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How can I tell if an equation is balanced correctly?


Vocabulary
chemical change*
law of conservation of mass*
A properly balanced equation conserves mass and charge. All of the following requirements must be met:

Each side of the equation must have the same number of atoms. Atoms aren't created or destroyed in chemical changes. You can very quickly spot an unbalanced equation this way. For example,
CH4(g) + O2(g) = CO2(g) + H2O(ell) Incorrect (7 atoms on the left, 6 atoms on the right)
CH4(g) + 2 O2(g) = CO2(g) + 2 H2O(ell) Correct (9 atoms on the left, 9 atoms on the right)

Each side of the equation must have the same number of atoms of each element. Atoms aren't converted into different types of atoms in chemical change (although this can happen in a nuclear change.) For example,
CuSO4·5 H2O(s) = CuSO4(s) + H2O(g) Incorrect. Each side has the same number of Cu and S, but different numbers of H and O.
CuSO4·5 H2O(s) = CuSO4(s) + 5 H2O(g) Correct. Each side has the same number of Cu, S, H, and O.
NaOH(aq) + HCl = H2O(ell) + KCl(aq) Incorrect. Each side has the same total number of atoms, but a sodium atom has changed into a potassium atom!
NaOH(aq) + HCl = H2O(ell) + NaCl(aq) Correct. Each side has the same total number of atoms and the same number of Na, H, Cl, and O atoms.

Each side of the equation must have the same total charge. Electrons aren't created or destroyed in chemical changes (although once again, this can happen in a nuclear reaction). For example:
Cu2+(aq) + Al(s) = Cu(s) + Al3+(aq) Incorrect. Each side has the same number of Cu and Al atoms, but the left side has a total charge of +2, but the right has a total charge of +3, implying that an electron is destroyed in the reaction.
3 Cu2+(aq) + 2 Al(s) = 3 Cu(s) + 2 Al3+(aq) Correct. Each side has the same number of Cu and Al atoms, and the same total charge (+6).

See Ten tips on balancing equations for more details.

Author: Fred Senese senese@antoine.frostburg.edu



General Chemistry Online! How can I tell if an equation is balanced correctly?

Copyright © 1997-2010 by Fred Senese
Comments & questions to fsenese@frostburg.edu
Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/reactions/faq/balancing-requirements.shtml