Problem 66 - Chemistry: An Atoms-Focused Approach

Question

Show, with appropriate net ionic reactions, how \(\mathrm{Cr}^{3+}\) and \(\mathrm{Cd}^{2+}\) can be removed from wastewater by treatment with solutions of sodium hydroxide.

Step-by-Step Solution

Answer

Answer: Cr³⁺ and Cd²⁺ ions are removed from wastewater by reacting with hydroxide ions (OH⁻) present in sodium hydroxide to form insoluble hydroxides that precipitate out of the solution.

Step 1: Write the balanced chemical equations

First, we need to write the balanced chemical equations for the reactions between sodium hydroxide and the metal ions. For \(\mathrm{Cr}^{3+}\) ions: \(\ce{Cr^{3+}}(aq) + 3\ce{OH^{-}}(aq) -> \ce{Cr(OH)3}(s)\) For \(\mathrm{Cd}^{2+}\) ions: \(\ce{Cd^{2+}}(aq) + 2\ce{OH^{-}}(aq) -> \ce{Cd(OH)2}(s)\)

Step 2: Write the net ionic equations

Now, we can write the net ionic equations for each reaction by removing the spectator ions. Since sodium and hydroxide ions are not involved in forming the precipitates, they are considered spectator ions. For \(\mathrm{Cr}^{3+}\) ions: \(\ce{Cr^{3+}}(aq) + 3\ce{OH^{-}}(aq) -> \ce{Cr(OH)3}(s)\) For \(\mathrm{Cd}^{2+}\) ions: \(\ce{Cd^{2+}}(aq) + 2\ce{OH^{-}}(aq) -> \ce{Cd(OH)2}(s)\) #Conclusion# By adding sodium hydroxide to the wastewater containing \(\mathrm{Cr}^{3+}\) and \(\mathrm{Cd}^{2+}\) ions, we can remove these metal ions from the wastewater. The net ionic reactions show that these metal ions react with hydroxide ions to form insoluble hydroxides, which precipitate out of the solution. This helps in the process of removing these metal ions from the wastewater.

Key Concepts

Chemical Reactions

Chemical reactions occur when substances interact to form new compounds. In the context of wastewater treatment, specific reactions are used to remove contaminants effectively. A chemical reaction can involve the rearrangement of atoms using a reactant or multiple reactants to form a product. The key components of these reactions include:

• Reactants: Initial substances that undergo change, such as metal ions and sodium hydroxide in the provided exercise.
• Products: New substances formed as a result of the reaction, like the insoluble hydroxides formed in this case.
• Balanced Equations: Balanced chemical equations ensure that the number of atoms of each element is conserved throughout the reaction.

In our exercise, chemical reactions between sodium hydroxide and metal ions (\(\mathrm{Cr}^{3+}\) and \(\mathrm{Cd}^{2+}\)) lead to the formation of products (\(\ce{Cr(OH)3}\) and \(\ce{Cd(OH)2}\)) which precipitate out. These reactions are fundamental in processes like wastewater treatment that seek to convert unwanted soluble ions into manageable forms.

Wastewater Treatment

Wastewater treatment is a process used to remove contaminants from water before it is released back into the environment. Addressing metal ions contamination is crucial because these can be hazardous to both human health and ecosystems. This involves various physical, chemical, and biological processes.

One of the common chemical methods involves precipitation reactions, as seen with our metal ions of interest. Wastewater treatment using precipitation reactions ensures:

• Removal of harmful metal ions by converting them into solid precipitates.
• Prevention of these metals entering the environment, thereby protecting aquatic life.

Thus, by adding reagents like sodium hydroxide, metal ions are prompted to form insoluble compounds, helping in their separation from the water stream. The overarching goal is to ensure that treated water is safe for release or even reuse, reducing pollution and resource consumption.

Precipitation Reactions

Precipitation reactions are a type of chemical reaction where two solutes react to form an insoluble solid, known as the precipitate. This is particularly useful in wastewater treatment as it allows for the removal of dissolved substances.

These reactions follow the general form:\[A^+(aq) + B^-(aq) \rightarrow AB(s)\]Here, \(A^+\) and \(B^-\) are dissolved ions, and \(AB(s)\) is the solid precipitate that forms.

In our exercise example:

• \(\mathrm{Cr}^{3+}\) ions react with hydroxide ions \(\ce{OH^-}\) to form \(\ce{Cr(OH)3}\), a solid.
• Similarly, \(\mathrm{Cd}^{2+}\) ions form \(\ce{Cd(OH)2}\) precipitates with hydroxide ions.

This method is effective because the precipitates can be filtered or settled out of the solution, simplifying the removal of contaminants. By removing specific ions, precipitation reactions enhance the purity of treated wastewater.