Question

Specify what ions are present in solution upon dissolving each of the following substances in water: (a) \(\mathrm{FeCl}_{2}\) ,\((\mathbf{b}) \mathrm{HNO}_{3,}(\mathbf{c})\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4},(\mathbf{d}) \mathrm{Ca}(\mathrm{OH})_{2.}\)

Short answer

Upon dissolving the given substances in water, the ions present in the solution are as follows: (a) FeCl₂: \(Fe^{2+}\) and \(Cl^-\) (b) HNO₃: \(H^+\) and \(NO_3^-\) (c) (NH₄)₂SO₄: \(NH_4^+\) and \(SO_4^{2-}\) (d) Ca(OH)₂: \(Ca^{2+}\) and \(OH^-\)

Step-by-step solution

(a) Dissociation of FeCl2

When iron(II) chloride (FeCl2) dissolves in water, it dissociates into its constituent ions. Since iron has a +2 charge and chloride has a -1 charge, the dissociation can be represented as: FeCl2(s) → Fe^2+(aq) + 2Cl^−(aq) Upon dissolving FeCl2 in water, the ions present in the solution are Fe^2+ and Cl^−.

(b) Dissociation of HNO3

When nitric acid (HNO3) dissolves in water, it completely ionizes, forming a strong acid. The dissociation can be represented as: HNO3(aq) → H^+(aq) + NO3^−(aq) Upon dissolving HNO3 in water, the ions present in the solution are H^+ and NO3^−.

(c) Dissociation of (NH4)2SO4

When ammonium sulfate ((NH4)2SO4) dissolves in water, it dissociates into ammonium (NH4^+) and sulfate (SO4^2−) ions. The dissociation can be represented as: (NH4)2SO4(s) → 2NH4^+(aq) + SO4^2−(aq) Upon dissolving (NH4)2SO4 in water, the ions present in the solution are NH4^+ and SO4^2−.

(d) Dissociation of Ca(OH)2

When calcium hydroxide (Ca(OH)2) dissolves in water, it dissociates into calcium (Ca^2+) and hydroxide (OH^−) ions. The dissociation can be represented as: Ca(OH)2(s) → Ca^2+(aq) + 2OH^−(aq) Upon dissolving Ca(OH)2 in water, the ions present in the solution are Ca^2+ and OH^−.

Key concepts

Dissociation of Electrolytes

Understanding the process of dissociation of electrolytes is fundamental in chemistry, especially when studying solutions. This process occurs when an ionic compound, such as a salt, acid, or base, dissolves in water and separates into individual ions. For instance, when FeCl₂ is dissolved in water, it undergoes dissociation into Fe²⁺ and Cl⁻ ions.

The extent of dissociation is an indicator of how well an electrolyte conducts electricity in solution. The more an electrolyte dissociates, the more ions are present to carry an electrical current. This is often visualized in equations showing the transformation from the solid state to separate aqueous ions, such as:
\( FeCl_2(s) \rightarrow Fe^{2+}(aq) + 2Cl^{-}(aq) \).

It's important to note that the subscript numbers indicate the number of ions produced upon dissociation, which allows us to balance the equation and maintain charge neutrality.

Ions in Aqueous Solution

Ions in aqueous solutions are the result of electrolytes dissolving in water. These solutions play a critical role in a range of chemical processes, including biological functions and industrial applications. Each ion retains its charge when dissolved, becoming solvated by water molecules. For example, dissolving HNO₃ in water produces H⁺ and NO₃⁻ ions.
\( HNO_3(aq) \rightarrow H^+(aq) + NO_3^{-}(aq) \).

These ions are the carriers of the electric current in the solution and are responsible for its conductivity. In the case of HNO₃, a strong acid, the ions provided are the reason why the solution becomes capable of conducting electricity very well.

Strong and Weak Electrolytes

Electrolytes are classified based on how completely they dissociate into ions in water. Strong electrolytes break apart completely, releasing a maximum number of ions and conducting electricity well, similar to the way HNO₃ does. Substances like NaCl and HCl also fall into this category.

In contrast, weak electrolytes only partially dissociate, resulting in fewer ions in solution and, therefore, reduced electrical conductivity. Acetic acid (CH₃COOH) is a common example of a weak electrolyte. Its limited ionization in water can be represented as:
\( CH_3COOH(aq) \rightleftharpoons CH_3COO^- (aq) + H^+ (aq) \).

The double arrow indicates a reversible reaction, characteristic of weak electrolytes, where some molecules remain undissociated in equilibrium with their ions.

Acid-Base Reactions

Acid-base reactions are crucial to understand when studying the behavior of ions in aqueous solutions. They occur when an acid donates a proton (H⁺) to a base. A common example is the reaction between HNO₃ and Ca(OH)₂, where HNO₃ acts as an acid giving up a proton, and Ca(OH)₂ acts as a base accepting it.

Such reactions can be represented by equations showing the transfer of H⁺ ions, as in the neutralization reaction:
\( 2HNO_3(aq) + Ca(OH)_2(aq) \rightarrow Ca(NO_3)_2(aq) + 2H_2O(l) \).

This reaction leads to the formation of a salt, in this case, calcium nitrate, and water, which is typical for acid-base interactions. The ability to identify and balance such reactions is a valuable skill in chemistry, enabling the prediction of the products formed in various acid-base scenarios.