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Chemical Equations
Describing Chemical Process
Chemical Equations
❧ Identify the substances involved in a chemical process
❧ Distinguish between the reactants and products in a chemical process
❧ Allow easy determination of quantities of substances involved in chemical processes
Verbal description of a common chemical process:
“Methane undergoes combustion in oxygen to form carbon dioxide and water.”
The substances:
❧ methane: CH4 ❧ Oxygen: O2 ❧ Carbon dioxide: CO2 ❧ Water: H2O
Reactant(s) Product(s)
The form of an equation:
Reactant(s) Product(s)
One or more substances -- the beginning “stuff”
The form of an equation:
Reactant(s) Product(s)
One or more substances -- the beginning “stuff”
Reaction arrow; points
toward the product(s)
The form of an equation:
Reactant(s) Product(s)
One or more substances -- the beginning “stuff”
One or more substances --
the final “stuff”
Reaction arrow; points
toward the product(s)
The form of an equation:
For the reaction of methane:
CH4 + O2 ! CO2 + H2O
The reaction arrow
Methane Oxygen Carbon dioxide
Water
Reactants Products
For the reaction of methane:
CH4 + O2 ! CO2 + H2O
C
H
H
H
H O=O ! O=C=O O H
H
For the reaction of methane:
CH4 + O2 ! CO2 + H2O
C
H
H
H
H O=O ! O=C=O O H
H
Note, there appear to be more oxygen atoms, fewer hydrogen atoms at the end that at the beginning!
2 H atoms, 3 oxygen atoms 4 H atoms, 2 oxygen atoms
For the reaction of methane:
CH4 + O2 ! CO2 + H2O
C
H
H
H
H O=O ! O=C=O O H
H
There are enough H atoms in one methane molecule to make two water molecules.
O H
H
2
…but, oxygen is worse off!
For the reaction of methane:
CH4 + 2 O2 ! CO2 + 2 H2O
C
H
H
H
H O=O ! O=C=O O H
H
Adjust the total number of oxygen atoms with more O2. Now, oxygen is OK.
O H
H O=O
For the reaction of methane:
CH4 + 2 O2 ! CO2 + 2 H2O
C
H
H
H
H O=O ! O=C=O O H
H
All atoms are in the same numbers before and after.
O H
H O=O
Propane reacts similarly with O2
C3H8 + O2 ! CO2 + H2O
None of the atoms appear in the same amounts before and after; the equation is UNBALANCED.
Propane
Propane reacts similarly with O2
C3H8 + O2 ! CO2 + H2O
There are enough H atoms in one propane molecule to make four water molecules.
None of the atoms appear in the same amounts before and after; the equation is UNBALANCED.
Propane reacts similarly with O2
C3H8 + O2 ! CO2 + 4 H2O
There are enough carbon atoms in propane to make three carbon dioxide molecules.
Hydrogens are OK, but carbons are unequal before and after, still.
Propane reacts similarly with O2
C3H8 + O2 ! 3 CO2 + 4 H2O
Adjust the total number of oxygen molecules...
Carbons are OK, but oxygen atoms are still unequal before and after.
Propane reacts similarly with O2
C3H8 + 5 O2 ! 3 CO2 + 4 H2O
The equation is BALANCED.
Now, all atoms are equal in number before and after.
Propane reacts similarly with O2
C3H8 + 5 O2 ! 3 CO2 + 4 H2O
The equation is BALANCED.
Now, all atoms are equal in number before and after.
A “picture” of this reaction: C3H8 + 5 O2 ! 3 CO2 + 4 H2O
C C
C
H
H
H H H
H
H
H
A “picture” of this reaction: C3H8 + 5 O2 ! 3 CO2 + 4 H2O
C C
C
H
H
H H H
H
H
H
O=O O=O O=O
O=O
O=O
A “picture” of this reaction:
C3H8 + 5 O2 ! 3 CO2 + 4 H2O
C C
C
H
H
H H H
H
H
H
O=O O=C=O O=C=O
O=C=O O=O O=O
O=O
O=O
A “picture” of this reaction:
C3H8 + 5 O2 ! 3 CO2 + 4 H2O
C C
C
H
H
H H H
H
H
H
O=O O=C=O O=C=O
O=C=O
H O H
H O H
H O H
H O H
O=O O=O
O=O
O=O
Suggestions to Balance Equations ❧ Work with elements that appear in the fewest
formulas first (in one formula on “each side” of the reaction arrow)
❧ Proceed to elements appearing in greater and greater numbers of formulas.
❧ NEVER, EVER change chemical formulas (subscripts)
❧ Use coefficients to balance the number of each element.
❧ Always check to see that elements are in same numbers on both sides.
Practice
C6H12O6 + O2 ! H2O + CO2
Practice
C6H12O6 + O2 ! H2O + CO2
Start with either C or H. Oxygen appears in every formula; save it till last.
Practice
C6H12O6 + O2 ! H2O + CO2
There are enough C atoms in C6H12O6 to form six CO2 molecules.
6
Practice
C6H12O6 + O2 ! H2O + 6 CO2
There are enough H atoms in C6H12O6 to form six H2O molecules.
6
Practice
C6H12O6 + O2 ! 6 H2O + 6 CO2
Six O2 molecules are required to provide enough total oxygen atoms to balance.
Six O atoms
Six O atoms Twelve O atoms
Twelve O atoms, 6 sets of two
6
Check
C6H12O6 + 6 O2 ! 6 H2O + 6 CO2
18 total O atoms 18 total oxygen atoms
Check
C6H12O6 + 6 O2 ! 6 H2O + 6 CO2
12 H atoms 12 total H atoms
Check
C6H12O6 + 6 O2 ! 6 H2O + 6 CO2
six C atoms six C atoms
Your turn...
Al2O3 + H2 ! H2O + Al
Your turn...
1 Al2O3 + 3 H2 ! 3 H2O + 2 Al
Your turn...
Al2O3 + 3 H2 ! 3 H2O + 2 Al Check:
● 2 Al atoms ● 3 Oxygen atoms ● 6 H atoms
● 2 Al atoms ● 3 Oxygen atoms ● 6 H atoms
Your turn...
Ca(OH)2 + HCl ! H2O + CaCl2
Your turn...
1 Ca(OH)2 + 2 HCl ! 2 H2O + 1 CaCl2
Your turn...
Check: ● 1 Ca atom ● 2 oxygen atoms ● 2 chlorine atoms ● 4 hydrogen atoms
● 1 Ca atom ● 2 oxygen atoms ● 2 chlorine atoms ● 4 hydrogen atoms
Ca(OH)2 + 2 HCl ! 2 H2O + CaCl2
Your turn...
H2O + Mg3N2 ! Mg(OH)2 + NH3
Your turn...
6 H2O + 1 Mg3N2 ! 3 Mg(OH)2 + 2NH3
Your turn...
6 H2O + Mg3N2 ! 3 Mg(OH)2 + 2 NH3
Check: ● 12 H atoms ● six O atoms ● 3 Mg atoms ● 2 N atoms
● 12 H atoms ● six O atoms ● 3 Mg atoms ● 2 N atoms
Your turn...
NH3 + O2 ! NO + H2O
Your turn...
4 NH3 + 5 O2 ! 4 NO + 6 H2O
Your turn...
4 NH3 + 5 O2 ! 4 NO + 6 H2O Check:
● 4 N atoms ● 12 H atoms ● 10 oxygen atoms
● 4 N atoms ● 12 H atoms ● 10 oxygen atoms
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