Stoichiometry and Balancing Chemical Equations

• Stoichiometry is about the calculations of relative amounts of reactants and products in chemical reactions
• There are 3 types of stoichiometry: gravimetric (masses), gas (volumes), and solution (solution volumes)

Chemical Reactions

• Chemical equations show the rearrangement of atoms/ions during a chemical change where reactants turn into products
• These equations are a shorthand method of representing chemical reactions

Balancing Chemical Equations

• Atoms, mass, and energy are conserved
• Chemical equations must represent the correct chemical formula and state of reactants/products
• Chemical equations must show that atoms/ions are conserved: Total # of each kind of atom in reactants = total # of each kind of atom in products

Hints for Balancing

• Balance polyatomic ions first
• Start with the element with the largest number of atoms if unsure of where to begin balancing
• Always double-check each time a number is applied
• For hydrocarbon combustion, balance in the order of carbon -> hydrogen -> oxygen

Reaction Types

• Reactions types include Simple Decomposition, Formation/Simple Composition, Single Replacement, Double Replacement, and Complete Combustion

Simple Decomposition

• Decomposition occurs when a compound is broken down into its elements
• General form: Compound -> Element + Element + ...
• Example: 2 HgO (s) → 2 Hg (l) + O2 (g)

Formation/Simple Composition

• Occurs when a compound is formed by the reaction between its elements (or if a single compound is formed from two or more compounds)
• General form: Element + Element + ... -> Compound
• Example: 2 Mg (s) + O2 (g) -> 2 MgO (s)

Single Replacement

• Occurs when a compound and an element react to form a different element and compound
• These reactions involve ions (hint: elements swap places)
• General form: Element 1 + Compound 1 -> Element 2 + Compound 2
• Example: 2 NaCl (aq) + Br2 (l) → 2 NaBr (aq) + Cl2 (g)
• Metal elements replace the cation, while Nonmetal elements replace the anion

Double Replacement

• Occurs when two compounds react and form two new compounds, involving all ions swapping places
• General form: Compound 1 + Compound 2 → Compound 3 + Compound 4
• Example: 2 AgBr (aq) + ZnCl2 (l) → ZnBr2 (aq) + 2 AgCl (g)
• In ionic compounds always write the cation first

Complete Combustion

• Occurs when a compound reacts with oxygen gas, specifically hydrocarbon combustion involving carbon and hydrogen
• The same two products are always formed: carbon dioxide and water
• General form: CxHy + O2 (g) -> CO2 (g) + H2O (g,l)
• Example: CH4 (aq) + 2 O2 (g) → CO2 (g) + 2 H2O (l)

Predicting Products

• Identify the reaction type by writing the chemical formulas for elements/compounds involved
• Write the chemical formulas for the appropriate product(s) (consider the ions "switching partners")
• Follow the steps for balancing equation

Reaction Details

• Ions essentially "switch partners" (positive of one reacts with negative of other) with double replacement reactions
• In ionic compounds always write the cation (metal or ammonium ion) first in the chemical formula followed by the anion
• Assume complex ions remain intact and are exchanged and balanced as complete units
• Include physical states of matter (use solubility chart) for solutions
• With combustion, oxygen reacts with a hydrocarbon to produce energy where the reactant is "burned"
• If combustion is complete, carbon dioxide and water vapor are products
• If combustion is incomplete, carbon monoxide and solid carbon are usually also produced

Important Notes

• Write water as HOH in replacement reactions and as H2O in other types
• All metallic elements are monatomic. Eg. Na and Pb
• Some nonmetallic elements are diatomic ie. the "HONorable Halogens," namely H2 O2 N2 F2 Cl2 Br2 12
• In Single Replacement Reactions, metal elements replace metal ions in ionic compounds or H in acids/water, while nonmetal elements replace nonmetal ions in ionic compounds
• In ionic compounds always write the cation (metal or ammonium ion) first in the chemical formula followed by the anion
• Polyatomic molecular elements are phosphorus = P4 and sulfur = S8

Acid Naming Rules

• hydrogen + ide becomes hydro__ic acid
• hydrogen + ite becomes __ous acid
• hydrogen + ate becomes __ic acid

Dissociation Equations

• Occurs when ionic compounds separate into individual aqueous ions when dissolved in water because the dissolving allows them to conduct electricity
• The total ionic equation involves breaking apart all soluble ionic compounds into their ions
• Then, any species that are the same on both sides can be cancelled, leaving the net ionic equation

Steps for Solving Stoichiometry Problems

• First write a balanced chemical equation, identify the required and given substances, and record the required and given data
• If not given moles, convert the given quantity (mass, volume or number of particles) to number of moles: n = m/M, n = v/V, n = N/NA
• Use the mole ratio to calculate the moles of the required substance from the moles of the given substance:
• If necessary, convert the moles of the required substance to the required quantity (mass, volume or number of particles): m = nM, v = nV, N = NNA
• Metric Conversions:
  • To convert mg, mL, mmol to g, L, mol, divide by 1000.
  • To convert kg, kL, kmol to g, L, mol, multiply by 1000.

Mole Ratio

• The most important information a balanced equation provides for stoichiometry is the mole ratio: the relative number of moles of reactants and products
• In equation form:
• Where:
  • nR = moles of required,
  • nG = mole of given,
  • R = the coefficient of required,
  • G = coefficient of given

Gravimetric Stoichiometry

• Carbon dioxide that is produced by astronauts can be removed by a reaction with lithium hydroxide, where the products are lithium carbonate and water
• Example: CO2(g) + 2 LiOH(aq) → Li2CO3(aq) + H2O(l)

Limiting and Excess Reagent Problems

• First, one reactant is completely used up where there may be more of the other reactant present, called the excess
• These problems can be recognized by the fact that the amounts of both reactants will be given
• Limiting Reagent:
  • Determines (limits) the amount of product(s) formed in a reaction as well as the amount of the other reactants(s) that is used in the reaction
  • Is completely used up in a reaction
• Excess Reagent:
  • Is added in excess of what can possibly react with the limiting reagent
  • Has some that is always remaining ("in excess") at the end of the reaction
  • Is used to ensure that all of the limiting reagent is used up in the reaction

General Steps

• Write a balanced chemical equation
• Calculate n for EACH reactant
• Use the mole ratio for each reactant to determine