Question

Assign an oxidation number to each element in the reaction.

Answer 1

CaF2(s) + H2SO4(aq) → CaSO4(aq) + 2HF(g)

In CaF2, the oxidation number of Ca is +2, and that of  F is -1

In H2SO4, the oxidation number of H is +1, that of S is +6, and that of O is -2

In CaSO4, the oxidation number of Ca is +2, that of S is +6, that of S is -2

In HF, the oxidation number of H is +1, and that of F is -1

Answer 2

Ans-A
In CaF2, the oxidation number of Ca is +2,& that of F is -1.

Ans-B
In H2SO4, the oxidation number of H is +1, that of S is +6,& that of O is -2.

Ans-C
In CaSO4, the oxidation number of Ca is +2, that of S is +6,& that of O is -2.

Ans-D
In HF, the oxidation number of H is +1,& that of F is -1.

Explanation:
Oxidation number is a number that is assigned to an element in a compound, which shows the number of electrons gained or lost by an atom.

Rules:
1. If the element is ALONE in the chemical equation, and it is NOT an ION, its oxidation number will ALWAYS be zero.

2. When there is an ION in the equation, its oxidation number will ALWAYS be its ionic number.

A(2 parts):
The oxidation number of Ca & F?

In what group Ca lies? Well in group 2 in the periodic table.
Is it alone? No, it is not. It's with Florine F.
Is it multiple of itself? No.

The second element is Florine(F).
In what group F lies? Well in group 17(in halogens) in the periodic table.
Is it multiple of itself? Yes. There are 2 Florine atoms.

Is there any net charge on the compound? No.

Blank 2: The oxidation number of F2 = 2 * (-1) = -2. Since the oxidation number of the single atom F is -1 for the halogens(group-17 members). In this compound there are 2 Florine atoms, therefore it should be -2. Although the F2 has the oxidation number of -2, the single atom of F has the oxidation number -1.

Blank 1: The oxidation number of Calcium= +2. Since the oxidation number of the single atom Ca is +2 for the group-2 members. Also, we can recheck it by using the Rule-2, mentioned above, but for compound. As I mentioned before, the net charge on the CaF2 is zero; therefore, the sum of the oxidation number of Ca and that of F2 has to be zero. Since the oxidation number of F2 is -2, the oxidation number of Ca has to be +2 to make the net charge equals to zero. Therefore, the oxidation number of Ca is +2.

B(3 parts):
The oxidation number of H, S & O?

In what group Hydrogen(H) lies? Well in group 1 in the periodic table.
Is it alone? No, it is not. It's with Sulfur(S) and Oxygen(O) .
Is it multiple of itself? Yes. It's H2; therefore, there are two hydrogen atoms.

The second element is Sulfur(S).
In what group S lies? Well in group 16(in chalcogens) in the periodic table.
Is it multiple of itself? No.

The third element is Oxygen(O).
In what group Oxygen(O) lies? Well in group-16 in the periodic table.
Is it alone? No, it is not. It's with Sulfur(S) and Hydrogen(H) .
Is it multiple of itself? Yes. It's O4; therefore, there are four oxygen atoms.

Is there any net charge on the compound? No.

Blank 3: The oxidation number of O4 = 4 * (-2) = -8. Since the oxidation number of the single atom O is -2 for the chalcogens (group-16 members). In this compound there are 4 Oxygen atoms, therefore it should be -8. Although the O4 has the oxidation number of -8, the single atom of O has the oxidation number -2.

Blank 1: The oxidation number of H2 = 2 * (+1) = +2. Since the oxidation number of the single atom H is +1 for the group-1 members. In this compound there are 2 hydrogen atoms, therefore it should be +2. Although the H2 has the oxidation number of +2, the single atom of H has the oxidation number +1.

Blank 2: As I mentioned before, the net charge on the H2SO4 is zero; therefore, the sum of the oxidation number of H2, S and that of O4 has to be zero. Since the oxidation number of H2 is +2, and the oxidation number of O4 is -8, the oxidation number of Sulfur has to be +6 to make the net charge equals to zero. Therefore, the oxidation number of S is +6.

C(3 parts):

Blank 3: The oxidation number of O4 = 4 * (-2) = -8. Since the oxidation number of the single atom O is -2 for the chalcogens (group-16 members). In this compound there are 4 Oxygen atoms, therefore it should be -8. Although the O4 has the oxidation number of -8, the single atom of O has the oxidation number -2.

Blank 1: The oxidation number of Calcium= +2. Since the oxidation number of the single atom Ca is +2 for the group-2 members. Although Sulfur in the compound is also a single element, but as Calcium comes first, therefore, we would consider Ca as an independent element. Hence, Ca has the oxidation number +2.

Blank 2: As I mentioned before, the net charge on the CaSO4 is zero; therefore, the sum of the oxidation number of Ca, S and that of O4 has to be zero. Since the oxidation number of Ca is +2, and the oxidation number of O4 is -8, the oxidation number of Sulfur has to be +6 to make the net charge equals to zero. Therefore, the oxidation number of S is +6.

D(2 parts):

Blank 2: The oxidation number of F =  -1. Since the oxidation number of the single atom F is -1 for the halogens(group-17 members).

Blank 1: The oxidation number of Hydrogen H = +1. As I mentioned before, the net charge on the HF is zero; therefore, the sum of the oxidation number of H and that of F has to be zero. Since the oxidation number of F is -1, the oxidation number of H has to be +1 to make the net charge equals to zero. Therefore, the oxidation number of H is +1.