Stoichiometry

What is stoichiometry? study of quantitative relationships in balanced chemical equations chemical equations illustrate chemical reactions

Physical vs. Chemical Change Physical Change: Chemical Change: change in form/appearance examples: dissolving phase changes change in identity change in chemical formula

Evidence of a Chemical Reaction 5 types of evidence to look for: temperature change endothermic/exothermic emission of light energy change in identifying property: color, mp, bp, density, H f, H v, … 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 productsmass 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 # atoms same on both sides of equation

Coefficients in Chemical Equations in front of# 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 word equationbegin with word equation –describes reactants & products –convert words to chemical formulas skeleton equationnext is skeleton equation –replace names of substances with chemical formulas balancebalance skeleton equation –balanced equation demonstrates law of conservation of mass

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

Balanced Equations co-efficients 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

Example 1 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 R A P 1 Fe 2 1 Fe 2 2 O 3 2 O 3 ✔

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

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