Question

Explain why these reactions cannot occur as written. (a) \(\mathrm{Fe}^{3+}(\mathrm{aq})+\mathrm{MnO}_{4}^{-}(\mathrm{aq})+\mathrm{H}^{+}(\mathrm{aq}) \longrightarrow\) \(\mathrm{Mn}^{2+}(\mathrm{aq})+\mathrm{Fe}^{2+}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(1)\) (b) \(\mathrm{H}_{2} \mathrm{O}_{2}(\mathrm{aq})+\mathrm{Cl}_{2}(\mathrm{aq}) \longrightarrow\) \(\mathrm{ClO}^{-}(\mathrm{aq})+\mathrm{O}_{2}(\mathrm{g})+\mathrm{H}^{+}(\mathrm{aq})\)

Short answer

Reaction (a) cannot occur as written because it violates the conservation of charge principle. Reaction (b) cannot occur as written because it violates the conservation of mass principle.

Step-by-step solution

Analyzing Reaction (a)

In reaction (a), the left side includes: Fe3+, MnO4-, and H+. The total charge of the particles on the left side is +3 - 1 + 1 = +3. The right side includes: Mn2+, Fe2+, and H2O. The total charge of the particles on the right side is +2 + 2 + 0 = +4. The total charge of the particles is not the same on both sides, meaning reaction (a) violates the conservation of charge principle and cannot occur as written.

Analyzing Reaction (b)

In reaction (b), the left side includes: H2O2(and Cl2. The total charge of the particles on the left side is 0 + 0 = 0. The right side includes: ClO-, O2, and H+.The total charge of the particles on the right side is -1 + 0 + 1 = 0. However, the amount of Cl and O atoms on the reactants side does not equal the Cl and O atoms on the products side. Because the reaction violates the conservation of mass principle, it cannot occur as written.

Key concepts

Conservation of Charge

The conservation of charge is a fundamental principle in chemistry that dictates the total electric charge in a closed system remains constant during a chemical reaction. Essentially, the sum of positive and negative charges must be the same before and after the reaction. In a chemical equation, this means that the total charges on the reactant side should equal the total charges on the product side.

• In the provided exercise, reaction (a) involves the conversion of iron and manganese ions along with hydronium ions into different products.
• Initially, the reactants have a collective charge of +3, due to the presence of Fe³⁺, MnO₄^-, and H⁺.
• Meanwhile, on the product side, the total charge sums up to +4 because of Mn²⁺, Fe²⁺, and water.

This discrepancy in charge violates the conservation of charge principle, since the initial and final charges are not equal. Therefore, the reaction as written is not possible without balancing the charges to ensure both sides have the same total charge.

Conservation of Mass

The conservation of mass is another key principle in chemistry, stating that mass in a closed system remains constant over time, irrespective of the processes acting inside the system. During a chemical reaction, the number of each type of atom present in the reactants must equal the number of each type in the products.

• In reaction (b) from the exercise, the reactants, which are hydrogen peroxide (H₂O₂) and chlorine gas (Cl₂), must interact to form hypochlorite ions (ClO^-), molecular oxygen (O₂), and hydrogen ions (H⁺).
• While the charge balance of zero on both sides is correct, the number of chlorine and oxygen atoms does not match between reactants and products.

This mismatch means that matter is not conserved, suggesting that the atoms are not correctly balanced. The reaction cannot proceed unless the atoms of each element are equal on both sides, maintaining the concept of conservation of mass.

Oxidation-Reduction Reactions

Oxidation-reduction reactions, also known as redox reactions, involve the transfer of electrons between two substances. These types of reactions are fundamental in processes such as metabolism, combustion, and corrosion. In a redox reaction, one substance loses electrons (oxidation), while another gains electrons (reduction). To identify these changes, it helps to track the oxidation states of all elements involved in the reaction.

• In reaction (a), the oxidation state of iron changes from +3 in Fe³⁺ to +2 in Fe²⁺, indicating a reduction as it gains an electron.
• Manganese in MnO₄^- changes from an oxidation state of +7 to +2 in Mn²⁺, highlighting a reduction process as a significant amount of electrons are gained.
• For a true redox reaction, the electron loss should equal the electron gain, providing a clear link with the conservation of charge.

Understanding oxidation and reduction processes is vital to grasping how these reactions translate into the movement and conservation of electrons, which is essential for the balancing and feasibility of chemical reactions.