Stoichiometry

Stoichiometry

1. Fundamentals

  • Stoichiometry is the measure of the quantitative relationship between the products and reactants of a given chemical reaction in terms of their relative ratios of mass.

  • Fundamental Equation of Law of Conservation of Mass:
    Input + Generation = Output + Consumption + Accumulation

  • In a steady state process, Accumulation is always equal to 0.

  • Since chemical reactions can neither create nor destroy matter, nor transmute one element into another, the amount of each element must be the same throughout the overall reaction.

  • A balanced chemical equation is very important as it indicates the quantitative relationships between the moles of reactants and products.

2. Predicting Chemical Reactions

  • Synthesis (Combination): Two or more reactants combine to form a single product.

  • Formula: A + B -> AB

  • Decomposition: A single reactant breaks down into two or more products.

  • Formula: AB -> A + B

  • Single Replacement: One element replaces another in a compound.

  • Formula: A + BC -> AC + B

  • Double Replacement: Two compounds exchange ions

  • Formula: AB + CD -> AD + CB

3. Activity Series of Metals & HalogensMetals Activity:

  • K, Ba, Sr, Ca, Na: React with cold water, replacing hydrogen.

  • Mg, Al, Zn, Cr, Fe, Cd, Co, Ni, Sn, Pb: React with steam, but not cold water, replacing hydrogen.

  • H: Reference point.

  • Cu, Hg, Ag, Pt, Au: Unreactive with water or acids.

Halogens Activity:

  • F2 > Cl2 > Br2 > I2 (More reactive halogens replace less reactive ones in single replacement reactions).

4. Predicting & Balancing Reactions (Practice Problems)Predict the products of the following reactions:

  • MgBr2 + AgNO3 ->

  • S8 + O2 ->

  • P4O10 + H2O ->

  • H2 + S8 ->

  • 2 Al + 3 I2 ->

  • CuCl2 + KOH ->

  • Mg + HCl ->

  • AlBr3 + Cl2 ->

Balance the following reactions:

  • CO2 + H2O -> C6H12O6 + O2

  • SiCl4 + H2O -> H4SiO4 + HCl

  • Al + HCl -> AlCl3 + H2

  • C3H8 + O2 -> CO2 + H2O

  • Al2(SO4)3 + Ca(OH)2 -> Al(OH)3 + CaSO4

5. Stoichiometric Calculations & Conversions

  • Solving Stoichiometry involves mastery of dimensional analysis.

  • Unit for atomic and molecular mass: g/mol

Fundamental Steps in Solving Stoichiometry Problems:

  1. Determine the Given and the Required.

  2. Always check your ratio for consistency.

  3. Always check your units for consistency.

  4. Always follow significant figures.

  5. Always write your units after the value.

Mole - Gram Conversion Formula:

  • mass = molecular mass x number of moles

  • m = MM x n

  • Note: Molar mass, molecular weight, and molecular mass refer to the same concept.

Conversion Pathway:

  • Grams of Known <-> Moles of Known <-> Moles of Desired <-> Grams of Desired

  • Moles of Known <-> Atoms/Molecules of Known

  • Moles of Desired <-> Atoms/Molecules of Desired

  • Use the Mole Ratio to convert between Moles of Known and Moles of Desired.

6. Problem Solving Examples (Stoichiometry)

  1. Acetylsalicylic acid (C9H8O4) is the active ingredient of aspirin.

  • a. What is the mass, in grams, of 0.509 moles of C9H8O4?

  • b. A 1g tablet of aspirin contains 75.2% by mass C9H8O4. How many moles of acetylsalicylic acid are in the tablet?

  1. How many molecules of SO3 are needed to react with 144 grams of Fe2O3 given this balanced chemical equation?

  • Fe2O3 + 3 SO3 -> Fe2(SO4)3

  1. You used a magnesium ribbon (pure magnesium) in the laboratory and subjected it to heat. The magnesium ribbon glowed into a very bright light, an indication that Magnesium Oxide was formed. You got carried away and used 15g of the magnesium ribbon. Calculate the amount of Magnesium Oxide produced.

  2. How many grams of water are produced when 4 grams of hydrogen gas (H2) react with excess oxygen?

  3. Given the following balanced chemical equation, what is the mole ratio needed to calculate the moles of KCl produced when 4.5 g of O2 are formed?

  • 2 KClO3(s) -> 2 KCl(s) + 3 O2(g)

  1. Combustion Reaction: How many grams of CO2 are produced when 10 grams of propane (C3H8) are burned completely?

  2. Combustion Reaction: How many moles of CO2 will be produced by the reaction of 5.5 g of glucose, given the following balanced equation?

  • C6H12O6 + 6 O2 -> 6 CO2 + 6 H2O

7. Limiting and Excess Reactants

  • Reactants are not usually mixed in exact ratios.

  • An excess of one reactant is often used (usually the less expensive one).

  • One reactant will limit the amount of product that will form.

Steps to Find the Limiting Reactant:

  1. Calculate the amount of product that will form if the first reactant were completely consumed.

  2. Repeat the calculation for the second reactant in the same way.

  3. Choose the smaller amount of product and relate it to the reactant that produced it. THIS IS THE LIMITING REACTANT. The resulting amount of product is the theoretical yield.

  4. From the Theoretical Yield, determine how much of the reactant in excess is used, and subtract from the starting amount.

Practice Problems (Limiting Reactants):

  1. How many moles of HCl can be produced from 4.0 mol of H2 and 3.5 mol of Cl2? What is the limiting reactant?

  • H2 + Cl2 -> 2 HCl

  1. How many grams of AgBr can be produced from 50.0 g MgBr2 and 100.0 g AgNO3? How much excess reactant remains?

  • MgBr2 + 2 AgNO3 -> 2 AgBr + Mg(NO3)2

  1. How many moles of Fe3O4 can be produced from 16.8 g Fe and 10.0 g H2O? What is the limiting reactant?

  • 3 Fe + 4 H2O -> Fe3O4 + 4 H2

  1. Given 10 grams of sodium (Na) and 10 grams of chlorine gas (Cl2), which reactant is limiting in the formation of sodium chloride (NaCl)?

  2. Consider the following reaction: 2 Sb(s) + 3 I2(s) -> 2 SbI3(s)

  • 1.20g of Sb and 2.40g of I2 were mixed. Determine the limiting reactant.

  1. Will 28.7g of SiO2 react completely with 22.6g of H2F2? If not, identify the limiting reagent:

  • SiO2 + 2 H2F2 -> SiF4 + 2 H2O

8. Reaction Yield

  • The amount of products formed calculated by stoichiometry are the maximum yields possible (100%).

  • Yields are often lower due to side reactions, loss of product while isolating/transferring material, etc.

  • Theoretical yield: Maximum possible yield for a reaction, calculated based on the balanced chemical equation.

  • Actual yield: Actual yield obtained from the reaction in a laboratory setup.

  • Percent yield Formula:

  • Percent Yield = (Actual yield / Theoretical yield) * 100

Practice Problems (Percent Yield):

  1. Calculate the percent yield of AgBr if 375.0 g of the compound are prepared from 200.0 g of MgBr2.

  • MgBr2 + 2 AgNO3 -> Mg(NO3)2 + 2 AgBr

  1. Calculate the percent yield of Al2O3 if 125.0 g of Al give 100.0 g of Al2O3.

  • 2 Al + 3 CrO -> Al2O3 + 3 Cr