Balance the following half-reactions. (b) (basic) CrO₄^2-(aq) → Cr(OH)₄^-(aq)

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Identify the elements that are being oxidized and reduced. In this case, chromium (Cr) is the element of interest.

Balance the chromium atoms on both sides of the equation. Since there is one Cr atom on each side, they are already balanced.

Balance the oxygen atoms by adding water (H2O) molecules. There are 4 oxygen atoms on the left and 4 on the right, so they are balanced.

Balance the hydrogen atoms by adding hydroxide ions (OH^-) to the side that needs more hydrogen. Add 4 OH^- to the left side to balance the 4 H in Cr(OH)4^- on the right.

Balance the charge by adding electrons (e^-). The left side has a charge of -6 (from CrO4^2- and 4 OH^-), and the right side has a charge of -1 (from Cr(OH)4^-), so add 3 e^- to the left side to balance the charges.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Half-Reaction Method

The half-reaction method is a technique used to balance redox reactions by separating the oxidation and reduction processes. Each half-reaction is balanced for mass and charge, allowing for a clearer understanding of electron transfer. This method is particularly useful in complex reactions, as it simplifies the balancing process by focusing on individual components.

Basic Medium Balancing

When balancing reactions in a basic medium, hydroxide ions (OH-) are added to neutralize any hydrogen ions (H+) that may appear during the balancing process. This is essential because it ensures that the reaction remains in a basic environment, which affects the species present and their interactions. The addition of water may also be necessary to balance oxygen atoms.

Chromate and Hydrochromate Species

Chromate (CrO4^2-) and hydrochromate (Cr(OH)4^-) are important species in redox chemistry involving chromium. Chromate is a strong oxidizing agent, while hydrochromate is formed in basic conditions. Understanding the interconversion between these species is crucial for balancing reactions involving chromium, as it influences the overall stoichiometry and electron transfer.

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