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WRITING AND. Martha Andreski. BALANCING. EQUATIONS. I. CHEMICAL EQUATIONS. …are expressions of reactions (rxns) They consist of 2 parts: REACTANTS PRODUCTS left "yields" right

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  • Martha Andreski





…are expressions of reactions (rxns)They consist of 2 parts:


left "yields" right

ie: lead (II) nitrate and potassium iodide react to form lead (II) iodide and potassium nitrate

Pb(NO3)2 + KI PbI2 + KNO3

other information
Other Information
  • we should also include other info, such as the form (state) in which reactants and products will be found
  • (aq) = aqueous ---> in solution
  • (l) = liquid ---> "melted" form
  • (s) = solid
  • (g) = gas = "is given off"
  • (cr) = crystal
  • Therefore
  • Pb(NO3)2(aq) + KI(aq) PbI2(s) + KNO3(aq)
  • NOTE: if a solid results from the rxn b/w 2 aq soln's, the solid is called a "precipitate" (ppt)
ii balancing equations
II. Balancing Equations
  • Law of Conservation of Matter:
  • we should have the same amount of atoms on the left (reactants) as we have on the right (products)
  • ie: H2 + O2 H2O does not have the same # of atoms on ea side
  • It is UNBALANCED and must be balanced
  • "Coefficients" are used in front of formulas to balance
  • H2 + O2 H2O
balance this equation
Balance This Equation
  • H2 + O2 H2O



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Before balancing, there are 2 H’s on the left and 2 H’s on the right;

There are 2 O’s on the left but only one O on the right.

So we must add oxygens to the right but we can only do this by changing the coefficient in front of the molecule “H2O”

/ 2

But now the H balance on the left is thrown off, so you must change that coefficient to a 2

practice balancing these equations copy these
Practice Balancing These Equations:(copy these)
  • KClO3 KCl + O2
  • Mg + O2 MgO
  • BaCl2 + KIO3 Ba(IO3)2 + KCl
  • Cr(NO3)3 + NaOH  Cr(OH)3 + NaNO3










Now, work on the Balancing Equations Practice Sheet. Finish for homework.

  • Did you notice that H2 and O2 (not H and O) were written? There are several elements which MUST be written with a subscript of 2 when written uncombined with other elements. We call these the “Diatomics”.
  • "ine"-"gen" rule: F2, Cl2, Br2, I2, At2 = "-ine"
  • H2, N2, O2 = "-gen"
  • NOTE: The diatomics rule is NEVER used for compounds!
  • (NOT: H2F2, H2Cl2, NaCl2 WHY? )
iii 4 types of chemical reactions
III. 4 Types of Chemical Reactions
  • A) Composition, or Synthesis -
  • when 2 or more simple substances combine to form a more complex substance
  • ie: Fe + S FeS
  • Mg + O2 MgO
  • Note: oxygen in the 2nd equation is diatomic ("-ine-gen" rule)
  • Note #2: when substances burn, or "undergo combustion", it means oxygen is a reactant! Memorize this!
b decomposition analysis
B. Decomposition (Analysis)
  • When 1 (usually) substance breaks down to form 2 or more simple substances
  • ie: KClO3 KCl + O2
  • MgCO3 MgO + CO2
  • Are these as predictable? Probably not! (Who would guess?)
  • Therefore, we will discuss the 6 general types of Decomposition Reactions. Following these rules you can predict products properly.
6 decomposition rules memorize these
6 Decomposition RulesMemorize these!
  • 1. Metallic carbonates form metallic oxides & CO2.
  • CaCO3 CaO + CO2
  • H2CO3 H2O + CO2
  • 2. Metallic hydroxides form metallic oxides & H2O.
  • Ca(OH)2 CaO + H2O
  • Pb(OH)2 PbO + H2O
  • 3. Metallic chlorates form metallic chlorides & O2.
  • KClO3 KCl + O2
  • Ca(ClO3)2 CaCl2 + O2
  • 4. Some acids* form non-metallic oxides & H2O.
  • H2CO3 CO2 + H2O
  • H3PO4 P2O5 + H2O
  • 5. Some oxides decompose (they just split up!)
  • HgO  Hg + O2
  • Li2O  Li + O2
  • 6. Some decompostitions occur b/c of an electric current.
  • NaCl  Na + Cl2
c single replacement or single displacement
C. Single Replacement or Single Displacement
  • the + and - parts of the reactants reorganize; "one atom and one compound trade places with each other”
  • ie: Fe + CuSO4 Cu + FeSO4
      • AgNO3 + Cu  Ag + CuNO3
  • Cl2 + KBr  KCl + Br2
d double replacement or double displacement ionic
D. Double Replacement or Double Displacement (“Ionic”)
  • 2 compounds have their components "changing places“
  • ie: NaCl + AgNO3 NaNO3 + AgCl
  • BaCl2 + KIO3 Ba(IO3)2 + KCl
  • NOTE: Just because we can write an equation for it, does that mean it can react? NO!
  • We need to look at an “Activity Series” and write "Net Ionic Equations“ (more on N.I.E.’s later)
activity series
Activity Series
  • Being able to write a reaction doesn’t necessarily make it happen!
  • In general, elements displace those below them from a compound but not those above it
  • Also – the farther apart the substances are on the series, the quicker the reaction tends to go
li rb k ba ca na mg al mn zn cr fe ni sn pb h 2 cu hg ag pt au

React with cold H2O and acids, replacing H2

React with acids or steam but usually not H2O(l), to replace H2

React with acids but not H2O(l) to replace H2

All react with O2 to form oxides

Mostly unreactive

how to use the activity chart
How to Use the Activity Chart
  • If you add Zn(s) to CuCl2(aq), will a reaction occur? (yes; Zn is higher than Cu so it does occur)
  • If you add Mg(s) to Pb(NO3)2(aq), will a reaction occur? (yes; Mg is higher than Pb so it does occur)
  • If you add Ni(s) to Al2(SO4)3(aq), will a reaction occur? (no; Ni is lower than Al so write “No Reaction”)
Now, work on the Writing and Balancing Equations worksheet. In front of each equation, write a symbol to represent whether the reaction is…
  • Composition C
  • Decomposition D
  • Single Replacement SR
  • Double Replacement DR