where the ratio of hydrogen to oxygen is always 2:1. Glucose (C6H12O6) and Sucrose (C12H22O11) are both carbohydrates whereas the compound C12H22O10 is not.

Plants make carbohydrates (glucose) by the process of photosynthesis.

During photosynthesis carbon dioxide and water are combined to produce glucose and oxygen.

carbon dioxide + water glucose + oxygen

6CO2 + 6H2O C6H12O6 + 6O2

The green pigment chlorophyll is needed to help the plants absorb the light energy from the sun. The glucose produced from photosynthesis is stored by the plant as a carbohydrate called starch.

The energy stored in the starch can be released by the process of respiration.

Glucose + Oxygen Carbon dioxide + water + energy

C6H12O6 + 6O2 6CO2 + 6H2O

Animals and plants obtain a supply of energy by breaking down carbohydrates (using oxygen). The energy released is used to keep the animal warm, to maintain body organs (e.g. heart beating) and for movement etc.

Analysis of carbohydrates (National 4)

Name: _________________________

Carbon dioxide

Water

LightGlucose

Oxygen

Chlorophyll

There are many 1000s of carbohydrates just like there are many 1000s of hydrocarbons. The most common examples are fructose, glucose, maltose, sucrose and starch. As chemists, we want to be able to tell the difference between these different carbohydrates

Iodine test

Iodine solution can be used to test for STARCH. Only starch turns Iodine solution a blue/black colour (from brown).

Solubility and light beam experiments

Sugars dissolve in water whereas starch does not dissolve well.

When a beam of light is passed through a solution of glucose (or other sugar), it is NOT reflected: we do not see the beam. With a starch “solution”, we DO see the beam. This is because starch molecules are very large and cause the light to be reflected whereas the other sugar molecules are so small that they don’t reflect the light. (The diagram shows how the starch does not form a true solution as it is so large and so will not dissolve.)

Benedict’s test

This test can be used to tell glucose from sucrose. Sugars which give a positive test with Benedict’s (i.e. a red colour is produced) are known as REDUCING SUGARS.

i.e. Sucrose and starch do not react with benedict’s.

Positive Benedict’s No reactionGlucoseMaltoseFructose

StarchSucrose

Burning carbohydrates (National 4)

The energy trapped in the carbohydrate is released slowly during respiration.

If, however, the carbohydrate undergoes combustion (burned), there is a rapid release of energy. The experiment shown opposite is used to show that, when carbohydrates burn, the products are carbon dioxide and water.

C6H12O6 + 6O2 6CO2 + 6H2O

Competing demands for carbohydrates (National 4)

As carbohydrates release energy when burned, scientists have investigated their potential use as fuels. This is a controversial move as many people think that it is wrong to use carbohydrates as a fuel when a large percentage of the world population are starving and live in food poverty.

Producing alcohol (National 4)

Starch and sugars are a source of food for animals, but they’re also a starting chemical for making alcoholic drinks. Almost any fruit or vegetable (these contain sugars and starch) can be used to make an alcoholic drink e.g. wine comes from glucose in grapes.

A natural reaction called FERMENTATION changes sugar solutions into alcohol. The alcohol produced is called ethanol. The other product is carbon dioxide.

For FERMENTATION to occur, a catalyst is needed. Yeast can be used as it contains an enzyme called ZYMASE which catalyses the reaction.

Enzyme in yeast GLUCOSE ETHANOL + CO2

Yeast is a living organism. It is a fungus which feeds on sugars. The fungus produces a catalyst which breaks down sugar into ethanol and CO2. As yeast is living, the catalyst is called an enzyme-a biological catalyst.

Increasing the concentration of ethanol (National 4)

The amount of ethanol produced during fermentation is fairly low. Yeast cells can be poisoned by ethanol. This means that fermentation stops after the concentration of ethanol becomes too high for the yeast cells to survive. Usually, fermentation cannot produce an ethanol concentration of more than 12%. To produce alcoholic drinks of higher concentration, distillation has to be used.

As ethanol boils at a lower temperature than water, heating a fermentation mixture causes most of the ethanol to evaporate (along with some flavours). If this vapour is cooled by a condenser, the liquid ethanol can be collected. This gives a very high concentration of ethanol. This is diluted with water before sale.

Alcohols/alkanols (National 5)

Alcohols are a homologous series of compounds which have the hydroxyl functional group (- OH). All alcohols have a name ending in –ol. Ethanol is the alcohol produced from the fermentation of sugar. Alcohols are often called alkanols.

When naming alcohols, the compound is named to give the OH group the lowest number e.g. butan-2-ol

Name: Full structural formula: Short structural formula:Methanol CH3OH

Ethanol CH3CH2OH

Propan-1-ol CH3CH2CH2OH

Propan-2-ol CH3CH(OH)CH3

Butan-1-ol CH3CH2 CH2CH2OH

Uses of alcohols

Fuels:

Alcohols are widely used as fuels. When an alcohol burns completely it releases carbon dioxide and water. This reaction is highly exothermic.

C2H5OH (l) + 3O2 (g) 2CO2 (g) + 3H2O (l)

Ethanol can be used as a fuel in its own right, or in mixtures with petrol (gasoline). "Gasohol" is a petrol / ethanol mixture containing about 10 - 20% ethanol.

Ethanol can be produced by fermentation of sugar cane. This is a useful way for countries without an oil industry to reduce imports of petrol (octane).

Made this way the ethanol is a renewable resource (it won’t run out). As green plants absorb carbon dioxide when they grow, the burning of ethanol is classified carbon neutral and does not contribute to the green-house effect.

Solvents:

Alcohols are widely used as solvents. They have a neutral pH and are relatively safe. They are often used as non-aqueous solvents to dissolve many compounds which are insoluble in water.

They are regularly used to dissolve compounds to make hand sanitizer, perfumes and cosmetics. As a result of their low boiling point, when applied to a hot surface (skin) the alcohol easily evaporates and is therefore a suitable and safe solvent.

Carboxylic acids/alkanoic acids (National 5)

Carboxylic acids are a homologous series of compounds which have the carboxyl functional group (- COOH) at the end of the chain. The oic in the name means that the compound has this COOH functional group. Carboxylic acids are often called alkanoic acids.

Name: Full structural formula: Short structural formula:Methanoic acid HCOOH

Ethanoic acid CH3COOH

Propanoic acid CH3CH2COOH

Butanoic acid CH3CH2CH2COOH

Properties of carboxylic acids

As the name suggests the carboxylic acids have a pH less than 7. As the carboxyl group is easy to ionise, the addition of carboxylic acids to water forms hydrogen (H+) and carboxylate ions.

This can be represented by the following equation:

Methanoic acid Methanoate ion + hydrogen ion

CHOOH (aq) CHOO- (aq) + H+ (aq)

Reactions of carboxylic acids

As acids carboxylic acids are acidic they can be readily neutralised by bases forming salts and water.

Carboxylic acid + alkali salt + waterMethanoic acid + sodium hydroxide sodium methanoate + waterCHOH (aq) + NaOH (aq) NaCHOO (aq) + H2O (l)Carboxylic acid + Metal carbonate salt + water + carbon dioxideEthanoic acid + Magnesium carbonate magnesium ethanoate + water + carbon dioxide

2CH3COOH (aq) + MgCO3 (s) Mg(CH3COO)2 (aq) + H2O (l) + CO2 (g)They are also able to form salts when reacted with reactive metals.

Carboxylic acid + reactive metal salt + hydrogen

Propanoic acid + calcium calcium propanoate + hydrogen

2C2H5COOH + Ca Ca(C2H5COOCOO)2 + H2 (g)

The salts formed during both types of reactions are known as carboxylate salts and can be named according to the parent carboxylic acid.

Parent carboxylic acid

Name ending of salt

Methanoic acid - methanoateEthanoic acid - ethanoatePropanoic acid - propanoateButanoic acid - butanoate

Uses of carboxylic acids

Vinegar is a solution of ethanoic acid. Vinegar is used in household cleaning products designed to remove limescale (a build up of insoluble carbonates on plumbing fixtures) and as a preservative in the food industry.

Esters (National 5)

Esters are a homologous series of compounds which can be identified from the ester functional group (COO) and the –oate ending.

Esters are formed when carboxylic acids combine with alcohols. This is known as a condensation reaction. In a condensation reaction a larger molecule is always formed from smaller molecules, water is also produced.

An ester takes its name from the alcohol and carboxylic acid from which it is made. E.g.

ethanol + methanoic acid ethyl methanoate

butan-1-ol + propanoic acid butyl propanoate

Uses of esters

Flavourings/cosmetics

Esters are compounds that are famous for having a strong smell. As a result they are often used to flavour food and in perfumes.

Examples of esters responsible for fruit smells include:

3-methyl-1-butyl ethanoate = banana methyl butanoate = apple propyl ethanoate = pear

Solvents

Esters are widely used as solvents. They can be used to dissolve many compounds which are insoluble in water.

Textile manufacture

Many of the clothes you are wearing will be made of synthetic fabrics. These compounds are known as polyesters as they contain many ester functional groups.

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