CHM112 Home Page Reference Materials Homework Assignments Lectures Exams & Quizzes Grades Study Aids URI Home Page


Answering "Complete and Balance" type questions:


1. Are any products given?
If so, check to see if there are changes in oxidation numbers. If so, this is an oxidation/reduction reaction. Use the rules for oxidation/reduction reactions.

2. Are there any acids or bases?
a. How do we recognize acids?

i. H is written first in the chemical formula. The common strong acids: nitric acid (HNO3); hydrochloric acid (HCl); hydrobromic acid (HBr); hydriodic acid (HI); perchloric acid (HClO4); sulfuric acid (H2SO4, first hydrogen only).

ii. Most cations (the exceptions: cations from strong bases).

iii. Organic acids with chemical formulas written as RCOOH or RCO2H.

b. How do we recognize bases?

i. Metal hydroxides. Group 1 (LiOH, NaOH, KOH, RbOH, and CsOH) and Group 2 (Mg(OH)2, Ca(OH)2, Sr(OH)2, Ba(OH)2) hydroxides are considered strong bases.

ii. Most anions (the exceptions: anions from strong acids).

iii. Amines, which are organic compounds having a nitrogen with an available lone pair, generally written as RN, RNH, or RNH2.


If acids or bases are present, write the reaction of the acid or base (or both) with water giving the conjugate species and hydronium ion or hydroxide ion.

If both hydronium ion and hydroxide ion are generated by the presence of acids and bases in the reactants, they must react with each other to form water:

H3O+(aq) + OH(aq) 2H2O(l)

The overall reaction must be an acid/base reaction. The stoichiometry must be adjusted for each individual reaction so that all of the hydronium ion and all of the hydroxide ion are removed from reaction.

If a strong acid or strong base is involved, the acid/base reaction will go to completion. If only weak species are present, a calculation using Ka and Kb must be done to determine the extent of reaction.

Acid/base reactions often create ionic salts as products. Check to see if any ionic salts can precipitate.


3. Are there ionic salts present?

If so, write the dissolution reaction (or reactions, if more than one salt is present) that generates the ions in solution.

a. Is water written as a reactant?

If water is written explicitly as a reactant, check for possible hydrolysis reactions with the cation and the anion. Hydrolysis reactions are always equilbria.

b. Look for cation-anion combinations.

See if a cation present in solution can combine with an anion present in solution to form a precipitate. Precipitation reactions go to completion.

c. Is there a common ligand present?

Common ligands include ammonia (NH3), ethylenediammine (en, H2NCH2CH2NH2), cyanide (CN), oxalate (ox2, C2O42), thiocyanate (SCN), and ethylenediamminetetracetate (EDTA4). Halides and hydroxides can also act as ligands. 

If a common ligand is present and a cation, this may be a complex ion formation reaction (Lewis acid/base). Check the table of complex ions to see if this combination of cation and ligand do form a complex ion and, if so, to determine the stoichiometry.



Examples done in class:

1. HBr(aq)  +  NH3(aq)

2. H3PO4(aq)  + F(aq)

3. Fe(NO3)2(aq) + H2O(l)

4. NaF(aq) + H2O(l)

5. ZnF2(aq) + H2O(l)

6. Pb(NO3)2(aq) + K2SO4(aq)

7. Fe(NO3)3(aq) + CsCl(aq)

8. Co(NO3)2(aq) + H2S(aq)

9. Ca(NO3)2(aq) + K2SO4(aq)

10. CaHPO4(s) + HCl(aq)

11. CaHPO4(s) + NaOH(aq)

12. Cu2+(aq) + NH3(aq)

13. Mg(s) + NO3(aq) Mg2+(aq) + NO2(g)

14. Cr2O72(aq) + SO2(g) Cr(OH)3(s) + SO42(aq)

15. ClO2(aq) ClO(aq) + ClO4(aq)

For many more problems, see the practice reactions.