Yr 12 IB Chemistry Percentage Yield and Percentage

Yr 12 IB Chemistry Percentage Yield and Percentage Atom Economy During a Reaction

Percentage Yield During a Reaction Actual Yield in Grams Produced During an Experiment % Yield = Theoretical Yield (as predicted by reaction equation) in Grams % yield will usually be less than 100% because : 1. The reaction may be incomplete 2. Other reactions may occur 3. Product is lost during separation and purification x 100

Steps to Calculate a Percentage Yield 1. Convert the mass (w. R) of main reactant (R) to a number (n. R) of moles using n. R = w. R/Mr of R 2. Calculate theoretical number (n. P) of moles of product (P) using the ratio from the balanced equation 3. Calculate theoretical mass (w. P) of product (P) using w. P = n. P x Mr of P 4. Calculate the percentage yield using : % Yield = Actual Yield in Grams Theoretical Yield (wp) in Grams x 100 N. B. This assumes all other reactants are present in excess, or at least in stoichiometric amounts

e. g 1 12. 0 g ethanol (C 2 H 5 OH) is reacted to produce 5. 3 g of ethene (C 2 H 4). Calculate the % yield. 1. Number of moles of ethanol used = 12. 0 / 46 = 0. 26 2. C 2 H 5 OH C 2 H 4 + H 2 O. Theoretical number of moles of ethene formed = 0. 26 since 1: 1 reaction 3. Theoretical mass of ethene formed = 0. 26 x 28 4. % Yield = = 7. 3 g 5. 3 7. 3 x 100 = 72. 5

e. g 2 8. 85 g of sodium hydroxide (Na. OH) is reacted to produce 9. 45 g of sodium carbonate (Na 2 CO 3). Calculate the % yield. 1. Number of moles of Na. OH used = 8. 85 / (23. 0 + 16. 0 + 1. 0) = 0. 221 2. 2 Na. OH + CO 2 Na 2 CO 3 + H 2 O. Theoretical number of moles of Na 2 CO 3 formed = 0. 221/2 (since 2: 1 reaction ) = 0. 111 3. Theoretical mass of Na 2 CO 3 formed = 0. 111 x (2(23)+12+3(16)) = 11. 73 g 4. % Yield = 9. 45 11. 73 x 100 = 80. 6

% Atom Economy (also called % Atom Utilisation) A measure of the percentage of the starting materials that actually ends up in the useful products. The greater the % atom economy of a reaction, the more 'efficient' or 'economic' it is likely to be, To calculate the % Atom Economy : 1. Write the balanced equation for the reaction 2. Calculate the total mass of the reactants for the numbers of moles in the reaction equation ; mreactants 3. Calculate the total mass of the “desired product” for the numbers of moles in the reaction equation ; mproduct 4. m product x 100 % Atom Economy = mreactants Note : reactions involving only the production of a SINGLE PRODUCT will have 100% atom economy e. g. H 2 + Cl 2 2 HCl C 2 H 4 + H 2 C 2 H 6 N 2 + 3 H 2 2 NH 3

Example 1 The equation for the production of lime (Ca. O) from limestone (Ca. CO 3) is: Ca. CO 3 Ca. O + CO 2 Calculate the % atom economy of this process. Mreactants = 40. 1 + 12. 0 + 3(16. 0) = 100. 1 g Mproduct = 40. 1 + 16. 0 = 56. 1 g % Atom Economy = 56. 1 x 100. 1 = 56. 0

Example 2 The equations for the extraction of titanium from titanium(IV) oxide are: Ti. O 2 + C + 2 Cl 2 Ti. Cl 4 + CO 2 Ti. Cl 4 + 2 Mg Ti + 2 Mg. Cl 2 Calculate the % atom economy of the overall process. Combining 2 equations for an overall equation gives: Ti. O 2 + C + 2 Cl 2 + Ti. Cl 4 + 2 Mg Ti. Cl 4 + CO 2 + Ti + 2 Mg. Cl 2 Mreactants = 47. 9 + 2(16. 0) + 12. 0 + 2(2 x 35. 5) + 2(24. 3) = 282. 5 g Mproduct = 47. 9 g % Atom Economy = 47. 9 x 100 282. 5 = 17. 0 Such a low % atom economy is typical of a MULTISTAGE process.

Example 3 The equation for the extraction of copper from copper sulphide is: Cu. S + O 2 Cu + SO 2 1 Calculate the % atom economy of the reaction. 2 If 9. 60 tonnes of copper sulphide produce 4. 80 tonnes of copper, calculate the % yield 1 Cu. S + O 2 Cu + SO 2 Mreactants = (63. 5 + 32. 1) + (2 x 16) = 127. 6 g Mproduct = 63. 5 g % Atom Economy = 63. 5 x 100 = 127. 6 49. 8 2. Theoretically, in excess oxygen, one mole Cu. S one mole Cu i. e. (63. 5 + 32. 1) g Cu. S 63. 5 g Cu i. e. 95. 6 g Cu. S 63. 5 g Cu i. e. 1. 0 g Cu. S 63. 5 / 95. 6 = 0. 664 g Cu i. e. 1. 0 tonnes Cu. S 0. 664 tonnes Cu i. e. 9. 60 tonnes Cu. S 9. 60 x 0. 664 = 6. 38 tonnes Cu % Yield Cu = 4. 80 x 100 6. 38 = 75. 2

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