Problem 54 - Chemistry The Molecular Nature of Matter

Question

Laundry bleach such as Clorox is a dilute solution of sodium hypochlorite, \(\mathrm{NaOCl}\). Write a balanced net ionic equation for the reaction of \(\mathrm{NaOCl}\) with \(\mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}\). The \(\mathrm{OCl}^{-}\) is reduced to chloride ion and the \(\mathrm{S}_{2} \mathrm{O}_{3}{\underline{\phantom{xx}}}^{2-}\) is oxidized to sulfate ion.

Step-by-Step Solution

Answer

The balanced net ionic equation is \[6\mathrm{H}^{+} + \mathrm{OCl}^{-} + \mathrm{S}_{2}\mathrm{O}_{3}^{2-} + 4\mathrm{e}^{-} \rightarrow \mathrm{Cl}^{-} + \mathrm{SO}_{4}^{2-} + 3\mathrm{H_2O}.\]

Step 1: Write the skeletal equation

Before balancing the net ionic equation, you must write the unbalanced form of the equation. You should include only those compounds and ions that undergo a chemical change. In this case, \(\mathrm{OCl}^{-}\) ions are reduced to \(\mathrm{Cl}^{-}\) ions, and \(\mathrm{S}_{2}\mathrm{O}_{3}^{2-}\) ions are oxidized to \(\mathrm{SO}_{4}^{2-}\) ions. The skeletal equation is: \[\mathrm{OCl}^{-} + \mathrm{S}_{2}\mathrm{O}_{3}^{2-} \rightarrow \mathrm{Cl}^{-} + \mathrm{SO}_{4}^{2-}\]

Step 2: Balance the atoms other than oxygen and hydrogen

Looking at the skeletal equation, balance the atoms that are neither oxygen nor hydrogen. In this case, ensure that the number of sulfur and chlorine atoms are the same on both sides of the equation. Currently, sulfur is already balanced with one sulfur atom on each side. However, chlorine has one atom on each side, which means they are also balanced.

Step 3: Balance the oxygen atoms

To balance the oxygen atoms, you need to add water molecules \(\mathrm{H_2O}\) on the side that needs oxygen. On the left side, there is one oxygen atom, while on the right side there are four. Therefore, we need to add water molecules on the left side: \[\mathrm{OCl}^{-} + \mathrm{S}_{2}\mathrm{O}_{3}^{2-} \rightarrow \mathrm{Cl}^{-} + \mathrm{SO}_{4}^{2-} + 3\mathrm{H_2O}\]

Step 4: Balance the hydrogen atoms

After adding water to balance the oxygen atoms, we have added hydrogen atoms to the right side and need to balance them by adding hydrogen ions \(\mathrm{H^+}\) to the left side of the equation. There are six hydrogen atoms on the right, so we need to add six hydrogen ions to the left: \[6\mathrm{H}^{+} + \mathrm{OCl}^{-} + \mathrm{S}_{2}\mathrm{O}_{3}^{2-} \rightarrow \mathrm{Cl}^{-} + \mathrm{SO}_{4}^{2-} + 3\mathrm{H_2O}\]

Step 5: Balance the charge

Lastly, we must ensure that the electrical charge is the same on both sides of the equation. The left side of the equation has a total charge of \(6+(-1)+(-2) = +3\), while the right side has a charge of \(2- + (-1) + 0 = +1\). To balance the charges, we need four electrons \(\mathrm{e}^{-}\) on the right side: \[6\mathrm{H}^{+} + \mathrm{OCl}^{-} + \mathrm{S}_{2}\mathrm{O}_{3}^{2-} + 4\mathrm{e}^{-} \rightarrow \mathrm{Cl}^{-} + \mathrm{SO}_{4}^{2-} + 3\mathrm{H_2O}\]

Step 6: Verify the balanced ionic equation

Confirm that the final equation is balanced by ensuring that the number of atoms and the charge is the same on both sides of the equation. If everything is balanced, the equation is correct.

Key Concepts

Chemical Reactions

Chemical reactions are processes where reactants convert into products through the breaking and formation of chemical bonds. They are essential for everything from the metabolic processes in our bodies to the creation of the various substances we use daily.

In our laundry bleach example involving sodium hypochlorite (NaOCl) and sodium thiosulfate (Na2S2O3), one species is reduced while another is oxidized, which is indicative of a redox reaction, a type of chemical reaction. To understand the outcome better, a clear picture of the reactants and expected products is crucial.

This knowledge supports our efforts to write the net ionic equation, focusing on the ions that are actively involved in the chemical change and disregarding spectator ions that do not participate in the reaction.

Oxidation-Reduction

Oxidation-reduction, or redox, reactions involve the transfer of electrons between substances. Oxidation is the loss of electrons, while reduction is the gain of electrons. An easy way to remember this is the acronym 'OIL RIG': Oxidation Is Loss, Reduction Is Gain.

The laundry bleach reaction showcases these processes beautifully: OCl^{-} is reduced to Cl^{-}, and S2O3^{2-} is oxidized to SO4^{2-}. During this redox reaction, the oxidation state of chlorine decreases (indicating reduction), while the oxidation state of sulfur increases (indicating oxidation). These changes in oxidation states are key to identifying the nature of the redox processes involved and are necessary for balancing the net ionic equation appropriately.

Balancing Chemical Equations

Balancing chemical equations is a necessary step in the study of chemistry to obey the Law of Conservation of Mass, stating that matter is neither created nor destroyed. When we write a chemical equation, we need to make sure that the number of atoms of each element is equal on both sides of the equation. This sometimes requires the addition of coefficients, which are numbers placed before compounds in an equation to balance the number of atoms.

Using the exercise as our example, once we've figured out the reactants' transformations, we must adjust the coefficients to ensure that atoms are balanced. The method involves first balancing atoms of elements other than oxygen and hydrogen, then balancing the oxygen atoms by adding water molecules (H2O), and finally hydrogen by adding hydrogen ions (H+). Balancing takes patience and practice but is vital for correctly representing a chemical reaction.

Stoichiometry

Stoichiometry is the quantitative study of the relationships between the amounts of reactants and products in a chemical reaction. It involves calculations based on the balanced chemical equation and allows chemists to predict the yield of products formed in a reaction from a given amount of reactants.

In the sodium hypochlorite reaction, understanding stoichiometry is helpful in determining how much sodium thiosulfate is required to neutralize a specific amount of bleach, or vice versa. In real-life applications, such as water treatment or laboratory analysis, stoichiometry plays a critical role in ensuring reactions are carried out with the correct proportions of chemicals, minimizing waste and optimizing effectiveness. In our step-by-step solution, once the net ionic equation is balanced for both mass and charge, stoichiometry ensures that any calculations based on the reaction are accurate.