How do you find the Oxidation Number?

Daily Dose 001 | Chemistry

How do you find the Oxidation Number?

Understanding how to find the Oxidation Number is crucial in CHEMISTRY, as it will help us determine the electron transfer that occurs during chemical reactions that we will see on the FE Exam.

Though finding the Oxidation number may seem challenging at first, understanding that the process can be simplified and replicated across the most complex scenarios will ease any potential negative anticipation to seeing them on your exam.

Key Definition

What is the OXIDATION NUMBER?

The OXIDATION NUMBER is a tool that we use to keep track of electrons as they move between molecules, atoms, and ions in chemical reactions, most commonly as we will see in oxidation-reduction reactions, or redox reactions.

So the Oxidation Number is a tool.

A good analogy to help grasp the concept of the purpose of Oxidation Numbers is to picture them in terms of a bookkeeper for a business.

A bookkeeper is responsible for keeping track of the money coming in to and going out of the business accounts…the Oxidation Number is the bookkeeper of chemical reactions…keeping track of the electrons going in to and out of eat element as they create new molecules through chemical reactions.

There are some general rules we can establish to help us identify the OXIDATION NUMBER or OXIDATION STATE of an element within a molecule.

The Process

How can we find the Oxidation Number?

Finding the Oxidation Number may seem intimidating at first, but the following steps will allow you to quickly and efficiently solve this type of Chemistry problem on the FE Exam:

• Step 1: Know the Basics

Oxidation numbers represent the hypothetical charge an atom would have if electrons were completely transferred. Here are a few fundamental rules to keep in mind:

• The oxidation number of an uncombined element is always zero.
• The sum of the oxidation numbers of all atoms in a neutral compound is zero.
• The sum of the oxidation numbers of all atoms in an ion is equal to the ion’s charge.
• Some elements have fixed oxidation numbers: hydrogen is typically +1, oxygen is usually -2, and the alkali metals (Group 1) are +1, among others.
• Step 2: Analyze the Compound

When approaching a compound or ion, identify any elements with known oxidation numbers.

For example, oxygen is commonly -2, and hydrogen is usually +1.

• Step 3: Assign Variables and Apply Rules

Now, let’s assign variables to unknown oxidation numbers.

Consider the following guidelines:

• Group 1 elements (alkali metals) generally have a +1 oxidation number.
• Group 2 elements (alkaline earth metals) commonly have a +2 oxidation number.
• Fluorine (F) is always assigned an oxidation number of -1.
• Hydrogen (H) typically has an oxidation number of +1, except when it forms hydrides (-1).
• Oxygen (O) usually has an oxidation number of -2, unless it forms a peroxide (-1) or superoxide (-1/2).
• The sum of oxidation numbers in a neutral compound must be zero.
• Step 4: Solve for Unknowns

Using the assigned variables, apply the rules above and solve for the unknown oxidation numbers by considering the overall charge of the compound or ion.

Here are a few quick examples:

Example 1: Find the oxidation number of chlorine (Cl) in HCl.

• Given: Hydrogen (H) has a known oxidation number of +1.
• Solution: Since the compound is neutral, the sum of oxidation numbers must be zero. Let’s assume the oxidation number of chlorine is ‘x’. Thus, +1 (from H) + x = 0. Solving for ‘x’, we find that chlorine (Cl) has an oxidation number of -1.

Example 2: Find the oxidation number of sulfur (S) in H2SO4.

• Given: Hydrogen (H) has a known oxidation number of +1, and oxygen (O) is typically -2.
• Solution: The compound is neutral, so the sum of oxidation numbers must be zero. Let’s assume the oxidation number of sulfur is ‘x’. The equation becomes 2(+1) + x + 4(-2) = 0. Simplifying, we get +2 + x – 8 = 0. Solving for ‘x’, we find that sulfur (S) has an oxidation number of +6.
• Step 5: Verify and Check

Once you’ve determined the oxidation numbers, double-check your work. Make sure the sum of the oxidation numbers equals the compound’s charge for ions or zero for neutral compounds.

Check out the video and see how to find the Oxidation Number in the most efficient manner come your FE Exam day.

As always, we are here to help, Prepineer

Video Review

[VIDEO] How to find the OXIDATION NUMBER