Question

Which member of each pair produces the more acidic aque- ous solution: (a) \(\mathrm{ZnBr}_{2}\) or \(\mathrm{CdCl}_{2},\) (b) \(\mathrm{CuCl}\) or \(\mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2},(\mathrm{c})\) \(\mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{2}\) or \(\mathrm{NiBr}_{2}\) ? Explain.

Short answer

(a) \(\mathrm{CdCl}_{2}\) is more acidic than \(\mathrm{ZnBr}_{2}\) because Cd is larger in size compared to Zn. (b) \(\mathrm{CuCl}\) and \(\mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2}\) have similar acidic tendencies as both anions (Cl and \(\mathrm{NO}_3\)) form neutral solutions with Cu. (c) \(\mathrm{NiBr}_{2}\) is more acidic than \(\mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{2}\) due to Ni being a more acidic cation compared to Ca.

Step-by-step solution

Determine if the given elements are cations or anions

First, we need to recognize the cations and anions within each given element. (a) \(\mathrm{ZnBr}_{2}\) - Zn is the cation and Br is the anion \(\mathrm{CdCl}_{2}\) - Cd is the cation and Cl is the anion (b) \(\mathrm{CuCl}\) - Cu is the cation and Cl is the anion \(\mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2}\) - Cu is the cation and \(\mathrm{NO}_3\) is the anion (c) \(\mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{2}\) - Ca is the cation and \(\mathrm{NO}_3\) is the anion \(\mathrm{NiBr}_{2}\) - Ni is the cation and Br is the anion

Determine their acidic, basic, or neutral tendencies

Now we need to determine whether the cations and anions will form acidic, basic, or neutral solutions. (a) Zn and Cd are both transition metals, which usually form acidic solutions. Br and Cl are both halogens, which form neutral solutions when paired with transition metals (since they are both weaker bases). (b) Cu is a transition metal, which usually forms acidic solutions. Cl is a halogen, which forms a neutral solution when paired with a transition metal. \(\mathrm{NO}_3\) is a polyatomic ion, which also forms a neutral solution when paired with a transition metal like Cu. (c) Ca and Ni are both transition metals, which usually form acidic solutions. \(\mathrm{NO}_3\) is a polyatomic ion, which forms a neutral solution when paired with a transition metal like Ca or Ni. Br is a halogen, which forms a neutral solution when paired with a transition metal like Ni.

Compare the acidic tendencies of the two elements

Now, let's compare the acidic tendencies of the elements. (a) Both \(\mathrm{ZnBr}_{2}\) and \(\mathrm{CdCl}_{2}\) should form acidic solutions due to their transition metal cations (Zn and Cd). However, Cd is more heavy and larger in size compared to Zn, so \(\mathrm{CdCl}_{2}\) has a greater tendency to form acidic solutions. (b) Both \(\mathrm{CuCl}\) and \(\mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2}\) should form acidic solutions due to their transition metal cation (Cu). Since both anions (Cl and \(\mathrm{NO}_3\)) form neutral solutions when combined with Cu, there is no significant difference in their acidic tendencies. Therefore, they both have similar acidic tendencies. (c) Both \(\mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{2}\) and \(\mathrm{NiBr}_{2}\) should form acidic solutions due to their transition metal cations (Ca and Ni). However, Ni is a more acidic cation (due to its greater charge and smaller ion size) compared to Ca, so \(\mathrm{NiBr}_{2}\) has a greater tendency to form acidic solutions. To sum up: (a) \(\mathrm{CdCl}_{2}\) is more acidic than \(\mathrm{ZnBr}_{2}\) (b) \(\mathrm{CuCl}\) and \(\mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2}\) have similar acidic tendencies (c) \(\mathrm{NiBr}_{2}\) is more acidic than \(\mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{2}\)

Key concepts

Transition Metals

Transition metals are elements found in the central block of the periodic table, known as the d-block. They are unique because of their ability to exhibit variable oxidation states and form colored compounds. One of their notable characteristics in an aqueous solution is their tendency to increase acidity. This occurs because transition metals often have high positive charges and small ion sizes. These qualities mean they can polarize the electron clouds of water molecules, making it easier for water to donate hydrogen ions, or protons, into the solution, thus increasing acidity.

In the context of the original exercise, the transition metals considered include Zinc (Zn), Cadmium (Cd), Copper (Cu), Nickel (Ni), and Calcium (Ca) - although Ca is typically not a transition metal, it can still affect the acidity due to similar type interactions. When comparing these metals in solutions, the more positively charged or smaller-sized cation will usually lead to a more acidic solution. For instance, Cadmium is noted to be heavier and larger than Zinc, giving rise to more prominent acidic solutions due to stronger cation-anion interactions.

Cation-Anion Interactions

Cation-anion interactions are crucial in determining the properties of a solution, including its acidity. Cations are positively charged ions, while anions carry a negative charge. The attraction between cations and anions can influence the extent to which a chemical solution is able to accept or donate protons, thereby affecting its pH level.

In the original exercise, this interaction is explored with compounds like \(\mathrm{ZnBr}_{2}\), \(\mathrm{CdCl}_{2}\), \(\mathrm{CuCl}\), and \(\mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2}\). Typically, strong cation-anion interactions are a factor that can lead to more acidic solutions, because the positive charge of the cation can promote the release of hydrogen ions from water molecules. Additionally, in cases where the cation is a transition metal, the electrons from the anion might interact with the partially filled d-orbitals of the cation, resulting in more complex changes that can increase acidity.

Understanding which cation-anion combinations lead to acidic solutions is essential for predicting the behavior of chemical solutions. Each specific pairing has its own unique impacts on the hydrogen-ion concentration equilibria.

Chemical Solutions

Chemical solutions are homogeneous mixtures of two or more substances. In the context of the given problem, we're looking at solutions that consist of a solute (such as \(\mathrm{ZnBr}_{2}\) or \(\mathrm{CdCl}_{2}\)) dissolved in a solvent, typically water. The properties of these solutions, including their acidity or basicity, can largely be influenced by the nature of the solute.

Acidity in chemical solutions is often determined by the ability of the solute to release or accept protons. This is especially pertinent in solutions with metallic ions, where the metal's behavior in solution affects protonation levels. Transition metal ions can alter the pH of a solution due to their tendency to hydrolyze and form complex ions with the solvent.

For instance, given the examples in the problem, \(\mathrm{ZnBr}_{2}\) and \(\mathrm{CdCl}_{2}\) in solution show a tendency to be more acidic due to the transition metals present. Recognizing how these metals interact within a chemical solution is crucial for predicting pH levels and understanding chemical reactivity and stability.