21. Aqueous & Organic Chemistry

14.1 What happens to water on the Earths surface? The water on the Earths surface is continually being re-cycled. As it falls, rain water contains only dissolved gases but once it reaches the ground water becomes contaminated in various ways. Water is the most abundant substance on the surface of our planet and is essential for all life. Water in rivers, lakes and the oceans is evaporated by the heat of the Sun. The water vapour formed rises into the atmosphere and forms clouds. The clouds cool as they rise further and produce rain. This is known as the water cycle. Water is an important raw material and has many uses. It is used as a solvent and as a coolant. It is used in many industrial processes including the manufacture of sulphuric acid. Drinking water is treated by passing water through filter beds to remove solid particles and then chlorine is added to kill bacteria. The use of artificial fertilisers results in many natural waters being contaminated with dissolved nitrate and ammonium ions. Dissolved nitrate ions can have harmful effects on babies and so the levels of nitrate are carefully monitored Many compounds dissolve in water. Some of these make the water hard. Most hard water contains dissolved calcium or magnesium compounds. The hard water is formed when natural waters flow over ground or rocks containing calcium or magnesium compounds. Soft water readily forms a lather with soap. Hard water does not easily form lather with soap. Hard water contains dissolved compounds that react with soap to form scum. Using hard water can increase costs because more soap is needed and hard water often leads to deposits (scale) forming in heating systems and kettles. Hard water can be made soft by removing the dissolved calcium and magnesium ions. The addition of sodium carbonate solution precipitates out calcium carbonate or magnesium carbonate. Ion exchange columns contain hydrogen ions or sodium ions which replace calcium and magnesium ions when hard water passes down the column. Hard water contains dissolved compounds that are good for health. Hard water often provides calcium compounds that help the development of strong bones and teeth and help to reduce heart illnesses. 14.2 How well do different gases and solids dissolve in water? Some gases and solid substances are more soluble in water than others and some are hardly soluble at all. The solubility of gases and solids in water also depends on the temperature of the water. Many gases are soluble in water. The solubility increases as the temperature decreases and as the pressure increases. Carbonated water is produced by dissolving carbon dioxide under high pressure. When the pressure is released the gas bubbles out of the solution. Carbonated water is used in fizzy drinks. Dissolved oxygen is essential for aquatic life. Hot water from power stations may be discharged into rivers or lakes. This discharge reduces the amount of oxygen dissolved in the water and this can damage aquatic life. Chlorine water is made by dissolving chlorine gas in water. Chlorine water is used to bleach materials and kill bacteria. Most ionic compounds are soluble in water. Most covalent compounds are insoluble in water. The solubility of a solute in water, or any other solvent, is usually given in grams of solute per 100 grams of water (or solvent) at that temperature. The solubility of most solutes increases as the temperature increases. A saturated solution is one in which no more solute will dissolve at that temperature. When a hot saturated solution cools some of the solute will separate from the solution. Candidates should be able to: - interpret solubility curves; - use solubility curves to explain crystallisation. 14.3 Why do some substances produce acidic or alkaline solutions? Some soluble substances produce acidic or alkaline solutions. This happens because these substances react in a special way with the water. Some compounds react with water to produce acidic or alkaline solutions. Water must normally be present for a substance to act as an acid or as a base. Acids, in aqueous solution, produce H+ ions. The H+ ion is a proton. In water this proton is hydrated and is represented as H+ (aq). Alkalis, in aqueous solution, produce OH ions. An acid can be defined as a proton donor. A base can be defined as a proton acceptor. ! Candidates should be able to: - describe the contributions of Arrhenius, Lowry and Brønsted to our understanding of acid-base behaviour; - explain why the work of Arrhenius took much longer to be accepted than the work of Lowry and Brønsted. Acids and alkalis are classified by the extent of their ionisation in water. A strong acid or alkali is one that is 100% ionised in water. Examples of strong acids are hydrochloric, sulphuric and nitric acids. Examples of strong alkalis are sodium and potassium hydroxide. A weak acid or alkali is only partially ionised in water. Examples of weak acids are ethanoic, citric and carbonic acids. An example of a weak alkali is ammonia solution. Candidates should be able to describe how to distinguish between strong and weak acids of the same concentration by using the pH scale or the rate of reaction with metals. 14.4 What different ways are there of making salts? Neutralising an acidic solution with an alkaline solution is one way of making salts. But salts can also be made using several other methods. There are several general methods of producing salts: - reaction of a metal with an acid; - reaction of an insoluble base with an acid; - reaction of a soluble base with an acid; - by mixing two solutions to form an insoluble salt (precipitation) - by direct combination of the elements to form anhydrous salts; limited to the chlorides of aluminium and iron. Candidates should be able to give practical details of salt preparations based on each of these methods. Higher Tier The volume of acid and alkali solutions which neutralise each other can be measured by titration using a suitable indicator. Candidates should be able to describe how a titration is carried out. 14.5 How can we work out the concentration of solutions? It is very useful to be know exactly how much of a dissolved substance is present in a certain volume of a solution. So we need a standard way of comparing the concentrations of solutions. Higher Tier The relative atomic mass of an element or the relative formula mass of a compound in grams is called one mole of that substance. Candidates should be able to: - calculate the number of moles of a substance using given formula and relative atomic or formula masses; - calculate the mass of a substance in a given mole quantity of that substance. The concentration of an aqueous solution is usually expressed in terms of mole per cubic decimetre (mol dm-3). Candidates should be able to: - calculate the number of moles or mass of substance in an aqueous solution of given volume and concentration; - calculate the concentration of an aqueous solution given the amount of substance and volume of water. Titrations can be used to find the concentration of an acid or alkali from the relative volumes used and the concentration of one of the two reactants. Candidates should be able to carry out calculations involving neutralisation reactions in aqueous solution (the balanced equation will be given). 14.6 What is produced when organic compounds are burned? Some organic compounds are used as fuels. Other organic compounds, including plastics, are burned as waste. Burning these organic compounds releases gases into the atmosphere. Coal, crude oil, natural gas and wood contain organic compounds. Organic compounds all contain carbon. When organic compounds are burned in a plentiful supply of air the carbon is oxidised to carbon dioxide and the hydrogen is oxidised to water. In a limited supply of air incomplete combustion occurs forming carbon monoxide and/or carbon. Carbon monoxide is poisonous because it reduces the capacity of blood to carry oxygen. ! Candidates should be able to use given data to compare the cost, efficiency and cleanliness of burning different fossil fuels. Higher Tier Plastics (and other organic compounds) which contain chlorine and nitrogen produce poisonous fumes when burnt (hydrogen chloride and hydrogen cyanide respectively) especially where there is a limited supply of air. The combustion products of carbon and hydrogen are also formed. 14.7 Why are there families of organic compounds? Organic compounds belong to different families. The compound in each family have a chemical structure and a similar chemical formula. Each family of organic compounds forms what is called a homologous series. Higher Tier An homologous series is a family of compounds which have a general formula and have similar chemical properties. The saturated hydrocarbons form an homologous series called alkanes with a general formula CnH2n+2. The unsaturated hydrocarbons form an homologous series called alkenes with a general formula CnH2n. Candidates should be able to represent and interpret alkane and alkene molecule diagrams. Isomerism occurs when two or more compounds have the same chemical formula but have different structures. Candidates should be able to draw the isomers of butane and pentane. Candidates should be able to show an understanding of isomerism by predicting some of the structures of the isomers of given higher alkanes. Isomers have different physical properties which depend upon the strength of the intermolecular forces. For isomers the strength of intermolecular forces increases as the carbon chain length increases and decreases as the amount of chain branching increases. Candidates should be able to predict and explain the differences in the boiling points of isomers of alkanes in terms of intermolecular forces arising from the ability of the molecules to pack closely. Alkanes and alkenes undergo combustion reactions. The alkenes are more reactive than alkanes because of the presence of the carbon carbon double bond. The alkenes undergo addition reactions in which one of the bonds in the carbon carbon double bonds breaks allowing each carbon atom to form a covalent bond with another atom. Examples of addition reactions are: - with hydrogen in the presence of a catalyst to form an alkane; - with bromine when it decolourises bromine water. Vegetable oils contain unsaturated fats and can be hardened to form margarine by adding hydrogen on to some of the carbon carbon double bonds. 14.8 What is ethanol and how can we make it? What we call alcohol in everyday life is a substance whose chemical name is ethanol. Ethanol is just one member of a family of substances called alcohols. Ethanol is used as a solvent, as a fuel and is present in alcoholic drinks. Ethanol can be produced by fermentation of sugars. The raw materials are mixed with water and yeast at just above room temperature. The yeast contains enzymes which are biological catalysts. The sugars react to form ethanol and carbon dioxide. The carbon dioxide is allowed to escape and air is prevented from entering the reaction vessel. When the reaction is over the ethanol is separated from the reaction mixture by fractional distillation. Higher Tier The alcohols form an homologous series with the functional group OH. It is the presence of this functional group that gives alcohols their characteristic properties. Candidates should be able to draw the structures of methanol and ethanol. Alcohols react, reversibly, with carboxylic acids to form esters and water. Ethyl ethanoate is formed by the reaction of ethanoic acid with ethanol. Alcohols react with sodium to form hydrogen. Ethanol can be produced by the reaction of steam and ethene in the presence of a strong acid catalyst (phosphoric acid). The reaction is carried out at a moderately high temperature and a high pressure. Candidates should be able to evaluate the two different methods of producing ethanol in terms of: - rate of reaction; - quality of product; - the use of finite resources; - a batch process versus a continuous process. Ethanol can be oxidised to form ethanoic acid. It is this oxidation of ethanol that results in alcoholic drinks turning sour. The steroid, cholesterol, contains the alcohol group OH. Cholesterol is an essential steroid to humans but if too much is produced it can cause heart disease. 14.9 What other families of organic compounds are there? The acids that we find in fruits and in vinegar belong to a homologous series called carboxylic acids. Polymers do not form a homologous series but they are all organic compounds having very long molecules. Higher Tier Carboxylic acids form an homologous series and have the functional group COOH. Candidates should be able to draw the structures of methanoic, ethanoic and propanoic acid. Vinegar contains ethanoic acid. Ethanoic acid is used in the manufacture of the fibre, acetate rayon. Oranges, lemons and many soft drinks contain a carboxylic acid, citric acid. Aspirin is a carboxylic acid. Aspirin is a drug used for pain relief and is taken regularly by those at risk from heart attacks. Ascorbic acid (vitamin C) is another carboxylic acid and is present in fresh fruit and vegetables. Carboxylic acids are weak acids. They are neutralised by alkalis and they react with carbonates and hydrogencarbonates to produce carboxylic acid salts, carbon dioxide and water. Carboxylic acids react with alcohols in the presence of concentrated sulphuric acid to form esters. Concentrated sulphuric acid acts as a catalyst in this reaction. Esters are widely used as fragrances and food flavourings. Polymers Most polymers (plastics) are made from compounds containing the C=C bond by addition polymerisation. Poly(chloroethene) is made from chloroethene, CH2=CHCl but the polymer is generally called polyvinylchloride, PVC. Polymers (plastics) consist of a tangled mass of very long molecules in which the atoms are joined by strong covalent bonds to form long chains. In a thermosoftening plastic the forces between the chains are weak so the plastic softens when heated and hardens again when cooled. When a thermosetting plastic is first heated covalent bonds are formed between adjacent chains. These strong cross-linkages prevent thermosetting plastics from being softened and therefore from being re-moulded. Poly(ethene), poly(propene) and PVC are examples of thermosoftening polymers. Melamine (used in furniture) and many glues are examples of thermosetting polymers.