Honors Chem Unit 12 Stoich Notes Stoichiometry- calculation of quantities in chemical reactions From the coefficients in a BALANCED equation you can get: N2+3H2 2NH3 1. # particles (molecules, formula units, atoms) 1 molecule N2: 3 molecules H2 : 2 molecules NH3 2. # moles 1 mole N2: 3 mole H2 : 2 mole NH3

3. Mass- Obeys Law of Conservation of Mass 1 mol N2 = 28.0 g +3 mol H2 = 6.0 g 2 mol NH3 = 34.0 g 4. Volume- GASES at STP 1 mol= 22.4L 3 mol H2 = 3(22.4)=67.2L 1 mol N2 = 1(22.4) = 22.4 L 2 mol NH3 = 2(22.4) = 44.8 L Notice that the number of atoms & mass are always conserved, but the volume & molecules may not be!!

Notes Mole-Mole calculations N2+3H2  2NH3 1mol N2 1moleN2 2 mol NH3 3 mol H2 2mol NH3 3 mol H2 These are your mole conversion factors How many moles of NH3 are in 0.60 mol N2?

Mass:Mass 1. Convert mass to moles (1mol/ g) 2. Convert moles of what you have to moles of what you want 3. Convert mol to mass ( g/1mol) How many g NH3 are produced when 5.40 g of H2 react w/ excess N2? For all problems Convert to mol, use mol:mol, convert to wanted units

How many moles O2 are produced when 29.2 g H2O decomposed into O2 & H2? Assuming STP, how many L O2 are needed to produce 19.8 L SO3? 2SO2(g) + O2(g)  2 SO3(g)

Use ratios from coefficients b/c the volumes relate from 22.4L=1mol 2NO(g) + O2 (g)  2 NO2(g) How many mL NO2 are produced when 3.4mL O2 react with excess NO at STP? 1mL O2=2 mL NO2=2 mL NO Write up Stoich lab pg 251

Limiting reactant & % Yield Limiting Reagent- determines amt. Of product formed, runs out 1st Steps: (if given 2 reactants) 1. Solve for same product 2. One w/least amount of product is limiting 3. Other is excess (left over) 4. Limiting determines the amount of product

2 Na(s) + Cl2 (g)  2NaCl (s) Suppose 8.70 mol Na react w/3.20 mol Cl2. What is limiting reactant? How many moles of NaCl are produced? How many g NaCl are produced? How much of the excess reactant is left?

2Cu (s) + S (s)  Cu2S(s) What is the limiting reactant when 80.0 g Cu reacts with 25.0 g S? How many g Cu2S can be formed? How much of the excess reactant is left?

% yield- measures the efficiency of reaction Normally can’t be larger than 100% Most are less than 100 % b/c: Reactions don’t go to completion Impure reactants Competing side reaction Loss of product during transfer or filtration Human error (bad measurement, miscalculation)

Actual yield- amt. of product formed in lab Theoretical yield- amt. product that could be formed from calculations % Yield: actual yield X 100 theoretical yield CaCO3 (s)  CaO (s) + CO2 (g) What is the theoretical yield of CaO if 24.8 g CaCO3 is heated? What is the % yield if 13.1 g CaO is produced?