Question

Which are the values of X and Y respectively?
$CrO_{4(aq)}^{2 - }\underset{{pH = Y}}{\overset{{pH = X}}{\longleftrightarrow}}C{r_2}O_{7(aq)}^{2 - }$
A.$6$ and $4$
B.$8$ and $5$
C.$6$ and $8$
D.$7$ and $7$

Answer 1

An oxidation-reduction (redox) reaction is a type of chemical reaction that involves a transfer of electrons between two species. An oxidation-reduction reaction is any chemical reaction in which the oxidation number of a molecule, atom, or ion changes by gaining or losing an electron. Oxidation is termed as an increase in the oxidation state while reduction is termed as a decrease in the oxidation state.

Complete answer:
When chromate ion changes to dichromate ion:
$CrO_{4(aq)}^{2 - } \to C{r_2}O_{7(aq)}^{2 - }$
The oxidation number of chromium changes from $+ 6$ to $+ 7$. This means that the reaction is an oxidation reaction (loss of electrons). Hence, the reaction will happen in an acidic environment. Therefore, $X < 7$.
Similarly, when dichromate ion changes back to chromate ion:
$C{r_2}O_{7(aq)}^{2 - } \to CrO_{4(aq)}^{2 - }$
The oxidation number of chromium changes from $+ 7$ to $+ 6$. This means that the reaction is a reduction process (gain of electrons). Hence, the reaction will happen in a basic environment. Therefore, $Y > 7$.
Since, $X < 7$ and $Y > 7$, therefore, $Y > X$. According to the given option, only option C fulfils these three requirements.
Therefore, the correct option is C.

Note:
Many reactions in organic chemistry are redox reactions due to changes in oxidation states but without distinct electron transfer. Although oxidation reactions are commonly associated with the formation of oxides from oxygen molecules, oxygen is not necessarily included in such reactions, as other chemical species can serve the same function. Analysis of bond energies and ionization energies in water allow calculation of the redox potentials.