Stoichiometry: Chapter 9

Chapter 9.1 Stoichiometry: The Arithmetic of Equations Branch of chemistry Deals with numerical relationships within a chemical reaction Chemical book keeping

Interpreting a chemical equation: N2 + 3H2  2NH3 1. Particles 2. Moles 3. Mass 4. Volume

Chapter 9.2: Mole to Mole Calculations: A balanced equation is essential for all calculations involving amounts of reactants and products.

Interpret: N2(g) + 3H2(g) 2NH3(g) How would we interpret this equation in terms of moles? 1 mol of nitrogen reacts with 3 moles of hydrogen, to yield 2 moles of ammonia

Use to create mole ratios (conversion factors) N2(g) + 3H2(g) 2NH3(g) Coefficients = the ratio in which the reactants react and the products formed Use to create mole ratios (conversion factors) 1 mol N2/ 3 mol H2 1 mol N2/ 2 mol NH3 3 mol H2/ 2 mol NH3

What are the other three conversion factors? N2(g) + 3H2(g) 2NH3(g) What are the other three conversion factors? Inverse 3 mol H2/ 1 mol N2 2 mol NH3/ 1 mol N2 2 mol NH3/ 3mol H2

Practice Problem: 4Al(s) + 3O2(g)  2Al2O3(s) How many moles of aluminum are needed to form 3.7 mol of Aluminum Oxide? What is the known (starting) value?: 3.7 mol of Al What is the unknown value?: Mole of aluminum oxide What is the mole ratio? (conversion factor) Unknown over known 2 mol Al2O3 / 4 mol Al

Practice Problem: Steps Step One: start with known value Step Two: convert from moles of know to moles of unknown by multiplying by mole ratio.

Must calculate molar mass Mass to Mass: From the given mass of a reactant or product, we can calculate the mass of any other substance in the reaction. Must calculate molar mass

Practice problem: CaC2(s) + 2H2O(l)  C2H2(g) + Ca(OH)2(aq) How many grams of acetylene are produced by adding water to 5.0g of Calcium Carbide? What is the known (starting) value?: 5.0 g of CaC2 What is the unknown value?: grams of C2H2 What is the mole ratio? (conversion factor) Unknown over known 1 mol C2H2 / 1 mol CaC2

Practice Problem Steps: Step One: convert know value to moles Divide by molar mass of known Step Two: convert from moles of know to moles of unknown by multiplying by mole ratio. Step Three: convert from moles of unknown to grams of unknown Multiply by molar mass of unknown

Other Stoichiometric Calculations Can calculate any unknown value of moles, volume, mass and particles from any known value of moles, volume, mass and particles. Think mole road map

For all of the previously mentioned problems How? For all of the previously mentioned problems 1. Convert the given quantity (known) to moles 2. Then use the mole ratio from the balanced equation to figure moles of unknown

How continued? 3. Then the moles are converted to any other unit of measurement the problem requires.

Chapter 9.3: Limiting Reagent and Percent Yield Determines the amount of product that can be formed in a chemical reaction Can only be a reactant The reactant that gets used up force Excess Reagent: The reactant that is left over Does not get used up

Practice Problem: Mg(s) + 2HCl(aq)  MgCl2(aq) + H2(g) Identify the limiting reagent when 6.00g of HCl reacts with 5.00g of Mg.

Practice Problem: Mg(s) + 2HCl(aq)  MgCl2(aq) + H2(g) How many grams of H2 can be produced when 6.00g of HCl is added to 5.0g of Mg?

Percent Yield: the ratio of actual yield to theoretical yield

Percent Yield: should = 100% Theoretical Yield: Maximum amount of product that can be formed Calculated yield Actual Yield: The amount of product actually formed Experimental value/ measured in the lab

Practice problem: SiO2(s) + 3C(s)  SiC(s) + 2CO(g) If 50g of SiO2 is heated with an excess of Carbon, 27.9g of SiC is produced. What is the percent yield? Actual yield = ?? Theoretical yield = ??