Stoichiometry

What is stoichiometry?

• study of quantitative relationships in balanced chemical equation

• Equations represent chemical reactions

Physical vs. Chemical Change

Physical Change =Physical Change =

Chemical Change = Chemical Change =

•Change in form/appearance•Examples:

•Dissolving & phase changes

•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, c …• Formation of Gas

– Bubbling, odor• Formation of Solid

– Precipitation

Chemical Equations

A + B C + DLeft 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)(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 each element same on both sides # atoms each element same on both sides

of equationof equation

Coefficients in Chemical Equations• # in front ofin 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 equationword equation – describes what happens

• Next is skeleton equationskeleton equation – replace names of substances with chemical

formulas• BalanceBalance skeleton equation

– balanced equation must demonstrate law of conservation of mass

Equation Balancing

• SurveySurvey 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

• COEFFICIENTSCOEFFICIENTS used to balance # each type atom • Make successive passes checking ONE ELEMENT Make successive passes checking ONE ELEMENT

AT A TIMEAT 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 + OFe + O22 Fe Fe22OO33

• 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 + Fe + 33 O O22 22 Fe Fe22OO33

• O’s are balanced - Now balance Fe44 Fe + 3 O Fe + 3 O22 2 Fe 2 Fe22OO33

• 4 Fe and 6 O each side

Example 2

Na + HNa + H22O O NaOH + H NaOH + H22

• 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 + Na + HH22O O 2 NaO 2 NaOHH + + HH22

• Put a 2 in front of Na to balance Na’s2 Na + H2 Na + H22O O 2 NaOH + H 2 NaOH + H22

• 2 in front of H2O to balance O and H2 Na + 2 H2 Na + 2 H22O O 2 NaOH + H 2 NaOH + H22

Example 3

AgNOAgNO33 + MgCl + MgCl22 Mg(NO Mg(NO33))22 + AgCl + 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 AgNO2 AgNO33 + MgCl + MgCl22 Mg(NO Mg(NO33))22 + AgCl + AgCl• Balance Ag’s and Cl with a 2

2 AgNO2 AgNO33 + MgCl + MgCl22 Mg(NO Mg(NO33))22 + 2 AgCl + 2 AgCl

Chemical Reactions

Types of Reactions

• Synthesis• Decomposition• Single Replacement• Double Replacement• Combustion

SynthesisFormat: A + B C

Identifying feature: 1 product only

Note: A and B may be elements or compounds, C is a compound

2 Fe(s) + 3 Cl2 Fe(s) + 3 Cl22(g) (g) 2 FeCl 2 FeCl33(s)(s)

CaO(s) + HCaO(s) + H22O(l) O(l) Ca(OH) Ca(OH)22(s)(s)

DecompositionFormat: AB A + B

Identifying feature: 1 reactant only

Note: A & B may be elements or compounds

2 NaN2 NaN33(s) (s) 2 Na(s) + 3 N 2 Na(s) + 3 N22(g)(g)

NHNH44NONO33(s) (s) N N22O(g) + 2 HO(g) + 2 H22O(g)O(g)

Single ReplacementFormat: A + BX AX + B

Identifying Feature: Element + Compound Different Element + Different Compound

2 Li(s) + 2 H2 Li(s) + 2 H22O(l) O(l) 2 LiOH(aq) + H 2 LiOH(aq) + H22(g)(g)

Cu(s) + 2AgNOCu(s) + 2AgNO33(aq) (aq) 2Ag(s) + Cu(NO 2Ag(s) + Cu(NO33))22(aq)(aq)

Double ReplacementFormat: AX + BY AY + BX

Identifying feature: 2 compounds yield 2 new compounds

Ca(OH)Ca(OH)22(aq) + 2HCl(aq) (aq) + 2HCl(aq) CaCl CaCl22(aq) +(aq) + 2 H2 H22O(l)O(l)

2 NaOH(aq) + CuCl2 NaOH(aq) + CuCl22(aq) (aq) 2 NaCl(aq) + 2 NaCl(aq) + Cu(OH)Cu(OH)22(s)(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

CHCH44(g) + 2 O(g) + 2 O22(g) (g) CO CO22(g) + 2 H(g) + 2 H22O(g)O(g)

For Regents For Regents synthesissynthesis TOPS combustion: TOPS combustion: 2 H2(g) + O2(g) 2 H2O(g)

C(s) + O2(g) CO2(g)

Identify the Reaction Type

• CaO + COCaO + CO22 CaCO CaCO33

• 2 H2 H22O O 2 H 2 H22 + O + O22

• NaOH + HCl NaOH + HCl NaCl + H NaCl + H22OO• Zn + 2 HCl Zn + 2 HCl ZnCl ZnCl22 + H + H22

• 2 Mg + O2 Mg + O22 2 MgO 2 MgO• Mg + HMg + H22SOSO44 MgSO MgSO44 + H + H22

• 2 KClO2 KClO33 2 KCl + 3 O 2 KCl + 3 O22

• AgNOAgNO33 + NaCl + NaCl NaNO NaNO33 + AgCl + AgCl

Synthesis

Synthesis

Decomposition

Decomposition

DR

DR

SR

SR

Stoichiometry!!!

Next chapter:Learn to use balanced chemical equations to

predict amount of given reactant or product under certain conditions

Remember: DON’T mess with the subscripts !!