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Balancing Chemical EquationsBalancing Chemical EquationsIf you are starting with a word equation, besure to write each chemical formulacorrectly.

You must remember that elements ending in - gen or - ine are diatomic!

Count how many of each type of atom ispresent on each side of the equation.

Begin by balancing elements that are found in

one substance on each side.

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Balance hydrogen and oxygen last as theysometimes balance out at the end.

If there is an odd number of an element onone side and an even number on the other,the odd number will need to be evened out sotry using a coefficient of 2 for that substance.If there are polyatomic ions that remaintogether as a unit during the reaction, countthe polyatomic ion as one unit.

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You are allowed to change coefficients butnever change subscripts!

Subscripts come from the valence (oxidation) numbers of the elements

and once a chemical formula is correctly written, the subscripts become etched

in stone!

After balancing, put in the correct physicalstates by using the solubility chart.

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States of Elements and States of Elements and CompoundsCompounds

Ionic Compounds NaCl(s), NH4Cl(s), MgCl2(s)

Nonpolar Covalent Compounds H2(g), F2(g), Br2(l), CO2(g), O2(g), I2(s)

Polar Covalent Compounds H2O(l), C12H22O11(s), NH3(g), SO2(g)

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Elements Na(s), Al(s), Cu(s)

Types of Bonds (difference in electronegativities)

0.0 – 0.3 nonpolar covalent 0.4 – 1.7 polar covalent 1.8 – 4.0 ionic

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Synthesis ReactionsSynthesis Reactions

A synthesis or a direct combination reaction has the general form:

A + BA + B → AB

2H2(g) + O2(g) → 2H2O(g) TwoTwo elements in theirelements in their elemental (free)elemental (free) states form a compound.states form a compound.

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Important Notes to RememberImportant Notes to Remember Note that hydrogen and oxygen because they end in - gen are diatomic. The reactant states are determined byThe reactant states are determined by what state hydrogen and oxygen are atwhat state hydrogen and oxygen are at room temperature (25°C, 298 K).room temperature (25°C, 298 K). Note that the subscripts are not changed in attempting to balance the equation. Water can be in either the liquid orWater can be in either the liquid or gaseous state depending on the ambientgaseous state depending on the ambient temperature.temperature.

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Decomposition ReactionsDecomposition Reactions

A decomposition or an analysis reaction has the general form:

AB AB → A + B

2H2O(l) → 2H2(g) + O2(g) A compound is decomposed into itsA compound is decomposed into its constituent elements.constituent elements. See important notes for synthesisSee important notes for synthesis reactions.reactions.

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Typical Decomposition ReactionsTypical Decomposition Reactions

When metallic carbonates are heated,metallic oxides and carbon dioxide areformed.

Metallic carbonates are ionic compounds containing the carbonate polyatomic (CO3

2-) ion. Metallic oxides are ionic compounds containing the oxide (O2-) ion.

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BaCO3(s) → BaO(s) + CO2(g)

When metallic hydroxides are heated,metallic oxides and water are formed.

Metallic hydroxides are ionic compounds containing the hydroxide (OH-) ion. Ba(OH)2(s) → BaO(s) + H2O(g)

Δ

Δ

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When metallic chlorates are heated,metallic chlorides and oxygen are formed.

Metallic chlorates are ionic compounds containing the chlorate (ClO3

-) ion. 2KClO3(s) → 2KCl(s) + 3O2(g)

Δ

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When some acids are heated, nonmetallicoxides and water are formed.

Nonmetallic oxides are covalent (molecular) compounds containing

oxide (O2-). Examples are NO2, SO3, and CO2. H2SO3(aq) → H2O(l) + SO2(g)

Δ

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When some metallic oxides are heated, a metal and oxygen are formed.

2HgO(s) → 2Hg(l) + O2(g)

Some decomposition reactions are produced

by electrolysis (an electric current). 2H2O(l) → 2H2(g) + O2(g) 2NaCl(l) → 2Na(l) + Cl2(g)

Δ

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Single Replacement ReactionsSingle Replacement Reactions

A single replacement reaction has thegeneral form:

A + BC → AC + B

Zn(s) + Cu(NO3)2(aq) → Zn(NO3)2(aq) + Cu(s)

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Important Notes to RememberImportant Notes to Remember Note that elements (Zn, Cu) in their free or uncombined state have no charge. Not all single replacement reactions occur. The elemental metal, Zn, must be a more reactive metal to replace the metal in the ionic compound. To determine if a single replacement occurs, check the Activity Series of the Elements.

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More Single Replacement More Single Replacement ReactionsReactions

Mg + HNO3 → ?

Mg(s) + 2HNO3(aq) → Mg(NO3)2(aq) + H2(g)

This reaction occurs because Mg is above H on the Activity Series indicating that it is a more active metal.

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More Single Replacement More Single Replacement ReactionsReactions

Ag + Pb(NO3)2 → ?

Ag + Pb(NO3)2 → NR

This reaction does not occur because Ag is below Pb on the Activity Series indicating that it is a less active metal.

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Double Replacement ReactionsDouble Replacement Reactions

A double replacement reaction has thegeneral form:

AB + CD AB + CD → AD + CB

Ba(OH)2(s) + H2SO4(aq) → CaSO4(s) + 2H2O(l)

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Important Notes to RememberImportant Notes to RememberNot all double replacement reactions occur. For such a reaction to occur, one of the following must take place:

A precipitate may be formed (i.e. AgCl(s)). A gas may be formed (i.e. CO2(g)) Water may be formed (i.e. H2O(l)).

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Combustion ReactionsCombustion Reactions

Organic compounds (compounds of carbon)contain either carbon and hydrogen(hydrocarbons) or carbon, hydrogen, andoxygen.

CO and CO2 are not considered organiccompounds.

When such compounds burn completely inexcess oxygen, combustion occurs and theproducts are always CO2(g) and H2O(l).

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Combustion ReactionsCombustion ReactionsWhen such compounds burn withoutsufficient oxygen, combustion occurs and theproducts are always CO(g) and H2O(l).

When balancing combustion reactions, followthese steps in order:

balance the carbons. balance the hydrogens. balance the oxygens.

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Balancing Combustion ReactionsBalancing Combustion Reactions

C6H12O6 + O2 →

C6H12O6 + O2 → CO2 + H2O

C6H12O6 + O2 → 6CO2 + H2O

C6H12O6 + O2 → 6CO2 + 6H2O

C6H12O6 + 6O2 → 6CO2 + 6H2O

C6H12O6(s) + 6O2(g) → 6CO2(g) + 6H2O(l)

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Balancing Combustion ReactionsBalancing Combustion Reactions

C8H18 + O2 →

C8H18 + O2 → CO2 + H2O

C8H18 + O2 → 8CO2 + H2O

C8H18 + O2 → 8CO2 + 9H2O

C8H18 + O2 → 8CO2 + 9H2O

2C8H18(g) + 25O2(g) → 16CO2(g) + 18H2O(l)

25 2

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What Must Balance In A Reaction?What Must Balance In A Reaction?ammonium carbonate + calcium choride → ?

ammonium carbonate + calcium choride → ammonium chloride + calcium carbonate

(NH4)2CO3 + CaCl2 → NH4Cl + CaCO3

(NH4)2CO3(aq) + CaCl2(aq) → 2NH4Cl(aq) + CaCO3(s)

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Atoms

2 N atoms → 2 N atoms

8 H atoms → 8 H atoms

1 C atom → 1 C atom

3 O atoms → 3 O atoms

1 Ca atom → 1 Ca atom

2 Cl atoms → 2 Cl atoms

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Mass

(NH4)2CO3(aq) + CaCl2(aq) → 2NH4Cl(aq) +

CaCO3(s)

96.11 g + 110.98 g = 107.00 g + 100.09 g

207.09 g = 207.09 g

Moles

2 mol reactant ≠ 3 mol product