Limiting Reactants & Percent Yield Section 12.3

If we have… how many sets can we make? Limiting reactant = the reactant that runs out first causing the reaction to stop Excess reactant = reactant that does not get completely used up during a reaction With this left over Excess

Steps to solve limiting reactants problems Split into 2 problems Solve each problem Pick smallest answer Smallest answer comes from limiting reactant Largest answer comes from excess reactant

Calculating product when reactant is limiting If we had 12 moles of nitrogen and 18 moles of hydrogen, what is the maximum number of moles of NH3 that could be produced? N2 + 3H2  2 NH3

Calculating product when reactant is limiting If we had 112 grams of nitrogen and 18 grams of hydrogen, what is the maximum number of grams of NH3 that could be produced? N2 + 3H2  2 NH3 2(14) 2(1) 14 + 3 28 2 17 Remember… Nitrogen Hydrogen Pick lowest number Therefore Hydrogen is your limiting reactant

Calculating product when reactant is limiting With 48 grams of magnesium and 48 grams of oxygen available, how much product can be formed? 2Mg + O2  2 MgO 24.3 2(16) 24.3 + 16 32 40.3 Oxygen Magnesium Pick lowest number Therefore Magnesium is your limiting reactant and oxygen is the excess reactant

Limiting Reactants Practice III How many grams of sodium chloride can be produced in the following reaction with 150 grams of sodium and 200 grams of chlorine? 2Na + Cl2  2 NaCl 23 2(35.5) 23 + 35.5 71 58.5 Sodium Chlorine Pick lowest number

Limiting Reactants Practice III In the above reaction, how many more grams of chlorine will be required to fully react with the remaining sodium? We need to figure out how many grams of chlorine is needed to react with 150 grams of sodium 2Na + Cl2  2 NaCl 23 2(35.5) 71 Sodium Chlorine needed = 231 g Chlorine given (from problem) = 200 g Additional Chlorine needed = 31 g

Percent Yield Actual yield = the actual amount of product formed during an experiment Theoretical yield = amount of product that could be produced according to calculations Percent yield = ratio of actual and theoretical yield

Percent Yield Practice I A student uses stoichiometry to calculate the predicted yield of CO2 in a chemical reaction as 323 g. When the student mixes the chemicals to make the CO2, he finds that the reaction only produces 308 g of CO2. What is the percent yield?

In the reaction, how many grams of potassium oxide can be produced with 78 grams of potassium and 140 grams of boron oxide? 6 K + B2O3  3 K2O + 2 B 39.1 2(10.8)+3(16) 2(39.1)+16 69.6 94.2 Potassium Boron Oxide Pick lower number

Problem 2 continued If the actual yield from the above reaction was 81 grams, what is the percent yield? Actual yield = 81 grams Theoretical yield = 93.95 g (from last slide)

Pick lower number N2 + 3H2  2 NH3 3. The actual yield in the above reaction was 39 grams of ammonium. Using 84 grams of nitrogen and 8 grams of hydrogen, what is the percent yield? -first we must calculate the theoretical yield of product which means we need to find our limiting reactant N2 + 3H2  2 NH3 2(14) 2(1) 14+ 3 28 2 17 Nitrogen Hydrogen Pick lower number

Problem 3 continued Actual yield = 39 grams Theoretical yield = 45.3 g (from last slide)