Problem 51 - Chemistry: The Central Science in SI Units, Global Edition

Question

Hydrogen phosphate \(\left(\mathrm{HPO}_{4}^{2-}\right)\) can be removed in water treatment by the addition of slaked lime, \(\mathrm{Ca}(\mathrm{OH})_{2} .\) Write a balanced chemical equation for the reaction (using ions as reactant), in which \(\mathrm{Ca}_{5}(\mathrm{OH})\left(\mathrm{PO}_{4}\right)_{3}\) forms as a precipitate.

Step-by-Step Solution

Answer

The balanced chemical equation for the reaction between hydrogen phosphate ions (HPO4^2-) and slaked lime, Ca(OH)2, to form the precipitate Ca5(OH)(PO4)3 is: \(Ca(OH)_{2} + 3HPO_{4}^{2-} \rightarrow 5Ca^{2+} + OH^- + Ca_{5}(OH)(PO_{4})_{3}\)

Step 1: Write the ionic equation for hydrogen phosphate (HPO4^2-)

Since hydrogen phosphate is in the form of an ion, its ionic equation will simply be: \(HPO_{4}^{2-}\)

Step 2: Write the ionic equation for slaked lime (Ca(OH)2)

Slaked lime, Ca(OH)2, ionizes in water to form calcium ions (Ca^2+) and hydroxide ions (OH^-). The ionic equation for this process is: \(Ca(OH)_{2} \rightarrow Ca^{2+} + 2OH^-\)

Step 3: Write the ionic equation for the formation of the precipitate (Ca5(OH)(PO4)3)

The precipitate is made up of five calcium ions (Ca^2+), one hydroxide ion (OH^-), and three hydrogen phosphate ions (HPO4^2-). The ionic equation for the formation of the precipitate will be: \(5Ca^{2+} + OH^- + 3HPO_{4}^{2-} \rightarrow Ca_{5}(OH)(PO_{4})_{3}\)

Step 4: Combine the individual ionic equations into the overall balanced chemical equation

Now we just need to combine the ionic equations from steps 1, 2, and 3 into the overall balanced chemical equation: \(Ca(OH)_{2} + 3HPO_{4}^{2-} \rightarrow 5Ca^{2+} + 2OH^- + Ca_{5}(OH)(PO_{4})_{3}\) Since 2OH^- ions are formed when slaked lime ionizes, and only 1OH^- ion is needed in the formation of the precipitate, 1OH^- ion will remain in solution at the end of the reaction: \(Ca(OH)_{2} + 3HPO_{4}^{2-} \rightarrow 5Ca^{2+} + OH^- + Ca_{5}(OH)(PO_{4})_{3}\) This equation is balanced, as there are equal numbers of each type of ion on both sides of the equation.

Key Concepts

Ionic Equations

An ionic equation is a chemical equation that represents the ions involved in a chemical reaction. Instead of showing all the chemical species, it only displays the active ions. This helps to focus on the actual chemical changes occurring in the reaction. Ionic equations are essential in understanding reactions in aqueous solutions, where ions are the primary participants.

A complete ionic equation includes all of the ions present in the reaction. For instance, the ionization of slaked lime, \(\text{Ca(OH)}_2\), in water is given by:

• \(\text{Ca(OH)}_2 \rightarrow \text{Ca}^{2+} + 2\text{OH}^-\)

Balanced ionic equations ensure that the number of each type of ion is the same on both sides. Therefore, they help in identifying the main species that participate in a reaction, such as the formation of a precipitate or the release of gas.

For any ionic reaction, begin by writing down the ions involved and then balance the equation to ensure the same number of ions on each side.

Precipitation Reactions

Precipitation reactions occur when two aqueous solutions are mixed and an insoluble solid, known as a precipitate, forms. This happens because the combination of the cations and anions in the solution creates a compound that does not dissolve in water. The precipitate is a key indicator that a chemical reaction has occurred.

In the context of water treatment, the precipitation of \(\text{Ca}_5(\text{OH})(\text{PO}_4)_3\) is the desired outcome. The combination of three hydrogen phosphate ions \(\text{3HPO}_4^{2-}\) and five calcium ions \(\text{5Ca}^{2+}\) from slaked lime results in this compound:

• \(\text{5Ca}^{2+} + \text{OH}^- + \text{3HPO}_4^{2-} \rightarrow \text{Ca}_5(\text{OH})(\text{PO}_4)_3\)

This solid precipitate can be then removed from the solution, effectively reducing the concentration of certain ions, like phosphates, which are often pollutants in water systems.

To identify a precipitation reaction, look for a change in the solution's clarity or the formation of a solid deposit.

Water Treatment

Water treatment is a process that purifies water by removing impurities and harmful substances. One common method involves using chemicals to induce reactions that form insoluble compounds, which can be easily removed from the water.

In the case of hydrogen phosphate removal, slaked lime \(\text{Ca(OH)}_2\) is used. It reacts with hydrogen phosphate ions \(\text{HPO}_4^{2-}\) to form a solid precipitate \(\text{Ca}_5(\text{OH})(\text{PO}_4)_3\), which can be filtered out. This step is crucial not just to ensure clean water but also to control the nutrient levels, such as phosphates, that can contribute to environmental issues like eutrophication.

Effective water treatment protects ecosystems and ensures public safety by delivering clean, drinkable water free of contaminants