Balancing Chemical ReactionsBalancing Chemical Reactions

Balancing Chemical ReactionsBalancing Chemical Reactions

Reactants: Zn + IReactants: Zn + I22 Product: Zn IProduct: Zn I22

Balancing Equations:Balancing Equations:At the conclusion of our time together, At the conclusion of our time together,

you should be able to:you should be able to:

1.1. List the various parts of a chemical List the various parts of a chemical equation and define eachequation and define each

2.2. Count the number of atoms on each side of Count the number of atoms on each side of an equationan equation

3.3. Use conservation of matter to balance a Use conservation of matter to balance a chemical equation.chemical equation.

IntroductionIntroduction

Chemical reactions occur when bonds Chemical reactions occur when bonds between the outermost parts of atoms are between the outermost parts of atoms are formed or brokenformed or broken

Chemical reactions involve changes in matter, Chemical reactions involve changes in matter, the making of new materials with new the making of new materials with new properties, and energy changes.properties, and energy changes.

Symbols represent elements, formulas Symbols represent elements, formulas describe compounds, chemical equations describe compounds, chemical equations describe a chemical reactiondescribe a chemical reaction

Chemical EquationsChemical Equations

Depict the kind of reactants and products and their relative amounts in a reaction.

4 4 Al (s) + 3 O2 (g) ---> 2 Al2O3 (s)

The blue numbers are called subscripts.

The numbers in the front are called coefficients.

The letters (s) and (g) are the physical states of compounds.

Parts of a Reaction EquationParts of a Reaction Equation

Chemical equations show the conversion Chemical equations show the conversion of of reactantsreactants (the molecules shown on the (the molecules shown on the left of the arrow) into left of the arrow) into productsproducts (the (the molecules shown on the right of the molecules shown on the right of the arrow).arrow).

A + sign separates molecules on the A + sign separates molecules on the same sidesame side

The arrow is read as “yields”The arrow is read as “yields” ExampleExample

2H2H22 (g) + O (g) + O22 (g) (g) 2H 2H22O (l)O (l)

This reads “hydrogen gas plus oxygen This reads “hydrogen gas plus oxygen gas reacts to yield dihydrogen gas reacts to yield dihydrogen monoxide liquid.”monoxide liquid.”

The charcoal used in a grill is basically carbon. The The charcoal used in a grill is basically carbon. The carbon reacts with oxygen to yield carbon dioxide. The carbon reacts with oxygen to yield carbon dioxide. The

chemical equation for this reaction, C + Ochemical equation for this reaction, C + O22 CO CO22, ,

contains the same information as the English sentence contains the same information as the English sentence but has quantitative meaning as well.but has quantitative meaning as well.

Chemical EquationsChemical Equations

Because of the principle of the Because of the principle of the conservation of matterconservation of matter, ,

an an equation must be equation must be balancedbalanced..

It must have the same It must have the same number of atoms of the number of atoms of the same kind on both sides.same kind on both sides.

Lavoisier, 1788Lavoisier, 1788

Symbols Used in EquationsSymbols Used in Equations

Solid (s)Solid (s) Liquid (l)Liquid (l) Gas (g)Gas (g) Aqueous solution (aq)Aqueous solution (aq) Catalyst Catalyst HH22SOSO44

Escaping gas (Escaping gas ()) Change of temperature (Change of temperature ())

Balancing EquationsBalancing Equations

When balancing a chemical reaction you When balancing a chemical reaction you may add coefficients in front of the may add coefficients in front of the compounds to balance the reaction, but compounds to balance the reaction, but you may you may notnot change the subscripts. change the subscripts.

Changing the subscripts changes the Changing the subscripts changes the compound. Subscripts are determined by compound. Subscripts are determined by the valence electrons (charges for ionic or the valence electrons (charges for ionic or sharing for covalent)sharing for covalent)

Subscripts vs. CoefficientsSubscripts vs. Coefficients

The subscripts The subscripts tell you how tell you how many atoms of a many atoms of a particular particular element are in a element are in a compound. The compound. The coefficient tells coefficient tells you about the you about the quantity, or quantity, or number, of number, of molecules of the molecules of the compound.compound.

Balancing Equations:Balancing Equations:Let’s see if you can:Let’s see if you can:

1.1. List the various parts of a chemical List the various parts of a chemical equation and define eachequation and define each

2.2. Count the number of atoms on each side of Count the number of atoms on each side of an equationan equation

Balancing Chemical Reactions IIBalancing Chemical Reactions II

Balancing Equations

Balancing Equations:Balancing Equations:At the conclusion of our time together, At the conclusion of our time together,

you should be able to:you should be able to:

1.1. Use conservation of matter to Use conservation of matter to balance a chemical equation.balance a chemical equation.

Chemical EquationsChemical EquationsChemical EquationsChemical Equations

4 Al(s) + 3 O2(g) ---> 2 Al2O3(s)

This equation means

4 Al atoms + 3 O2 molecules ---produces--->

2 molecules of Al2O3

Ag + Cl2 AgClAg + Cl2 AgCl

1 2 1 1

Both sides must be equalfor ALL atoms.

Ag + Cl2 AgClAg + Cl2 AgCl

1 2 1 1

Both sides must be equalfor ALL atoms.

22

2 2

Ag + Cl2 AgClAg + Cl2 AgCl

1 2 1 1

Both sides must be equalfor ALL atoms.

22

2 2

22

2

CH4 + O2 CO2+ H2O 1 2 1 24 2 1

CH4 + O2 CO2+ H2O 1 2 1 24 2 1

22

24

CH4 + O2 CO2+ H2O 1 2 1 24 2 1

22

24

22

4

Steps to Balancing EquationsSteps to Balancing Equations

There are four basic steps to balancing a There are four basic steps to balancing a chemical equation.chemical equation.

1.1. Write the correct formula for the reactants and Write the correct formula for the reactants and the products. DO NOT TRY TO BALANCE IT the products. DO NOT TRY TO BALANCE IT YET! You must write the correct formulas first. YET! You must write the correct formulas first. And most importantly, once you write them And most importantly, once you write them correctly DO NOT CHANGE THE FORMULAS!correctly DO NOT CHANGE THE FORMULAS!

2. Find the number of atoms for each element on 2. Find the number of atoms for each element on the left side. Compare those against the number the left side. Compare those against the number of the atoms of the same element on the right of the atoms of the same element on the right sideside..

Steps to Balancing EquationsSteps to Balancing Equations

3. Determine where to place coefficients in front of 3. Determine where to place coefficients in front of formulas so that the left side has the same formulas so that the left side has the same number of atoms as the right side for EACH number of atoms as the right side for EACH element in order to balance the equation.element in order to balance the equation.

4. Check your answer to see if:4. Check your answer to see if: The numbers of atoms on both sides of the The numbers of atoms on both sides of the

equation are now balanced.equation are now balanced. The coefficients are in the lowest possible The coefficients are in the lowest possible

whole number ratios. (reduced)whole number ratios. (reduced)

Balancing EquationsBalancing EquationsBalancing EquationsBalancing Equations

___ H___ H22(g) + ___ O(g) + ___ O22(g) ---> (g) ---> _____ H_ H22O(l)O(l)2 2

This equation is not balanced!

Two hydrogen atoms from a hydrogen molecule (H2) combines with one of the oxygen atoms from an oxygen molecule (O2) to form H2O. Then, the remaining oxygen atom combines with two more hydrogen atoms (from another H2 molecule) to make a second H2O molecule.

Balancing Balancing Equations EquationsBalancing Balancing Equations Equations

___ Al(s) + ___ Br___ Al(s) + ___ Br22(l) ---> ___ AlBr(l) ---> ___ AlBr33(s)(s)2 3 2

Some Suggestions to Help YouSome Suggestions to Help You

Look for singleton elements in Look for singleton elements in compounds and determine the compounds and determine the relationships.relationships.

Do the hardest element to Do the hardest element to balance first keeping in mind balance first keeping in mind the relationships from #1.the relationships from #1.

Balance the lone elements last.Balance the lone elements last.

Balancing Balancing EquationsEquationsBalancing Balancing EquationsEquations

____C3H8(g) + _____ O2(g) ---->

_____CO2(g) + _____ H2O(g)

____B4H10(g) + _____ O2(g) ---->

___ B2O3(g) + _____ H2O(g)

A

3A 4A

1

3 4

5

A

2A 5A

2

4 10

11

Balancing Practice Balancing Practice ProblemsProblems

Challenges #1Challenges #1

Balancing Practice Balancing Practice ProblemsProblems

Challenges #1Challenges #1

____C7H6O2 + _____ O2 ---->

_____CO2(g) + _____ H2O(g)

A

7A 3A

2

14 6

15

Balancing Equations:Balancing Equations:Let’s see if you can:Let’s see if you can:

1.1. Use conservation of matter to Use conservation of matter to balance a chemical equation.balance a chemical equation.

Balance Each of the Following Balance Each of the Following EquationsEquations

#6#6

Balance Each of the Following Balance Each of the Following EquationsEquations

#6#6

____Al4C3 + _____ H2O ---->

_____CH4 + _____ Al(OH)3

A

3A 4A

1

3 4

12

Balance Each of the Following Balance Each of the Following EquationsEquations

#3#3

Balance Each of the Following Balance Each of the Following EquationsEquations

#3#3

____H3AsO4 ---->

_____As2O5 + _____ H2O

2A

A 3A

2

1 3

Balancing Chemical Reactions from WordsBalancing Chemical Reactions from Words

Balancing Equations:Balancing Equations:At the conclusion of our time together, At the conclusion of our time together,

you should be able to:you should be able to:

1.1. List the various parts of a chemical List the various parts of a chemical equation and define eachequation and define each

2.2. Count the number of atoms on each side of Count the number of atoms on each side of an equationan equation

3.3. Use conservation of matter to balance a Use conservation of matter to balance a chemical equation.chemical equation.

4.4. Go from a word equation to a balanced Go from a word equation to a balanced chemical equation.chemical equation.

Converting and Balancing Word Converting and Balancing Word EquationsEquations

Converting and Balancing Word Converting and Balancing Word EquationsEquations

Sodium phosphate + iron (III) oxide Sodium phosphate + iron (III) oxide sodium oxide + iron (III) phosphatesodium oxide + iron (III) phosphate

Na3PO4 + Fe2O3 ---->

Na2O + FePO4

2

3 2

Converting Word Equations into Converting Word Equations into Chemical Equations #10Chemical Equations #10

Converting Word Equations into Converting Word Equations into Chemical Equations #10Chemical Equations #10

Strontium iodide + Lead (II) phosphate Strontium phosphate + lead (II) iodide

SrI2 + Pb3(PO4)2 ---->

Sr3(PO4)2 + PbI2

3

3

Balancing Equations:Balancing Equations:Let’s see if you can:Let’s see if you can:

1.1. List the various parts of a chemical List the various parts of a chemical equation and define eachequation and define each

2.2. Count the number of atoms on each side of Count the number of atoms on each side of an equationan equation

3.3. Use conservation of matter to balance a Use conservation of matter to balance a chemical equation.chemical equation.

4.4. Go from a word equation to a balanced Go from a word equation to a balanced chemical equation.chemical equation.

Converting and Balancing Word Converting and Balancing Word EquationsEquations

Converting and Balancing Word Converting and Balancing Word EquationsEquations

Silver phosphate + aluminum oxide Silver phosphate + aluminum oxide silver oxide + aluminum phosphatesilver oxide + aluminum phosphate

Ag3PO4 + Al2O3 ---->

Ag2O + AlPO4

2

3 2

Converting Word Equations into Converting Word Equations into Chemical EquationsChemical Equations

Converting Word Equations into Converting Word Equations into Chemical EquationsChemical Equations

Calcium iodide + tin (II) phosphate calcium phosphate + tin (II) iodide

CaI2 + Sn3(PO4)2 ---->

Ca3(PO4)2 + SnI2

3

3