Chapter 4, Problem 1PE

Interpretation Introduction

Interpretation:

Equations that show the dissociation of ${\text{CaI}}_{\text{2}}$ and potassium phosphate in water are to be written.

Concept Introduction:

An ionic compound completely dissociates in water to form ions.

Polyatomic ions do not dissociate into individual atoms.

While writing the chemical equation for dissociation, the formula of the ionic compound and the ions formed from it are written along with their charges.

The number of ions formed by one formula unit of the compound is written in the equation. Usage of the symbol (aq) after a particle means that it is a hydrated ion (surrounded by water molecules in the solution).

Criteria for balanced ionic and net ionic equations:

Material balance: There must be the same number of atoms of each kind on both sides of the arrow.

Electrical balance: The net electrical charge on the left must equal the net electrical charge on the right (the charge does not necessarily have to be zero).

Answer to Problem 1PE

Solution: The equations are:

a) ${\text{CaI}}_{\text{2}}\left(s\right)\to {\text{Ca}}^{\text{2+}}\left(aq\right)+2{\text{I}}^{\text{-}}\left(aq\right)$.

b) ${\text{K}}_{\text{3}}{\text{PO}}_{\text{4}}\left(s\right)\to 3{\text{K}}^{\text{+}}\left(aq\right)+{\text{PO}}_{\text{4}}{}^{\text{3-}}\left(aq\right)$.

Explanation of Solution

a) ${\text{CaI}}_{\text{2}}$

${\text{CaI}}_{\text{2}}$ consists of calcium and iodine atoms. Calcium is in the group $2\text{A}$ and has a charge of $2+$ and iodine belongs to the group $7\text{A}$ and has a charge of $1-$.

Thus, formula unit of the compound is ${\text{CaI}}_{\text{2}}$ and on dissociation it releases one ${\text{Ca}}^{\text{2+}}$ and two ${\text{I}}^{\text{-}}$ ions.

The chemical equation for the dissociation of ${\text{CaI}}_{\text{2}}$ is as follows:

${\text{CaI}}_{\text{2}}\left(s\right)\to {\text{Ca}}^{\text{2+}}\left(aq\right)+2{\text{I}}^{\text{-}}\left(aq\right)$

${\text{CaI}}_{\text{2}}$ is a solid, so the symbol $\left(s\right)$ is written after it and the ions are in aqueous solution, so the symbol $\left(aq\right)$ is written after the ions.

b) Potassium phosphate

The compound potassium phosphate consists of cation ${\text{K}}^{\text{+}}$ and anion ${\text{PO}}_{\text{4}}{}^{3-}$. Thus, according to the charges, the formula of the compound is ${\text{K}}_{\text{3}}{\text{PO}}_{\text{4}}$. The equation is expressed by writing the formula of potassium phosphate on the left side and the ions on the right side.

The reaction is as follows:

${\text{K}}_{\text{3}}{\text{PO}}_{\text{4}}\left(s\right)\to 3{\text{K}}^{\text{+}}\left(aq\right)+{\text{PO}}_{\text{4}}{}^{\text{3-}}\left(aq\right)$

${\text{K}}_{\text{3}}{\text{PO}}_{\text{4}}$ is a solid, so the symbol $\left(s\right)$ is written after it and the ions are in an aqueous solution, so the symbol $\left(aq\right)$ is written after the ions.