What are redox or oxidation numbers?
Redox comes from the combination of the word reduction and oxidation. The first thing we have to learn in this lesson is what they call the redox numbers. They are numbers that are assigned, much like ions. In fact, they mean the same thing as the ion charges but they are assigned to different elements in a slightly different way. So if I say an element has a redox number of +2, then that means the element is positively charged and is therefore missing 2 electrons. Look at the picture below for the rules to assign redox number to different elements in different compounds or molecules.
The order that I listed these rules REALLY DOES MATTER. The reason why is if you have a conflict between the rules you automatically go with the rule that is higher (or more toward the beginning) on the list. The only way to understand these rules is to test them out and see what they really mean. These rules are meant to be used if you are given some kind of compound, molecule, or element. Just like the when we were figuring out ionic compounds before, the compound should add up to zero unless the entire compound or element is charged. Make sure you open up a separate web page with my rules in the center. This way you can look back at the redox rules as you are looking through the example problems.
How do I use redox or oxidation numbers?
I also organize my redox numbers as I apply the rules to each compound I come across. Above each element in the compound I put the INDIVIDUAL redox number (the redox number for only one of that element). Below each element in the compound I put the TOTAL redox number (individual redox number multiplied by amount of that element). To the RIGHT of the total redox numbers I put what they should all add up to. This is shown in the redox number table below. If you don’t understand it right away try looking at the examples that follow.
| Individual = | +2 | +4 | -2 | |
| Mg | C | O3 | ||
| Total = | +2 | +4 | – 6 | = zero |
VIDEO Redox Numbers Demonstrated Example 1: What are the redox numbers for each element in the molecule below? Use the redox rules.
H2O
Is it a compound or individual element?
Answer: Compound, therefore set up a redox number table
| Individual = | |||
| H2 | O | ||
| Total = |
What do I fill in first in the table?
Answer: The entire compound adds up to zero because it does not show a charge.
| Individual = | |||
| H2 | O | ||
| Total = | = zero |
What is the first of my rules that apply?
Answer: Rule 3 Oxygen = -2
| Individual = | -2 | ||
| H2 | O | ||
| Total = | = zero |
How much oxygen do you have?
Answer: 1, So 1 * -2 = -2
| Individual = | -2 | ||
| H2 | O | ||
| Total = | -2 | = zero |
What do all the hydrogen have to add up to?
Answer: +2, because the total O are -2 and the compound is a zero charge. (+2 + -2 = zero)
| Individual = | -2 | ||
| H2 | O | ||
| Total = | +2 | -2 | = zero |
What is the charge on each hydrogen?
Answer: (total H charge / amount of H) = +2 / 2 = +1
| Individual = | +1 | -2 | |
| H2 | O | ||
| Total = | +2 | -2 | = zero |
What is the redox number for each element?
COMPLETE ANSWER: (the individual numbers) H = +1, O = -2
VIDEO Redox Numbers Demonstrated Example 2: What are the redox numbers for each element in the molecule below? Use the redox rules.
H2SO4
Is it a compound or individual element?
Answer: Compound, therefore set up a redox number table
| Individual = | ||||
| H2 | S | O4 | ||
| Total = |
What do I fill in first in the table?
Answer: The entire compound adds up to zero because it does not show a charge.
| Individual = | ||||
| H2 | S | O4 | ||
| Total = | = zero |
What is the first of my rules that apply?
Answer: Rule 3 Oxygen = -2
| Individual = | -2 | |||
| H2 | S | O4 | ||
| Total = | = zero |
How much oxygen do we have?
Answer: 4, So 4 * -2 = -8
| Individual = | -2 | |||
| H2 | S | O4 | ||
| Total = | -8 | = zero |
What is the next rule that applies?
Answer: Rule 4 First column of the periodic table = +1 (H = +1)
| Individual = | +1 | -2 | ||
| H2 | S | O4 | ||
| Total = | -8 | = zero |
How much hydrogen do we have?
Answer: 2, So 2 * +1 = +2
| Individual = | +1 | -2 | ||
| H2 | S | O4 | ||
| Total = | +2 | -8 | = zero |
What do all the sulfur have to add up to?
Answer: +6, because the total H are +2 and the total O are -8 and the compound is a zero charge. (+2 + +6 + -8 = zero)
| Individual = | +1 | -2 | ||
| H2 | S | O4 | ||
| Total = | +2 | +6 | -8 | = zero |
What is the charge on each sulfur?
Answer: (Total S charge / amount of S) = +6 / 1 = +6
| Individual = | +1 | +6 | -2 | |
| H2 | S | O4 | ||
| Total = | +2 | +6 | -8 | = zero |
What are the redox numbers for each element?
COMPLETE ANSWER: (the individual numbers) H = +1, S = +6, O = -2
VIDEO Redox Numbers Demonstrated Example 3: What are the redox numbers for each element in the molecule below? Use the redox rules.
NaBrO3
Is it a compound or individual element?
Answer: Compound, therefore set up a redox number table
| Individual = | ||||
| Na | Br | O3 | ||
| Total = |
What do I fill in first in the table?
Answer: The entire compound adds up to zero because it does not show a charge.
| Individual = | ||||
| Na | Br | O3 | ||
| Total = | = zero |
What rule applies first?
Answer: Rule 3 Oxygen = -2
| Individual = | -2 | |||
| Na | Br | O3 | ||
| Total = | = zero |
How much oxygen do we have?
Answer: 3, So 3 * -2 = -6
| Individual = | -2 | |||
| Na | Br | O3 | ||
| Total = | -6 | = zero |
What is the next rule that applies?
Answer: Rule 4 First column of the periodic table = +1 (Na = +1)
| Individual = | +1 | -2 | ||
| Na | Br | O3 | ||
| Total = | -6 | = zero |
How much sodium do we have?
Answer: 1, So 1 * +1 = +1
| Individual = | +1 | -2 | ||
| Na | Br | O3 | ||
| Total = | +1 | -6 | = zero |
What is the total charge of all the Cl?
Answer: Cl is +5, because the total Na are +1 and the total O are -6 and the compound is a zero charge. (+1 + +5 + -6 = zero)
| Individual = | +1 | -2 | ||
| Na | Br | O3 | ||
| Total = | +1 | +5 | -6 | = zero |
What is the charge of each Cl?
Answer: (Total Cl charge / amount of Cl) = +5 / 1 = +5
| Individual = | +1 | +5 | -2 | |
| Na | Br | O3 | ||
| Total = | +1 | +5 | -6 | = zero |
What are the redox numbers for each element?
COMPLETE ANSWER: (the individual numbers) Na = +1, Cl = +5, O = -2
Notice the last element in the compound that I assign a charge to no longer has to follow my rules. The last element I assign a charge to simply has to be a charge or redox number that makes the whole compound add up to zero or whatever the total charge of the compound is.
VIDEO Redox Numbers Demonstrated Example 4: What are the redox numbers for each element in the molecule below? Use the redox rules.
SO32-
Is it a compound or individual element?
Answer: Compound, therefore set up a redox number table
| Individual = | |||
| S | O3 | ||
| Total = |
What do I fill in first in the table?
Answer: The entire compound adds up to -2.
| Individual = | |||
| S | O3 | ||
| Total = | = -2 |
What is the first rule that we apply?
Answer: Rule 3Oxygen = -2
| Individual = | -2 | ||
| S | O3 | ||
| Total = | = -2 |
How much oxygen do we have?
Answer: 3, So 3 * -2 = -6
| Individual = | -2 | ||
| S | O3 | ||
| Total = | -6 | = -2 |
What is the total charge of all the S?
Answer: Cl is +4, because the total O are -6 and the compound is a -2 charge. (+4 + -6 = -2)
| Individual = | -2 | ||
| S | O3 | ||
| Total = | +4 | -6 | = -2 |
What is the charge of each Cl?
Answer: (Total S charge / amount of S) = +4 / 1 = +4
| Individual = | +4 | -2 | |
| S | O3 | ||
| Total = | +4 | -6 | = -2 |
What are the redox numbers for each element?
COMPLETE ANSWER: S = +4, O = -2
VIDEO Redox Numbers Demonstrated Example 5: What are the redox numbers for each element in the molecule below? Use the redox rules.
Fe2+
Is it a compound or individual element?
Answer: Element, therefore we DO NOT set up a redox number table
What is the first rule that applies?
Answer: Rule 1 A charged element = the redox number of its charge
What is the charge on Fe?
Answer: +2
What are the redox numbers for each element?
COMPLETE ANSWER: Fe = +2
VIDEO Redox Numbers Demonstrated Example 6: What are the redox numbers for each element in the molecule below? Use the redox rules.
Br2
Is it a compound or individual element?
Answer: Element, therefore we DO NOT set up a redox number table
What is the first rule that we can apply?
Answer: Rule 2 Elements alone = zero
What is the charge of Br?
Answer: zero
What are the redox numbers for each element?
COMPLETE ANSWER: Br= Zero
PRACTICE PROBLEMS: Give the redox numbers for each element in the compound.
| CH4 | C = -4, H = +1 |
| O2 | O = zero |
| CaCO3 | Ca = +2, C = +4, O = -2 |
| NO3– | N = +5, O = -2 |
| Cr2O72- | Cr = +6, O = -2 |
| BBr3 | B = +3, Br= -1 |
| Pb4+ | Pb = +4 |
| N2O2F2 | N = +3, O = -2, F= -1 |