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

MethaneOxygen Carbon

dioxideWater

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 atoms4 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

HO=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

HO=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

CC

C

H

H

HHH

H

H

H

A “picture” of this reaction:

C3H8 + 5 O2 3 CO2 + 4 H2O

CC

C

H

H

HHH

H

H

H

O=OO=OO=O

O=O

O=O

A “picture” of this reaction:

C3H8 + 5 O2 3 CO2 + 4 H2O

CC

C

H

H

HHH

H

H

H

O=OO=C=O

O=C=OO=C=O

O=OO=O

O=O

O=O

A “picture” of this reaction:

C3H8 + 5 O2 3 CO2 + 4 H2O

CC

C

H

H

HHH

H

H

H

O=OO=C=O

O=C=OO=C=O

HO

H

HO

HH

OH

HO

H

O=OO=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.

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 atomsTwelve 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 AlCheck:

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 + 2 NH3

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 H2OCheck:

4 N atoms 12 H atoms 10 oxygen atoms

4 N atoms 12 H atoms 10 oxygen atoms