Problem 86 - Chemistry: An Atoms-Focused Approach

Question

In one stage of the nitrogen cycle, Nitrosomonas bacteria convert ammonia and oxygen into nitrite ions. a. What is the change in oxidation state of nitrogen during the reaction? b. Write a balanced net ionic equation for the reaction in acidic groundwater.

Step-by-Step Solution

Answer

Answer: The change in oxidation state of nitrogen during the reaction is +6. The balanced net ionic equation for the reaction in acidic groundwater is 2NH3 + 3O2 + 12H+ -> 2NO2- + 6H2O.

Step 1: Determine the oxidation states of nitrogen in ammonia and nitrite ions

To find the change in oxidation state, we first need to know the initial and final oxidation states of nitrogen. In ammonia (NH3), nitrogen has an oxidation state of -3. In nitrite ions (NO2-), nitrogen has an oxidation state of +3.

Step 2: Calculate the change in oxidation state of nitrogen

Now that we have the initial and final oxidation states of nitrogen, we can calculate the change in oxidation state: Change in oxidation state = Final oxidation state - Initial oxidation state Change in oxidation state = (+3) - (-3) Change in oxidation state = +6 So, the change in oxidation state of nitrogen during the reaction is +6. Answer to (a): The change in oxidation state of nitrogen during the reaction is +6.

Step 3: Write the half-reactions

For the reaction in acidic groundwater, we'll need to write the half-reactions for both the oxidation and reduction processes. The oxidation half-reaction involves ammonia (NH3) being oxidized to nitrite ions (NO2-) and the reduction half-reaction involves oxygen being reduced to water (H2O). Oxidation half-reaction: NH3 -> NO2- + 6e- Reduction half-reaction: O2 + 4e- + 4H+ -> 2H2O

Step 4: Balance the half-reactions

To balance the half-reactions, we need to multiply each half-reaction by an appropriate factor so that the number of electrons lost in the oxidation half-reaction is equal to the number of electrons gained in the reduction half-reaction: Oxidation half-reaction (multiplied by 2): 2NH3 -> 2NO2- + 12e- Reduction half-reaction (multiplied by 3): 3O2 + 12e- + 12H+ -> 6H2O

Step 5: Combine the balanced half-reactions

Now we can combine the balanced half-reactions to form the balanced net ionic equation for the reaction: 2NH3 + 3O2 + 12H+ -> 2NO2- + 6H2O Answer to (b): The balanced net ionic equation for the reaction in acidic groundwater is 2NH3 + 3O2 + 12H+ -> 2NO2- + 6H2O.

Key Concepts

Oxidation State

The oxidation state of an element in a compound is crucial for understanding chemical reactions. It represents the number of electrons an atom gains, loses, or shares when it forms chemical bonds. In the nitrogen cycle, calculating the change in oxidation state can help us understand what happens at a microscopic level when Nitrosomonas bacteria convert ammonia to nitrite ions.

• In ammonia (\( ext{NH}_3 \)), nitrogen has an oxidation state of \(-3\).
• In nitrite ions (\( ext{NO}_2^- \)), nitrogen holds an oxidation state of \(+3\).

The change in oxidation state is calculated as follows: \[\text{Change in oxidation state} = \text{Final oxidation state} - \text{Initial oxidation state}\]This results in a change of \((+3) - (-3) = +6\), revealing that nitrogen undergoes a significant increase in its oxidation number, signifying a loss of electrons and indicating oxidation.

Nitrosomonas Bacteria

Nitrosomonas bacteria play a pivotal role in the nitrogen cycle, particularly in the first stage called nitrification. These bacteria specialize in oxidizing ammonia (\( ext{NH}_3 \)) into nitrite (\( ext{NO}_2^- \)), a process that requires precise biochemical machinery and environmental conditions. It’s essential to understand the conditions under which they thrive and how they contribute to the nitrogen available in ecosystems.

• They perform the oxidation process using a specific enzyme known as ammonia monooxygenase.
• This process is aerobic, meaning it requires the presence of oxygen.
• The conversion of ammonia to nitrite is the first step in making nitrogen compounds bioavailable for plant uptake.

As these bacteria oxidize ammonia, they facilitate the conversion of nitrogen from a relatively inert form into a readily usable state within the ecosystem, playing an intimate role in sustaining life.

Net Ionic Equation

The net ionic equation provides a simplified representation of a chemical reaction by omitting spectator ions that don’t change during the reaction. In the exercise, the interaction in acidic conditions is addressed using net ionic equations. This makes the underlying chemical changes evident by focusing solely on the reacting ions.

Steps to determine the net ionic equation:

• Identify the oxidation and reduction half-reactions.
• Balance each half-reaction by ensuring that electrons lost and gained are equal.
• Combine the balanced half-reactions to form a complete reaction.

For the given reaction: - Oxidation half-reaction: \( 2\text{NH}_3 \rightarrow 2\text{NO}_2^- + 12e^- \) - Reduction half-reaction: \( 3\text{O}_2 + 12e^- + 12\text{H}^+ \rightarrow 6\text{H}_2\text{O} \) The final balanced net ionic equation becomes: \[ 2\text{NH}_3 + 3\text{O}_2 + 12\text{H}^+ \rightarrow 2\text{NO}_2^- + 6\text{H}_2\text{O} \]

Acidic Conditions

In chemistry, the pH of the environment significantly influences the course and outcome of reactions. When performing reactions under acidic conditions, hydrogen ions (\( ext{H}^+ \)) play a substantial role in the chemical transformations occurring during the reaction. They can directly affect the charges of reactive species and the overall mechanism.

For the conversion of ammonia to nitrite by Nitrosomonas bacteria:

• The presence of \( ext{H}^+ \) ions is crucial to balance the charges in the net ionic equation.
• Acidic conditions help in the stabilization of certain intermediates formed during the oxidation process.
• They also contribute to the completion of the reduction half-reaction by enabling the formation of water from oxygen.

Thus, understanding how these conditions influence the chemical environment is essential, as they ensure the reaction progresses efficiently towards completion, contributing to the continuous cycling of nitrogen in ecosystems.