Balancing Chemical Reaction Equations

The Law of Conservation of Mass requires that the number and type of atoms be the same on both sides of a reaction arrow. Students are expected to deduce the reaction coefficients if they are not given. This is called balancing the reaction. There are lots of tricks to balancing reaction equations, but they all rely on ‘trial and error.’ You must fill in coefficients and count atoms to see if the atom counts on both sides of the equation match.

!Warning! When you balance chemical reaction equations, you can adopt a systematic method that helps reduced the time involved in the trial and error process, or you can leap right in. In either case, watch for these common problems:

Do not change subscripts! You balance a reaction equation by adjusting coefficients only. If you are given a verbal description, double check to make sure that your chemical formulae are correct before you start to balance the equation. A tiny mistake with a subscript can result in a reaction that is impossible to balance no matter how hard you try.
Do not forget that hydrogen, nitrogen, oxygen, and the halogens are all diatomic elements.
When counting atom totals, remember that subscripts after a parenthesis indicate a multiple of everything inside the parenthesis. Ca₃(PO₄)₂ has two phosphorus and eight oxygen atoms.
When counting atom totals, remember that coefficients indicate multiples of the entire compound. 2Ca₃(PO₄)₂ has six calcium, four phosphorus, and 16 oxygen atoms.
When an element appears in more than one reactant or product, you have to include ALL of the atoms in the count. Ca₃(PO₄)₂ + NaNO₃ has 11 oxygen atoms if both appear on the same side of the reaction arrow.

Concept Check: Balance these reaction equations:

K₂O_(s) + H₂O_(l) à KOH_(aq)

C₅H_10(l)+ O_2(g) à CO_2(g)+ H₂O_(l)

Barium nitrate aqueous solution reacts with sulphuric acid aqueous solution to form nitric acid aqueous solution and barium sulphate precipitate.

Answer:

K₂O_(s) + H₂O_(l) à 2KOH_(aq)

2C₅H_10(l) + 15O_2(g) à 10CO_2(g)+ 10H₂O_(l)

Ba(NO₃)_2(aq) +H₂SO_4(aq) à 2HNO_3(aq)+ BaSO_4(ppt)

This site was last updated 09/15/06