Which reactions are redox reactions?
(a) Ba(NO3)2(aq) + K2SO(aq) → BaSO4(s) + 2 KNO3(aq)
(b) Ca(s) + Cl2(g) → CaCl2(s)
(c) HCl(aq) + NaOH(aq) → H2O(l) + NaCl(aq)
(d) Zn(s) + Fe2+(aq) → Zn2+(aq) + Fe(s)
First let’s see what is a redox reactions :
An oxidation-reduction (redox) reaction is a type of chemical reaction that involves a transfer of electrons between two species. Redox is a chemical reaction which involves both a reduction process and a complementary oxidation process, in which the oxidation states of atoms are changed.
We identify the reaction if its redox by the change in oxidation numbers. If an atom’s oxidation number increases, it is an oxidation reaction and if an atom’s oxidation number decreases, then it is a reduction reaction.
Now, let’s see rules for assigning oxidation numbers :
1. By convention the cation is written first in a formula, followed by the anion.
Example : in NaH, the H is H- , in HCl, the H is H+.
2. The oxidation number of a free element is always 0. This is also true for elements found in nature as diatomic (two-atom) elements such as H2, O2, N2, F2, Cl2, Br2 or I2
Example : The atoms in He and N2 have oxidation numbers of 0.
3. The oxidation number of a monatomic ion equals the charge of the ion.
Example : The oxidation number of Na+ is +1;
The oxidation number of N3- is -3.
4. The usual oxidation number of hydrogen is +1.
The oxidation number of hydrogen is -1 in compounds containing elements that are less electronegative than hydrogen, as in CaH2.
5. The oxidation number of oxygen in compounds is usually -2. Exceptions include OF2, since F is more electronegative than O, and BaO2, due to the structure of the peroxide ion, which is [O-O]2-.
6. The oxidation number of a Group IA element in a compound is +1.
7. The oxidation number of a Group IIA element in a compound is +2.
8. The oxidation number of a Group VIIA element in a compound is -1, except when that element is combined with one having a higher electronegativity.
The oxidation number of Cl is -1 in HCl, but the oxidation number of Cl is +1 in HOCl.
9. The sum of the oxidation numbers of all of the atoms in a neutral compound is 0.
10. The sum of the oxidation numbers in a polyatomic ion is equal to the charge of the ion.
For example, the sum of the oxidation numbers for SO42- is -2.