Problem 67 - Chemistry: Principles and Reactions

Question

Write balanced net ionic equations for the following reactions in acid solution. (a) Liquid hydrazine reacts with an aqueous solution of sodium bromate. Nitrogen gas and bromide ions are formed. (b) Solid phosphorus \(\left(P_{4}\right)\) reacts with an aqueous solution of nitrate to form nitrogen oxide gas and dihydrogen phosphate \(\left(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\right)\) ions. (c) Aqueous solutions of potassium sulfite and potassium permanganate react. Sulfate and manganese(II) ions are formed.

Step-by-Step Solution

Answer

Question: Write balanced net ionic equations for the following reactions in acidic solutions: (a) Liquid hydrazine (N2H4) reacts with an aqueous solution of sodium bromate (NaBrO3) to form nitrogen gas (N2) and bromide ions (Br-). Balanced net ionic equation: 4 N2H4 + 10 NaBrO3 + 48 H+ -> 4 N2 + 20 Br- + 24 H2O (b) Solid phosphorus (P4) reacts with an aqueous solution of nitrate (NO3-) to form nitrogen oxide gas (NO) and dihydrogen phosphate ions (H2PO4-). Balanced net ionic equation: P4 + 5 NO3- + 20 H+ -> 4 H2PO4- + 5 NO (c) Aqueous solutions of potassium sulfite (K2SO3) and potassium permanganate (KMnO4) react to form sulfate ions (SO4^2-) and manganese(II) ions (Mn^2+). Balanced net ionic equation: 2 MnO4- + 2 SO3^2- + 16 H+ -> 2 SO4^2- + 2 Mn^2+ + 4 H2O

Step 1: Identify the half-reactions

Decompose the reaction into its half-reactions by identifying the elements that change oxidation numbers. We can notice N in N2H4 is oxidized, and Br in NaBrO3 is reduced. Oxidation half-reaction: N2H4 -> N2 Reduction half-reaction: NaBrO3 -> Br-

Step 2: Balance the atoms

Balance the atoms other than O and H in each half-reaction. For the oxidation half-reaction, N is balanced: N2H4 -> N2 For the reduction half-reaction, we need 2 moles of NaBrO3 to balance Br: 2 NaBrO3 -> 2 Br-

Step 3: Balance oxygen and hydrogen

Add water molecules to balance oxygen atoms and add H+ ions to balance hydrogen atoms in each half-reaction. Oxidation half-reaction: N2H4 -> N2 + 4 H+ Reduction half-reaction: 2 NaBrO3 + 12 H+ -> 2 Br- + 6 H2O

Step 4: Balance the charges

Add electrons to balance the charges in the half-reactions. Oxidation half-reaction: N2H4 -> N2 + 4 H+ + 4 e- Reduction half-reaction: 2 NaBrO3 + 12 H+ + 10 e- -> 2 Br- + 6 H2O

Step 5: Combine half-reactions

Multiply the half-reactions to equalize the electrons and add them to obtain the net ionic equation. 4 N2H4 + 10 NaBrO3 + 48 H+ -> 4 N2 + 20 Br- + 24 H2O + 20 e- + 20 e- Balanced net ionic equation: 4 N2H4 + 10 NaBrO3 + 48 H+ -> 4 N2 + 20 Br- + 24 H2O (b) Solid phosphorus \(\left(P_{4}\right)\) reacts with an aqueous solution of nitrate to form nitrogen oxide gas and dihydrogen phosphate \(\left(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\right)\) ions.

Steps 1-4: Achieve balanced half-reactions

Do steps 1-4 for this reaction to achieve the balanced half-reactions: Oxidation half-reaction: P4 + 12 H2O -> 4 H2PO4- + 20 H+ + 20 e- Reduction half-reaction: 5 NO3- + 20 H+ + 20 e- -> 5 NO + 10 H2O

Step 5: Combine half-reactions

Multiply the half-reactions to equalize the electrons and add them to obtain the net ionic equation. Balanced net ionic equation: P4 + 5 NO3- + 20 H+ -> 4 H2PO4- + 5 NO (c) Aqueous solutions of potassium sulfite and potassium permanganate react. Sulfate and manganese(II) ions are formed.

Steps 1-4: Achieve balanced half-reactions

Do steps 1-4 for this reaction to achieve the balanced half-reactions: Oxidation half-reaction: 2 SO3^2- + 2H2O -> 2 SO4^2- + 4 H+ + 2 e- Reduction half-reaction: MnO4- + 8 H+ + 5 e- -> Mn^2+ + 4 H2O

Step 5: Combine half-reactions

Multiply the half-reactions to equalize the electrons and add them to obtain the net ionic equation. 2 MnO4- + 2 SO3^2- + 9 H2O + 16 H+ -> 2 SO4^2- + 2 Mn^2+ + 12 H2O + 10 e- + 10 e- Balanced net ionic equation: 2 MnO4- + 2 SO3^2- + 16 H+ -> 2 SO4^2- + 2 Mn^2+ + 4 H2O

Key Concepts

Acid-Base Reactions

Understanding acid-base reactions is fundamental in chemistry. These reactions involve the transfer of protons (H+) from the acid to the base. A classic representation of an acid-base reaction is the neutralization where an acid reacts with a base to form water and a salt. For instance, when hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH), they produce sodium chloride (NaCl) and water (H2O).
In net ionic equations for acid-base reactions, spectator ions—those not participating in the reaction—are omitted to focus on the actual chemical change. Here's an example using our exercise: when hydrazine (N2H4) acting as a base reacts with protons, it releases nitrogen gas and forms water as an acidic solution is neutralized in the process.

Redox Reactions

Redox reactions, or oxidation-reduction reactions, are processes where electrons are transferred between substances. They consist of two parts: oxidation, where an element loses electrons, and reduction, where an element gains electrons.
In our exercise, we observed this with the transformation of sodium bromate (NaBrO3) to bromide ions (Br-), which is the reduction half-reaction, and the oxidation of hydrazine (N2H4) to nitrogen gas (N2). Identifying the elements that change oxidation states is crucial, enabling us to separate the overall reaction into two half-reactions, each representing either reduction or oxidation.

Oxidation States

Oxidation states indicate the degree of oxidation of an atom in a molecule. They help in understanding redox reactions by showing how many electrons are lost or gained by each atom. An increase in oxidation state means oxidation, while a decrease signals reduction.
For example, in the reduction half-reaction of our exercise, bromine goes from an oxidation state of +5 in sodium bromate (NaBrO3) down to -1 in bromide ions (Br-). This change indicates a gain of electrons, characterizing it as a reduction. Learning to assign oxidation states precisely is a necessary step in writing complete redox reactions and balancing them correctly.

Balancing Chemical Equations

Balancing chemical equations is a process to ensure that the same number of atoms for each element is present on the reactant side as on the product side, complying with the law of conservation of mass. This procedure often involves adjusting coefficients preceding the chemical formulas.
In the context of net ionic equations, once atoms and charge are balanced separately in each half-reaction, the next step is to multiply the half-reactions by appropriate coefficients to equalize the electron transfer. Then these half-reactions are added together to form the balanced net ionic equation. This final balanced equation for the redox reaction between hydrazine and sodium bromate was achieved by ensuring that the electrons lost in oxidation equal the electrons gained in reduction.