1. Chemical Synthesis Module C6

2. Chemical synthesis: chemical reactions and processes used to get a desired product using starting materials called reagents. The products can be useful for a variety of purposes but tend to be either… fine chemicals a chemical product that is made in relatively small quantities and is typically high in cost, e.g. a flavouring or vitamin, drugs etc fine chemicals a chemical product that is made in relatively small quantities and is typically high in cost, e.g. a flavouring or vitamin, drugs etc bulk chemicals bulk chemicals A chemical product that is made in large amounts, very cheaply and often used to make other chemicals or to process other materials e.g. bleach, solvents, sulphuric acid etc

3. Useful chemicals include… Useful chemicals include… Food additives, fertilisers, dyestuffs, paints, pigments and pharmaceuticals;

4. Learn the formulae of… Gases: chlorine, hydrogen, nitrogen, oxygen Acids: hydrochloric acid, nitric acid, sulfuric acid, Alkalis: sodium hydroxide, magnesium hydroxide Salts: – –Simple salts… [MX] sodium chloride, magnesium oxide, potassium chloride [MX 2 ]calcium chloride, – –More complex salts… [MgCO 3 ] magnesium carbonate, [MgSO 4 ] magnesium sulfate, [Na 2 CO 3 ] sodium carbonate, [CaCO 3 ] calcium carbonate Task: For each salt show the ions that are present

5. More ions… If magnesium forms Mg 2+ ions and sulfate forms SO 4 2- ions then identify the charges on the other ions in the following compounds… If magnesium forms Mg 2+ ions and sulfate forms SO 4 2- ions then identify the charges on the other ions in the following compounds… –MgO –MgCl 2 –MgNO 3 –Na 2 SO 4 –Al 2 (SO 4 ) 3 –CaSO 4

6. Acidic substances… Dissolve in water to form H + ions giving a pH of less than 7 Dissolve in water to form H + ions giving a pH of less than 7 Can be… Can be… –solids e.g. citric acid, tartaric acid –Liquids e.g. sulfuric acid, nitric acid, ethanoic acid –Gases e.g. hydrogen chloride Form salts with many other substances such as alkalis, hydroxides, carbonates, oxides, metals Form salts with many other substances such as alkalis, hydroxides, carbonates, oxides, metals

7. Alkalis: Dissolve in water to form OH - ions giving a pH of GREATER than 7 Dissolve in water to form OH - ions giving a pH of GREATER than 7 Best examples are sodium hydroxide, potassium hydroxide, calcium hydroxide Best examples are sodium hydroxide, potassium hydroxide, calcium hydroxide Form salts with acids Form salts with acids Like acids they can attack skin and other materials Like acids they can attack skin and other materials Corrosive when concentrated and irritants when dilute

8. Salt formation… Metal + acid  salt + hydrogen Metal + acid  salt + hydrogen Metal carbonate + acid  salt + water + carbon dioxide Metal carbonate + acid  salt + water + carbon dioxide Metal hydroxide + acid  salt + water Metal hydroxide + acid  salt + water Metal oxide + acid  salt + water Metal oxide + acid  salt + water Alkali + acid  salt + water Alkali + acid  salt + water Task: Give an example of each type of reaction for salt formation

9. Neutralisation… When the number of H + ions is exactly matched by the number of OH - ions to form a pH of 7 When the number of H + ions is exactly matched by the number of OH - ions to form a pH of 7 H + + OH -  H 2 O H + + OH -  H 2 O An alkali can cancel out an acid to form a salt and the water (shown above) An alkali can cancel out an acid to form a salt and the water (shown above)

10. Stages in a chemical synthesis of a compound: choosing the reaction or series of reactions risk assessment (chemical and procedural) working out the quantities of reactants to use carrying out the reaction in suitable apparatus in the right conditions (such as temperature, concentration or the presence of a catalyst) separating the product from the reaction mixture purifying the product measuring the yield and checking the purity of the product

11. Useful techniques: Dissolving… forming solutions to allow easy mixing of reactants, Crystallisation… to purify a sample by the formation of pure crystals from a cooled (often saturated) solution, Filtration… to separate solid impurities from a solution OR to obtain the pure crystals, evaporation… to remove excess solvent from a solution, drying in an oven or dessicator… to remove water which might otherwise bump up the yield and cause an error! Titration… to find the concentration of an acid (or alkali) using an alkali (or acid) of a known concentration AND an indicator

12. Titration: Open the tap to let the acid run into the flask Open the tap to let the acid run into the flask Stop the tap at the first sign of a colour change Stop the tap at the first sign of a colour change calculate the volume delivered calculate the volume delivered Repeat but add drop by drop near the volume where the colour change occurred to get a greater accuracy Repeat but add drop by drop near the volume where the colour change occurred to get a greater accuracy Use the volumes of both solutions and the concentration of the acid to find the concentration of the alkali using a given formula Use the volumes of both solutions and the concentration of the acid to find the concentration of the alkali using a given formula (This could be a solid dissolved in water)

13. Purity and Yield: Purity can be checked by… –Melting point/ boiling point –Chromatography Yield can be calculated by… actual mass of pure sample X 100 = % yield theoretical mass expected

14. Calculating theoretical yields: Start with a balanced equation Start with a balanced equation Divide the mass of a reactant by its relative formula mass Divide the mass of a reactant by its relative formula mass Compare ratios of reactant to product Compare ratios of reactant to product Use relative formula mass of product to find theoretical mass (yield) Use relative formula mass of product to find theoretical mass (yield) Worked example… Worked example…

15. Mg(s) + 2HCl(aq)  MgCl 2 (aq) +H 2 (g) 2.4g of magnesium (Relative atomic mass A r =24) is used to make magnesium chloride (relative formula mass M r =95). How much of this salt should we expect? 2.4g of magnesium (Relative atomic mass A r =24) is used to make magnesium chloride (relative formula mass M r =95). How much of this salt should we expect? 2.4/24 = 0.1 2.4/24 = 0.1 Ratio is 1:1 for magnesium to magnesium chloride Ratio is 1:1 for magnesium to magnesium chloride 0.1 X 1 X 95 = 9.5g yield of magnesium chloride 0.1 X 1 X 95 = 9.5g yield of magnesium chloride

16. 2Mg(s) + O 2 (g)  2MgO(s) 16g of oxygen (Relative formula mass M r =32) is used to make magnesium oxide (relative formula mass M r =40). How much magnesium oxide should we expect? 16g of oxygen (Relative formula mass M r =32) is used to make magnesium oxide (relative formula mass M r =40). How much magnesium oxide should we expect? 16/32 = 0.5 16/32 = 0.5 Ratio is 1:2 for oxygen to magnesium oxide Ratio is 1:2 for oxygen to magnesium oxide 2 X 0.5 X 40 = 40g yield of magnesium oxide 2 X 0.5 X 40 = 40g yield of magnesium oxide

17. Rates of Reaction: The rate or speed of a reaction depends on… The rate or speed of a reaction depends on… –Concentration of the reactants –Particle size (surface area) –Temperature –If catalysts are present The rate is important because… The rate is important because… –If too fast = safety –If too slow = too little profit (economic reasons)

18. Measuring rates of reaction: By monitoring the products formed OR the reactants used up By monitoring the products formed OR the reactants used up The gradient gives the rate The gradient gives the rate We can monitor We can monitor –The volume of gas formed –Change in colour –Change in mass –The formation of a precipitate (solid from two solutions) Formation of a product fast (pink) Formation of a product slowly (yellow) Using up of a reactant (blue) Measured property e.g. volume of gas

19. Explaining Rates: It’s all to do with particles and their collisions It’s all to do with particles and their collisions If a collision has enough energy (the activation energy) then bonds are broken allowing a reaction to occur If a collision has enough energy (the activation energy) then bonds are broken allowing a reaction to occur More collisions mean more chance of a reaction More collisions mean more chance of a reaction Faster moving particles mean more chance of having enough kinetic energy to react Faster moving particles mean more chance of having enough kinetic energy to react We can increase the number of collisions by… We can increase the number of collisions by… –Increasing the concentration (more particles) –Increasing the temperature (faster moving particles) –Adding a catalyst

20. Increasing the concentration: What would you expect to happen to the rate if we… Doubled the concentration Doubled the concentration Increased it by 100 X Increased it by 100 X Decreased it by one third Decreased it by one third

21. Catalysts: Speed up the rate by bring reactants together or by making a simpler reaction path Speed up the rate by bring reactants together or by making a simpler reaction path Are called enzymes in biological systems Are called enzymes in biological systems Are often transition metals in inorganic reactions (non-biological) Are often transition metals in inorganic reactions (non-biological) Are not used up during the reaction… so can be used again Are not used up during the reaction… so can be used again