Problem 33 - General Chemistry: Principles and Modern Applications

Question

Which of the following would you expect to react as a Bronsted-Lowry acid: \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+},\left[\mathrm{FeCl}_{4}\right]^{-},\) \(\left[\mathrm{Al}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}, \operatorname{or}\left[\mathrm{Zn}(\mathrm{OH})_{4}\right]^{2-} ? \mathrm{Why} ?\)

Step-by-Step Solution

Answer

\(\left[\mathrm{Al}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}\) is the compound that can act as a Bronsted-Lowry acid

Step 1: Identify Potential Proton Donors

The first step is to examine each of the given chemical substances and identify which ones have hydrogen atoms that they might be able to donate. These substances are \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+}, \left[\mathrm{FeCl}_{4}\right]^{-}, \left[\mathrm{Al}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}, \mathrm{and} \left[\mathrm{Zn}\left(\mathrm{OH}\right)_{4}\right]^{2-}\).

Step 2: Analyze Each Chemical Substance

\(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+}, \left[\mathrm{FeCl}_{4}\right]^{-}, \mathrm{and} \left[\mathrm{Zn}\left(\mathrm{OH}\right)_{4}\right]^{2-}\) do not have any H+ ions to donate. The first substance has its H atoms tied up in ammonia ligands which are not likely to be donated, while the second substance does not even have any H atoms. The third substance, while it does have H atoms in hydroxide ions, is unlikely to donate them as this would form water and subsequently form an unstable and highly reactive Zn4+ ion. The remaining substance, however, \(\left[\mathrm{Al}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}\), could potentially donate a H+ ion from one of its water ligands, forming a hydroxide ion, and is therefore the only substance that can act as a Bronsted-Lowry acid.

Step 3: Determine the Bronsted-Lowry Acid

Based on the analysis in steps 1 and 2, the only substance among \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+}, \left[\mathrm{FeCl}_{4}\right]^{-}, \left[\mathrm{Al}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}, \mathrm{and} \left[\mathrm{Zn}\left(\mathrm{OH}\right)_{4}\right]^{2-}\) that appears to be capable of donating a H+ ion and thus acting as a Bronsted-Lowry acid is \(\left[\mathrm{Al}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}\).

Key Concepts

Proton Donors

In the world of chemistry, proton donors play a pivotal role in acid-base reactions. To comprehend this, let's first clarify that a proton is simply a hydrogen atom that has lost its electron, leaving behind a positively charged particle, symbolized as H+. A Bronsted-Lowry acid is characterized by its ability to donate a proton to another substance.

When assessing a chemical substance's potential to act as a proton donor, we look for readily releasable hydrogen atoms. In our exercise, the only compound with hydrogen atoms that can feasibly be donated is \(\left[\mathrm{Al}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}\) because it contains water molecules as ligands. These water molecules can lose a hydrogen ion, transforming into hydroxide ions (OH-), thereby exhibiting the acid behavior of proton donation. As a comparison, the other substances lack this ability either because their hydrogen atoms are not in a position to be released easily or because they're part of a structure that doesn't typically give up a hydrogen ion.

It's essential for students to visualize the structure of such complexes to determine if hydrogen is present in a form that can be easily detached and donated. This approach simplifies the identification of potential acid candidates.

Chemical Substance Analysis

Chemical substance analysis involves the detailed examination of the composition and structure of chemical compounds to understand their behavior in various reactions. In our exercise, we closely analyze the given substances to predict which can act as a Bronsted-Lowry acid. This systematic scrutiny is crucial as it helps to rationalize why certain compounds can donate a proton, and others cannot.

For the purpose of identifying acids, we examine the bonding and stability of hydrogen within the compound. For example, in \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+}\), the hydrogen atoms are part of ammonia (NH3) and are not available for donation. Similarly, \(\left[\mathrm{FeCl}_{4}\right]^{-}\) lacks hydrogen completely. In the case of \(\left[\mathrm{Zn}\left(\mathrm{OH}\right)_{4}\right]^{2-}\), although there are hydrogen atoms, loss of H+ would leave behind an unstable Zn4+ ion, thus not favoring proton donation. Analyzing chemical stability and reactivity helps in predicting chemical behavior accurately, which is a necessary skill for students studying chemistry.

Acid-base Reactions

Acid-base reactions are a foundational concept in chemistry, involving the transfer of protons between substances. These reactions are described by the Bronsted-Lowry theory, where acids are proton donors and bases are proton acceptors. Simply put, when an acid loses a proton, it becomes a conjugate base, while a base gaining this proton becomes a conjugate acid.

Understanding the nature of these interactions helps students predict the outcome of a reaction. For instance, our analysis shows that \(\left[\mathrm{Al}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}\) can act as an acid, donating a proton to become \(\left[\mathrm{Al}\left(\mathrm{OH}\right)_{5}\right]^{2+}\) and leaving a hydroxide ion (OH-) as the conjugate base. It's crucial for learners to recognize that reaction conditions, such as solvent and temperature, can also affect the propensity of a substance to donate or accept protons, reinforcing the importance of context in chemical reactions.