Chapter 8Chapter 8::Balancing Chemical Balancing Chemical

ReactionsReactions

ObjectivesObjectivesby the end of this chapter, you should be able to:by the end of this chapter, you should be able to:

Write equations describing chemical Write equations describing chemical reactions using appropriate symbolsreactions using appropriate symbols

Write balanced chemical equations Write balanced chemical equations when given the names or formulas of when given the names or formulas of the reactants and products in a the reactants and products in a chemical reactionchemical reaction

Define chemical equation, catalyst, Define chemical equation, catalyst, aqueous solution, skeleton equation, aqueous solution, skeleton equation, coefficients, and balanced equation coefficients, and balanced equation

Chemical EquationsChemical Equations Chemical equations – using chemical formulas Chemical 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 Reactants Reactants Products Products Catalyst (a substance that speeds up the rate of Catalyst (a substance that speeds up the rate of

the reaction but that is not used up in the the reaction but that is not used up in the reaction) should be written above the arrowreaction) should be written above the arrow

CC66HH1212OO66 + CO + CO2 2 O O2 2 + H + H22O + energyO + energy(s)(s) (g)(g) (g)(g) (l)(l)

Can indicate the physical state of a Can indicate the physical state of a substance in the equation by putting substance in the equation by putting a symbol after each formulaa symbol after each formula

Solid – (Solid – (ss)) Liquid – (Liquid – (ll)) Gas – (Gas – (gg)) Aqueous solution: a substance Aqueous solution: a substance

dissolved in water – (dissolved in water – (aqaq)) Refer to the Table on page 266 for Refer to the Table on page 266 for

explanations of other symbols used explanations of other symbols used in chemical equations in chemical equations

Skeleton EquationSkeleton Equation A chemical equation that does A chemical equation that does notnot

indicate the relative amounts of the indicate the relative amounts of the reactants and products involved in the reactants and products involved in the reaction reaction

Examples:Examples:

a. Fe(a. Fe(ss) + O) + O22((gg) ) Fe Fe22OO33((ss))

b. Hb. H22OO22((aqaq) ) H H22O(O(ll) + O) + O22((gg) ) Manganese(IV) oxide is a catalyst, so Manganese(IV) oxide is a catalyst, so

MnOMnO22 should be written above the arrow. should be written above the arrow.

Write a Skeleton EquationWrite a Skeleton Equation Solid sodium hydrogen carbonate reacts Solid sodium hydrogen carbonate reacts

with hydrochloric acid to produce with hydrochloric acid to produce aqueous sodium chloride, water, and aqueous sodium chloride, water, and carbon dioxide gas. Include appropriate carbon dioxide gas. Include appropriate symbols.symbols.

1. Write the correct formula for each 1. Write the correct formula for each substance in the reaction.substance in the reaction. 2. Separate the reactants from the 2. Separate the reactants from the products.products. 3. Indicate the physical state of each 3. Indicate the physical state of each substance.substance.

AnswerAnswer

NaHCONaHCO33((ss) + HCl() + HCl(aqaq) ) NaCl( NaCl(aqaq) + ) + HH22O(O(ll) + CO) + CO22((gg))

A Balanced EquationA Balanced Equation An equation that gives the correct An equation that gives the correct

quantity of each reactant and productquantity of each reactant and product Coefficients (numbers placed in front of Coefficients (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 same Each side of the equation has the same number of atoms of each elementnumber of atoms of each element

Example: A standard bicycle is Example: A standard bicycle is composed of one frame, two wheels, composed of one frame, two wheels, one handlebar, and two pedalsone handlebar, and two pedals

F + 2W + H + 2P F + 2W + H + 2P FW FW22HPHP22

Rules for Balancing EquationsRules for Balancing Equations 1. Determine the correct formulas for all of 1. Determine the correct formulas for all of

the reactants and products. In some cases, the reactants and products. In some cases, also list in parenthesis the physical state of also list in parenthesis the physical state of matter.matter.

2. List reactants on the left side of the 2. List reactants on the left side of the arrow (Use plus sign (+) when there is more arrow (Use plus sign (+) when there is more than one reactant)than one reactant)

3. List the products on the right side of the 3. List the products on the right side of the arrow (Use plus sign (+) when there is more arrow (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 CO CO22))

5. Count the number of atoms of each 5. Count the number of atoms of each element in the reactants and products. element in the reactants and products. For simplicity, a polyatomic ion appearing For simplicity, a polyatomic ion appearing unchanged on both sides of the arrow is unchanged on both sides of the arrow is counted as a single unit.counted as a single unit.

6. Balance the elements one at a time by 6. Balance the elements one at a time by using coefficients. DO NOT CHANGE THE using coefficients. DO NOT CHANGE THE SUBSCRIPTS.SUBSCRIPTS.

7. Check each atom or polyatomic ion to 7. Check each atom or polyatomic ion to be sure that the equation is balanced.be sure that the equation is balanced.

8. Make sure that all the coefficients are 8. Make sure that all the coefficients are in the lowest possible ratio that balances.in the lowest possible ratio that balances.

Problem: Hydrogen and oxygen react to Problem: 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) ) H H22O(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 Use coefficient to get 2 oxygen on the right

side: side: HH22((gg) + O) + O22((gg) ) 2 H 2 H22O(O(ll) ) Left side Right sideLeft side Right side H – 2 H – 4H – 2 H – 4 O – 2 O – 2 O – 2 O – 2

Need 4 hydrogen atoms, so place a Need 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 H 2 H22O(O(ll) ) Left side Right sideLeft side Right side H – 4 H – 4H – 4 H – 4 O – 2 O – 2 O – 2 O – 2 Check number of atomsCheck number of atoms Check that the coefficients are in the Check that the coefficients are in the

lowest possible ratiolowest possible ratio The equation is balancedThe equation is balanced

ProblemsProblems 1. Balance the following equations:1. Balance the following equations:

a. SOa. SO22 + O + O22 SO SO33

b. Al + Ob. Al + O22 Al Al22OO33

2. Rewrite the word equation as a 2. Rewrite the word equation as a balanced chemical equation:balanced chemical equation:

Aluminum sulfate and calcium Aluminum sulfate and calcium hydroxide react to form aluminum hydroxide react to form aluminum

hydroxide and calcium sulfate.hydroxide and calcium sulfate.

AnswersAnswers

1a) 2SO1a) 2SO2 2 + O+ O22 2SO 2SO33

1b) 4Al + 3O1b) 4Al + 3O22 2Al 2Al22OO33

2) Word equation to balanced chemical 2) Word equation to balanced chemical

equation:equation:

AlAl22(SO(SO44))3 3 + 3Ca(OH)+ 3Ca(OH)22 2Al(OH) 2Al(OH)33 + 3CaSO + 3CaSO44

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 + __H2O O

5. __Mg + __Mn5. __Mg + __Mn2OO3 __MgO + __Mn__MgO + __Mn

Types of Chemical ReactionsTypes of Chemical Reactions

ObjectivesObjectives:: 1. Identify a reaction as 1. Identify a reaction as

combination, decomposition, single-combination, decomposition, single-replacement, double-replacement, or replacement, double-replacement, or combustioncombustion

2. Predict the products of 2. Predict the products of combination, decomposition, single-combination, decomposition, single-replacement, double-replacement, replacement, double-replacement, and combustion reactionsand combustion reactions

Classifying ReactionsClassifying Reactions For combination (synthesis: combination of For combination (synthesis: combination of

parts into a whole) and decomposition, parts into a whole) and decomposition, compare the number of reactants and compare the number of reactants and productsproducts

For combustion, check for oxygen (OFor combustion, check for oxygen (O22) )

For single- and double-replacement, look for a For 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 Not all chemical reactions fit uniquely into only one of these classesonly one of these classes

Combination ReactionsCombination Reactions(AKA- Synthesis Reaction)(AKA- Synthesis Reaction)

Two or more substances combine to Two or more substances combine to form a single substanceform a single substance

Reactants are usually either two Reactants are usually either two elements or two compounds elements or two compounds

The product is always a compound The product is always a compound (Can be an ionic compound or a (Can be an ionic compound or a molecular compound)molecular compound)

A + B ABA + B AB

Decomposition ReactionsDecomposition Reactions A single compound is broken down into A single compound is broken down into

two or more productstwo or more products

The products can be any combination of The products can be any combination of elements and compoundselements and compounds

Most decomposition reactions require Most decomposition reactions require energy in the form of heat, light, or energy in the form of heat, light, or electricityelectricity

Extremely rapid decomposition reactions Extremely rapid decomposition reactions that produce gaseous products and heat that produce gaseous products and heat are often the cause of explosionsare often the cause of explosions

AB A + BAB A + B

Single-replacement ReactionsSingle-replacement Reactions (Also called single-displacement reactions)(Also called single-displacement reactions) One element replaces a second element in a One element replaces a second element in a

compoundcompound

Remember: either the Remember: either the anionsanions or or cationscations will will switch with each other. They cannot be paired switch 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

Whether one metal will displace Whether one metal will displace another metal from a compound can another metal from a compound can be determined by the relative be determined by the relative reactivities of the two metals. reactivities of the two metals. (Memorize the symbols and activity (Memorize the symbols and activity series of metals on page 286.)series of metals on page 286.)

A reactive metal will replace any metal A reactive metal will replace any metal listed below it in the activity series listed below it in the activity series

Examples: Examples: Iron will displace copper from a Iron will displace copper from a

copper compound in solution.copper compound in solution. Magnesium does not replace lithium Magnesium does not replace lithium

from aqueous solutions of their from aqueous solutions of their compounds. compounds.

The Activity Series of MetalsThe Activity Series of Metals

Refer to Table 8.2 on page 217Refer to Table 8.2 on page 217

Will magnesium displace zinc from a zinc Will magnesium displace zinc from a zinc compound in solution?compound in solution?

Will magnesium displace silver from a Will magnesium displace silver from a silver compound in solution?silver compound in solution?

Important NoteImportant Note: :

1. Metals from lithium to lead will replace 1. Metals from lithium to lead will replace

hydrogen from acids.hydrogen from acids.

2. Metals from lithium to sodium will also 2. Metals from lithium to sodium will also

replace hydrogen from water.replace hydrogen from water.

Single-Replacement (cont’d)Single-Replacement (cont’d) A nonmetal can also replace another A nonmetal can also replace another

nonmetal from a compoundnonmetal from a compound This replacement is usually limited to the This replacement is usually limited to the

halogens (Group 7A):halogens (Group 7A):

FF2 2 (most activity)(most activity)

ClCl2 2 ..

BrBr2 2 ..

II2 2 (least activity)(least activity) The activity of the halogens decreases as The activity of the halogens decreases as

you go down group 7A on the periodic tableyou go down group 7A on the periodic table

Double-replacement ReactionsDouble-replacement Reactions Involves an exchange of positive ions Involves an exchange of positive ions

between two reacting compoundsbetween two reacting compounds

Often characterized by the production of a Often characterized by the production of a precipitate (ppt.-insoluble substance that precipitate (ppt.-insoluble substance that “falls out” of a solution)“falls out” of a solution)

Product may be a gas that “bubbles” out Product may be a gas that “bubbles” out of the mixtureof the mixture

Product may be a molecular compound, Product may be a molecular compound, such as watersuch as water

AB + CD AD + CBAB + CD AD + CB

Combustion ReactionsCombustion Reactions An hydrocarbon reacts with oxygen (often An hydrocarbon reacts with oxygen (often

producing energy) with water and carbon producing energy) with water and carbon dioxide as productsdioxide as products

Commonly involve hydrocarbons Commonly involve hydrocarbons (compounds of hydrogen and carbon)(compounds of hydrogen and carbon)

The complete combustion of a hydrocarbon The complete combustion of a hydrocarbon produces carbon dioxide and waterproduces carbon dioxide and water

If the supply of oxygen during a reaction is If the supply of oxygen during a reaction is insufficient, combustion will be incompleteinsufficient, combustion will be incomplete

CCxxHHyy + O + O22 CO CO22 + H + H22OO

During incomplete combustion, During incomplete combustion, elemental carbon and toxic carbon elemental carbon and toxic carbon monoxide may be additional productsmonoxide may be additional products

Reaction between some elements Reaction between some elements and oxygenand oxygen

Example: Both magnesium and Example: Both magnesium and sulfur will burn by reaction with sulfur will burn by reaction with oxygenoxygen

Refer to worksheet handout.Refer to worksheet handout. Identify the combustion reactions. Identify the combustion reactions.

Make a Chemistry FoldableMake a Chemistry Foldable 1. Fold a sheet of notebook paper to the 1. Fold a sheet of notebook paper to the

red margin line.red margin line. 2. Using scissors, cut the folded section 2. Using scissors, cut the folded section

into five equal parts.into five equal parts. 3. Label each section with the name of 3. Label each section with the name of

one of the five types of reactions.one of the five types of reactions. 4. Open each flap and put in three 4. Open each flap and put in three

characteristics and one example (include characteristics and one example (include balanced equation).balanced equation).

5. Write the title : 5. Write the title : Types of Chemical Types of Chemical ReactionsReactions on the top of the sheet. Add on the top of the sheet. Add your name and class.your name and class.

6. Use the chemistry foldable as a study 6. Use the chemistry foldable as a study guide. guide.

Predicting Products of a Chemical Predicting Products of a Chemical ReactionReaction

Recognize the possible type of reaction that Recognize the possible type of reaction that the reactants can undergothe reactants can undergo

Some reactions do not fit any one of the five Some reactions do not fit any one of the five general types (Example: redox reactions)general types (Example: redox reactions)

Oxidation-reduction (redox) reactions will be Oxidation-reduction (redox) reactions will be discussed during the second semesterdiscussed during the second semester

OIL RIGOIL RIG – – ooxidation xidation iis the s the lloss of electrons oss of electrons and and rreduction eduction iis the s the ggain of electrons ain of electrons LEO the lion says GERLEO the lion says GER – – lloss of oss of eelectrons lectrons is is ooxidation and xidation and ggain of ain of eelectrons is lectrons is rreductioneduction

Reactions in Aqueous SolutionReactions in Aqueous Solution

ObjectivesObjectives::

1. Write and balance net ionic 1. Write and balance net ionic equationsequations

2. Use solubility rules to predict the 2. Use solubility rules to predict the precipitateprecipitate formed in double formed in double replacement reactionsreplacement reactions

Net Ionic EquationsNet Ionic Equations Most ionic compounds dissociate, or Most ionic compounds dissociate, or

separate, into cations and anions when separate, into cations and anions when they dissolve in water.they dissolve in water.

Refer to question #21 on the worksheet Refer to question #21 on the worksheet handout. Use this equation to answer #22 handout. Use this equation to answer #22 on the worksheet handout.on the worksheet handout.

Write a Write a complete ionic equationcomplete ionic equation that that shows dissolved ionic compounds as their shows dissolved ionic compounds as their free ions.free ions.

Eliminate ions that do not participate in Eliminate ions that do not participate in the reaction by canceling ions that appear the reaction by canceling ions that appear on both sides of the equation. These are on both sides of the equation. These are called spectator ions.called spectator ions.

Ions that are not directly involved in a Ions that are not directly involved in a reaction are called reaction are called spectator ions. spectator ions.

Rewrite the equation, leaving out the Rewrite the equation, leaving out the canceled spectator ions.canceled spectator ions.

Balance the atoms and the charges of the Balance the atoms and the charges of the ions. (In this case, the number of atoms and ions. (In this case, the number of atoms and the net ionic charge on each side of the the net ionic charge on each side of the equation is zero and it is therefore balanced.)equation is zero and it is therefore balanced.)

A net ionic equation indicates only those A net ionic equation indicates only those particles that actually take part in the particles that actually take part in the reaction.reaction.

Record your answer to #23 on the worksheet Record your answer to #23 on the worksheet handout. handout.

Practice ProblemPractice Problem Write a balanced net ionic equation for the Write a balanced net ionic equation for the

following reaction:following 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 2. The number of atoms balance, but the charges

on 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 ) to

balance 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 ) rebalances

the 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

Predicting the Formation of a Predicting the Formation of a PrecipitatePrecipitate

Use the general rules for solubility of Use the general rules for solubility of ionic compounds (Table 8.3 on page 227)ionic compounds (Table 8.3 on page 227)

Examples:Examples:

1. Sodium nitrite will not form a 1. Sodium nitrite will not form a precipitate because alkali metal salts precipitate because alkali metal salts and nitrate salts are soluble (Rules 1 and and nitrate salts are soluble (Rules 1 and 2)2)

2. Rule 3 (Exceptions) indicates that 2. Rule 3 (Exceptions) indicates that barium sulfate is insoluble and therefore barium sulfate is insoluble and therefore will precipitate.will precipitate.

Solubility Rules for Ionic CompoundsSolubility Rules for Ionic Compounds

CompoundsCompounds SolubilitySolubility ExceptionsExceptions1. Salts of alkali 1. Salts of alkali metals and metals and ammoniaammonia

SolubleSoluble Some lithium Some lithium compoundscompounds

2. Nitrate salts and 2. Nitrate salts and chlorate saltschlorate salts

SolubleSoluble Few ExceptionsFew Exceptions

3. Sulfate salts3. Sulfate salts SolubleSoluble Compounds of Pb, Compounds of Pb, Ag, Hg, Ba, Sr, and Ag, Hg, Ba, Sr, and CaCa

4. Chloride salts4. Chloride salts SolubleSoluble Compounds of Ag Compounds of Ag and some and some compounds of Hg compounds of Hg and Pband Pb

5. Carbonates, 5. Carbonates, phosphates, phosphates, chromates, sulfides, chromates, sulfides, and hydroxides and hydroxides

Most are insolubleMost are insoluble Compounds of the Compounds of the alkali metals and of alkali metals and of ammoniaammonia

Practice ProblemPractice Problem Identify the precipitate formed and Identify the precipitate formed and

write the net ionic equation for the write the net ionic equation for the reaction of aqueous potassium reaction of aqueous potassium carbonate with aqueous strontium carbonate with aqueous strontium chloride.chloride.

1. Write the reactants showing each as 1. Write the reactants showing each as dissociated free ions. Balance the dissociated free ions. Balance the charges.charges.

2. Using solubility rules, look at 2. Using solubility rules, look at possible new pairings of cation and possible new pairings of cation and anion that give an insoluble substance.anion that give an insoluble substance.

3. Eliminate the spectator ions and 3. Eliminate the spectator ions and write the net ionic equation. write the net ionic equation.

AnswerAnswer 1. Reactants as dissociated free ions1. Reactants as dissociated free ions2K2K++ ( (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 2. Of the two possible combinations, KCl is soluble (Rules 1 and 4) and SrCOsoluble (Rules 1 and 4) and SrCO33 is insoluble is insoluble (Rule 5)(Rule 5)

3. The net ionic equation must be balanced for 3. The net ionic equation must be balanced for the number of atoms of each element and the the number of atoms of each element and the charges on the ions.charges on the ions.

SrSr2+2+ ( (aqaq) + CO) + CO332-2- ( (aqaq) ) SrCO SrCO33 ( (ss))

Note: Ignore Sample Problem 8-11 on page Note: Ignore Sample Problem 8-11 on page 228. There is a textbook error. 228. There is a textbook error.