Describing Chemical Reactions

An equation is balanced by changing coefficients in a somewhat trial-and-error fashion. Please note that the coefficients can be changed, but the subscripts can ____________ be changed.

Hints:

• Balance elements that exist in compounds with odd number of atoms

• Balance the element that occurs only once on each side

• If an element exists as a free element on one side, balance that element last

Three quick examples to try:

(a) 2 H₂ = ___ H atoms

(b) 2 H₂O = ___ H atoms, ___ O atoms

(c) 2 (NH₄)₂S = ___ N atoms, ___H atoms, ___ S atoms
Two things you CANNOT do when balancing an equation.

1) You cannot change a subscript.

Example: You cannot change the oxygen's subscript in water from one to two, as in:

Balance: H₂ + O₂  H₂O

H₂ + O₂  H₂O₂

Whereas this balances the equation, you have changed the compound in it. H₂O₂ is a completely different substance than H₂O.

2) You cannot place a coefficient in the middle of a chemical formula.

The coefficient must go at the beginning of a formula, not in the middle, as in:

Example:

4 H₂ + 2 O₂  4 H₂O
This equation is balanced (having the same number of atoms of each element on both sides). However, all the coefficients have the common factor of two. Divide through to eliminate common factors like this and to have a CORRECTLY balanced equation.
___ H₂ + ___ O₂  ___ H₂O