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Chapter 4

Q. 4.7

Write the balanced (a) molecular, (b) overall ionic, and (c) net ionic equations that describe the reaction that takes place when an aqueous solution of nitric acid is neutralized by an aqueous solution of calcium hydroxide.

Step-by-Step

Verified Solution

Collect and Organize Solutions of nitric acid and calcium hydroxide take part in a neutralization reaction. Our task is to write three different equations describing the neutralization reaction.

Analyze We know that calcium hydroxide [Ca(OH)_{2}] is the base and nitric acid (HNO_{3}) is the acid. The products of a neutralization reaction are water and a salt. (a) All species are written as neutral compounds in the molecular equation. The products are water and calcium nitrate, the salt made from the cation of the base and the anion of the acid. (b) We can write each substance in the balanced molecular equation in ionic form to generate the overall ionic equation. (c) Removing spectator ions from the overall ionic equation gives us the net ionic equation.

Solve
a. The molecular equation is

HNO_{3} (aq) + Ca(OH)_{2}(aq) → Ca(NO_{3})_{2}(aq) + H_{2}O(\ell )                                   (unbalanced)
2 HNO_{3} (aq) + Ca(OH)_{2}(aq) → Ca(NO_{3})_{2}(aq) + 2 H_{2}O(\ell )                                 (balanced)

b. The overall ionic equation is

2 H^{+}(aq) + 2 NO_{3}^{-}(aq) + Ca^{2+}(aq) + 2 OH^{-}(aq) → Ca^{2+}(aq) + 2 NO_{3} ^{-}(aq) + 2 H_{2}O(\ell )

c. The spectator ions are Ca^{2+} and NO_{3} ^{-}. Removing them gives us the net ionic equation:

2 H^{+}(aq) + 2 OH^{-}(aq) → 2 H_{2}O(\ell)

or

H^{+}(aq) + OH^{-}(aq) → H_{2}O(\ell)

Think About It We treat chemical equations just like algebraic equations, canceling out spectator ions that are unchanged in the reaction and appear on both sides of the reaction arrow in the overall ionic equation. After canceling the spectator ions, we are left with the net ionic equation, which focuses on the species that were changed as a result of the reaction. It is also important to remember that nitrate is a polyatomic ion (see Table 2.3)

Table 2.3  Names, Formulas, and Charges of Some Common Polyatomic Ions
Name Chemical Formula
Acetate CH_{3}COO^{-}
Carbonate CO_{3}^{2-}
Hydrogen carbonate or bicarbonate HCO_{3}^{-}
Cyanide CN^{-}
Hypochlorite CIO^{-}
Chlorite CLO_{2}^{-}
Chlorate CLO_{3}^{-}
Perchlorate CLO_{4}^{-}
Dichromate Cr_{2}O_{7}^{2-}
Chromate CrO_{4}^{2-}
Permanganate MnO_{4}^{-}
Azide N_{3}^{-}
Ammonium NH_{4}^{+}
Nitrite NO_{2}^{-}
Nitrate NO_{3}^{-}
Hydroxide OH^{-}
Peroxide O_{2}^{2-}
Phosphate PO_{4}^{3-}
Hydrogen phosphate HPO_{4}^{2-}
Dihydrogen phosphate H_{2}PO_{4}^{-}
Disulfide S_{2}^{2-}
Sulfate SO_{4}^{2-}
Hydrogen sulfate or bisulfate HSO_{4}^{-}
Sulfite SO_{3}^{2-}
Hydrogen sulfite or bisulfite HSO_{3}^{-}
Thiocyanate SCN^{-}

that retains its identity in solution and does not separate into atoms and ions. Note that the net ionic equation is the same as in the reaction of an aqueous solution of HCl with aqueous NaOH.