Chemical Reactions, Chemical Equations, and Stoichiometry Brown, LeMay Ch 3 AP Chemistry

3.2: Types of reactions Precipitation: formation of insoluble precipitate(s); more of this in Chapter 4 Combustion: rapid reactions that produce a flame; hydrocarbon combustion always produces CO2 and H2O. If insufficient O2, CO is produced; more of this in Chapter 25 Acid/base: exhibit no change in oxidation number for any species (ionic or molecular) involved; many special cases; more in Ch. 15-17

Redox: electrons are transferred from one reactant to the other; change in oxidation number; more in Ch. 20 Complexation: a central metal ion has a characteristic number of surrounding molecules or ions bonded to it; more in Ch. 24

3.3: Atomic, Molecular & Formula Weights Atomic mass units: 1 amu = 1.66054 x 10-24 g or 1 g = 6.02214 x 1023 amu 1 atom of 12C isotope defined as weighing exactly 12 amu Average atomic mass (or atomic weight): 12C: 98.892% x 12 amu = 13C: 1.108% x 13.00335 amu = 11.867 + 0.1441 AW = 12.011 amu Therefore 12.011 g C = 1 mol C

Molecular mass or weight (MM or MW): sum of atomic masses of each atom in the chemical formula. Ex: H2SO4 MW = (2)(1.0079) + 32.06 + (4)(16.00) = 98.08 amu Formula weight (FW): same as MW, except for ionic substances in which no “molecule” exists. Then, FW is the simplest integer ratio of moles of each element (ions) present. Ex: NaCl is a 3D array of ions FW = 22.99 + 35.453 = 58.44 amu

3.4 – 3.7: Mass Relationships Percent Composition: Ex: What is the percentage of oxygen in phosphoric acid?

The Mole and Molar Mass (MM) From Latin moles: “a mass” Avogadro’s Number = 6.022 x 1023 atoms in exactly 12.000 g of 12C = 1 mol Converting: grams → moles → molecules Ex: How many molecules of H2O in 100.0 g H2O? = 3.343 x 1024 molecules H2O

C:H mole ratio is 6.7:20. ≈ 1:3. So, empirical formula is CH3, Empirical Formula: simplest integer ratio of moles (or atoms) of each element present Ex: What is the empirical formula of a compound that is composed of 80.% Carbon and 20.% Hydrogen? If the molecular weight is found to be 30 g/mol, what is the molecular formula? Strategy: Assume 100. g of unknown C:H mole ratio is 6.7:20. ≈ 1:3. So, empirical formula is CH3, and the molecular formula is C2H6.

Limiting Reagent (or reactant): Ex: If 6.0 g hydrogen gas reacts with 40.0 g oxygen gas, what mass of water will be produced? 2 H2 (g) + O2 (g) → 2 H2O (g) O2 is limiting reagent, H2 is excess.

Percent yield & Percent error: Ex: If in the previous example, only 40.0 g water were formed, what is the percent yield and percent error?