Balancing by inspection (to assist Introductory Chemistry, Steven S. Zumdahl, 1990, D. C. Heath & Co.)
The chemical symbol HgO represents:
a molecule
a compound
an ion
a cation
an anion
The chemical symbol HgO represents:
a formula
mercury oxide
a compound of hydrogen, gold, and oxygen
an atom of hydrogen and oxygen
a molecule of hydrogen and oxide
The number in this forumla (commonly written below the line) O2 is:
a subscript
the number of atoms of oxygen in the molecule
the unit charge on an oxygen molecule
a coefficient
an exponent
Molecules can be told from ions because molecules have:
a zero net charge
a positive charge
a negative charge
a variable charge
The chemical symbol H+ is:
a cation
a hydrogen ion
an anion
an atom
a molecule
The chemical symbol OH- is:
an anion
a hydroxide ion
a hydride anion
a hydrogen ion
Chemical reactions are accurately and concisely described:
by chemical equations
by the physical state of reactants and products
by the changes observed
by the identities (formulas) of the reactants and products
by the relative number of each type of molecule
A correctly written (conventional) chemical equation is:
reactants ----> products
reactants <---- products
products ----> reactants
products <---- reactants
A complete chemical equation:
is balanced
has the same number of atoms of each element on both sides
includes word descriptions of each molecule
has an equal number of molecules on both sides
The atoms in the products are ( ) the atoms in the reactants.
rearranged from
grouped differently from
bound more weakly than in
carry more energy than
carry more mass than
Balancing chemical equations by trial and error:
is called by inspection
requires keeping track of both molecules and atoms
requires no system of approach
results in a correctly balanced equation every time
permits replacing one atom (element) for another
is a random process requiring good luck
The goal of balancing equations by inspection is to add ( ) to the unbalanced equation until the number of atoms of each element is the same for the reactants and the products.
molecules
atoms
compounds
ions
To balance CH4 + O2 --> CO2 + H20 or C 1=1, H 4>2, O 2<3, one could add:
1 H2O and 1 O2 or H 4=4, O 4=4
1 H2O or H 4=4, O 2>4
1 CH4 or C 2<1, H 8<2, O 2>3
1 CO2 or C 1>2, H 4<2, O 2>5
To balance HgO --> Hg + O2 or Hg 1=1, O 1<2, one could:
add 1 HgO for Hg 2=2 Hg, O 2=2
add 1 HgO for 2HgO --> 2Hg + O2
double the number of molecules of each kind
remove the subscript from O2
To balance C + O2 --> CO2, one:
needs do nothing, it is balanced
could add 1 C for C 2=2, O 2=2
needs to double the number of the reactant molecules
To balance Zn + HCl --> H2 + ZnCl2 or Zn 1=1 , H 1<2, Cl 1<2, write the equation on the left side and the atom tally on the right side of your page. On the next line down add and tally:
1 HCl for H 2=2, Cl 2=2 atom tally
1 HCl for Zn + 2HCl --> H2 + ZnCl2 molecule tally
1 Zn for 2 Zn = 2 Zn
1 HCl for H 2=2, Cl 2=2 molecule tally
1 HCl for Zn + 2HCl --> H2 + ZnCl2 atom tally
To balance H2 + O2 --> H2O or H 2=2, O 2>1, write the equation on the left side and the atom tally on the right side of your page. On the next line down add and tally:
1 H20 for H 2<4, O 2=2
1 H20 for H2 + O2 --> 2H2O for a partial solution
a minus O for H2 + O --> H2O
1 H2O for H2 + O2 --> 2H2O for a balanced equation
To balance C2H5OH + O2 --> CO2 + H2O, write the equation on the left side and the atom tally on the right side of your page: (step 1)
C 2>1, H 6>2, O 3=3
C 1=1, H 3>1, O 7>4
To balance C2H5OH + O2 --> CO2 + H2O or C 2>1, H 6>2, O 3=3, write the equation on the left side and the atom tally on the right side of your page. Add and tally: (step 2)
1 CO2 for C 2=2, H 6>2, O 3<5
1 CO2 for C2H5OH + O2 --> 2CO2 + H2O partial solution
1 CO2 for C2H5OH + O2 --> 2CO2 + H20 balanced equation
To balance C2H5OH + O2 --> 2CO2 + H2O or C 2=2, H 6>2, O 3<5, list the molecule tally on the left side and the atom tally on the right side of the page: (step 3). Add and tally:
To balance C2H5OH + O2 --> 2CO2 + 3H2O or C 2=2, H 6=6, O 3<7, list the molecule tally on the left side and the atom tally on the right side of the page: (step 4 or 5). The next step in balancing:
is to add 2 O2 molecules for a balanced equation
is to check that the smallest integer coefficients have been used
is to tally the atoms correctly
is to tally the molecules correctly
To balance K + H2O --> H2 + KOH, list the molecule tally on the left side and the atom tally on the right side of the page. (step 1)
K 1=1, H 2<3, O 1=1 is the atom tally.
Adding 1 H2O leaves the H and O atoms unbalanced but is the proper thing to do.
Add 1/2 H2O to balance the H.
Add 1 H2O for K + H2O --> H3O + KOH.
To balance K + 2H2O --> H2 + KOH, list the molecule tally on the left side and the atom tally on the right side of the page. (step 2)
K 1=1, H 4>3, O 2>1 for the atom tally.
The low H and O count can be made up by adding KOH.
Add an OH- ion to balance the H and O atoms.
Adding KOH will balance the equation.
To balance NH3 + O2 --> NO + H2O, (step1):
N 1=1, H 3>2, O 2=2 for the atom tally
H comes in sets of 3 and goes in sets of 2 (apply trick of cross multiplying: 3 x 2 = 2 x 3 to balance hydrogen)
add 1 H2O for best next move
add H+ for NH3 + O2 --> NO + H3O to balance
To balance 2NH3 + O2 --> NO + 3H2O (step 2):
N 2>1, H 6=6, O 2<4 is the atom tally.
Add 1 NO for 2NH3 + O2 --> 2NO + 3H2O.
Add 1 O2 which is a less complicated molecule than NO.
Add 1 NO and 1 O2 to balance equation.
To balance 2NH3 + O2 --> 2NO + 3H2O (step 3):
N 2=2, H 6=6, O 2<5 is the atom tally.
O comes in sets of 2 and goes in sets of 5 (cross multiply: 2 x 5 = 5 x 2 to balance).
Add 1 O2 as next best step.
Add 2 O2 as next best step.
2NH3 + 5O2 --> 4NO + 6H2O is balanced equation.
To balance SiO2 + HF --> SiF4 + H20 (step 1):
Si 1=1, O 2>1, H 1<2, F 1<4 is the atom tally.
Pick F to balance as it is the most complicated molecule and has the highest ratio of atoms.
Pick O to balance as it is in excess in the reactants.
To balance SiO2 + 4HF --> SiF4 + H20 (step 2):
Si 1=1, O 2>1, H 4>2, F 4=4 is the atom tally.
Add 1 H2O to balance.
2SiO2 + 8HF --> 2SiF4 + 4H2O is the balanced equation.
At each step in balancing equations, the atom tally shows:
which atoms need adjusting
if the equation is in balance
the formula for the reactants
the formula for the products
At each step in balancing a chemical reaction:
sum the atoms of added molecules with the old atom tally for the new atom tally
write added molecules below their position in the chemical equation
select the least complicated molecules to balance first
write the atom tally below the molecule tally (the equation)
