How to Calculate Oxidation Numbers Introduction

Summary

This educational video script offers a detailed guide on determining oxidation numbers in chemical compounds. It outlines basic rules, such as elements in their pure form having an oxidation number of 0, and monatomic ions having an oxidation number equal to their ion charge. The script progresses to examples like KCl, MgO, CO, NH3, and NO2-, explaining how to apply these rules and the importance of the sum of oxidation numbers equaling zero in neutral compounds. It also covers exceptions, like halogens with oxygen, and ends with a teaser for more practice problems in a follow-up video.

Takeaways

• 📚 The video explains how to determine oxidation numbers for elements in a chemical compound.
• 🔢 An element by itself has an oxidation number of 0.
• ⚡ Monatomic ions have an oxidation number equal to their ion charge.
• 📐 The sum of oxidation numbers in a neutral compound equals 0.
• 📋 Group 1A elements (like potassium) have an oxidation number of +1.
• 🌀 Halogens usually have an oxidation number of -1, unless combined with oxygen.
• 💡 For compounds like CO, use the known oxidation number of oxygen (-2) to find the unknown (carbon in this case).
• 🌿 Hydrogen typically has a +1 oxidation number when combined with nonmetals.
• 🔄 For polyatomic ions, the sum of oxidation numbers equals the ion charge.
• 🧪 In challenging examples like BrO3-, halogens can have positive oxidation numbers when paired with oxygen.
• 📈 Practice is key to mastering the process of determining oxidation numbers.

Q & A

What is the oxidation number of an element by itself?

The oxidation number of an element by itself is always 0.

What is the oxidation number of monatomic ions?

The oxidation number of monatomic ions is the same as their ion charge.

How do you write oxidation numbers?

Oxidation numbers are written with the sign first (positive or negative) followed by the number.

What is the sum of oxidation numbers for a neutral compound?

The sum of oxidation numbers for a neutral compound equals 0.

What is the oxidation number of potassium (K) in the compound KCl?

The oxidation number of potassium (K) in KCl is +1.

What is the oxidation number of chlorine (Cl) in KCl?

The oxidation number of chlorine (Cl) in KCl is -1.

Why is the oxidation number of hydrogen in NH3 +1?

Hydrogen is +1 with nonmetals, and nitrogen is a nonmetal, so hydrogen in NH3 has an oxidation number of +1.

What is the oxidation number of nitrogen in NH3?

The oxidation number of nitrogen in NH3 is -3, calculated by the rule that the sum of oxidation numbers in a neutral compound must be 0.

How do you determine the oxidation number of carbon in CO?

The oxidation number of carbon in CO is +2, determined by the rule that the sum of oxidation numbers in a neutral compound must be 0, with oxygen being -2.

What is the oxidation number of calcium in CaC2?

The oxidation number of calcium in CaC2 is +2, calculated by the rule that the sum of oxidation numbers in a neutral compound must be 0, with each carbon being -1.

What is the oxidation number of nitrogen in NO2-?

The oxidation number of nitrogen in NO2- is +3, determined by the rule that the sum of oxidation numbers in a polyatomic ion equals the ion charge.

Why is the oxidation number of bromine in BrO3- not -1?

The oxidation number of bromine in BrO3- is +5, not -1, because it is paired with oxygen, which changes the typical oxidation number for halogens.

What is the oxidation number of chlorine in ClF3?

The oxidation number of chlorine in ClF3 is +3, determined by the rule that fluorine is always -1, and the sum of oxidation numbers in a neutral compound must be 0.

Outlines

00:00

🔢 Understanding Oxidation Numbers

The paragraph introduces the concept of oxidation numbers, explaining their significance in chemistry. It outlines the rules for determining oxidation numbers for elements in chemical compounds. The video promises to guide viewers through these rules, starting with the basics such as elements in their pure form having an oxidation number of zero. It also covers monatomic ions, where the oxidation number equals the ion charge. The paragraph sets the stage for working through practice problems to solidify understanding.

05:01

🧪 Applying Oxidation Number Rules

This section delves into applying the rules for oxidation numbers to specific compounds. It uses examples like KCl, MgO, CO, NH3, and CaC2 to demonstrate how to assign oxidation numbers based on group affiliations in the periodic table and the sum of oxidation numbers in neutral compounds. The paragraph also introduces the concept of polyatomic ions and how their oxidation numbers relate to their charge, using NO2- as an example. The discussion reinforces the importance of practice in mastering the calculation of oxidation numbers.

10:03

🌟 Advanced Oxidation Number Calculations

The final paragraph tackles more complex scenarios involving elements like zinc, bromine, and chlorine. It explains how to determine oxidation numbers for elements that do not have straightforward rules, such as halogens when combined with oxygen or other halogens. The examples include ZnH2, BrO3-, and ClF3, where the video script clarifies exceptions to the general rules and emphasizes the need for careful consideration of compound neutrality and ion charges. The paragraph concludes by encouraging further practice to enhance proficiency in calculating oxidation numbers.

Keywords

💡Oxidation Numbers

Oxidation numbers, also known as oxidation states, are a way of keeping track of the number of electrons that atoms in a molecule, ion, or elemental form 'appear' to have. In the video, the concept is central to understanding how to calculate and assign these numbers to different elements in a chemical compound. For instance, the video explains that an element by itself has an oxidation number of 0, and monatomic ions have an oxidation number equal to their ion charge.

💡Element by itself

The video uses the term 'element by itself' to refer to a chemical element that is not combined with any other element. For example, Cl- as an element by itself has an oxidation number of 0. This concept is foundational for understanding how oxidation numbers work, as it establishes a baseline for elements in their pure form.

💡Monatomic Ions

Monatomic ions are ions that consist of a single atom. The video emphasizes that the oxidation number of a monatomic ion is the same as its ion charge. For example, K+ has an oxidation number of +1, and N3- has an oxidation number of -3. This rule is crucial for calculating the oxidation numbers of elements in ionic compounds.

💡Neutral Compound

A neutral compound is a chemical compound that is electrically neutral; it has no overall charge. The video explains that the sum of oxidation numbers for a neutral compound equals 0. This rule is used to solve for unknown oxidation numbers in compounds like KCl and MgO.

💡Group 1A Elements

Group 1A elements, also known as the alkali metals, are located in the first column of the periodic table. The video mentions that these elements always have an oxidation state of +1, as seen with potassium (K) in KCl. Understanding the oxidation states of elements based on their group affiliation is key to determining oxidation numbers.

💡Halogens

Halogens are a group of nonmetal elements in the periodic table. The video explains that halogens usually have an oxidation state of -1, except when combined with oxygen or fluorine. For example, chlorine (Cl) in KCl has an oxidation number of -1.

💡Peroxide

Peroxides are compounds containing an ion of oxygen in which the oxidation state of oxygen is -1. The video clarifies that oxygen typically has an oxidation state of -2, except in peroxides like H2O2, where it is -1. This exception is important for correctly assigning oxidation numbers in peroxides.

💡Polyatomic Ion

A polyatomic ion is a group of two or more atoms that are chemically bonded together and have a net charge. The video explains that the sum of oxidation numbers for a polyatomic ion equals the ion charge, as seen with NO2- where nitrogen has an oxidation number of +3.

💡Ionic Charge

The ionic charge refers to the electrical charge of an ion. The video points out that when writing oxidation numbers, the sign (positive or negative) is written first, followed by the number, which is the opposite of how ionic charges are written. This distinction is important for clarity when dealing with ions.

💡Sum of Oxidation Numbers

The sum of oxidation numbers is a principle used to determine the oxidation state of an unknown element in a compound. The video uses this principle to solve for the oxidation numbers of elements in compounds like CO, where the sum of the oxidation numbers of carbon and oxygen must equal zero.

💡Practice Problems

Practice problems are used throughout the video to illustrate how to apply the rules for determining oxidation numbers. These problems serve as examples to help viewers understand and apply the concepts discussed, such as calculating the oxidation number of nitrogen in NH3 or zinc in ZnH2.

Highlights

Oxidation numbers represent the charge an element has in a compound.

An element by itself has an oxidation number of 0.

Monatomic ions have an oxidation number equal to their ion charge.

The sum of oxidation numbers in a neutral compound equals 0.

Elements in group 1a have an oxidation number of +1.

Halogens usually have an oxidation number of -1, except with oxygen.

Oxygen typically has an oxidation number of -2, except in peroxides.

Hydrogen has a +1 oxidation number when combined with nonmetals.

In polyatomic ions, the sum of oxidation numbers equals the ion charge.

Carbon monoxide is an example of how to calculate oxidation numbers.

Ammonia (NH3) demonstrates calculating oxidation numbers for nitrogen.

Calcium carbonate (CaCO3) shows how to divide oxidation numbers among multiple atoms.

NO2- ion example illustrates calculating oxidation numbers for nitrogen and oxygen.

HNO3 example shows how to determine oxidation numbers for hydrogen, nitrogen, and oxygen.

Zinc hydride (ZnH2) example explains how to find oxidation numbers for metals with hydrogen.

Bromate ion (BrO3-) is an exception where halogens can have a positive oxidation number.

Chlorine trifluoride (ClF3) example shows how halogens can have different oxidation numbers.

Fluorine is always -1, regardless of the compound it's in.

Practice problems are recommended to improve understanding of oxidation numbers.

Transcripts

in this video we're going to learn how

to figure out the oxidation numbers for

the different elements in a chemical

compound the oxidation numbers are the

numbers that I've written here above

each one of the elements now if you want

to learn more about what oxidation

numbers are or why they're important

check out my video called what are

oxidation numbers in this video we're

going to work through the process of how

you figure out what these numbers are so

here are the rules that we're going to

use to figure out oxidation numbers now

just so you know every teacher and

textbook has their own version of these

rules but they all work in pretty much

the same way so if you learn mine you'll

still get the answer right 100% of the

time even if these are a little bit

different from your teachers I'm going

to talk through a few of these rules

right now and then I'll introduce the

rest just as we work through practice

problems the first is this an element by

itself always has an oxidation number of

0 here's what I mean by that there are a

lot of chemical compounds that have just

one element that element is not combined

with any other elements that's what I'm

calling an element by itself so that's

something like CL - it doesn't matter

how many atoms of that element you have

just as long as it's only that element

and none others so CL - an element by

itself oxidation number of 0 sodium na

by itself 0 s 6 just sulfur nothing else

P for phosphorous and nothing else

zero so that's probably the easiest rule

here it's always zero for the oxidation

state if you have an element by itself

with nothing else the other rule here is

about monatomic ions these are ions that

are made of only one and so like this

for monatomic ions their oxidation

number is the same as their ion charge

so for k+ here it's oxidation number is

going to be plus 1 for n 3 - it'll have

an oxidation number of minus 3 and

mg 2 plus here is going to have an

oxidation number of plus 2 now keep this

in mind when we write oxidation numbers

we write this the sign first so plus

minus and then the number after this is

the opposite of how we write ionic

charges so just keep that in mind the

charge might be 2 Plus on magnesium but

the oxidation number is plus 2 now for

the rest of these rules we usually use

more than one together so I'll just talk

about these as we use them in example

problems here is our first example KCl

we want to figure out the oxidation

numbers of the elements in this compound

ok so let's take a look at our rules

here K potassium is in this column in

the periodic table it's in group 1a so

there's this rule that elements in group

1a are always plus 1 so that is

potassium Jacque sedation number then we

have CL over here that is one of the

halogens halogens usually minus 1

positive with oxygen well CL certainly

isn't with oxygen here so we'll give it

a minus 1 oxidation number now I want to

use this to show you a third rule that's

this rule right here the sum of

oxidation numbers for a neutral compound

equals 0 KCl is a neutral compound here

it doesn't have a charge after so that

means that the sum of these oxidation

numbers is going to equal 0 and that's

definitely true here plus 1 minus 1

equals 0

we'll use this later on when we do other

examples but just keep that in mind the

sum of oxidation numbers for a neutral

compound should always equal 0 mg o mg

is in this column in the periodic table

group 2a

and group 2a elements are always plus 2

so there's that oxygen here we have a

rule for oxygen

it is usually minus 2 it is minus 1

in peroxide h2o2 hydrogen peroxide is

the most common peroxide is probably the

only one you'll ever see but anyway this

oxygen is definitely not in hydrogen

peroxide so it's fair to say that it's

oxidation number will be minus 2 plus 2

4 mg minus 2 4 oh and they add together

to make 0 because this is a neutral

compound co carbon monoxide let's figure

this one out ok so see here there isn't

any rule for carbon so we'll have to

figure it out based on what we do know

okay so we do know oxygen oxygen is

usually minus 2 unless we're in a

peroxide definitely not a peroxide so we

can safely say that oxygens oxidation

number is minus 2 now let's use this

other piece of information that we know

and that's the sum of oxidation numbers

for a neutral compound should equal 0

so whatever carbons oxidation number is

should add together with oxygens to make

0 so we can figure out the carbons

oxidation number should be plus 2 minus

2 from oxygen gives us 0 nh3 ok and

there isn't any information about that

so just like in the last example we'll

have to figure out its oxidation number

using what we do know here we've got

hydrate there's a rule for hydrogen

hydrogen is plus 1 with nonmetals and

minus 1 with metals so nitrogen is

definitely a nonmetal which means it in

this case hydrogen is going to have a

plus 1 oxidation number ok but there are

3 hydrogen's each one of them has a plus

1 so what we have to do is we have to

multiply this plus 1 times 3 for the 3

hydrogen's and that's going to give us

plus 3 now let's keep this rule in mind

that the sum of oxidation numbers for a

neutral compound should equal 0 NH 3 is

definitely a neutral compound we don't

see any charge here so

whatever nitrogens number here is needs

to add together with plus three to make

zero so that means that nitrogen here

has to be minus three that adds there to

get zero so it's oxidation number is

minus three hydrogen's is plus one CAC -

so calcium here is in this column group

2a so we know that it's number is always

plus two we don't know what carbons

number is but we can figure it out what

we do know is that calcium's number plus

whatever carbons number is needs to

equal zero because this is a neutral

compound it doesn't have a charge that

means that the total number on carbon is

going to be negative two but there are

two carbons here so we have to divide

this number between the two of them so

we're going to take this and divide it

by two to get an oxidation number of

minus one for each one plus two for

calcium - one for carbon no.2 one -

check this out it is an ion so this is

going to be a little bit different in

this case the sum of oxidation numbers

for a polyatomic ion equals the ion

charge so let's see how we'll use that

here nitrogen we don't know what its

oxidation number is going to be but

oxygen here is usually minus two so we

have minus two here but we have two

oxygens so you're going to minus 2 times

2 is going to give us minus four

altogether now

whatever nitrogens is when we combine it

with minus four we need to get not zero

but we need to get minus one because

that's what the ion charge of no.2 1

minuses so this means that we will have

a 3 plus 3 plus 3 minus 4 gives us minus

1 which means that oxidation number here

on night

is going to be +3 hno3

okay let's take a look at the rules we

got hydrogen hydrogen here is combined

with nonmetals nitrogen and oxygen so

that means that it's oxidation number is

going to be plus one oxygen over here is

going to be minus two but we have three

oxygens so we got to take this minus 2

times 3 to get minus 6 ok so now we have

plus 1 from the hydrogen plus whatever

nitrogen is minus 6 has got to give us 0

because this is a neutral compound here

so that means that in this case

nitrogens oxidation number will be plus

5 1 plus 5 minus 6 equals 0 now I'm

going to start talking about some more

challenging examples that use some rules

here that are a little less common ZN h

2 ok zinc here we don't know what its

charge will be but hydrogen we have a

rule for that now we have been used two

hydrogen's number being plus 1 because

it's been with nonmetals but here it's

with a metal with zinc so that means

that it's oxidation number is going to

be minus 1 but we have 2 of these

hydrogens so it's going to be minus 1

times 2 give us minus 2 since this is a

neutral compound whatever zinc is plus

minus 2 has to give us 0 so that means

that zinc's oxidation number must be

plus 2 here so we got plus 2 for zinc

and minus 1 for hydrogen because a

hydrogen was with metal so it has a

minus 1 oxidation number BR o 3 1 - this

is a tricky one but it's pretty common

so BR bromine is one of the halogens the

rule for the halogens is they are

usually minus 1 but they are positive

with oxygen so I'm not going to put 1

minus in here because it's with oxygen

so let's go to oxygen instead oxygen is

usually minus 2

is not in peroxide so let me put that

here - - now we have three oxygens so

that's going to be times three to give

us - six

now whatever bromine is - six has got to

give us minus one because this whole

thing is a polyatomic ion that has a

charge of minus one so that means that

bromine is charge must be plus five plus

five minus 6 gives us minus one so this

is an exception where the halogens

instead of having an oxidation number of

minus one have a positive oxidation

number because they're paired up with

oxygen keep that in mind

clf3 okay so here we have two halogens

in the same compound we have fluorine

and chlorine how are we going to figure

this out well this is one of these

things that sometimes happens where

fluorine combines with another one of

the halogens we're going to zero in on

this rule first that fluorine is always

always always minus one so I'm going to

put a minus one right there multiply it

by three because I have three of them

and get minus three

now chlorine the other halogen says that

these other halogens are usually minus

one but not always they can be positive

with oxygen they can also be positive

with fluorine here so fluorine gives us

minus three chlorine must be plus three

so that these can add together to give

us zero since this is a neutral compound

which means that chlorines oxidation

number must be plus three and it's plus

three instead of minus 1 because it's

paired with fluorine which is always

always always minus one no matter what

so that's how to work through these

rules to figure out what the oxidation

numbers of elements are in a comp

if you want a little bit more practice

I'm going to have another video with

more practice problems if you just are a

little bit shaky on this doing more and

more practice problems is the best way

to get good really fast