Stoichiometry

What is stoichiometry? The study of quantitative relationships in a balanced chemical equation Equations represent chemical reactions

Physical vs. Chemical Change Physical Change = Chemical Change = Change in form/appearance Examples: Dissolving & phase changes Change in identity Change in formula representing a substance

Evidence of a Chemical Reaction Temperature change Emission of Light Energy Change in identifying property: Color, mp, bp, density, Hf, Hv, c … Formation of Gas Bubbling, odor Formation of Solid Precipitation

Chemical Equations A + B  C + D Left Side = Reactants (starting materials) Right Side = Products (ending materials) “” is 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 a chemical reaction Mass reactants = Mass products Chemical bonds in reactants may break New bonds may form to produce products Number of atoms of each element is “constant” # of atoms of each element is the same on both sides of the equation

Coefficients in Chemical Equations Numbers in front of formulas called coefficients apply to everything following in formula Connect microscopic world with macroscopic world Microscopic: Coefficients represent numbers of individual atoms or molecules Macroscopic: Coefficients give mole ratios! Moles  connected to mass (MOLE MAP!)

Writing Chemical Equations Begin with word equation describes what happens Next go to skeleton equation replace names of substances with chemical formulas Balance skeleton equation balanced equation must demonstrate law of conservation of mass

Equation Balancing Survey skeleton equation (left to right) Count up # of each type of atom on reactant side Count up # of each type of atom on product side Use COEFFICIENTS to balance the numbers of each type of 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 Do one last pass to check that all # are same on both sides Use table to keep track of # each element

Example 1 Fe + O2  Fe2O3 Left Side: 1 Fe and 2 O Right Side: 2 Fe and 3 O Hint: LCM of 2 and 3 = 6 Get O to 6 Fe + 3 O2  2 Fe2O3 O’s are balanced - Now balance Fe 4 Fe + 3 O2  2 Fe2O3 4 Fe and 6 O each side

Example 2 Na + H2O  NaOH + H2 Notice: even # H’s on left, odd # on right Have to make # H’s on right even Put a 2 in front of NaOH Na + H2O  2 NaOH + H2 Put a 2 in front of Na to balance Na’s 2 Na + H2O  2 NaOH + H2 2 in front of H2O to balance O and H 2 Na + 2 H2O  2 NaOH + H2

Example 3 AgNO3 + MgCl2  Mg(NO3)2 + AgCl Hint: Treat NO3-1 as one unit since appears on both sides of equation Balance NO3-1 on the left with a 2 2 AgNO3 + MgCl2  Mg(NO3)2 + AgCl Balance Ag’s and Cl with a 2 2 AgNO3 + MgCl2  Mg(NO3)2 + 2 AgCl

Chemical Reactions

Types of Reactions Synthesis Decomposition Single Replacement Double Replacement Combustion

Synthesis Format: A + B  C Identifying feature: 1 product only Note: A and B may be elements or compounds, C is a compound Ex: 2 Fe(s) + 3 Cl2(g)  2 FeCl3(s) 2 Na(s) + Cl2(g)  2 NaCl(s) CaO(s) + H2O(l)  Ca(OH)2(s)

Decomposition Format: AB  A + B Identifying feature: 1 reactant only Note: A & B may be elements or compounds Ex: 2 NaN3(s)  2 Na(s) + 3 N2(g) NH4NO3(s)  N2O(g) + 2 H2O(g)

Single Replacement Format: A + BX  AX + B Identifying Feature: Element + Compound  Different Element + Different Compound Ex: 2 Li(s) + 2 H2O(l)  2 LiOH(aq) + H2(g) Cu(s) + 2AgNO3(aq)  2Ag(s) + Cu(NO3)2(aq)

Double Replacement Format: AX + BY  AY + BX Identifying feature: 2 compounds yield 2 new compounds Ex: Ca(OH)2(aq) + 2HCl(aq)  CaCl2(aq) + 2 H2O(l) 2 NaOH(aq) + CuCl2(aq)  2 NaCl(aq) + Cu(OH)2(s)

Combustion or Reaction with O2 Format: A + O2  B ( + C + …) Identifying feature: One of reactants is O2 Note: A can be an element or a compound, usually more than 1 product Ex: CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(g) For Regents synthesis tops combustion: 2 H2(g) + O2(g)  2 H2O(g) C(s) + O2(g)  CO2(g)

Identify the Reaction Type CaO + CO2  CaCO3 2 H2O  2 H2 + O2 NaOH + HCl  NaCl + H2O Zn + 2 HCl  ZnCl2 + H2 2 Mg + O2  2 MgO Mg + H2SO4  MgSO4 + H2 2 KClO3  2 KCl + 3 O2 AgNO3 + NaCl  NaNO3 + AgCl Synthesis Decomposition DR SR Synthesis SR Decomposition DR

Stoichiometry!!! Use balanced chemical equations to predict amount of given reactant or product under certain conditions Remember: DON’T mess with the subscripts !!