The oxidation number of chromium (Cr) in the dichromate ion (Cr2O72-) is
A. +2
B. +4
C. +6
D. +7
E. -2
http://dbhs.wvusd.k12.ca.us/webdocs/Redox/Redox-Rules.html
To determine the oxidation number of chromium (Cr) in the dichromate ion (Cr2O72-), you can follow these steps:
Step 1: Assign the oxidation number of oxygen (O) atom in the dichromate ion as -2. This is a common assumption since oxygen almost always has an oxidation number of -2 in compounds.
Step 2: Since there are seven oxygen atoms in the dichromate ion (Cr2O72-), the total oxidation number contributed by oxygen is -2 × 7 = -14.
Step 3: The total charge of the dichromate ion is -2. Therefore, the sum of the oxidation numbers of chromium and oxygen must equal -2.
Step 4: Given that the oxidation number of each oxygen atom is -2 and the overall negative charge is -2, subtract the total oxidation number contributed by oxygen from -2.
-2 - (total oxidation number of oxygen) = -2 - (-14) = +12
Step 5: Since there are two chromium atoms in the dichromate ion, divide the value obtained in Step 4 by 2.
+12 ÷ 2 = +6
The oxidation number of chromium (Cr) in the dichromate ion (Cr2O72-) is +6.
Therefore, the correct answer is C. +6.
To determine the oxidation number of chromium in the dichromate ion (Cr2O72-), we need to assign oxidation numbers to the other elements in the compound and then use the principle that the sum of the oxidation numbers in a neutral compound is zero, while the sum of the oxidation numbers in an ion is equal to the charge of the ion.
1. The oxygen atoms in the dichromate ion (O2-) have a fixed oxidation number of -2. In this case, there are seven oxygen atoms, so the combined oxidation number is -2 x 7 = -14.
2. The total charge of the dichromate ion (Cr2O72-) is -2. This means that the oxidation numbers of chromium and oxygen must add up to -2.
3. Let's assign the oxidation number of x to each chromium atom.
- The sum of the oxidation numbers must equal the charge of the ion: 2x + (-14) = -2.
4. Simplifying the equation: 2x - 14 = -2.
5. Solving for x: 2x = 12, x = 6.
Therefore, the oxidation number of chromium (Cr) in the dichromate ion (Cr2O72-) is +6.
So the correct answer is C. +6.