Question 3.EP.3: When aqueous solutions of acetic acid and potassium hydroxid...
When aqueous solutions of acetic acid and potassium hydroxide are combined, a neutralization reaction will occur. Write molecular, total ionic, and net ionic equations for this process.
Strategy
This is the reaction of a weak acid with a strong base. The reaction between an acid and a base always produces water as a product, along with an ionic compound formed from the remaining ions. (This second product is often called a salt.) We can begin by using this idea to write the molecular equation. To generate the ionic equations, then, we account for the dissociation of strong electrolytes into their constituent ions.
A hydrogen atom from the acid and the hydroxide ion from the base will produce water. The remaining ions will be an acetate anion (CH_{3}COO^{-} from the acid) and a potassium ion (K^{+}, from the base). Combining these gives us potassium acetate, KCH_{3}COO. This lets us write the molecular equation:
CH_{3}COOH+KOH\longrightarrow H_{2}O+KCH_{3}COO
\ To produce the total ionic equation, we must determine which of these species to write as dissociated ions. According to Table 3.2,
| Strong and weak acids and bases | |||||
| Strong Acids | Strong Bases | ||||
| HCL | Hydrochloric acid | LiOH | Lithium hydroxide | ||
| HNO_{3} | Nitric acid | NaOH | Sodium hydroxide | ||
| H_{2}SO_{4} | Sulfuric acid | KOH | Potassium hydroxide | ||
| HClO_{4} | Perchloric acid | Ca(OH)_{2} | Calcium hydroxide | ||
| HBr | Hydrobromic acid | Ba(OH)_{2} | Barium hydroxide | ||
| HI | Hydriodic acid | Sr(OH)_{2} | Strontium hydroxide | ||
| Weak Acids | Weak Bases | ||||
| H_{3}PO_{4} | Phosphoric acid | NH_{3} | Ammonia | ||
| HF | Hydrofluoric acid | CH_{3}NH_{2} | Methylamine | ||
| CH_{3}COOH | Acetic acid | ||||
| HCN | Hydrocyanic acid | ||||
| Note: All common strong acids and bases are shown, but only representative examples of weak acids and bases are listed. | |||||
acetic acid is a weak acid. This means that it is only partially dissociated in solution, and so we write it as an intact molecule. KOH, on the other hand, is a strong base, and so it will dissociate completely. So we write this as a pair of ions, K^{+} and OH^{-}. According to Table 3.1,
| Solubility guidelines for ionic compounds in water at room temperature | |||||
| Usually Soluble | Exceptions | ||||
| Group 1 cations
(Li^{+},Na^{+},K^{+},Rb^{+},Cs^{+}), |
No common exceptions | ||||
| Nitrates (NO_{3}^{-}), nitrites (NO_{2}^{-}) | Moderately soluble: AgNO_{2} | ||||
| Chlorides, bromides, iodides (Cl^{-},Br^{-},I^{-}) |
AgCl,Hg_{2}Cl_{2},PbCl_{2},AgBr,Hg_{2}Br_{2}, PbBr_{2}, AgI, Hg_{2}I_{2}, and PbI_{2} |
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| Fluorides (F^{-}) | Insoluble: MgF_{2}, CaF_{2}, SrF_{2}, BaF_{2}, PbF_{2} | ||||
| Sulfates (SO_{4}^{ \ 2-}) | Insoluble: BaSO_{4}, PbSO_{4}, HgSO_{4} Moderately soluble: CaSO_{4}, SrSO_{4},Ag_{2}SO_{4} |
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| Chlorates (ClO_{3}^{-}), perchlorates (ClO_{4}^{-}) | No common exceptions | ||||
| Acetates (CH_{3}COO^{-}) | Moderately soluble: AgCH_{3}COO | ||||
| Usually Insoluble | Exceptions | ||||
| Phosphates (PO_{4}^{ \ 3-}) | Soluble: (NH_{4})_{3}PO_{4}, Na_{3}PO_{4}, K_{3}PO_{4} | ||||
| Carbonates (CO_{3}^{ \ 2-}) | Soluble: (NH_{4})_{2}CO_{3}, Na_{2}CO_{3}, K_{2}CO_{3} | ||||
| Hydroxides (OH^{-}) | Soluble: LiOH, NaOH, KOH,Ba(OH)_{2} Moderately soluble: Ca(OH)_{2}, Sr(OH)_{2} |
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| Sulfides (S^{2-}) | Soluble: (NH_{4})_{2}S, Na_{2}S, K_{2}S,MgS,CaS | ||||
both potassium ions and acetate ions tend to produce soluble compounds. So KCH_{3}COO will be soluble and hence will dissociate into its constituent ions. Putting all of this together lets us write the total ionic equation:
CH_{3}COOH(aq)+K^{+}(aq)+OH^{-}(aq)\longrightarrow H_{2}O(l)+K^{+}(aq)+CH_{3}COO^{-}(aq)
Looking at this equation, we see potassium ions on both sides. Thus potassium is a spectator ion and can be deleted to give the net ionic equation:
CH_{3}COOH(aq)+OH^{-}(aq)\longrightarrow H_{2}O(l)+CH_{3}COO^{-}(aq)