Question

Which member of each pair produces the more acidic aqueous solution: (a) ${\mathrm{ZnBr}}_2$ or ${\mathrm{CdCl}}_2$, (b) $\mathrm{CuCl}$ or $\mathrm{Cu}{\left({\mathrm{NO}}_3\right)}_2$, (c) $\mathrm{Ca}{\left({\mathrm{NO}}_3\right)}_2$ or ${\mathrm{NiBr}}_2$ ? Explain.

Short answer

In summary, (a) $CdC{l}_2$ produces a more acidic aqueous solution than $ZnB{r}_2$ due to the higher acidity of ${\mathrm{Cd}}^{2+}$ compared to ${\mathrm{Zn}}^{2+}$; (b) $Cu(N{O}_3{)}_2$ produces a more acidic aqueous solution than $CuCl$ because of the absence of a basic anion; and (c) $NiB{r}_2$ produces a more acidic aqueous solution than $Ca(N{O}_3{)}_2$ because of the higher polarizing power and acidity of ${\mathrm{Ni}}^{2+}$ compared to ${\mathrm{Ca}}^{2+}$.

Step-by-step solution

Identify the ionic species formed in aqueous solutions for each compound

When the given compounds are dissolved in water, they dissociate into their respective ions. Identifying these ions helps us to understand their acidic or basic nature. (a) $ZnB{r}_2$ dissociates into ${\mathrm{Zn}}^{2+}$ and ${\mathrm{Br}}^{-}$ ions. $CdC{l}_2$ dissociates into ${\mathrm{Cd}}^{2+}$ and ${\mathrm{Cl}}^{-}$ ions. (b) $CuCl$ dissociates into ${\mathrm{Cu}}^{+}$ and ${\mathrm{Cl}}^{-}$ ions. $Cu(N{O}_3{)}_2$ dissociates into ${\mathrm{Cu}}^{2+}$ and ${{\mathrm{NO}}_3}^{-}$ ions. (c) $Ca(N{O}_3{)}_2$ dissociates into ${\mathrm{Ca}}^{2+}$ and ${{\mathrm{NO}}_3}^{-}$ ions. $NiB{r}_2$ dissociates into ${\mathrm{Ni}}^{2+}$ and ${\mathrm{Br}}^{-}$ ions.

Determine the acidic or basic nature of each species

We will now consider the nature of the ions formed from the compounds. This will help us infer whether the compound produces an acidic or basic solution. (a) $ZnB{r}_2$ ${\mathrm{Zn}}^{2+}$ ion: acts as an acid since it has a positive charge due to its ability to attract electrons. ${\mathrm{Br}}^{-}$ ion: acts as a base since it is the conjugate base of a strong acid (HBr). $CdC{l}_2$ ${\mathrm{Cd}}^{2+}$ ion: acts as an acid since it has a positive charge due to its ability to attract electrons. ${\mathrm{Cl}}^{-}$ ion: acts as a base since it is the conjugate base of a strong acid (HCl). (b) $CuCl$ ${\mathrm{Cu}}^{+}$ ion: acts as an acid since it has a positive charge due to its ability to attract electrons. ${\mathrm{Cl}}^{-}$ ion: same as in $CdC{l}_2$ case. $Cu(N{O}_3{)}_2$ ${\mathrm{Cu}}^{2+}$ ion: acts as an acid since it has a positive charge due to its ability to attract electrons. ${{\mathrm{NO}}_3}^{-}$ ion: acts as a neutral species since it is the conjugate base of a strong acid (HNO_3) and does not show any basic character. (c) $Ca(N{O}_3{)}_2$ ${\mathrm{Ca}}^{2+}$ ion: acts as an acid since it has a positive charge due to its ability to attract electrons. ${{\mathrm{NO}}_3}^{-}$ ion: same as in $Cu(N{O}_3{)}_2$ case. $NiB{r}_2$ ${\mathrm{Ni}}^{2+}$ ion: acts as an acid since it has a positive charge due to its ability to attract electrons. ${\mathrm{Br}}^{-}$ ion: same as in $ZnB{r}_2$ case.

Compare the acidity of the compounds

Now, we can compare the acidity of the given pairs based on the nature of their ions and their position on the periodic table. (a) In the case of $ZnB{r}_2$ and $CdC{l}_2$, both compounds release metal cations ${\mathrm{Zn}}^{2+}$ and ${\mathrm{Cd}}^{2+}$, which can act as acids, and their respective anions ${\mathrm{Br}}^{-}$ and ${\mathrm{Cl}}^{-}$, which can act as bases. Since both anions are weak bases, we will look at the metal cations to compare the acidity. Comparing the acidity of ${\mathrm{Zn}}^{2+}$ and ${\mathrm{Cd}}^{2+}$, we can see that ${\mathrm{Cd}}^{2+}$ has a higher acidity, as it is lower on the periodic table and has a larger size, which enhances its polarizing power. Therefore, $CdC{l}_2$ produces a more acidic aqueous solution than $ZnB{r}_2$. (b) For $CuCl$ and $Cu(N{O}_3{)}_2$, the main difference lies in the anions. ${\mathrm{Cl}}^{-}$ is a weak base, while ${{\mathrm{NO}}_3}^{-}$ is a neutral species. So, the $Cu(N{O}_3{)}_2$ solution will be more acidic due to the absence of a basic anion. Therefore, $Cu(N{O}_3{)}_2$ produces a more acidic aqueous solution than $CuCl$. (c) Comparing $Ca(N{O}_3{)}_2$ and $NiB{r}_2$, we can see that both compounds contain ${{\mathrm{NO}}_3}^{-}$ and ${\mathrm{Br}}^{-}$ anions, respectively. In this case, the ${\mathrm{Ni}}^{2+}$ cation has a higher polarizing power (and hence higher acidity) due to its smaller size and higher charge density compared to ${\mathrm{Ca}}^{2+}$. Therefore, $NiB{r}_2$ produces a more acidic aqueous solution than $Ca(N{O}_3{)}_2$.

Key concepts

Ionic Dissociation

When compounds dissolve in water, they break apart into ions. This process is called ionic dissociation. It is crucial for understanding how substances behave in solutions, such as their acidity or basicity. For instance:

• ZnBr₂ dissociates into ${\mathrm{Zn}}^{2+}$ and ${\mathrm{Br}}^{-}$ ions.
• CdCl₂ becomes ${\mathrm{Cd}}^{2+}$ and ${\mathrm{Cl}}^{-}$ ions.
• CuCl generates ${\mathrm{Cu}}^{+}$ and ${\mathrm{Cl}}^{-}$.
• Cu(NO₃)₂ dissociates into ${\mathrm{Cu}}^{2+}$ and ${\mathrm{NO}}_3^{-}$ ions.
• Ca(NO₃)₂ gives ${\mathrm{Ca}}^{2+}$ and ${\mathrm{NO}}_3^{-}$ ions.
• NiBr₂ splits into ${\mathrm{Ni}}^{2+}$ and ${\mathrm{Br}}^{-}$ ions.

Recognizing these ions helps predict if a solution will act as an acid or a base.

Cation Acidity

Cations are positively charged ions. They often influence the acidity of a solution. Metal cations can act as acids. This is because they attract electrons, which impacts the acidity.

• Higher charge on a cation enhances its ability to increase acidity. For example, ${\mathrm{Cu}}^{2+}$ has a higher charge than ${\mathrm{Cu}}^{+}$, making it more acidic.
• Cation size matters. Smaller cations have higher charge density, leading to greater acidity. ${\mathrm{Ni}}^{2+}$ is smaller and more polarizing than ${\mathrm{Ca}}^{2+}$, giving it a higher acidity.
• The position in the periodic table can indicate acidity. ${\mathrm{Cd}}^{2+}$'s larger atomic radius compared to ${\mathrm{Zn}}^{2+}$ suggests it is more acidic due to stronger polarization effects.

These properties help us understand the role of cations in determining acidity in solutions.

Anion Basicity

Anions are negatively charged ions, and their basicity can neutralize acidity. They result from the loss of hydrogen ions by acids.

• The basicity of an anion is often influenced by its origin. If it comes from a strong acid like $\mathrm{HCl}$ or $\mathrm{HBr}$, it tends to be weak in basicity. Thus, ${\mathrm{Cl}}^{-}$ and ${\mathrm{Br}}^{-}$ are weak bases.
• An anion from a strong acid typically contributes less to overall basicity, as seen with ${\mathrm{NO}}_3^{-}$, the conjugate base of ${\mathrm{HNO}}_3$.

Understanding these relationships helps us predict how anions can influence a solution's overall acidic or basic character.

Comparative Acidity

Comparing the acidity between substances involves evaluating both cations and anions. Cations contribute acidity while anions can mitigate it.

• CdCl₂ vs. ZnBr₂: Both cations (${\mathrm{Cd}}^{2+}$ and ${\mathrm{Zn}}^{2+}$) act as acids, but ${\mathrm{Cd}}^{2+}$'s larger size makes it more acidic. Both ${\mathrm{Cl}}^{-}$ and ${\mathrm{Br}}^{-}$ are weak bases, so $\text{Prime causes double exponent: use braces to clarify}$ effect dominates.
• Cu(NO₃)₂ vs. CuCl: Here, ${\mathrm{Cu}}^{2+}$'s strong acidity is not counterbalanced by ${\mathrm{NO}}_3^{-}$, a neutral anion. ${\mathrm{Cl}}^{-}$ in $\mathrm{CuCl}$ provides some basicity, making ${\mathrm{Cu}(\mathrm{NO}}_3{{)}_2}^{\prime }s$ solution more acidic.
• NiBr₂ vs. Ca(NO₃)₂: ${\mathrm{Ni}}^{2+}$ is smaller and more polarizing than ${\mathrm{Ca}}^{2+}$, leading to enhanced acidity for ${\mathrm{NiBr}}_2$. The ${\mathrm{Br}}^{-}$ is weakly basic, while ${\mathrm{NO}}_3^{-}$ is neutral.

Through these comparisons, we understand how different metal salts produce varying acidity in aqueous solutions.