Potassium dichromate
Potassium dichromate is an ionic compound made of potassium ions and the dichromate polyatomic ion, giving the formula \(K_2Cr_2O_7\).
Ions and formula unit
The dichromate ion carries a \(2-\) charge, \(Cr_2O_7^{2-}\), while potassium forms \(K^+\). Electrical neutrality in the formula unit requires two \(K^+\) ions for each dichromate ion:
\[ 2 \times (+1) + (-2) = 0. \]
| Species | Ion formula | Charge | Count in one formula unit |
|---|---|---|---|
| Potassium ion | \(K^+\) | \(+1\) | 2 |
| Dichromate ion | \(Cr_2O_7^{2-}\) | \(-2\) | 1 |
Oxidation state in the dichromate ion
The average oxidation state of chromium in \(Cr_2O_7^{2-}\) follows from charge accounting with oxygen at \(-2\). Let each chromium be \(x\):
\[ 2x + 7 \times (-2) = -2 \quad \Rightarrow \quad 2x - 14 = -2 \quad \Rightarrow \quad x = +6. \]
Potassium dichromate therefore contains chromium(VI), a common description in general chemistry and analytical chemistry contexts.
Molar mass (formula mass)
The molar mass is the sum of the atomic masses in \(K_2Cr_2O_7\). Using standard atomic masses (in \(\text{g mol}^{-1}\)) gives:
\[ M(K_2Cr_2O_7) = 2 \times 39.0983 + 2 \times 51.9961 + 7 \times 15.999 \approx 294.18~\text{g mol}^{-1}. \]
Aqueous behavior and redox chemistry
In water, potassium dichromate dissociates into ions:
\[ K_2Cr_2O_7(s) \rightarrow 2K^+(aq) + Cr_2O_7^{2-}(aq). \]
The dichromate/chromate system depends on acidity. In acidic solution, dichromate is favored:
\[ 2CrO_4^{2-}(aq) + 2H^+(aq) \rightleftharpoons Cr_2O_7^{2-}(aq) + H_2O(l). \]
Chromium(VI) in \(Cr_2O_7^{2-}\) is a strong oxidizing agent in acidic solution. A widely used reduction half-reaction is:
\[ Cr_2O_7^{2-} + 14H^+ + 6e^- \rightarrow 2Cr^{3+} + 7H_2O. \] \]
Naming and recognition in ionic compounds
The name “potassium dichromate” reflects the cation name (potassium) and the polyatomic anion name (dichromate). The “di-” prefix is part of the anion’s conventional name and does not indicate two dichromate ions in the formula unit.