Problem 14 - Chemistry and Chemical Reactivity

Question

The following compounds are water-soluble. What ions are produced by each compound in aqueous solution? (a) KI (c) \(\mathrm{K}_{2} \mathrm{HPO}_{4}\) (b) \(\mathrm{Mg}\left(\mathrm{CH}_{3} \mathrm{CO}_{2}\right)_{2}\) (d) NaCN

Step-by-Step Solution

Answer

(a) K⁺ and I⁻, (b) Mg²⁺ and 2 CH₃CO₂⁻, (c) 2 K⁺ and HPO₄²⁻, (d) Na⁺ and CN⁻.

Step 1: Dissociation of KI

Potassium iodide (KI) is a salt composed of potassium (K) and iodide (I) ions. In an aqueous solution, it dissociates completely into its constituent ions. Therefore, the dissociation equation is: \[ \text{KI}_{(aq)} \rightarrow \text{K}^+_{(aq)} + \text{I}^-_{(aq)} \] This indicates that one mole of KI produces one mole of \(\text{K}^+\) ions and one mole of \(\text{I}^-\) ions.

Step 2: Dissociation of K₂HPO₄

Potassium hydrogen phosphate (\(\mathrm{K}_{2} \mathrm{HPO}_{4}\)) is a salt formed from two potassium ions, one hydrogen ion, and one phosphate ion. In water, it dissociates as follows: \[ \text{K}_2\text{HPO}_4_{(aq)} \rightarrow 2\text{K}^+_{(aq)} + \text{HPO}_4^{2-}_{(aq)} \] The dissociation produces two \(\text{K}^+\) ions for each molecule and one \(\text{HPO}_4^{2-}\) ion.

Step 3: Dissociation of Mg(CH₃CO₂)₂

Magnesium acetate \(\text{Mg(CH}_3\text{CO}_2)_2\) dissociates into magnesium ions and acetate ions in water. The dissociation equation is: \[ \text{Mg(CH}_3\text{CO}_2)_2_{(aq)} \rightarrow \text{Mg}^{2+}_{(aq)} + 2\text{CH}_3\text{CO}_2^-_{(aq)} \] Thus, one mole of magnesium acetate yields one mole of \(\text{Mg}^{2+}\) ions and two moles of \(\text{CH}_3\text{CO}_2^-\) ions.

Step 4: Dissociation of NaCN

Sodium cyanide (NaCN) dissociates into sodium and cyanide ions in an aqueous solution. The dissociation equation is: \[ \text{NaCN}_{(aq)} \rightarrow \text{Na}^+_{(aq)} + \text{CN}^-_{(aq)} \] This means that one mole of NaCN produces one mole of \(\text{Na}^+\) ions and one mole of \(\text{CN}^-\) ions.

Key Concepts

Aqueous Solutions

In chemistry, an aqueous solution is simply a solution where the solvent is water. Water is a universal solvent, meaning it can dissolve a large variety of substances. This happens because water molecules are polar — they have positive and negative charges. Therefore, they can surround different ions or molecules and pull them apart, allowing them to "dissolve" in the water.
A key feature of aqueous solutions is their ability to facilitate chemical reactions by allowing ions to move freely. These solutions are crucial in many chemical processes, both industrial and biological. When substances dissolve in water, they can create an environment where new bonds form or where existing bonds are broken. This is why understanding aqueous solutions is essential for comprehending many principles in chemistry.
In our case, when compounds like KI, K₂HPO₄, Mg(CH₃CO₂)₂, and NaCN dissolve in water, they are said to dissociate, meaning they split into their component ions.

Ionic Compounds

Ionic compounds are chemical compounds composed of ions that are held together by electrostatic forces, often known as ionic bonds. These compounds are typically formed when metals, which lose electrons easily, interact with non-metals, which gain electrons easily. This exchange of electrons forms charged particles, or ions. Metals form positively charged ions (cations), and non-metals form negatively charged ions (anions).
When dissolved in an aqueous solution, ionic compounds dissociate into their constituent ions. For example:

• Potassium iodide (KI) dissociates into K⁺ and I⁻ ions.
• Potassium hydrogen phosphate (K₂HPO₄) yields K⁺ and HPO₄⁻ ions.
• Magnesium acetate [Mg(CH₃CO₂)₂] forms Mg²⁺ and CH₃CO₂⁻ ions.
• Sodium cyanide (NaCN) splits into Na⁺ and CN⁻ ions.

When you understand how ionic compounds behave in water, predicting the behavior of solutions becomes easier. Ionic compounds often have high melting and boiling points, and they conduct electricity when dissolved in water, thanks to their free-moving ions.

Chemical Equations

Chemical equations are symbolic representations of a chemical reaction where reactants are shown on the left, products on the right, and the arrow signifies the direction of the reaction. In the context of ionic dissociation in aqueous solutions, chemical equations are used to show how ionic compounds separate into individual ions.
For instance, for potassium iodide (KI), the chemical equation is:
\[ \text{KI}_{(aq)} \rightarrow \text{K}^+_{(aq)} + \text{I}^-_{(aq)} \]
This highlights how one formula unit of KI produces one potassium ion and one iodide ion in solution.
Chemical equations often maintain a balance between the number and type of each atom on both sides. They help chemists understand the quantitative aspects of chemistry, predicting the amount of product formed in a reaction or how much reactant is needed. Balanced equations are crucial for laboratory preparations and understanding natural processes.
By interpreting these equations for dissociation, you can grasp the fundamental concept of molecular and ionic interactions in solutions, providing a clear picture of what occurs change-wise when compounds dissolve in water.