Which atom has a change in oxidation number of –3 in the following redox reaction? K2Cr2O7 + H2O + S - - > KOH + Cr2O3 + SO2

Cr^+3 - - > Cr2O7^-2
Fe^+2 - - > Fe^+3
Fe^+3 - - > Fe^+2
Cr2O7^-2 - -> Cr^+3***

right

To determine which atom has a change in oxidation number of -3 in the given redox reaction, we need to compare the initial and final oxidation states of each atom.

In the reaction:
K2Cr2O7 + H2O + S -> KOH + Cr2O3 + SO2

We can identify the atom that undergoes a change in oxidation number by looking at the oxidation states of the atoms involved. Let's start by assigning the oxidation states to the atoms:

K has a fixed oxidation state of +1.
H in H2O has an oxidation state of +1.
O in H2O has an oxidation state of -2.
S in S has an unknown oxidation state.
Cr in K2Cr2O7 has an unknown oxidation state.
O in K2Cr2O7 has an oxidation state of -2.
K in K2Cr2O7 has a fixed oxidation state of +1.
Cr in Cr2O3 has an unknown oxidation state.
O in Cr2O3 has an oxidation state of -2.
S in SO2 has an unknown oxidation state.
O in SO2 has an oxidation state of -2.

From the given options, we can eliminate Fe^+2 - -> Fe^+3 since no iron (Fe) is present in the reaction.

Now let's calculate the oxidation states of Cr2O7^2- in K2Cr2O7 and Cr in Cr2O3.

In K2Cr2O7:
2K(+1) + Cr(unknown) + 7O(-2) = 0
We can solve for Cr's oxidation state: 2 + Cr + (-14) = 0
Cr = +12
Therefore, Cr in K2Cr2O7 has an oxidation state of +12.

In Cr2O3:
2Cr(unknown) + 3O(-2) = 0
We can solve for Cr's oxidation state: 2Cr + (-6) = 0
Cr = +3
Therefore, Cr in Cr2O3 has an oxidation state of +3.

Since the oxidation state of Cr changes from +12 in K2Cr2O7 to +3 in Cr2O3, we can conclude that the atom with a change in oxidation number of -3 is Cr.

Therefore, the correct answer is Cr2O7^-2 - -> Cr^+3.