Problem 65 - Chemistry An Atoms First Approach

Question

Hydrazine is somewhat toxic. Use the half-reactions shown below to explain why household bleach (a highly alkaline solution of sodium hypochlorite) should not be mixed with household ammonia or glass cleansers that contain ammonia. $$\begin{array}{c} \mathrm{ClO}^{-}+\mathrm{H}_{2} \mathrm{O}+2 \mathrm{e}^{-} \longrightarrow 2 \mathrm{OH}^{-}+\mathrm{Cl}^{-} \quad \quad \quad \mathscr{E}^{\circ}=0.90 \mathrm{V} \\ \mathrm{N}_{2} \mathrm{H}_{4}+2 \mathrm{H}_{2} \mathrm{O}+2 \mathrm{e}^{-} \longrightarrow 2 \mathrm{NH}_{3}+2 \mathrm{OH}^{-} \quad \mathscr{E}^{\circ}=-0.10 \mathrm{V} \end{array}$$

Step-by-Step Solution

Answer

When household bleach (containing ClO⁻) is mixed with ammonia (NH₃) or cleansers containing ammonia, the reaction leads to the formation of toxic hydrazine (N₂H₄) in an alkaline environment: \[ \mathrm{ClO}^{-}+\mathrm{N}_{2} \mathrm{H}_{4}+2 \mathrm{H}_{2} \mathrm{O} \longrightarrow\mathrm{Cl}^{-}+2 \mathrm{NH}_{3}+4 \mathrm{OH}^{-} \] Hence, mixing bleach with ammonia-containing substances is hazardous and should be avoided.

Step 1: Balancing the electron transfer

Since the number of electrons in both half-reactions is equal (2), the reactions can be added directly without the need to multiply these reactions based on the least common multiple.

Step 2: Combining the half-reactions

Combine the given half-reactions by adding them. Remember that the electrons will cancel each other out when combining them: \[\mathrm{ClO}^{-}+\mathrm{H}_{2} \mathrm{O}+2 \mathrm{e}^{-} \longrightarrow 2\mathrm{OH}^{-}+\mathrm{Cl}^{-}\] \[\mathrm{N}_{2} \mathrm{H}_{4}+2 \mathrm{H}_{2} \mathrm{O}+2 \mathrm{e}^{-} \longrightarrow 2 \mathrm{NH}_{3}+2 \mathrm{OH}^{-}\] Adding them together, we get: \[\mathrm{ClO}^{-}+\mathrm{N}_{2} \mathrm{H}_{4}+4 \mathrm{H}_{2} \mathrm{O} \longrightarrow 2\mathrm{OH}^{-}+\mathrm{Cl}^{-}+2 \mathrm{NH}_{3}+2 \mathrm{OH}^{-}\]

Step 3: Simplifying the reaction

Now we can simplify the resulting equation by canceling out equal components on both sides of the reaction: \[\mathrm{ClO}^{-}+\mathrm{N}_{2} \mathrm{H}_{4}+2 \mathrm{H}_{2} \mathrm{O} \longrightarrow\mathrm{Cl}^{-}+2 \mathrm{NH}_{3}+4 \mathrm{OH}^{-}\]

Step 4: Analyzing the products

In simpler terms, when household bleach (containing ClO⁻) is mixed with ammonia (NH₃) or cleansers containing ammonia, the reaction produces chloride ions (Cl⁻), ammonia (NH₃), and hydroxide ions (OH⁻): \[ \mathrm{ClO}^{-}+\mathrm{N}_{2} \mathrm{H}_{4}+2 \mathrm{H}_{2} \mathrm{O} \longrightarrow\mathrm{Cl}^{-}+2 \mathrm{NH}_{3}+4 \mathrm{OH}^{-} \] The product of concern is hydrazine (N₂H₄), which is a toxic compound. This reaction occurs in an alkaline environment, making this a hazardous outcome when bleach and ammonia-containing substances are mixed. In conclusion, household bleach should not be mixed with household ammonia or glass cleansers containing ammonia because the reaction produces toxic hydrazine in an alkaline environment.

Key Concepts

Balancing Electron Transfer

To fully grasp the essence of redox reactions in chemistry, it's crucial to understand the principle of balancing electron transfer. The process involves ensuring that the electrons lost by the oxidizing agent are equal to the electrons gained by the reducing agent. In the context of our exercise, we look at two half-reactions involving hydrazine and sodium hypochlorite, each having a transfer of 2 electrons.

This balanced electron transfer signifies that the stoichiometry of the reaction is correct and no further adjustments are needed for the half-reactions to be combined. The alignment of electrons ensures the law of conservation of matter is met, as the total number of electrons remains consistent throughout the reaction.

Combining Half-Reactions

Once the electron transfer is balanced, the next step is combining half-reactions. This involves adding the two half-reactions together while ensuring that the electrons cancel out. In doing so, we're able to see the bigger picture: how reactants transform into products in a full redox reaction.

By combining the half-reactions from our given exercise, we observe that the electrons indeed cancel out, confirming that the previous balancing was done correctly. This step is akin to putting together pieces of a puzzle, ensuring that all components fit to display the complete chemical process.

Reaction Simplification

Reaction simplification serves to streamline the chemical equation, making it easier to interpret and understand. During this phase, we eliminate identical species from both sides of the equation to distill the reaction to its most fundamental components.

In our exercise, the simplification step cancels out water molecules and hydroxide ions that appear on both sides of the combined reaction. What remains is a more concise representation of the transformation, highlighting the key reactants and products without extraneous information. Simplification not only aids clarity but can also be essential in calculating quantities for the reaction.

Hazardous Chemical Reactions

Finally, recognizing hazardous chemical reactions is critical for safety in chemistry. In our textbook example, the mixing of household bleach and ammonia is exceptionally dangerous due to the production of toxic hydrazine.

This reaction underscores the importance of understanding chemical properties and the potential risks when certain substances interact. It's pivotal for students to learn not just the mechanics of chemical reactions, but also their real-world implications and the precautionary measures that should be taken to prevent harm.