Question

The chemical reaction between two substances is characterised by change in colour from orange to green, these two substances are most likely to be :
A) Potassium dichromate solution and sulphur dioxide.
B) Potassium permanganate solution and sulphur dioxide.
C) Potassium permanganate solution and lemon juice.
D) Potassium dichromate solution and carbon dioxide.

Answer 1

Hint: The Potassium permanganate ($KMn{O_4}$) solution is dark purple. So, the possibility of a chemical reaction between $KMn{O_4}$ and sulfur dioxide; and $KMn{O_4}$ and lemon juice should be ruled out. Also, the Potassium dichromate ${K_2}C{r_2}{O_7}$ solution is orange in color.

Complete step by step answer:
The reaction between Potassium dichromate ${K_2}C{r_2}{O_7}$ solution and sulphur dioxide is given as
${K_2}C{r_2}{O_7} + 3S{O_2} + {H_2}S{O_4} \to C{r_2}{(S{O_4})_3} + {K_2}S{O_4} + {H_2}O$
Here, the color $C{r_2}{(S{O_4})_3}$ is green.
The reaction between Potassium dichromate (${K_2}C{r_2}{O_7}$) solution and carbon dioxide does not take place as ${K_2}C{r_2}{O_7}$ acts as a catalyst without changing itself.

The reaction between Potassium permanganate $KMn{O_4}$ solution and sulphur
dioxide $S{O_2}$ is a given as –
$2KMn{O_4} + 3S{O_2} + 3{H_2}O \to {K_2}O + 3Mn{O_2} + 3{H_2}O$
To find the Oxidation States of the reactant and product of the reaction between Potassium dichromate ${K_2}C{r_2}{O_7}$ solution and sulphur dioxide,
The oxidation state of Cr in Potassium dichromate ${K_2}C{r_2}{O_7}$ is $ + 6$ as follows,
$2(K) + 2(Cr) + 7(O) = 0$
$2( + 1) + 2(Cr) + 7( - 2) = 0$
$2 + 2(Cr) - 14 = 0$
$2(Cr) = 12$
$Cr = + 6$
Hence, the oxidation state of Chromium in Potassium dichromate ${K_2}C{r_2}{O_7}$ is $ + 6$
B) Now, the oxidation state of Chromium in Chromic sulphate $C{r_2}{(S{O_4})_3}$ is $ + 3$
$2(Cr) + 3(S{O_4}) = 0$
$2(Cr) + 3( - 2) = 0$
$Cr = + 3$
Hence, the oxidation state of chromium in Chromic sulphate $C{r_2}{(S{O_4})_3}$ is $ + 3$
Now, the color of Cr ($ + 6$) ion is orange and the color of Cr ($ + 3$) ion is green
According to the properties of d and f block elements, The two substances changing the color of the solution from orange to green are potassium dichromate solution and sulphur dioxide.

Hence, The answer to the question is Option (A) potassium dichromate solution and sulphur dioxide.

Note: To find gram molecular mass atomic mass of each element should be noticed. Multiply the number of atoms by the element’s atomic mass. If there is no number of atoms mentioned,it means there is only one atom of that element in the molecule. To find the gram molecular mass must add all of the values together.