Ammonium dichromate is heated to produce a green residue, the oxidation state of Cr in the residue is +x then x is -

To determine the oxidation state of chromium (Cr) in the residue resulting from the thermal decomposition of ammonium dichromate, we can follow these steps:

1. ### Break Down the Problem

The problem requires us to find the oxidation state of chromium in the green residue formed when ammonium dichromate is heated. The known product of this decomposition is chromium(III) oxide (Cr₂O₃), which is the green residue.

2. ### Relevant Concepts

• The oxidation state of an element in a compound can be determined using the known oxidation states of other elements.
• In chromium(III) oxide (Cr₂O₃), oxygen has an oxidation state of -2.
• The total charge of the compound must equal zero.

3. ### Analysis and Detail

Let's analyze the compound Cr₂O₃:

1. Let the oxidation state of Cr be $+x$.
2. In Cr₂O₃, there are 2 chromium atoms, so the total contribution of chromium to the charge would be $2x$.
3. There are 3 oxygen atoms and since each oxygen has an oxidation state of -2, the total contribution from oxygen is $3 \times (-2) = -6$.
4. Therefore, the equation for the total charge is:
   $2x + (-6) = 0$

4. ### Verify and Summarize

Now, let’s solve for $x$: $2x - 6 = 0 \implies 2x = 6 \implies x = +3$ Thus, the oxidation state of chromium in the green residue, Cr₂O₃, is +3.

Final Answer

The oxidation state of Cr in the residue is $+3$.