Pre-University Chemistry

1. Chapter 8Chapter 8::
Balancing ChemicalBalancing Chemical
ReactionsReactions

2. ObjectivesObjectives
by the end of this chapter, you should be able to:by the end of this chapter, you should be able to:
 Write equations describing chemicalWrite equations describing chemical
reactions using appropriate symbolsreactions using appropriate symbols
 Write balanced chemical equations whenWrite balanced chemical equations when
given the names or formulas of thegiven the names or formulas of the
reactants and products in a chemicalreactants and products in a chemical
reactionreaction
 Define chemical equation, catalyst,Define chemical equation, catalyst,
aqueous solution, skeleton equation,aqueous solution, skeleton equation,
coefficients, and balanced equationcoefficients, and balanced equation

3. Chemical EquationsChemical Equations
 Chemical equations – using chemical formulasChemical equations – using chemical formulas
to write equationsto write equations
 Reactants (left side of arrow)Reactants (left side of arrow)
 Products (right side of arrow)Products (right side of arrow)
 Arrow means yields, gives, or reacts to produceArrow means yields, gives, or reacts to produce
 ReactantsReactants  ProductsProducts
 Catalyst (a substance that speeds up the rate ofCatalyst (a substance that speeds up the rate of
the reaction but that is not used up in thethe reaction but that is not used up in the
reaction) should be written above the arrowreaction) should be written above the arrow
CC66HH1212OO66 + CO+ CO22 OO22 + H+ H22O + energyO + energy(s)(s) (g)(g) (g)(g) (l)(l)

4.  Can indicate the physical state of aCan indicate the physical state of a
substance in the equation by putting asubstance in the equation by putting a
symbol after each formulasymbol after each formula
Solid – (Solid – (ss))
Liquid – (Liquid – (ll))
Gas – (Gas – (gg))
Aqueous solution: a substance dissolvedAqueous solution: a substance dissolved
in water – (in water – (aqaq))
 Refer to the Table on page 266 forRefer to the Table on page 266 for
explanations of other symbols used inexplanations of other symbols used in
chemical equationschemical equations

5. Skeleton EquationSkeleton Equation
 A chemical equation that doesA chemical equation that does notnot
indicate the relative amounts of theindicate the relative amounts of the
reactants and products involved in thereactants and products involved in the
reactionreaction
 Examples:Examples:
a. Fe(a. Fe(ss) + O) + O22((gg))  FeFe22OO33((ss))
b. Hb. H22OO22((aqaq))  HH22O(O(ll) + O) + O22((gg))
Manganese(IV) oxide is a catalyst, soManganese(IV) oxide is a catalyst, so
MnOMnO22 should be written above the arrow.should be written above the arrow.

6. Write a Skeleton EquationWrite a Skeleton Equation
 Solid sodium hydrogen carbonate reactsSolid sodium hydrogen carbonate reacts
with hydrochloric acid to produce aqueouswith hydrochloric acid to produce aqueous
sodium chloride, water, and carbonsodium chloride, water, and carbon
dioxide gas. Include appropriate symbols.dioxide gas. Include appropriate symbols.
1. Write the correct formula for each1. Write the correct formula for each
substance in the reaction.substance in the reaction.
2. Separate the reactants from the2. Separate the reactants from the
products.products.
3. Indicate the physical state of each3. Indicate the physical state of each
substance.substance.

7. AnswerAnswer
 NaHCONaHCO33((ss) + HCl() + HCl(aqaq))  NaCl(NaCl(aqaq) + H) + H22O(O(ll))
+ CO+ CO22((gg))

8. A Balanced EquationA Balanced Equation
 An equation that gives the correct quantityAn equation that gives the correct quantity
of each reactant and productof each reactant and product
 Coefficients (numbers placed in front ofCoefficients (numbers placed in front of
the symbols) are usedthe symbols) are used
 Must obey law of conservation of mass:Must obey law of conservation of mass:
Each side of the equation has the sameEach side of the equation has the same
number of atoms of each elementnumber of atoms of each element
 Example: A standard bicycle is composedExample: A standard bicycle is composed
of one frame, two wheels, one handlebar,of one frame, two wheels, one handlebar,
and two pedalsand two pedals
F + 2W + H + 2PF + 2W + H + 2P  FWFW22HPHP22

9. Rules for Balancing EquationsRules for Balancing Equations
 1. Determine the correct formulas for all of the1. Determine the correct formulas for all of the
reactants and products. In some cases, also listreactants and products. In some cases, also list
in parenthesis the physical state of matter.in parenthesis the physical state of matter.
 2. List reactants on the left side of the arrow2. List reactants on the left side of the arrow
(Use plus sign (+) when there is more than one(Use plus sign (+) when there is more than one
reactant)reactant)
 3. List the products on the right side of the3. List the products on the right side of the
arrow (Use plus sign (+) when there is morearrow (Use plus sign (+) when there is more
than one product)than one product)
 4. Steps 1-3 provide a skeleton equation.4. Steps 1-3 provide a skeleton equation.
(Note: Sometimes this is also the balanced(Note: Sometimes this is also the balanced
equation. For example: C + Oequation. For example: C + O22  COCO22))

10.  5. Count the number of atoms of each element5. Count the number of atoms of each element
in the reactants and products. For simplicity, ain the reactants and products. For simplicity, a
polyatomic ion appearing unchanged on bothpolyatomic ion appearing unchanged on both
sides of the arrow is counted as a single unit.sides of the arrow is counted as a single unit.
 6. Balance the elements one at a time by using6. Balance the elements one at a time by using
coefficients. DO NOT CHANGE THEcoefficients. DO NOT CHANGE THE
SUBSCRIPTS.SUBSCRIPTS.
 7. Check each atom or polyatomic ion to be7. Check each atom or polyatomic ion to be
sure that the equation is balanced.sure that the equation is balanced.
 8. Make sure that all the coefficients are in the8. Make sure that all the coefficients are in the
lowest possible ratio that balances.lowest possible ratio that balances.

11. Problem: Hydrogen and oxygen react toProblem: Hydrogen and oxygen react to
form water. Write a balanced equation.form water. Write a balanced equation.
Reactants: HReactants: H22((gg) + O) + O22((gg))
Products: HProducts: H22O(O(ll))
HH22((gg) + O) + O22((gg))  HH22O(O(ll))
Count the atomsCount the atoms
Left side Right sideLeft side Right side
H – 2 H – 2H – 2 H – 2
O – 2 O – 1O – 2 O – 1
 Use coefficient to get 2 oxygen on the right side:Use coefficient to get 2 oxygen on the right side:
HH22((gg) + O) + O22((gg))  2 H2 H22O(O(ll))
Left side Right sideLeft side Right side
H – 2 H – 4H – 2 H – 4
O – 2 O – 2O – 2 O – 2

12.  Need 4 hydrogen atoms, so place aNeed 4 hydrogen atoms, so place a
coefficient of 2 in front of Hcoefficient of 2 in front of H22
2H2H22((gg) + O) + O22((gg))  2 H2 H22O(O(ll))
Left side Right sideLeft side Right side
H – 4 H – 4H – 4 H – 4
O – 2 O – 2O – 2 O – 2
 Check number of atomsCheck number of atoms
 Check that the coefficients are in theCheck that the coefficients are in the
lowest possible ratiolowest possible ratio
 The equation is balancedThe equation is balanced

13. ProblemsProblems
 1. Balance the following equations:1. Balance the following equations:
a. SOa. SO22 + O+ O22  SOSO33
b. Al + Ob. Al + O22  AlAl22OO33
 2. Rewrite the word equation as a2. Rewrite the word equation as a
balanced chemical equation:balanced chemical equation:
Aluminum sulfate and calciumAluminum sulfate and calcium
hydroxide react to form aluminumhydroxide react to form aluminum
hydroxide and calcium sulfate.hydroxide and calcium sulfate.

14. AnswersAnswers
1a) 2SO1a) 2SO22 + O+ O22  2SO2SO33
1b) 4Al + 3O1b) 4Al + 3O22  2Al2Al22OO33
2) Word equation to balanced chemical2) Word equation to balanced chemical
equation:equation:
AlAl22(SO(SO44))33 + 3Ca(OH)+ 3Ca(OH)22  2Al(OH)2Al(OH)33 + 3CaSO+ 3CaSO44

15. Practice ProblemsPractice Problems
 1. __NaCl + __BeF1. __NaCl + __BeF2 __NaF + __BeCl__NaF + __BeCl2
 2. __FeCl2. __FeCl3 + __Be+ __Be3(PO(PO4)) 2 __BeCl__BeCl2 + __FePO+ __FePO4
 3. __AgNO3. __AgNO3 + __LiOH+ __LiOH __AgOH + __LiNO__AgOH + __LiNO3
 4. __CH4. __CH4 + __O+ __O2 __CO__CO2 + __H+ __H2OO
 5. __Mg + __Mn5. __Mg + __Mn2OO3 __MgO + __Mn__MgO + __Mn

16. Types of Chemical ReactionsTypes of Chemical Reactions
ObjectivesObjectives::
 1. Identify a reaction as combination,1. Identify a reaction as combination,
decomposition, single-replacement,decomposition, single-replacement,
double-replacement, or combustiondouble-replacement, or combustion
 2. Predict the products of combination,2. Predict the products of combination,
decomposition, single-replacement,decomposition, single-replacement,
double-replacement, and combustiondouble-replacement, and combustion
reactionsreactions

17. Classifying ReactionsClassifying Reactions
 For combination (synthesis: combination of partsFor combination (synthesis: combination of parts
into a whole) and decomposition, compare theinto a whole) and decomposition, compare the
number of reactants and productsnumber of reactants and products
 For combustion, check for oxygen (OFor combustion, check for oxygen (O22))
 For single- and double-replacement, look for aFor single- and double-replacement, look for a
cation swap or the formation of a precipitatecation swap or the formation of a precipitate
 Not all chemical reactions fit uniquely into onlyNot all chemical reactions fit uniquely into only
one of these classesone of these classes

18. Combination ReactionsCombination Reactions
(AKA- Synthesis Reaction)(AKA- Synthesis Reaction)
 Two or more substances combine to formTwo or more substances combine to form
a single substancea single substance
 Reactants are usually either two elementsReactants are usually either two elements
or two compoundsor two compounds
 The product is always a compound (CanThe product is always a compound (Can
be an ionic compound or a molecularbe an ionic compound or a molecular
compound)compound)
A + B ABA + B AB

19. Decomposition ReactionsDecomposition Reactions
 A single compound is broken down into two orA single compound is broken down into two or
more productsmore products
 The products can be any combination ofThe products can be any combination of
elements and compoundselements and compounds
 Most decomposition reactions require energy inMost decomposition reactions require energy in
the form of heat, light, or electricitythe form of heat, light, or electricity
 Extremely rapid decomposition reactions thatExtremely rapid decomposition reactions that
produce gaseous products and heat are oftenproduce gaseous products and heat are often
the cause of explosionsthe cause of explosions
AB A + BAB A + B

20. Single-replacement ReactionsSingle-replacement Reactions
(Also called single-displacement reactions)(Also called single-displacement reactions)
 One element replaces a second element in aOne element replaces a second element in a
compoundcompound
 Remember: either theRemember: either the anionsanions oror cationscations willwill
switch with each other. They cannot be pairedswitch with each other. They cannot be paired
together since their charges repel each other.together since their charges repel each other.
How do we tell which is which? Use the periodicHow do we tell which is which? Use the periodic
table to predict their oxidation numbers.table to predict their oxidation numbers.
A + BC AC + BA + BC AC + B

21.  Whether one metal will displace anotherWhether one metal will displace another
metal from a compound can bemetal from a compound can be
determined by the relative reactivities ofdetermined by the relative reactivities of
the two metals. (Memorize the symbolsthe two metals. (Memorize the symbols
and activity series of metals on page 286.)and activity series of metals on page 286.)
 A reactive metal will replace any metalA reactive metal will replace any metal
listed below it in the activity serieslisted below it in the activity series
 Examples:Examples:
Iron will displace copper from a copperIron will displace copper from a copper
compound in solution.compound in solution.
Magnesium does not replace lithium fromMagnesium does not replace lithium from
aqueous solutions of their compounds.aqueous solutions of their compounds.

22. The Activity Series of MetalsThe Activity Series of Metals

23. Refer to Table 8.2 on page 217Refer to Table 8.2 on page 217
 Will magnesium displace zinc from a zincWill magnesium displace zinc from a zinc
compound in solution?compound in solution?
 Will magnesium displace silver from a silverWill magnesium displace silver from a silver
compound in solution?compound in solution?
 Important NoteImportant Note::
1. Metals from lithium to lead will replace1. Metals from lithium to lead will replace
hydrogen from acids.hydrogen from acids.
2. Metals from lithium to sodium will also2. Metals from lithium to sodium will also
replace hydrogen from water.replace hydrogen from water.

24. Single-Replacement (cont’d)Single-Replacement (cont’d)
 A nonmetal can also replace another nonmetalA nonmetal can also replace another nonmetal
from a compoundfrom a compound
 This replacement is usually limited to theThis replacement is usually limited to the
halogens (Group 7A):halogens (Group 7A):
FF22 (most activity)(most activity)
ClCl22 ..
BrBr22 ..
II22 (least activity)(least activity)
 The activity of the halogens decreases as youThe activity of the halogens decreases as you
go down group 7A on the periodic tablego down group 7A on the periodic table

25. Double-replacement ReactionsDouble-replacement Reactions
 Involves an exchange of positive ions betweenInvolves an exchange of positive ions between
two reacting compoundstwo reacting compounds
 Often characterized by the production of aOften characterized by the production of a
precipitate (ppt.-insoluble substance that “fallsprecipitate (ppt.-insoluble substance that “falls
out” of a solution)out” of a solution)
 Product may be a gas that “bubbles” out of theProduct may be a gas that “bubbles” out of the
mixturemixture
 Product may be a molecular compound, such asProduct may be a molecular compound, such as
waterwater
AB + CD AD + CBAB + CD AD + CB

26. Combustion ReactionsCombustion Reactions
 An hydrocarbon reacts with oxygen (oftenAn hydrocarbon reacts with oxygen (often
producing energy) with water and carbonproducing energy) with water and carbon
dioxide as productsdioxide as products
 Commonly involve hydrocarbons (compounds ofCommonly involve hydrocarbons (compounds of
hydrogen and carbon)hydrogen and carbon)
 The complete combustion of a hydrocarbonThe complete combustion of a hydrocarbon
produces carbon dioxide and waterproduces carbon dioxide and water
 If the supply of oxygen during a reaction isIf the supply of oxygen during a reaction is
insufficient, combustion will be incompleteinsufficient, combustion will be incomplete
CCxxHHyy + O+ O22 COCO22 + H+ H22OO

27.  During incomplete combustion, elementalDuring incomplete combustion, elemental
carbon and toxic carbon monoxide may becarbon and toxic carbon monoxide may be
additional productsadditional products
 Reaction between some elements andReaction between some elements and
oxygenoxygen
Example: Both magnesium and sulfur willExample: Both magnesium and sulfur will
burn by reaction with oxygenburn by reaction with oxygen
 Refer to worksheet handout.Refer to worksheet handout.
 Identify the combustion reactions.Identify the combustion reactions.

28. Make a Chemistry FoldableMake a Chemistry Foldable
 1. Fold a sheet of notebook paper to the red1. Fold a sheet of notebook paper to the red
margin line.margin line.
 2. Using scissors, cut the folded section into five2. Using scissors, cut the folded section into five
equal parts.equal parts.
 3. Label each section with the name of one of3. Label each section with the name of one of
the five types of reactions.the five types of reactions.
 4. Open each flap and put in three4. Open each flap and put in three
characteristics and one example (includecharacteristics and one example (include
balanced equation).balanced equation).
 5. Write the title :5. Write the title : Types of Chemical ReactionsTypes of Chemical Reactions
on the top of the sheet. Add your name andon the top of the sheet. Add your name and
class.class.
 6. Use the chemistry foldable as a study guide.6. Use the chemistry foldable as a study guide.

29. Predicting Products of a ChemicalPredicting Products of a Chemical
ReactionReaction
 Recognize the possible type of reaction thatRecognize the possible type of reaction that
the reactants can undergothe reactants can undergo
 Some reactions do not fit any one of the fiveSome reactions do not fit any one of the five
general types (Example: redox reactions)general types (Example: redox reactions)
 Oxidation-reduction (redox) reactions will beOxidation-reduction (redox) reactions will be
discussed during the second semesterdiscussed during the second semester
OIL RIGOIL RIG –– ooxidationxidation iis thes the lloss of electronsoss of electrons
andand rreductioneduction iis thes the ggain of electronsain of electrons
LEO the lion says GERLEO the lion says GER –– lloss ofoss of
eelectrons islectrons is ooxidation andxidation and ggain ofain of eelectrons islectrons is

30. Reactions in Aqueous SolutionReactions in Aqueous Solution
 ObjectivesObjectives::
1. Write and balance net ionic equations1. Write and balance net ionic equations
2. Use solubility rules to predict the2. Use solubility rules to predict the
precipitateprecipitate formed in double replacementformed in double replacement
reactionsreactions

31. Net Ionic EquationsNet Ionic Equations
 Most ionic compounds dissociate, or separate,Most ionic compounds dissociate, or separate,
into cations and anions when they dissolve ininto cations and anions when they dissolve in
water.water.
 Refer to question #21 on the worksheet handout.Refer to question #21 on the worksheet handout.
Use this equation to answer #22 on theUse this equation to answer #22 on the
worksheet handout.worksheet handout.
 Write aWrite a complete ionic equationcomplete ionic equation that showsthat shows
dissolved ionic compounds as their free ions.dissolved ionic compounds as their free ions.
 Eliminate ions that do not participate in theEliminate ions that do not participate in the
reaction by canceling ions that appear on bothreaction by canceling ions that appear on both
sides of the equation. These are called spectatorsides of the equation. These are called spectator
ions.ions.

32.  Ions that are not directly involved in a reactionIons that are not directly involved in a reaction
are calledare called spectator ions.spectator ions.
 Rewrite the equation, leaving out the canceledRewrite the equation, leaving out the canceled
spectator ions.spectator ions.
 Balance the atoms and the charges of the ions.Balance the atoms and the charges of the ions.
(In this case, the number of atoms and the net(In this case, the number of atoms and the net
ionic charge on each side of the equation is zeroionic charge on each side of the equation is zero
and it is therefore balanced.)and it is therefore balanced.)
 A net ionic equation indicates only thoseA net ionic equation indicates only those
particles that actually take part in the reaction.particles that actually take part in the reaction.
 Record your answer to #23 on the worksheetRecord your answer to #23 on the worksheet
handout.handout.

33. Practice ProblemPractice Problem
 Write a balanced net ionic equation for the followingWrite a balanced net ionic equation for the following
reaction:reaction:
Pb(Pb(ss) + AgNO) + AgNO33 ((aqaq))  Ag (Ag (ss) + Pb(NO) + Pb(NO33))22 ((aqaq))
Answer:Answer:
1. The nitrate ion is the spectator ion.1. The nitrate ion is the spectator ion.
2. The number of atoms balance, but the charges on2. The number of atoms balance, but the charges on
the ions do not balance.the ions do not balance.
3. Place a coefficient 2 in front of Ag3. Place a coefficient 2 in front of Ag++
((aqaq) to balance) to balance
the charges.the charges.
4. A coefficient of 2 in front of Ag (4. A coefficient of 2 in front of Ag (ss) rebalances the) rebalances the
atoms.atoms.
5. Pb(5. Pb(ss) + 2Ag) + 2Ag++
((aqaq))  2Ag (2Ag (ss) + Pb) + Pb2+2+
((aqaq) is the) is the
balanced net ionic equationbalanced net ionic equation

34. Predicting the Formation of aPredicting the Formation of a
PrecipitatePrecipitate
 Use the general rules for solubility of ionicUse the general rules for solubility of ionic
compounds (Table 8.3 on page 227)compounds (Table 8.3 on page 227)
 Examples:Examples:
1. Sodium nitrite will not form a precipitate1. Sodium nitrite will not form a precipitate
because alkali metal salts and nitrate saltsbecause alkali metal salts and nitrate salts
are soluble (Rules 1 and 2)are soluble (Rules 1 and 2)
2. Rule 3 (Exceptions) indicates that2. Rule 3 (Exceptions) indicates that
barium sulfate is insoluble and thereforebarium sulfate is insoluble and therefore
will precipitate.will precipitate.

35. Solubility Rules for Ionic CompoundsSolubility Rules for Ionic Compounds
CompoundsCompounds SolubilitySolubility ExceptionsExceptions
1. Salts of alkali metals1. Salts of alkali metals
and ammoniaand ammonia
SolubleSoluble Some lithiumSome lithium
compoundscompounds
2. Nitrate salts and2. Nitrate salts and
chlorate saltschlorate salts
SolubleSoluble Few ExceptionsFew Exceptions
3. Sulfate salts3. Sulfate salts SolubleSoluble Compounds of Pb, Ag,Compounds of Pb, Ag,
Hg, Ba, Sr, and CaHg, Ba, Sr, and Ca
4. Chloride salts4. Chloride salts SolubleSoluble Compounds of Ag andCompounds of Ag and
some compounds of Hgsome compounds of Hg
and Pband Pb
5. Carbonates,5. Carbonates,
phosphates,phosphates,
chromates, sulfides,chromates, sulfides,
and hydroxidesand hydroxides
Most are insolubleMost are insoluble Compounds of theCompounds of the
alkali metals and ofalkali metals and of
ammoniaammonia

36. Practice ProblemPractice Problem
 Identify the precipitate formed and writeIdentify the precipitate formed and write
the net ionic equation for the reaction ofthe net ionic equation for the reaction of
aqueous potassium carbonate withaqueous potassium carbonate with
aqueous strontium chloride.aqueous strontium chloride.
1. Write the reactants showing each as1. Write the reactants showing each as
dissociated free ions. Balance thedissociated free ions. Balance the
charges.charges.
2. Using solubility rules, look at possible2. Using solubility rules, look at possible
new pairings of cation and anion that givenew pairings of cation and anion that give
an insoluble substance.an insoluble substance.
3. Eliminate the spectator ions and write3. Eliminate the spectator ions and write
the net ionic equation.the net ionic equation.

37. AnswerAnswer
 1. Reactants as dissociated free ions1. Reactants as dissociated free ions
2K2K++
((aqaq) + CO) + CO33
2-2-
((aqaq) + Sr) + Sr2+2+
((aqaq) + 2Cl) + 2Cl--
((aqaq))
Charges must be balanced to equal 0.Charges must be balanced to equal 0.
 2. Of the two possible combinations, KCl is soluble2. Of the two possible combinations, KCl is soluble
(Rules 1 and 4) and SrCO(Rules 1 and 4) and SrCO33 is insoluble (Rule 5)is insoluble (Rule 5)
 3. The net ionic equation must be balanced for the3. The net ionic equation must be balanced for the
number of atoms of each element and the chargesnumber of atoms of each element and the charges
on the ions.on the ions.
SrSr2+2+
((aqaq) + CO) + CO33
2-2-
((aqaq))  SrCOSrCO33 ((ss))
 Note: Ignore Sample Problem 8-11 on page 228.Note: Ignore Sample Problem 8-11 on page 228.
There is a textbook error.There is a textbook error.