Consider the redox titration (Section 4.13) of 120.0 mL of 0.100 M FeSO4 with 0.120 M K2Cr2O7 at 25 °C, assuming that the pH of the solution is maintained at 2.00 with a suitable buffer. The solution is in contact with a platinum electrode and constitutes one half-cell of an electrochemical cell. The other half-cell is a standard hydrogen electrode. The two half-cells are connected with a wire and a salt bridge, and the progress of the titration is monitored by measuring the cell potential with a voltmeter. (a) Write a balanced net ionic equation for the titration reaction, assuming that the products are Fe3+ and Cr3+.

Verified step by step guidance

1. Identify the oxidation states of the reactants and products in the reaction. Fe in FeSO4 is in the +2 oxidation state and needs to be oxidized to Fe3+. Cr in K2Cr2O7 is in the +6 oxidation state and needs to be reduced to Cr3+.
2. Write the half-reactions for both the oxidation and reduction processes. For Fe2+ being oxidized to Fe3+, the half-reaction is: Fe2+ -> Fe3+ + e-. For Cr6+ being reduced to Cr3+, the half-reaction is: Cr2O7^2- + 14H+ + 6e- -> 2Cr3+ + 7H2O.
3. Balance the electrons in the half-reactions to combine them into a full redox reaction. Since the oxidation half-reaction releases 1 electron and the reduction half-reaction consumes 6 electrons, multiply the oxidation half-reaction by 6 to balance the electrons.
4. Combine the balanced half-reactions. The combined balanced reaction will be: 6Fe2+ + Cr2O7^2- + 14H+ -> 6Fe3+ + 2Cr3+ + 7H2O.
5. Check the final equation to ensure mass and charge balance. Confirm that the number of atoms for each element and the total charges on both sides of the equation are equal.

Key Concepts

Redox Reactions

Redox reactions involve the transfer of electrons between two species, where one species is oxidized (loses electrons) and the other is reduced (gains electrons). In this titration, Fe2+ ions from FeSO4 are oxidized to Fe3+, while Cr2O7^2- ions from K2Cr2O7 are reduced to Cr3+. Understanding the oxidation states and electron transfer is crucial for writing the balanced net ionic equation.

Ionic Equations

Ionic equations represent the species that participate in a chemical reaction in their ionic forms. In a redox titration, it is important to write the net ionic equation, which includes only the ions and molecules that undergo a change, excluding spectator ions. This helps in simplifying the reaction and focusing on the actual chemical changes occurring during the titration.

Electrochemical Cells

Electrochemical cells consist of two half-cells where oxidation and reduction reactions occur. The cell potential, measured in volts, indicates the driving force of the redox reaction. In this scenario, the platinum electrode serves as the site for the oxidation of Fe2+, while the standard hydrogen electrode provides a reference for measuring the cell potential, which is essential for understanding the progress of the titration.