This module is probably the most critical of the semester because it introduces chemical reactions, the units we use to describe them, and the relationship between inputs (reactants) and yields (products). This material is covered in Chapter 3 of Brown. Please bring your lab manual to class as I'll be referring to the periodic table frequently.
Topics for Lecture 3 [Links to PowerPoint slide sets]
1. Moles are the essential “counting” unit (and conversion factor) for chemistry (6.02 EE23 atoms or molecules/1 mole).
2. Chemical reactions conserve but rearrange (or reconnect) atoms in reactants to make products).
3. Stoichiometry uses molar coefficients to balance chemical equations and obey the laws of conservation of mass.
a. Moles are the central unit of chemical unit conversion. Use Avrogado’s number to convert from moles to
molecules (or atoms) & vice versa.
b. Chemical equations are balanced by iterative adjustment of stoichiometric coefficients, small whole
numbers representing moles.
c. Many chemical reactions can be categorized as: combination, decomposition, combustion, or
d. The amount of the limiting reactant determines the yield of all products.
i. Units of atomic mass can be expressed as amu (atomic mass units) or grams/1 mole.
ii. Molar mass (aka molecular weight (MW) is calculated as the sum of the atomic masses of all atoms in the molecule. Use molar mass to
convert from mass to moles & vice versa.
iii. Percent composition describes the percentage of a molecules mass made up of a single element: % C
in glucose = (mass all carbon atoms/mass glucose molecule)(100)
iv. Empirical formulas can be calculated from % composition: 1) calculate grams of each element in a 100
g sample; 2) convert g to moles; 3) calculate molar ratios; 4) use ratios as formula subscripts.
v. Molecular formulas can be calculated from empirical formulas: divide molar mass by empirical mass
and multiply all formula subscripts by that factor.
vi. Combination reactions combine atoms, ions or molecules to create fewer molecules; the number of
products is less than the number of reactants.
vii. Decomposition reactions break reactants down into smaller products; the number of products is
greater than the number of reactants.
viii. Combustion reactions combine carbon-based molecules with oxygen gas and produce carbon dioxide
ix. Exchange reactions combine two ionic compounds that switch ionic partners: cation 1 ends up with
anion 2 and cation 2 ends up with anion 1.
x. Stoichiometric coefficients can be used to “convert” one compound to another:
g of A --MW of A à moles of A –coefficient ratio à moles of B --MW of B à g of B
xi. The limiting reactant is the reactant that can produce the least product (via stoichiometric
xii. Some of the excess reactant remains after the limiting reactant is used up and products are made.
xiii. Percent yield = (theoretical yield/actual yield)(100)
Links and items of interest:
Resources for students: