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Oxidation and Reduction Reactions - Basic Introduction

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Summary

Core Theme

Oxidation-reduction (redox) reactions involve the transfer of electrons between substances, characterized by changes in oxidation states. Understanding these changes is key to identifying oxidized and reduced species, as well as oxidizing and reducing agents.

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in this video we're going to talk about

oxidation and reduction reactions

and here's the example we're going to

start with magnesium plus oxygen gas produces

produces

magnesium oxide

an oxidation reduction reaction or

simply a redox reaction electrons are

being transferred from one element to another

now perhaps you found or watched my

video on oxidation numbers if you have

then you know that the oxidation number

of any pure element is zero

now in magnesium oxide magnesium being

an alkaline earth metal has a charge of

plus two

oxygen has a charge of minus two

notice that the oxidation state of

magnesium went from zero to plus two

so the oxidation state increased

whenever the oxidation state goes up the

substance is set to be oxidized

in the case of oxygen the oxidation

number decreased from zero to negative two

two

whenever the oxidation number decreases

the substance

now you need to know that oxidation

always occurs with

a loss

of electrons reduction

reduction

is associated with a gain

of electrons

metals they like to give away electrons

they like to form

metal cations as they give away

electrons they will acquire a positive charge

charge

non-metals like oxygen

they like to acquire electrons and so

they will develop a negative charge

the substance that is oxidized

is known as the reducing agent

and the substance that is reduced

is known as

metals are reducing agents because

they will cause the other substance to

be reduced

not metals like oxygen gas fluorine

they're oxidizing agents because they

cause another substance to be oxidized

and that's where you got to see it

so these are some terms that you want to

be familiar with when dealing with redox reactions

reactions

the reaction between magnesium and

now magnesium

is changing into the magnesium ion

this magnesium ion has a plus two charge

and in order to become

a plus two cation it has to lose two electrons

electrons

now oxygen

i'm gonna write atomic oxygen because

this reaction is not balanced

individually oxygen acquires two electrons

electrons

now because magnesium lost electrons

this reaction it's which is known as a

half reaction

that's the oxidation part of the reaction

reaction

anytime a substance loses electrons it's

being oxidized

this second half reaction is the

reduction part

since oxygen is acquiring electrons

whenever a substance gain electrons or

if the oxidation number decreases

decreases

its reduction

now for half reactions anytime you have

electrons on the right side it's going

to be an oxidation half reaction

whenever the electrons are on the left

side it's a reduction half reaction

that's another way in which you can see

it too

consider this reaction

zinc metal reacts with hydrochloric acid

to produce hydrogen gas

identify the substance that is oxidized

and the substance that is reduced

and also identify the oxidizing agent

and reduce an agent

now if you get a question like this on

the test to find a substance that is

oxidized or reduced always look at the

reactants it's one of these two

now let's find the oxidation states of

everything the oxidation state of any

pure element is always zero

now whenever hydrogen is bonded to

a non-metal

it's going to have a positive one

oxidation state

which means chlorine has to be minus one

now in this one two chlorine is still

negative one which means zinc has to

have a plus two charge

now the oxidation of zinc changes from

zero to positive two

so zinc

is being oxidized

hydrogen changes from one to zero

so hcl as a substance is being reduced

even though only the hydrogen portion of

that substance is being reduced

so typically they actually for the

substance even though it's only the hydrogen

hydrogen

element that's being reduced

you would say the whole substance is

being reduced if you have to choose an

answer on a multiple choice test

now the substance that is oxidized

is to reduce an agent

and typically metals tend to be reducing agents

agents

the substance that is reduced

and this is a common question that you

might see on a typical chemistry exam

now let's try one more example for the

sake of practice methane

methane

reacts with oxygen gas

and it produces carbon dioxide and water

so feel free to pause the video identify

the oxidation states of every element in

this reaction

and then find a substance that is

oxidized reduce and identify the

oxidizing agent and reduce an

agent so let's identify the pure elements

elements

which is only oxygen gas that's going to

be zero

now we said that whenever hydrogen is

bonded to a non-metal

it's going to have a plus one oxidation state

and typically when oxygen is found in a

compound the oxidation state is negative two

two

except when it's bonded to fluorine or

except when it's in the form of peroxide

or superoxide it's different

whenever you hear the word oxide

the oxidation state of oxygen is

negative two if you hear the word

peroxide it's negative one if you hear

the word superoxide

it's negative one-half

now we gotta find the oxidation state of carbon

carbon

so let's start with methane

so we have one carbon atom and four

hydrogen atoms

which has to add up to zero because

methane is neutral in charge

now each hydrogen atom has an oxidation

state of one

so therefore c plus four

must equal zero therefore carbon has to

have an oxidation state of negative four

in methane

now let's calculate the oxidation state

of carbon in co2

so it's c

plus two oxygen atoms

which equals a net charge of zero

and each oxygen has a charge of negative two

so in this case carbon is going to have

so carbon changes from negative four to

positive four

therefore the oxidation number of carbon

is increasing which means carbon

is oxidized or technically methane is oxidized

oxidized

but oxygen

changes from zero to negative two

so therefore oxygen gas

the oxidation number is decreasing

so it's being reduced

which means that methane

is the reducing agent

and oxygen gas

it's being reduced which makes it the

so keep in mind even though the element

carbon is being oxidized you still

describe the whole substance methane as

being oxidized because carbon is part of methane

methane

now the next thing that you need to be

able to do

is you need to take a look at a reaction

and tell if it's an oxidation reduction reaction

reaction

so consider this reaction aluminum metal

reacts with copper chloride

to produce aluminum chloride

and copper metal is this reaction a

redox reaction

so this reaction is a single replacement reaction

reaction

and if there's a transfer of electrons

then it's going to be a redox reaction

aluminum has an oxidation state of zero

but in alcl3 it has an oxidation state

of clustering

so if the oxidation number changes

then it's a redox reaction

all single replacement reactions

are redox reactions

now if you don't want to look for the

oxidation numbers there's a quick way to

tell if something is going to be a redox reaction

reaction

if you see a pure element on one side

and then that element being part of a compound

compound

on the other side it's always going to

be a redox reaction there's a transfer

of electrons

and i want you to determine

so both of these examples

are synthesis reactions also known as

combination reactions

sometimes a combination reaction can be

a redox reaction and sometimes it won't be

now if you look at the first example

there are no pure elements all we have

is compounds when you see that chances

are it's not a redox reaction

so if you look at the second example we

have a pure element zinc

and then that same element is in a

compound so the second example is

a redox reaction and let's prove it so

let's start with the first example

oxygen has a negative two charge

magnesium in magnesium oxide has a plus

two charge

hydrogen has a positive one charge

now magnesium hydroxide

is composed of mg plus two

and two hydroxide ions

but as you can see magnesium still has

an oxidation state

of positive two

now let's focus on hydroxide

hydroxide is oxide

if you want to solve it you know that

an oxygen

is going to have a negative two charge

when you add up negative two and one

you're going to get the net charge of a

hydroxyl ion which is negative one

so in magnesium hydroxide oxygen is

still negative two and hydrogen is plus one

one

so notice that the oxidation state of

magnesium did not change

it's still positive two the oxidation

state of hydrogen is still the same

it's positive one

and the oxidation state of oxygen has

not changed

if the oxidation state does not change

that means there was no transfer of

electrons and so this is not

now let's analyze the second example

zinc and chlorine has an oxidation state

of zero

in zinc chloride

chlorine has a negative one oxidation

state zinc has a positive two oxidation

state so notice that zinc is being oxidized

oxidized

therefore there's a transfer of

electrons which makes this reaction a

redox reaction

now consider these two

decomposition reactions

when mercury oxide is heated it

decomposes into mercury and oxygen gas

and if you were to heat calcium

carbonate it will decompose into calcium

oxide plus carbon dioxide gas

so which of these decomposition

reactions is the redox reaction

looking at the first example

we have oxygen as a pure element on the

right side and then that same element is

within a compound so the second example

i mean the first example is a redox reaction

reaction

in the second example there are no pure elements

elements

all we have are just compounds

so the second example is not a redox

reaction that's a quick way to tell if

it's a redox or not

so now i'm going to give you a list of

reactions and determine which ones are

so feel free to pause the video if you

so the first example

is it a redox reaction

combustion reactions are always redox reactions

reactions

here we have a pure element on one side

and then that same element is inside a

compound so the first example is a redox reaction

reaction

what about the second

example notice that there are no pure

elements in this example

acid-base neutralization reactions are

not redox anytime you have a double

replacement reaction is never going to

be a redox reaction but all single

replacement reactions will be a redox reaction

reaction

the third example is a combination reaction

reaction

and there are no pure elements so

therefore this is not going to be a

redox reaction the last one is a double

replacement reaction specifically a

precipitation reaction

and as we can see

all we have are just four compounds in

this reaction there are no pure elements

so it's not a redox reaction

so to review

all combustion reactions

are redox reactions

all single replacement reactions are

redox reactions

now synthesis and combination reactions

sometimes the redox sometimes are not

and the same is true for decomposition reactions

reactions

double replacement reactions are never

redox reactions so that includes

acid-base reactions

precipitation reactions and other types

of double replacement reactions

and a quick way to tell

is if you see a pure element on one side

and then the same element within a

compound on the other side

it's going to be a redox reaction that's

the easiest way to tell

so that's it for this video if you want

to find more chemistry videos just check

out my channel

and you can find other videos on like

physics calculus algebra trig

trig precal

precal

and other stuff like that so thanks for watching

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Oxidation and Reduction Reactions - Basic Introduction

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