Arrhenius and Brønsted-Lowry Acids and Bases - 1.12.1

Hi. My name is Nick Chatterton.

I'm an associate lecturer at Burt Beck College and

also a senior lecturer at the Open University,

where I'm course leader for chemistry.

Today's lecture is gonna focus on the theory of acids and bases.

So first of all,

let's think about some common acids and bases that

you may have come across already in your study.

So, for example, hydrochloric acid, H C l sulfuric

acid, H two s 04

and nitric acid.

These are very, very common acids you'll find at school,

and they're known as the bench acids.

But there are other types of acidic material.

For example,

the molecules containing carboxylic acid functional groups such as ethanol

acid,

which is found in vinegar.

Citric acid, which you find in Citrus fruits and lactic acid, which, which is,

which is produced in the body.

But what about bases well again?

There are common bases you might have come across

at schools such as sodium hydroxide and potassium hydroxide,

so that both of these contain the hydroxide ions.

But there are other materials are basic, such as magnesium oxide and ammonium.

So let's now look at some of the theories that have been used to

explain acid and base behaviour and as often as the case with chemistry,

it's historical.

So let's go back to one of the earliest

acid and base theories and this is developed by

AUS

And he described acids as being substances that dissolve

in water to increase the concentration of H plus ions

and bases. On the other hand were substances that

Anius said dissolve in water to release hydroxide ions,

so it's very specific to H plus and O H minus,

so that's got a couple of examples here. So if we take H g l gas

and we dissolve that in water, it dissociates to form H plus ions and chloride ions.

So H G L releases H plus so increases the concentration of H plus ions, so it's an

anius acid.

Similarly, if you take Ethan OIC acid, which is a carboxylic acid molecule,

you dissolve that in water as you will see later on.

It releases some H plus ions, but maybe less than H C l.

Um But it still does release H plus ions,

so thereby increases the concentration of hydrogen ions making an

aus acid.

What about same hydroxide or the bases well again, you dissolve that in water.

It will break up into its constituent ions form n a plus and O H minus. So thereby is an

AUS base because it releases hydroxide ions.

So that's that was an early definition.

But what then happened is it was then looked at in the early 20th century by

two separate scientists, Bronte and Lowry,

and they looked at a more general definition for acid and base.

So that was all in water, what we were talking about here.

So things that dissolve in water to release H plus ions and O H minus ions.

So

what?

What Branson and Lowry realise is that actually,

a lot of acid based reactions don't necessarily have to occur in water,

and we can define them based just on the movement of H plus in reactions.

So they defined an acid as a substance that will donate H plus ions

OK, whereas a base is a substance that will accept

H plus ions. So we've got an example here.

We've got ammonia, NH three, reacting with water to form H four plus O H minus.

So we're going to show why this is a

what? What? What the what the acid. What the Bronte

Larry acid based chemistry here is.

Well, if you look at the starting point

NH three

gains and H

H plus from from water. So NH three is accepting a proton.

So, therefore is a is a is a Bronte

Lowry base, whereas water is donating h plus. So it's a Bronte

Larry acid.

So that's the That's the way we can sort of subclass those sorts of reactions.

We've got another example here slightly, slightly more,

slightly more complicated molecules.

We've got C six h 50 h reacting with the N H two minus ion

to form C six, h 50 minus and N h. Three. So let's have a look at what's happened here.

Well, c six h 50 h has been converted into C six h 50 minus during this reaction.

So therefore it has lost a H plus iron.

So therefore it is a Bronte

acid.

Similarly, N h two minus has been converted into N H three, So N.

H two minus has gained an H plus ion. So therefore it's a Bronte

base.

OK, so we now as we've seen, what brons that

um, acid based behaviour is, um it turns out that many acid base

are actually an equilibrium.

So therefore the reaction go both forward and backwards.

OK, so what we can do is we can look at the reaction in two directions,

both forward and back.

So we're gonna again look at the reaction between

N H three and water to form N H four plus and O H minus.

In the four directions we saw, water is an acid

and N H three is is a base,

whereas in the backward reaction, it's the other way around.

So NH four plus, if you like, reacts with o minus. So NH four plus is donating a proton.

So is an acid and o minus is accepting a proton. So it's a base.

So in some ways we have two pairs of reactions here.

So we've got

the water and the O. H minus is one pair

and the n h three and the n H four plus is one pair.

We call the products the conjugate of what is on the on on the reactant side.

So water and O H minus are a pair a conjugate acid base pair,

so water is the acid and we say that the O. H.

Minus, because we've already seen it as a base, is its conjugate base.

Similarly,

N H three and N H four plus are a pair.

N. H three is the base and N H four plus because we know it's an acid

is it's conjugate acid. So we've got these acid base pairs.

So let's now look at practising assigning acid

base pairs for a couple of equilibrium.

So the first one we're going to look at is C

three c

reacting with H2O to form C three, C

O minus and H two plus

so going forward. First of all,

C

three c

is

donating a proton to water, so it is an acid.

Water is accepting the proton, so it's a base.

So that's the That's the the reactant side dealt with.

But on the product side, so going backwards,

H 30 plus

is going to be donating a proton to C H three COO minus,

So H 30 plus is the conjugate acid of H2O

and C H three COO minus is the conjugate base of C H three COO h.

So you see, they just differ by one proton.

OK,

so the conjugate acid of something has one more proton than

the base. And the conjugate base has one less proton than the conjugate acid.

So let's just do one more example.

Let's do the H two s 04 plus h n 03 going to form H s 04 minus and h two n three plus

in the forward sense H two s 04 is donating a proton because it had

two at the start and one when it reacted when on the product side.

So H two s 04 is the acid

and h n three therefore must be accepting a proton so it must be the base.

So H s 04 is the acid

H and 03 is the base So we can then look to the other side.

So if h two s 04 was an acid on the other side,

H s 04 minus must be its base Must be the conjugate base

and similarly

h 23 plus must be the conjugate acid And you can check

We can consider H two n three plus reacting with h

04 minus.

So

h two n three plus has donated a proton to H

04

minus. So is the acid or the conjugate acid,

and H s 04 has minus. Has accepted that proton. So is the conjugate base.