Stoichiometry What is stoichiometry study of quantitative relationships

Stoichiometry

What is stoichiometry? • study of quantitative relationships in balanced chemical equation • chemical quations represent chemical reactions

Physical vs. Chemical Change Physical Change: • change in form/appearance • examples: • dissolving one substance in another • phase changes Chemical Change: • change in identity • change in formula representing substance

Evidence of a Chemical Reaction • temperature change • emission of light energy • change in identifying property: color, mp, bp, density, Hf, Hv, … • formation of gas – bubbling, odor • formation of solid – precipitate forms

Chemical Equations A+B C + D Left Side = Reactants (starting materials) Right Side = Products (ending materials) “ ” read as produces/yields How do we show the physical state of the reactants & products? (s), (l), (g), (aq)

Law of Conservation of Matter • Matter is neither created nor destroyed in chemical rxns • Mass reactants = Mass products • Chemical bonds in reactants may break; new bonds may form to produce products • # atoms of each element is “constant” – # atoms same on both sides of equation

Coefficients in Chemical Equations • # in front of formulas called coefficients – apply to everything following in formula • connect microscopic world with macroscopic world – microscopic: coefficients represent # individual atoms/molecules – macroscopic: coefficients represent mole ratios! • moles: connected to mass (MOLE MAP!)

Writing Chemical Equations • begin with word equation – describes reactants & products – convert words to chemical formulas • next is skeleton equation – replace names of substances with chemical formulas • Balance skeleton equation – balanced equation demonstrates law of conservation of mass

Equation Balancing • Survey skeleton equation (left to right) – count # each type atom on reactant side – count # each type atom on product side • COEFFICIENTS used to balance # each type atom • make successive passes checking ONE ELEMENT AT A TIME • NEVER CHANGE SUBSCRIPTS IN FORMULAS – would change identity of reactant/product

Balanced Equations • coefficients must be in lowest possible ratios • double check your work – always do one last pass to check numbers • use RAP table to keep track of # each element

R Example 1 P 1 Fe 2 2 Fe + O 2 Fe 2 O 3 Fe + 3 O 2 2 Fe 2 O 3 • O’s balanced - Now balance Fe 4 Fe + 3 O 2 2 Fe 2 O 3 • do one last check: 4 Fe, 6 O A ✔ O 3

Example 2 Na + H 2 O Na. OH + H 2 R 1 2 1 A Na H O • even # H’s on left, odd # on right (make H’s on right even) Na + H 2 O 2 Na. OH + H 2 • balance Na’s 2 Na + H 2 O 2 Na. OH + H 2 • balance O’s 2 Na + 2 H 2 O 2 Na. OH + H 2 ✔ P 1 3 1

R A 1 Ag 1 NO 3 1 Mg 2 Cl Example 3 P 1 2 1 1 Ag. NO 3 + Mg. Cl 2 Mg(NO 3)2 + Ag. Cl • treat NO 3 -1 as one unit since on both sides of equation 2 Ag. NO 3 + Mg. Cl 2 Mg(NO 3)2 + Ag. Cl • balance Ag & Cl 2 Ag. NO 3 + Mg. Cl 2 Mg(NO 3)2 + 2 Ag. Cl ✔