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Chapter 22: Problem 55

Write complete balanced half-reactions for (a) oxidation of nitrous acid to nitrate ion in acidic solution, (b) oxidation of \(\mathrm{N}_{2}\) to \(\mathrm{N}_{2} \mathrm{O}\) in acidic solution.

Short Answer

Expert verified
(a) HNO鈧� + H鈧侽 鈫� NO鈧冣伝 + 2H鈦� + e鈦� (b) N鈧� + H鈧侽 鈫� N鈧侽 + 2H鈦� + 2e鈦�

Step by step solution

01

Identify the Oxidation States

First, we need to determine the oxidation states of nitrogen in the reactants and products. In nitrous acid (HNO鈧�), nitrogen has an oxidation state of +3. In the nitrate ion (NO鈧冣伝), nitrogen has an oxidation state of +5. In N鈧�, the oxidation state of nitrogen is 0, and in N鈧侽, it is +1.
02

Write the Oxidation Half-Reactions

For the oxidation of nitrous acid (HNO鈧�) to nitrate ion (NO鈧冣伝), the half-reaction can be written as: \[ \text{HNO}_2 \rightarrow \text{NO}_3^- \] For the oxidation of \( \text{N}_2 \) to \( \text{N}_2\text{O} \), the half-reaction can be written as: \[ \text{N}_2 \rightarrow \text{N}_2\text{O} \]
03

Balance the Oxygen Atoms

In acidic solutions, add water (H鈧侽) molecules to balance the oxygen atoms. - For \( \text{HNO}_2 \rightarrow \text{NO}_3^- \), add one water molecule to the left side: \[ \text{HNO}_2 + \text{H}_2\text{O} \rightarrow \text{NO}_3^- \] - For \( \text{N}_2 \rightarrow \text{N}_2\text{O} \), add one water molecule to the left to balance with one oxygen atom: \[ \text{N}_2 + \text{H}_2\text{O} \rightarrow \text{N}_2\text{O} \]
04

Balance the Hydrogen Atoms

Add H鈦� ions to balance the hydrogen atoms on both sides. - For \( \text{HNO}_2 + \text{H}_2\text{O} \rightarrow \text{NO}_3^- \), add two H鈦� ions to the right: \[ \text{HNO}_2 + \text{H}_2\text{O} \rightarrow \text{NO}_3^- + 2\text{H}^+ \] - For \( \text{N}_2 + \text{H}_2\text{O} \rightarrow \text{N}_2\text{O} \), add two H鈦� ions to the right: \[ \text{N}_2 + \text{H}_2\text{O} \rightarrow \text{N}_2\text{O} + 2\text{H}^+ \]
05

Balance the Charges with Electrons

Balance the charges by adding electrons. - For \( \text{HNO}_2 + \text{H}_2\text{O} \rightarrow \text{NO}_3^- + 2\text{H}^+ \), add one electron to the right: \[ \text{HNO}_2 + \text{H}_2\text{O} \rightarrow \text{NO}_3^- + 2\text{H}^+ + e^- \] - For \( \text{N}_2 + \text{H}_2\text{O} \rightarrow \text{N}_2\text{O} + 2\text{H}^+ \), add two electrons to the right: \[ \text{N}_2 + \text{H}_2\text{O} \rightarrow \text{N}_2\text{O} + 2\text{H}^+ + 2e^- \]

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Oxidation Half-Reactions
Oxidation half-reactions are a critical part of redox (reduction-oxidation) reactions. In these reactions, the oxidation state of an element increases, meaning it loses electrons. The half-reaction specifically focuses on the part of the redox process where oxidation occurs, without initially considering the reduction part.
To construct an oxidation half-reaction, follow these steps:
  • Identify the initial and final substances in the reaction.
  • Determine the changes in oxidation states for the atoms involved.
  • Write down how the atoms or ions transform, focusing only on the loss of electrons.
In the example given in the exercise, nitrous acid (HNO鈧�) is oxidized to nitrate ion (NO鈧冣伝), and molecular nitrogen (N鈧�) is oxidized to nitrous oxide (N鈧侽).
Oxidation States
Oxidation states, sometimes referred to as oxidation numbers, indicate the degree of oxidation of an atom in a chemical compound. These numbers help in tracking electron transfer during a chemical reaction.
Factors to consider when determining oxidation states:
  • Oxidation states can be positive, negative, or zero, reflecting the hypothetical charge on an atom if all bonds were ionic.
  • Common rules include
    • an oxidation state of 0 for atoms in elemental form (e.g., N in N鈧�),
    • a -2 oxidation state for oxygen (with some exceptions), and
    • usually a +1 oxidation state for hydrogen.
In the given problems, determining oxidation states helps track how nitrogen changes between +3 to +5 in nitrous acid and 0 to +1 in molecular nitrogen. Ensuring these changes are reflected in the half-reactions is key to balancing the chemical equations.
Acidic Solutions
In chemical reactions, the solvent can greatly affect the behavior and outcome of the reaction. An acidic solution is typically one in which acid (often H鈦� ions) is present, providing a medium that allows certain reactions to take place.
Acidic environments are characterized by:
  • The tendency to release H鈦� ions, which are often crucial for balancing half-reactions by adding them where needed to equalize hydrogen atoms.
  • The ability to use water molecules to counterbalance oxygen discrepancies, which is more efficiently done in acidic environments.
In the exercise, the reactions are carried out in acidic conditions, supported by adding H鈦� ions to balance out hydrogen atoms effectively. Using acidic solutions ensures that the reaction progress smoothly and can be balanced with all necessary atoms.
Electrons
Electrons play a fundamental role in oxidation and reduction reactions. They are the primary charge carriers and are either lost or gained by atoms in chemical processes.
Key points about electrons in these processes:
  • In oxidation, electrons are lost by the atom or ion involved. This is seen as the oxidation state increases.
  • The number of electrons lost must always match the change in net charge to maintain the reaction's charge balance.
  • Electrons can also be used to balance charges in reactions, making them crucial for writing balanced half-reactions.
For the reactions of nitrous acid and nitrogen, electrons must be added to the product side to balance the oxidation process fully. By understanding electron transfer, it's possible to determine how substances change during a chemical reaction.

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Most popular questions from this chapter

What does hydrogen have in common with the halogens? Explain.

When aluminum replaces up to half of the silicon atoms in \(\mathrm{SiO}_{2}\), a mineral class called feldspars result. The feldspars are the most abundant rock-forming minerals, comprising about \(50 \%\) of the minerals in Earth's crust. Orthoclase is a feldspar in which Al replaces one-fourth of the Si atoms of \(\mathrm{SiO}_{2},\) and charge balance is completed by \(\mathrm{K}^{+}\) ions. Determine the chemical formula for orthoclase.

Consider the elements \(\mathrm{Ba}, \mathrm{Na}, \mathrm{O}, \mathrm{B}, \mathrm{P},\) and \(\mathrm{Kr}\). From this list, select the element that (a) is most electronegative, (b) has the greatest metalliccharacter, \((\mathbf{c})\) most readily forms a positive ion, \((\mathbf{d})\) exhibits a maximum oxidation sate of +5 , (e) exists as monoatomic gas at room temperature, (f) has multiple allotropes.

Which of the following statements are true? (a) Si can form an ion with six fluorine atoms, \(\mathrm{SiF}_{6}^{2-}\), whereas carbon cannot. (b) Si can form three stable compounds containing two Si atoms each, \(\mathrm{Si}_{2} \mathrm{H}_{2}, \mathrm{Si}_{2} \mathrm{H}_{4},\) and \(\mathrm{Si}_{2} \mathrm{H}_{6}\) (c) In \(\mathrm{HNO}_{3}\) and \(\mathrm{H}_{3} \mathrm{PO}_{4}\) the central atoms, \(\mathrm{N}\) and \(\mathrm{P}\), have different oxidation states. (d) \(\mathrm{S}\) is more electronegative than Se.

Give a reason why hydrogen might be placed along with the group 1 elements of the periodic table.
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