Choose the item that correctly describes the oxidizing agent and the reducing ag
ID: 1006609 • Letter: C
Question
Choose the item that correctly describes the oxidizing agent and the reducing agent in the reaction shown. CaS(s) + 4H2O2(aq) CaSO4(s) + 4H2O()
A. S changes from oxidation number -2 to +6. The CaS is the reducing agent.
O changes from oxidation number 0 to -2. The H2O2 is the oxidizing agent.
B. S changes from oxidation number -2 to +4. The CaS is the reducing agent.
O changes from oxidation number -1 to -2. The H2O2 is the oxidizing agent.
C. S changes from oxidation number -2 to +6. The CaS is the reducing agent.
O changes from oxidation number -1 to -2. The H2O2 is the oxidizing agent.
D. S changes from oxidation number -2 to +4. The CaS is the reducing agent.
O changes from oxidation number -0 to -2. The H2O2 is the oxidizing agent.
Explanation / Answer
Solution:-
The given equation is...
CaS(s) + 4H2O2(aq) ----------> CaSO4(s) + 4H2O(l)
Let's calculate the oxidation states for all the atoms on both sides by using oxidation number rules.
(1) oxidation number of second group metals is +2 in their compounds and the sum of oxidation numbers of all the atoms of a compound is 0.
So, oxidation number of Ca in CaS is +2 and that of S is -2.
(2) in peroxides the oxidation number of oxygen is -1. H2O2 is hydrogen peroxide. So, oxidation number of O is -1 and that of H is +1 in H2O2.
Oxidation number of Ca in CaSO4 is +2, oxidation number of oxygen in its compounds is -2. So, the oxidation number of oxygen in CaSO4 is -2. Now we could calculate the oxidation number of S by using algebra.
Let's say the oxidation number of S in CaSO4 is X, then......
+2 + X + 4(-2) = 0
2 + X - 8 = 0
X - 6 = 0
X = 6
So, oxidation number of S in CaSO4 is +6.
Oxidation number of O in H2O is -2 and that of H is +1.
So, what we notice is, S is changing from -2 to +6 and also CaS is acting as a reducing agent.
H2O2 is acting as the oxidizing agent but since the oxidation number of O is changing from -1 to -2 but the statement says 0 to -2 so it is not correct.
Now, let's check statement B.
B is also not correct since it says, S is changing from -2 to +4.
Let's check statement C.
Statement C is perfect as it is in accordance what we notice. S is changing from -2 to +6 and CaS is the reducing agent. O is changing from -1 to -2 and H2O2 is the oxidizing agent.
D is also not correct since the oxidation numbers are not same that we have calculated.
So, the only correct statement is (C).