1. Chemistry Lecture Notes
Writing and Balancing
Chemical Equations

3. 3H2(g) + N2(g)  2NH3(g) balanced (Dalton)
Chemical Equations
coefficients
3H2(g) + N2(g)  2NH3(g) balanced (Dalton)
reactants
products
general:
A(l) + B(s) C(g) + D(aq) + heat
(s) = solid heat produced: exothermic
(l) = liquid heat consumed: endothermic
(g) = gas
(aq) = aqueous other info: temperature, pressure, catalyst, etc.
other
info

4. Balancing Chemical Equations
Balancing chemical equations is generally done “by inspection,” but here are some common steps:
Write the unbalanced equation (often it is written for you).
Start by balancing the most complex substance (often but not always).
Choose an element that appears in only one reactant and product (always).
Balance by adjusting the coefficients so that there are the same number of atoms of each element on both sides.
Balance polyatomic ions (if present) as a unit.
Balance the remaining atoms, usually ending with the least complex substance, using fractional coefficients if necessary.
If fractional coefficients have been used, multiply both sides of the equation by the denominator to obtain whole numbers.
Do not alter any formulas (change subscripts).
Do not add or delete any reactants or products.
Use the smallest set of whole numbers.
Double check to be sure there are the same number of atoms of each kind on both sides of the equation.

5. Cu(s) + AgNO3(aq)  Cu(NO3)2(aq) + Ag(s)
Balancing Chemical Equations
P4O10(s) H2O(l)  H3PO4(aq)
Cu(s) AgNO3(aq)  Cu(NO3)2(aq) Ag(s)
Ca(s) NH3(g)  CaH2(s) Ca3N2(s)
K(s) H2O(l)  KOH(aq) H2(g)
The combustion of butane (C4H10)

6. Balancing Chemical Equations
Common polyatomic ions
OH–
hydroxide
NH4+
ammonium
CO32–
carbonate
HCO3–
hydrogen carbonate (bicarbonate)
SO32–
sulfite
HSO3–
hydrogen sulfite (bisulfite)
SO42–
sulfate
HSO4–
hydrogen sulfate (bisulfate)
PO33–
phosphite
PO43–
phosphate
HPO32–
hydrogen phosphite
HPO42–
hydrogen phosphate
H2PO3–
dihydrogen phosphite
H2PO4–
dihydrogen phosphate
NO2–
nitrite
NO3–
nitrate
CrO42–
chromate
Cr2O72–
dichromate
MnO4–
permanganate
SCN–
thiocyanate
C2O42–
oxalate
C2H3O2–
acetate
CN–
cyanide
OCN–
cyanate
ClO–
hypochlorite
ClO2–
chlorite
ClO3–
chlorate
ClO4–
perchlorate
BrO–
hypobromite
BrO2–
bromite
BrO3–
bromate
BrO4–
perbromate
IO–
hypoiodite
IO2–
iodite
IO3–
iodate
IO4–
periodate

7. Double Displacement Reactions
Double displacement (metathesis) reactions: reactions in which anions and cations exchange partners
general: A+B(aq) + C+D(aq)  A+D(aq) + CB(?)
if: CB(s) precipitate
CB(aq) weak electrolyte or nonelectrolyte
CB(g) gas
C+B(aq) ionic (soluble) no net reaction
net reaction
has occurred

8. Double Displacement Reactions
Precipitate-forming reactions
Solubility Rules:
Soluble compounds
all salts of alkali metal ions and the NH4+ ion
all salts of NO3, C2H3O2, ClO4 ions
most salts of Cl, Br, and I ions
except Ag+, Pb+2, and Hg2+2 salts
most salts of the SO42 ion
except Sr+2, Ba+2, Pb+2, and Hg2+2 salts
(CaSO4 is slightly soluble)
Insoluble compounds
most salts of CO32, PO43, OH, O2, and S2 ions
except alkali metal and NH4+ salts
(hydroxides and sulfides of Ca2+, Sr2+, and Ba2+ are slightly to moderately soluble)

9. Double Displacement Reactions
Precipitate-forming reactions
Complete and balance the following reactions in aqueous solution, indicating the state of each reactant and product:
AgNO NaBr
CuCl KOH
Ba(NO3) Na2SO4
Zn(C2H3O2) Na2S

10. Double Displacement Reactions
Acids and bases
Arrhenius: acid - produces H3O+(aq) in water (formula starts with an H)
base - produces OH(aq) in water (OH– salts or amines)
HCl(aq) H3O+(aq) + Cl(aq) 100% dissociated
strong acid
HC2H3O2(aq) H3O+(aq) + C2H3O2(aq) 0.4% dissociated
weak acid
NaOH(aq) Na+(aq) + OH(aq) 100% strong base
NH3(aq) NH4+(aq) + OH(aq) 0.4% weak base

11. Double Displacement Reactions
Acid-base neutralization reactions
HCl(aq) + NaOH(aq)  NaCl(aq) + H2O(l)
acid base salt water (nonelectrolyte)
Complete and balance the following reactions:
HClO4(aq) + KOH(aq) 
HNO3(aq) + Ca(OH)2(aq) 

12. Double Displacement Reactions
Gas-forming reactions
Reactions that produce gases: H2S, NH3, CO2
K2S(aq) + 2HNO3(aq)  H2S(g) + 2KNO3(aq)
NH4Cl(aq) + KOH(aq)  NH4OH(aq) + KCl(aq)
but: NH4OH(aq)  NH3(g) + H2O(l)
 NH4Cl(aq) + KOH(aq)  NH3(g) + H2O(l) + KCl(aq)
Na2CO3(aq) + 2HCl(aq)  H2CO3(aq) + 2NaCl(aq)
but: H2CO3(aq)  CO2(g) + H2O(l)
 Na2CO3(aq) + 2HCl(aq)  CO2(g) + H2O(l) + 2NaCl(aq)
correct
equation
correct
equation

13. Double Displacement Reactions
Gas-forming reactions
Complete and balance the following reactions.
CuS(s) HCl(aq) 
HCl(aq) + BaCO3(s) 
NaOH(aq) + NH4NO3(aq) 