Stoichiometry & Equations

What is stoichiometry?

• The study of quantitative relationships in a balanced chemical equation.

• Equations represent reactions.

Physical vs. Chemical Change

• Physical Change =Physical Change =

• Chemical Change = Chemical Change =

Change in form or appearance.Dissolving and phase changes are 2 kinds of physical changes.

Change in identity.Change in formula representing substance.

Chemical Change

2 H2O 2 H2 + O2

• How do I know this is a chemical change?

• The formulas DO NOT match.

Physical Change

• H2O (s) H2O(l)

• NaCl(s) NaCl(l)

• The formulas match! The identity of the substance does not change. Only the form or appearance is altered.

Names of some common reactions

• Corrosion or rusting• Burning• Rotting• Fermentation• Growing• Most cooking• Generating electricity• Running car engines• Decomposition

Evidence of a Chemical Reaction

• Temperature change– Increase or Decrease

• Emission of Light Energy• Change in identifying property:

Color, mp, bp, density, Hf, Hv, C …• Formation of a Gas

– Bubbling, odor• Formation of a Solid

– Precipitation

Ammonium Dichromate Volcano

Chemical Equations

A + B C + DLeft Side = Reactants or starting materials

Right Side = Products or ending materials

“” is read as produces or 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 a chemical reaction.

• Mass reactants = Mass productsMass reactants = Mass products• Chemical bonds in reactants may

break.• New bonds form to produce products• Number of atoms of each element is

“constant.”• # of atoms of each element is the # of atoms of each element is the

same on both sides of the equation.same on both sides of the equation.

Coefficients in Chemical Equations

•Coefficients are the numbers in front ofin front of the formulas.

•They apply to everything in the given formula.

Coefficients in Chemical Equations

• Coefficients connect the microscopic world with the macroscopic world.– Microscopic: Coefficients represent

numbers of individual atoms or molecules

– Macroscopic: Coefficients give mole ratios!

– Moles connected to mass through formula mass

Writing Chemical Equations

• Begin with word equationword equation – describe what happens

• Next go to skeleton equationskeleton equation – replace names of substances with chemical formulas

• BalanceBalance the skeleton equation. The balanced equation must demonstrate the law of conservation of mass.

Equation Balancing

• SurveySurvey the skeleton equation– Count up the # of each type of atom on reactant side– Count up the # of each type of atom on product side

• Use COEFFICIENTSCOEFFICIENTS to adjust the numbers of each type of atom.

• CheckCheck atom counts after every change.• NEVER CHANGE SUBSCRIPTS IN FORMULAS

– That’s like changing the identity of the reactant or product.

Balanced Equations

• Coefficients should be in their lowest possible ratios

• Double check your work. Count up the atoms of each element on both sides – they should be equal

• Get in the habit of using a table to keep track of elements

Example 1Fe + 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 both O to 6

Fe + 3 OFe + 3 O22 2 Fe 2 Fe22OO33

• O’s are balanced. Fix Fe.

4 Fe + 3 O4 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 left even. Put a 2

in front of NaOH

Na + HNa + H22O O 2 NaOH + H 2 NaOH + H22

• Put a 2 in front of Na to balance Na’s

2 Na + H2 Na + H22O O 2 NaOH + H 2 NaOH + H22

• 2 in front of H2O to balance O and H

2 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 the NO3- as a unit, because

it appears unchanged on both sides of eq. Balance nitrates 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 + 2 AgClAgCl

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

2 Na(s) + Cl2 Na(s) + Cl22(g) (g) 2 NaCl(s) 2 NaCl(s)

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

Synthesis Reactions

• Burning of coal – sulfur impurities are oxidized to form SO2 (Also combustion)

• RainS1O2.movAl + Br2

Fe + Cl2

DecompositionFormat: AB A + B

Identifying feature: 1 reactant only

Note: A & B may be elements or compounds

Ex: 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 New Element + New CompoundEx: 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)

Cu + 2AgNO3

Zn + 2AgNO3

Double ReplacementFormat: AX + BY AY + BX

Identifying feature: 2 compounds yield 2 new compounds

Ex: 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)

Double Replacement Rxns

• Cu(NO3)2 + NaOH

• NaI + KNO3 No rxn

Combustion or Reaction with O2

Format: A + O2 B ( + C + …)

Identifying feature: One reactant is O2.

Note: “A” can be an element or a compound.Usually more than 1 product.Ex: CHCH44(g) + 2 O(g) + 2 O22(g) (g) CO CO22(g) + 2 H(g) + 2 H22O(g)O(g)

2 H2(g) + O2(g) 2 H2O(g)

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

For the Regents, For the Regents, synthesis trumps synthesis trumps combustion! combustion!

Identify the Rxn 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

Dbl. Rep.

Dbl. Rep.

Single Rep.

Single Rep.

Stoichiometry!!!

• Use the balanced chemical equation to predict the amount of a given reactant or product under certain conditions.