Balancing Chemical Equations

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Balancing Chemical Equations
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Balanced Equation

• Atoms can not be created or destroyed
• All atoms we start with we must end up with
• A balanced equation has the same number of atoms
  of each element on both sides of the equation.

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• C O2 ? CO2
• This equation is already balanced
• But what if it isnt already?

4

• C O2 ? CO
• We need one more oxygen in the products.
• Cant change the formula, because it describes
  what is.

5

• The other oxygen must be used to make another CO
• But where does the other C come from?

6

• Must have started with two Cs
• 2 C O2 ? 2 CO

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Rules for Balancing

• Write the correct formulas for all the reactants
  and products
• Count the number of atoms of each type appearing
  on both sides.
• Balance the elements one at a time by adding
  coefficients (the numbers in front)
• 2 C O2 ? 2 CO
• Check to see if it is balanced

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Never

• Never change a subscript to balance an equation
  C O2 ? CO2
• If you change the formula you are describing a
  different reaction.
• H2O is a different compound than H2O2
• Never put a coefficient in the middle of a
  formula
• 2 NaCl is ok Na2Cl is not.

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Example

• H2 O2 ? H2O
• Make a table to keep track of atoms

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Example

• H2 O2 ? H2O

Need twice as much O in the product
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Example
H2 O2 ? 2H2O
Changes the O
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Example
H2 O2 ? 2H2O
2
Also changes the H
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Example
H2 O2 ? 2H2O
4
2
Now we need twice as much H in the reactant
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Example
2H2 O2 ? 2H2O
4
2
Recount to check
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Example
2H2 O2 ? 2H2O
Your answer
4
4
2
Recount to check
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Types of Reactions

• Millions of reactions
• Too many to remember
• They fall into several categories
• We will focus on Double Replacement in todays lab

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Double Replacement

• Two things replace each other
• Reactants two ionic compounds or acids.
• Usually in aqueous solution
• NaOH FeCl3 ?
• The positive ions change place
• NaOH FeCl3 ? Fe3OH- Na1Cl-1
• NaOH FeCl3 ? Fe(OH)3 NaCl
• 3NaOH FeCl3 ? Fe(OH)3 3NaCl

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Double Replacement

• Will only happen if one of the products
• Doesnt dissolve in water and forms a solid
• (look at solubility rules)
• Or is a gas that bubbles out
• Or is a covalent compound usually water

After adding lead nitrate
Potassium iodide
2KI(aq) Pb(NO3)2 (aq) ? 2KNO3(aq) PbI2 (s)
PbI2 lead (II) iodide is insoluble
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General Rules for the Water Solubilities of
Common Ionic Compounds

• Compounds that are mostly soluble
• All nitrates
• Alkali metal (group 1A) and ammonium compounds
• Chlorides, bromides, and iodides, except for
  those of Pb2, Ag, Hg2
• Sulfates except for those of Sr2, Ba2, Pb2,
  and Hg2
• CaSO4 is slightly soluble

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General Solubility Rules

• Compounds that are mostly insoluble
• Carbonates, hydroxides, and sulfides, except for
  ammonium compounds and those of the group 1A
  metals. (The hydroxides and sulfides of Ca2,
  Sr2, and Ba2 are slightly to moderately
  soluble.)

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EXAMPLE 1

• Na2S(aq) Cd(NO3)2(aq) ?
• What will the products be?
• Na2S(aq) Cd(NO3)2(aq) ? CdS(s) NaNO3(aq)
• Double replacement occurs because cadmium sulfide
  is insoluable forms solid and no longer
  participates in reaction.
• Balanced equation?
• Na2S(aq) Cd(NO3)2(aq) ? CdS(s) 2NaNO3(aq)

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Ionic Equations Showing the Ions

• Ionic equations show all free ions (ions that are
  free to move around when dissolved in a liquid
  usually water).
• Ions that are not directly involved in the
  reaction are spectator ions.
• A net ionic equation is one in which spectator
  ions are NOT shown.

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EXAMPLE 2

• NaOH (aq) MgSO4 (aq) ?
• What will the products be?
• NaOH (aq) MgSO4 (aq) ? Mg(OH)2 (s) Na2SO4
  (aq)
• Double replacement occurs because hydroxides of
  2A metals are insoluable.
• Balanced Chemical Equation?
• 2NaOH (aq) MgSO4 (aq) ? Mg(OH)2 (s) Na2SO4
  (aq)

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Ionic Equations of Example 2

• 2NaOH (aq) MgSO4 (aq) ? Mg(OH)2 (s) Na2SO4
  (aq)
• Ionic Equation
• 2Na 2OH- Mg2 SO42- ? Mg(OH)2 (s) 2Na1
  SO42-
• Net Ionic Equation (w/o spectator ions)
• 2Na 2OH- Mg2 SO42- ? Mg(OH)2 (s) 2Na1
  SO42-
• 2OH- Mg2 ? Mg(OH)2 (s)