Starter Complete the starter for ten

Learning outcomes Describe the atom economy of a chemical reaction State how an equation is used to calculate an atom economy Describe the percentage yield of a chemical reaction. Calculate percentage yields. Specification reference

Atom economy Expressed as a percentage Once you know a balanced equation, you can calculate the theoretical amount of product you should be able to make Most reactions make more that one product, often only one is useful This is not ideal if you are using limited resources

Atom economy Found directly from the balanced equation It is a theoretical quantity rather than a practical Atom economy is defined as: % atom economy = mass of desired product x 100 total mass of reactants

Worked example Chlorine, Cl 2, reacts with sodium hydroxide, NaOH, to form sodium chloride, NaCl, water, H 2 O, and sodium chlorate, NaOCl. Sodium chlorate is used as household bleach – this is the useful product. Step 1: write the equation for the reaction 2NaOH + Cl 2  NaCl + H 2 O + NaOCl

Step 2: work out the mass of each reactant and product involved. 2NaOH + Cl 2  NaCl + H 2 O + NaOCl 2 mol 1 mol  1 mol 1 mol 1 mol 80.0g 71.0g  58.5g 18.0g 74.5g Total 151.0g Step 3: calculate the atom economy % atom economy = mass of desired product x 100 total mass of reactants = (74.5/151)x100 = 49.3%

Questions 1. Lime (calcium oxide, CaO) is made by heating limestone (calcium carbonate, CaCO 3 ) to drive off carbon dioxide gas, CO 2. Calculate the atom economy of the reaction. 2. Sodium sulfate can be made from sulfuric acid and sodium hydroxide. If sodium sulfate is the required product, calculate the atom economy of the reaction. 3. Ethanol C 2 H 5 OH can be made by reacting ethene, C 2 H 4 with steam. Without doing a calculation, state the atom economy of the reaction. Explain your answer.

Answers 56.0% 79.8% 100%. All the reactants are changed into the only product

Percentage yield Tells us the actual loss of the reaction as a result of: The practical process of obtaining a product and As a result of reactions that do not go to completion. Reversible reactions For example: 2KI(aq) + Pb(NO 3 ) 2  PbI 2 (s) + 2KNO 3 (aq) 2 mol 1 mol 1 mol 2 mol 3.32g 3.31g 4.61g 2.02g

So if 3.32g potassium iodide is added to 3.31g of lead nitrate in aqueous solution should produce 4.61g of a precipitation of lead iodide which can be filtered off and dried. This is only in theory – solution can be lost when transferred from beakers, solid will be left on filter paper etc. % yield = number of moles of specified product x 100 theoretical maximum number of moles Or = mass of product x 100 theorectical mass of product

If you had obtained 4.00g of lead iodide, the yield would have been: (4.00/4.61) x 100 = 86.8% Extra info – yields of multi-step reactions can be very low because the overall yield is the yield of each step multiplied together. If a four step reaction with each yield 80%, the total yield would be: 80% x 80% x 80% x 80% = 41% What would the overall yield of a three step process if the yield of each separate step were 80%, 60%, and 75% respectively?

Questions Consider the thermal decomposition of calcium carbonate. Calculate the theoretical maximum number of moles of calcium oxide, CaO, that can be obtained from 1 mole of calcium carbonate. Starting from 10g calcium carbonate, calculate the theoretical maximum number of grams of calcium oxide that can be obtained. If 3.6g of calcium oxide was obtained, calculate the yield of the reaction