The reaction between dichromate (Cr2O7^2-) and iodide (I-) ions is an oxidation-reduction (redox) reaction, where the dichromate ion acts as an oxidizing agent and the iodide ion acts as a reducing agent. The overall reaction produces chromium (III) ions (Cr3+), iodine (I2), and water (H2O). The reaction can be represented by the following chemical equation:

Cr2O7^2- (aq) + 6 I- (aq) + 14 H+ (aq) → 2 Cr3+ (aq) + 3 I2 (s) + 7 H2O (l)

In this reaction, the dichromate ion undergoes reduction, where its oxidation state changes from +6 to +3. Meanwhile, the iodide ion undergoes oxidation, where its oxidation state changes from -1 to 0 in elemental iodine (I2).

The reaction involves several steps and proceeds through the formation of intermediate products. Initially, one of the chromium atoms in the dichromate ion is reduced from +6 to +5, resulting in the formation of a chromium (V) intermediate species. This intermediate species then reacts with iodide ions to form iodine and Cr3+ ions. The iodine atoms produced may further react to form diatomic iodine (I2) molecules, which can be observed as a brown-orange precipitate.

The reaction between dichromate and iodide is commonly used in analytical chemistry, particularly for the qualitative detection of iodide ions. The formation of a brown-orange precipitate of iodine can be visually observed, indicating the presence of iodide in a sample. Additionally, the kinetics of this reaction have been extensively studied and find applications in studying reaction rates and mechanisms.