STIOCHIOMETRY

CHEMISTRY CHAPTER 9

  

9.1 What is Stiochiometry

 A. The study of quantitative relationships between amounts of reactants used and product formed by chemical reactions.

  B. Mole-Mass relationships in chemical reactions

  1. Stiochiometry and balanced chemical

equations.

 

  4 Fe(s) + 3 O2(g) -------- 2 Fe2O3(s)

 

  2. Calculate Reactants and Products

4 mol Fe x 55.85 g Fe = 223.4g

Fe 1 mol Fe

 

  3 mol O2 x 32.00 g O2 = 96.0g O2

1 mol O2

 

  Total Mass in grams of Reactants = 319.4 g

 

 

 

  2 mol Fe2O3 x 158.7g Fe2O3 = 319.4g Product 1 mol Fe2O3

 Relationship derived from a balance relationship

  Iron Oxygen Iron III Oxide

 4 Fe(s)  + 3O2(g) = 2 Fe2O3(s)

 4 atoms Fe  3 molecules O2  2 Formula Units Fe2O3

 4 moles Fe  3 moles O2  2 moles Fe2O3

 223.4g Fe  96.0g O2  319.4g Fe2O3

   319.4 g Reactants = 319.4 g Product  Example Problem Page 355

 Practice Problems 1 (A-E) Page 356

3. Moles Ratio's    2 Al(s) + 3Br2(l) ------------ 2AlBr3(s)     2 mol Al and 2 mol Al   3 mol Br2 2 mol AlBr3

    3 mol Br2 and  3 mol Br2

  2 mol Al 2 mol AlBr3

    2 mol AlBr3 and 2 mol AlBr3

   2 mol Al 3 mol Br2

   4. Practice Problems 2&3 Page 357 9.2 Stiochiometric Calculations  A. Using Stiochiometry   1. Stiochiometric Moles-to-Mole Conversions   2. Example    2K(s) + 2H2O(l) -------------- 2KOH(aq) + H2

    Given= 0.0400 moles potassium Unknown= moles

of Hydrogen 

1 mol of H2

  2 mol K  moles of known x moles of unknown = moles of unknown  moles of known   0.0400 mol K x 1 mol of H2 = 0.0200 mol H2

  2 mol of K   If you put 0.0400 mol of K into water, 0.0200 mol of H2 will be produced. Example Problem 12-2 Page 359 Sample Problems 9&10 Page 359   3. Stiochiometric mole-to-mass conversion   Known = moles of Chlorine = 1.25 mol Cl2   Unknown = mass of Sodium Chloride = ? g NaCl 

Example Problem: Determine the mass of sodium chloride or table salt (NaCl) produced when 1.25 moles of chlorine gas reacts vigorously with sodium.   Known = 1.25 mol Cl2  Unknown = mass of Sodium chloride  Step 1 Write a balanced equation    2Na(s) + Cl2(g) ----------- 2NaCl(s)

  Step 2 Write the mole ratio that relates mol NaCl to Cl2 2 mol NaCl

1 mol Cl2  Step 3 Multiply the number of moles of Cl2 by the mole ratio  1.25 mol Cl2 x 2 mol NaCl = 2.50 mol NaCl  1 mol Cl2  Step 4 Multiply mol NaCl by the molar mass of NaCl   2.50 mol NaCl x 58.44 g NaCl = 146 g NaCl  1 mol NaCl

4. Stiochiometric mass-to-mass conversion  Example Problem: Ammonium nitrate (NH4NO3), an important fertilizer, produces N2O gas and H2O when it decomposes. Determine the mass of H2O produced from the decomposition of 25.0 g of solid ammonium nitrate.   Known = 25.0 g NH4NO3 Unknown = mass of H20  Step 1 Write a balanced equation   NH4NO3(s) ---------- N2O(g) + 2H2O(l)   Step 2 Convert grams to moles      25.0g NH4NO3 x 1mol NH4NO3 = 0.312 mol NH4NO3

80.4g NH4NO3   Step 3 Mole Relationship

Step 4 Multiply mol NH4NO3 by the mole ratio 0.312 mol NH4NO3 x 2 mol H20   = 0.624 mol H20  1 mol NH4NO3

 Step 5 Calcualte the mass of H20 using molar mass conversion factor   0.624 mol H20 x 18.0 g H2O = 11.2 g H2O  1 mol H2O   Practice Problems 13-14 Page 362

B. Steps in Stiochiometric Calculations   1. Write a balanced chemical equation. Interpret the equation in

terms of moles.  2. Determine the moles of the given substance using a mass-to-

mole conversion. Use the inverse of the molar mass as the conversion factor.3. Determine the moles of the unknown substance from the moles of the given substance. Use the appropriate mole ratio

from the balanced chemical  equation as the conversion factor.  4. From the moles of the unknown substance, determine the

mass of the unknown substance using a mole-to-mass conversion. Use the molar mass as the conversion  factor.

  

9.3 Limiting Reactants  A. Limiting reactants - limits the extent of the reaction and, thereby,

determines the amount of product.   1. A portion of all of the other reactants remains after the

reaction stops.   2. Excess reactants - the left over reactants.   Before reaction  3 Nitrogen molecules  3 Hydrogen molecules   2 Amonia   2 Nitrogen Molecules (6-Nitrogen atoms) (6-Hydrogen atoms) Molecules  (4-Nitrogen atoms)

  B. Calculating the product when a reactant is limited   1. Formation of disulfur dichloride (S2Cl2)

a) S8(l) + 4Cl2(aq) --------------- 4S2Cl2(l)

   2. If 200-g of sulfur reacts with 100-g of chlorine, what mass of disulfur dichloride is produced?

   a) You must first determine which one is the limiting reactant because the reaction will stop producing

product when the limiting reactant is used up.

b) Identify the limiting reactants involves finding the number of moles of each reactant.

  100g Cl2 x 1 mol Cl2 = 1.410 mol of Cl2

70.91g Cl2

 200g S8 x 1 mol S8 = 0.7791 mol S8

  265.5g S8

c) Next step is to determine if the two reactants are in correct mole ratio as given in the balanced chemical equation.  d) Equation shows a 4:1 ratio (4 moles of Cl and 1 mole of S)  e) To determine the actual ratio, divide the available moles of chlorine by the availible moles of sulfur.

  1.410 mol of Cl2 available = 1.808 mol of Cl2 available 0.7797 mol of S8 available 1 mol S8 available

  f) Only 1.808 mol of Chlorine is actually available for every 1 mol of sulfur instead of the 4 moles of the chlorine required by the balanced chemical equation. (Therefore chlorine is the limiting reactant)

12.4 Percent of YieldA. How much product?

1. Theoretical Yield – is the maximum amount of product that can be produced from a given amount of reactant.

2. Actual Yield – is the amount of product actually produced when the chemical reaction is carried out in an experiment.

3. Percent Yield – is the ratio of the actual yield to the theoretical yield expressed as a percent.

Percent Yield = actual yield (from an experiment) x 100 theoretical yield (from stiochiometric calculations)

Example: 75 shots / made 49 = 49/75 x 100 = 65%

B. Example Problem When potassium chromate (K2CrO4) is added to a solution

containing 0.500-g of silver nitrate (Ag(NO3)2) is formed.

a) Determine the theoretical yield of the silver chromate precipitate.

b) If 0.455-g of silver chromate is obtained, calculate the percent yield.

Known:Mass of silver nitrate = 0.500-g AgNO3

Actual yield = 0.455-g Ag2CrO4

Unknown:Theoretical yield = ? Ag2CrO4

Percent yield = ? Ag2CrO4

1. Write the balanced chemical equation and indicate the known and unknown quantities.

2AgNO3(aq) + K2CrO4(aq) Ag2CrO4(s) + 2KNO3(aq)

.500-g ?

2. Convert grams of AgNO3 to moles of AgNO3, using the inverse of molar mass.

.500-g AgNO3 x 1 mol AgNO3 = 2.94 x 10-3 mol AgNO3

1.69-g AgNO3

3. Use the appropriate mole ratio to convert mol AgNO3 to mol of Ag2CrO4.

2.94 x10-3 mol AgNO3 x 1mol Ag2CrO4 = 1.47 x 10-3 mol AgCrO4

2 mol AgNO3

4. Calculate the mass of Ag2CrO4 (theoretical yield) by multiplying mol of Ag2CrO4 by the molar mass.

1.47 x 10-3 mol Ag2CrO4 x 331.7-g Ag2CrO4 = 0.488-g Ag2CrO4

1 mol Ag2CrO4

5. Divide the actual yield by the theoretical yield and multiply by 100.

0.455-g Ag2CrO4 x 100 = 93.2% Ag2CrO4

0.488-g Ag2CrO4

Practice Problems (27-29) Page 372

3. Use the appropriate mole ratio to convert mol AgNO3 to mol of