Stoichiometry – Chemical Quantities Notes

Stoichiometry Stoichiometry – Study of quantitative relationships that can be derived from chemical formulas and chemical equations Mole ─ Mole Relationship need a balanced equation Mole Ratio – the ratio of moles of one substance to moles of another substance in a balanced chemical equation The coefficients in a balanced equation give the relative numbers of molecules, as well as, the relative number of moles. CO (g) + 2H 2(g)  CH 3 OH (l) Ex: How many moles of O 2 are required to produce 10. moles of CO 2 ? 2 CO + O 2  2 CO 2 1 mol CO = 2 mol H 2 = 1 mol CH 3 OH 10. mol CO 2 x __________ mol CO 2 mol O = 5.0 mol O 2

What other relationships do we have for the mole? 1 mol = 6.02 x atoms / molecules / particles 1 mol = [molar mass] g 1 amu = 1.66x g We can add these mole relationships on either end of the mole ratio: # unit A x 1 mol A x mol B x __ unit B = # unit B _ unit A _ mol A 1 mol B mole relationship mole ratio mole relationship (switch units) (switch substances) (switch units) *A is the GIVEN substance & B is the WANTED substance

Mass A – Mole B Ex: Calculate moles of O 2 produced if 2.50 g KClO 3 decomposes completely: 2 KClO 3  2 KCl + 3 O 2 x ______________ mol O 2 mol KClO x ________________ mol KClO g KClO g KClO 3 = mol O 2 K 1 x = Cl 1 x = O 3 x = g/mol

Mass A – Mass B Ex: Determine the mass of NaCl that decomposes to yield 355 g Cl 2 2NaCl  2 Na + Cl g Cl 2 mol Cl 2 g Cl x _____________ x ______________ mol Cl 2 mol NaCl 1 2 x ______________ mol NaCl g NaCl = 585 g NaCl Cl 2 x = g/mol Na 1 x = Cl 1 x = g/mol

Mole A – Mass B Ex: Calculate the number of grams of oxygen required to react exactly with 4.30 mol of propane, C 3 H 8, in the reaction by the following balanced equation: C 3 H 8(g) + 5O 2(g)  3CO 2(g) + 4H 2 O (g) 4.30 mol C 3 H 8 mol O 2 mol C 3 H x _____________ x __________ mol O 2 g O = 688 g O 2 O 2 x = g/mol

Learning Check How many grams of water are needed to produce 9.23 moles of oxygen? ____ Na 2 O 2 + ____ H 2 O  ____ NaOH + ____ O 2

2. Limiting Reactant & Percent Yield

Background Knowledge Check Label the reactant(s) and product(s) in the following reaction: 2 Mg + O 2  2MgO Reactant(s): Product(s): Mg and O 2 MgO

Limiting Reactant Manufacturers of cars and bicycles order parts in the same proportion as they are used in their product. Car manufacturers order four times as many wheels as engines and bicycle manufacturers order twice as many pedals as seats. In the same manner, when chemicals are mixed together so they can undergo a reaction, they are often mixed in stoichiometric quantities – exactly the correct amounts so that all the reactants “run out” at the same time. If the chemicals aren’t mixed to run out at the same time, one of the chemicals will limit or halt the reaction from taking place any further. The reactant that “runs out” or limits the reaction is called the limiting reactant. The reactant that still remains or is extra is called the excess reactant. In any stoichiometric problem, where reactants are not mixed in stoichiometric quantities, it is essential to determine which reactant is limiting in order to calculate correctly the amounts of products that will be formed.

Analogy: Baking Cookies A recipe calls for 2 cups of flour for every egg. You have 5 cups of flour and 3 eggs. What is your limiting ingredient? What is your excess ingredient? Steps for Solving Stoichiometry Problems Involving Limiting Reactants 1.Write and balance the equation for the reaction, if necessary. 2.For each reactant, convert grams reactant to grams product. 3.Compare grams of product: The smaller grams of product is the theoretical yield and is the amount of product made The smaller grams of product came from the limiting reactant The larger grams of product came from the excess reactant flour eggs

x ______________ mol Mg mol MgO2 2 x _____________ mol O 2 mol MgO mol Mg = = 3.86 mol O g MgO Ex: 7.24 moles of Mg and 3.86 moles of O 2 react to form MgO. 2 Mg + O 2  2MgO How many grams of MgO are formed ? What is the limiting reactant ?What is the excess reactant ? MgO2O2 Mg 1 x = O 1 x = g/mol x ____________ mol MgO g MgO x ____________ mol MgO g MgO g MgO

x ___________ g N 2 mol N x ___________ mol N 2 mol NH x ___________ g H 2 mol H x ______________ mol NH 3 g NH x ___________ mol H 2 mol NH x 10 4 g N x 10 4 g NH 3 = = 5.00 x 10 3 g H x 10 4 g NH 3 Ex: Suppose 2.50 x 10 4 g of N 2 and 5.00 x 10 3 g of H 2 are mixed and reacted to form ammonia. Calculate the mass of ammonia produced when the reaction is run to completion. N H 2  2 NH 3 What is the limiting reactant? What is the excess reactant? N2N2 H2H2 N 2 x = g/mol H 2 x 1.01 = 2.02 g/mol N 1 x = H 3 x 1.01 = g/mol x ______________ mol NH 3 g NH

Percent Yield Theoretical yield – amount of product predicted from the amounts of reactants used, calculated from the limiting reactant Actual yield – amount of product actually obtained through experiment Percent yield – comparison of actual and theoretical yield Percent Yield = Actual yield X 100% Theoretical yield

Example:Methanol, CH 3 OH, can be produced by the reaction between carbon monoxide and hydrogen. Suppose 6.85 x 10 4 g of CO is reacted with 8.60 x 10 3 g of hydrogen. CO + 2H 2  CH 3 OH 1.Calculate the theoretical yield of methanol. 2.If 3.57 x 10 4 g of CH 3 OH is actually produced, what is the percent yield of methanol? x ___________ g CO mol CO x ___________ mol CO mol CH 3 OH 1 1 x ___________ g H 2 mol H x ___________ mol H 2 mol CH 3 OH x 10 4 g CO 7.83 x 10 4 g CH 3 OH = = 8.60 x 10 3 g H x 10 4 g CH 3 OH C 1 x = O 1 x = g/mol H 2 x 1.01 = 2.02 g/mol C 1 x = H 4 x 1.01 = 4.04 O 1 x = g/mol % Yield = 3.57 x 10 4 g 6.82 x 10 4 g X 100% = 52.3 % 6.82 x 10 4 g CH 3 OH x ______________ mol CH 3 OH g CH 3 OH x ______________ mol CH 3 OH g CH 3 OH

Learning Check Learning Check - If 1.30 grams of oxygen and 3.10 grams of iron are reacted, then what is the theoretical yield of iron (III) oxide. What is the limiting reactant? If 3.00 grams of iron (III) oxide is produced, then what is the percent yield? Balanced equation: 4 Fe + 3 O 2  2 Fe 2 O 3 Theoretical Yield? Limiting Reactant? Excess Reactant? Percent Yield?