7. Patterns of Chemical Change

 12.1 How can we speed up or slow down chemical reactions? Being able to control the speed of chemical reactions is important both in everyday life (for example in cooking) and when making new materials on an industrial scale. The speed (rate) of a chemical reaction increases: - if the temperature increases; - if the concentration of dissolved reactants or the pressure of gases increases; - if solid reactants are in smaller pieces (greater surface area); - if a catalyst is used. A catalyst increases the rate of a chemical reaction but it is not used up during the reaction. It is used over and over again to speed up the conversion of reactants to products. Different reactions need different catalysts. Increasing the rates of chemical reactions is important in industry because it helps to reduce costs. The rate of a chemical reaction can be followed by measuring the rate at which the products are formed or the rate at which the reactants are used up. This allows a comparison to be made of the changing rate of a chemical reaction under different conditions. Candidates should be able to interpret graphs showing the amount of product formed (or reactant used up) with time in terms of the above principles. Chemical reactions can only occur when reacting particles collide with each other and with sufficient energy. The minimum amount of energy particles must have to react is the activation energy. Increasing the temperature increases the speed of the reacting particles so that they collide more frequently and more energetically. This increases the rate of reaction. Increasing the concentration of reactants in solutions and increasing the pressure of reacting gases also increases the frequency of collisions and so increases the rate of reaction. 12.2 How can we use living things to do our chemistry for us? Living things produce catalysts called enzymes which allow chemical reactions to occur quite quickly at ordinary temperatures and pressures. Enzymes are widely used in the food industry and are being used more and more to manufacture many other chemicals. Living cells use chemical reactions to produce new materials. Yeast cells convert sugar into carbon dioxide and alcohol. This process is called fermentation and is used: - to produce the alcohol in beer and wine; - to produce the bubbles of carbon dioxide which make bread dough rise. A simple laboratory test for carbon dioxide is that it turns limewater milky. Bacteria are used to produce yoghurt from milk. The bacteria convert the sugar in milk (lactose) to lactic acid. The chemical reactions brought about by living cells are quite fast in conditions that are warm rather than hot. This is because the cells use catalysts called enzymes. Enzymes are protein molecules which are usually damaged by temperatures above about 45 C. Different enzymes work best at different pH values. Enzymes are involved in the following processes: + in the home: - biological detergents may contain protein-digesting and fatdigesting enzymes (proteases and lipases); + in industry: - proteases are used to pre-digest the protein in some baby foods; - carbohydrases are used to convert starch syrup into sugar syrup; - isomerase is used to convert glucose syrup into fructose syrup, which is much sweeter and therefore can be used in smaller quantities in slimming foods. In industry, enzymes are used to bring about reactions at normal temperatures and pressures that would otherwise require expensive, energy demanding equipment. ! Candidates should be able, when provided with appropriate information, to evaluate the advantages and disadvantages of using microorganisms and enzymes to bring about chemical reactions. Higher Tier Successful industrial processes depending on enzymes usually: - stabilise the organism to keep it functioning for a long period; - immobilise the enzyme by trapping it in an inert solid support or carrier such as alginate beads; - allow a continuous process rather than a batch process. 12.3 Do chemical reactions always release energy? Chemical reactions, like anything else that happens, involve energy transfers. Many chemical reactions involve the release of energy. For other chemical reactions to occur energy must be supplied. [Even chemical reactions which release energy sometimes need to be supplied with energy to get them started.] Some chemical reactions are reversible. When fuels burn, energy is released as heat. Whenever chemical reactions occur, energy is usually transferred to or from the surroundings. An exothermic reaction is one which transfers energy, often as heat, to the surroundings. An endothermic reaction is one which takes in energy, often as heat, from the surroundings. If a reversible reaction is exothermic in one direction it is endothermic in the opposite direction. The same amount of energy is transferred in each case. For example: hydrated copper sulphate (blue) + heat energy --> anhydrous copper sulphate (white) + water The reverse reaction can be used as a test for water. In some chemical reactions, the products of the reaction can react to produce the original reactants. How such reactions are called reversible reactions and are represented. For example: ammonium chloride ammonia + hydrogen chloride (white solid) (colourless gases) Higher Tier During a chemical reaction: - energy must be supplied to break bonds; - energy is released when bonds are formed. In an exothermic reaction, the energy released from forming new bonds is greater than the energy needed to break existing bonds. In an endothermic reaction, the energy needed to break existing bonds is greater than the energy released from forming new bonds. Candidates should be able to: - interpret simple energy level diagrams in terms of bond breaking and bond formation (including the idea of activation energy and the effect on this of catalysts); - calculate the nett energy transfer in reactions, using simple energy level diagrams or supplied bond energies. 12.4 How do chemicals produce the fertiliser we need to grow food? Chemists use nitrogen from air to make nitrogen fertiliser. The processes they use to do this involve several types of chemical reaction and many important chemical ideas. Air is almost 80% nitrogen. The nitrogen can be used to manufacture several important chemicals, including nitrogen-based fertilisers. Nitrogen-based fertilisers are important in agriculture for increasing the yields of crops. Nitrates can, however, create problems if they find their way into streams, rivers or groundwater and so contaminate our drinking water. ! Candidates should be able, when provided with appropriate information, to reach balanced judgements concerning the benefits of using nitrate fertilisers and the contamination of drinking water they can cause. Ammonia is manufactured in the Haber process. The raw materials are nitrogen from the air and hydrogen obtained from natural gas. The purified gases are passed over a catalyst of iron at a high temperature (about 450 C) and a high pressure (about 200 atmospheres). Some of the hydrogen and nitrogen reacts to form ammonia. The reaction is reversible. This means that ammonia also breaks back down again into nitrogen and hydrogen: nitrogen + hydrogen ammonia The reaction conditions are chosen to produce a reasonable yield of ammonia quickly. On cooling the ammonia liquefies and is removed. The remaining hydrogen and nitrogen is re-cycled. Ammonia can be oxidised to produce nitric acid. Ammonia gas reacts with oxygen in air in the presence of a hot platinum catalyst. This oxidation reaction forms nitrogen monoxide which is then cooled and reacted with water and more oxygen to form nitric acid. Ammonium nitrate fertiliser is made by the neutralisation reaction between ammonia and nitric acid. Higher Tier Candidates should be able to outline and evaluate the economic factors associated with the conditions under which the Haber process is normally carried out. Candidates should be able to explain the details of these processes in terms of chemical principles from this specification including: - energy transfers during the reaction; - the rates of the reactions; - equilibrium conditions in reversible reactions. When a reversible reaction occurs in a closed system, an equilibrium is reached when the reactions occur at exactly the same rate in each direction. The relative amounts of all the reacting substances at equilibrium depend on the conditions of the reaction. If the forward reaction is endothermic, and the temperature is increased, the yield of products is increased; if the temperature is decreased, the yield of products is decreased. If the forward reaction is exothermic, and the temperature is increased, the yield of products is decreased; if the temperature is decreased, the yield of products is increased. In gaseous reactions, an increase in pressure will favour the reaction which produces the least number of molecules as shown by the symbol equation for that reaction. These factors, together with reaction rates, are important when determining the optimum conditions in industrial processes, including the Haber process. 12.5 How do we know how much of each reactant to use in a chemical reaction? If we know the formula of a chemical compound and the relative masses of all the atoms involved we can calculate the formula mass of the compound and the percentage of each element in the compound. [If we also know the symbol equation for a chemical reaction we can calculate how much of each reactant we need to produce a certain amount of product. From the masses of reactants and products we can also calculate the empirical formulae of chemical compounds.] Atoms of different elements have different masses. To be able to work out exactly what is happening in chemical reactions we need to know how the masses of atoms compare with each other, i.e. their relative atomic masses (Ar). Candidates should be able to: - calculate the relative formula mass (Mr) of compounds whose formulae are supplied; - calculate the percentage of an element in a compound whose formula is supplied. [See Data Sheet for Ar of elements.] Higher Tier Candidates should be able to use supplied balanced symbol equations and supplied data about the masses/volumes of some reactants/products: - to calculate the masses/volumes of other reactants/products; [Candidates may use moles in their calculations but are not required to do so. The volume of the Mr in grams, of a gas, will be given.] - to determine the ratios of atoms in compounds from supplied masses or percentage composition (empirical formulae). Candidates should be able to use given half equations for reactions occurring at the electrodes and given data about the mass/volume of one of the products to calculate the mass/volume of the other product.