Question

Write an equation to show how the following acids or bases react with water. (a) \(\mathrm{HCl}(g)\) (b) \(\mathrm{HClO}_{4}(l)\) (c) \(\mathrm{CH}_{3} \mathrm{CO}_{2}^{-}(a q)\)

Short answer

The reactions of these compounds with water can be written as follows:\n\n(a) \(\mathrm{HCl}(g) + \mathrm{H}_{2} \mathrm{O}(l) \rightarrow \mathrm{H}_{3} \mathrm{O}^{+}(a q) + \mathrm{Cl}^{-}(a q)\)\n\n(b) \(\mathrm{HClO}_{4}(l) + \mathrm{H}_{2} \mathrm{O}(l) \rightarrow \mathrm{H}_{3} \mathrm{O}^{+}(a q) + \mathrm{ClO}_{4}^{-}(a q)\)\n\n(c) \(\mathrm{CH}_{3} \mathrm{CO}_{2}^{-}(a q) + \mathrm{H}_{2} \mathrm{O}(l) \rightarrow \mathrm{CH}_{3}\mathrm{COOH}(a q) + \mathrm{OH}^{-}(a q)\)

Step-by-step solution

- HCl(g) Reaction

Hydrochloric acid (\(\mathrm{HCl}\)) is a strong acid. When it reacts with water, it donates a proton (\(H^{+}\)) to form a hydronium ion (\(H_{3}O^{+}\)) and a chloride ion (\(Cl^{-}\)). The equation would therefore be: \(\mathrm{HCl}(g) + \mathrm{H}_{2} \mathrm{O}(l) \rightarrow \mathrm{H}_{3} \mathrm{O}^{+}(a q) + \mathrm{Cl}^{-}(a q)\).

- HClO4 Reaction

Similar to the above, perchloric acid (\(\mathrm{HClO}_{4}\)) is also a strong acid and donates a proton to form a hydronium ion (\(H_{3}O^{+}\)) and a perchlorate ion (\(ClO_{4}^{-}\)). The balanced equation becomes: \(\mathrm{HClO}_{4}(l) + \mathrm{H}_{2} \mathrm{O}(l) \rightarrow \mathrm{H}_{3} \mathrm{O}^{+}(a q) + \mathrm{ClO}_{4}^{-}(a q)\).

- CH3CO2- Reaction

\(\mathrm{CH}_{3} \mathrm{CO}_{2}^{-}(a q)\) acts like a base and when it reacts with water, it accepts a proton to form acetic acid (\(\mathrm{CH}_{3}\mathrm{COOH}\)) and hydroxide ion (\(\mathrm{OH}^{-}\)). The reaction can be written as: \(\mathrm{CH}_{3} \mathrm{CO}_{2}^{-}(a q) + \mathrm{H}_{2} \mathrm{O}(l) \rightarrow \mathrm{CH}_{3}\mathrm{COOH}(a q) + \mathrm{OH}^{-}(a q)\).

Key concepts

strong acids

Strong acids are powerful in that they completely dissociate in water. This means when a strong acid, like hydrochloric acid (\(\mathrm{HCl}\)), is mixed with water, it fully splits into ions. This process happens with virtually no remaining undissociated molecules. Strong acids are always ready to donate their protons.
Some characteristics of strong acids include:

• They have a high tendency to donate protons (\(H^+\)).
• They completely ionize in water, leaving no acid molecules behind.
• They form strong electrolytes, conducting electricity efficiently in solution.
• Examples include hydrochloric acid (\(\mathrm{HCl}\)) and perchloric acid (\(\mathrm{HClO}_{4}\)).

Strong acids play a crucial role in various chemical reactions, essentially setting up the next steps in reaction processes.

hydronium ion

The hydronium ion, \(\mathrm{H}_{3}\mathrm{O}^{+}\), is a special ion that results when an acid interacts with water. In acid-base reactions, when an acid donates a proton (or \(H^+\)), this proton does not exist freely in water. Instead, it combines with a water molecule (\(\mathrm{H}_{2}\mathrm{O}\)) to form the hydronium ion.
Creating hydronium ions involves the following:

• When acids dissolve in water, they release \(H^+\) ions.
• These \(H^+\) ions quickly join with water to form \(\mathrm{H}_{3}\mathrm{O}^{+}\).

So, in the presence of strong acids like \(\mathrm{HCl}\) and \(\mathrm{HClO}_{4}\), you get a plentiful amount of hydronium ions, which makes the solution highly acidic. Hydronium ions help us gauge the acidity level of a solution, as more hydronium ions mean a lower pH and hence, a stronger acidic environment.

proton donation

Proton donation is a key step in acid-base reactions and is central to what acids do. When we talk about acids, we often describe them as substances capable of donating protons (\(H^+\) ions). This is because an acid's strength and functionality largely depend on their ability to donate these protons when they interact with other substances, like water.
Here's how proton donation works:

• An acid will surrender a \(H^+\) ion when it reacts with a base or water.
• This donated proton then associates with a water molecule to form \(\mathrm{H}_{3}\mathrm{O}^{+}\).

Strong acids, which we discussed earlier, are the best proton donors because they freely and completely donate their protons in solution. Consequently, this donation is what leads to the creation of hydronium ions and contributes to the acidic nature of the solution. Understanding proton donation helps explain the behavior of acids in different scenarios, such as titrations and buffer solutions.