the brønsted-lowry theory of acids two important theories have been developed to explain the...

TheBrønsted-

Lowry Theory of Acids

Two important theories have been developed to explain the properties of acids, bases, and salts. These are the Arrhenius Theory and the Bronsted-Lowry Theory. Here, we’ll look at the Bronsted-Lowry theory.

This Theory of Acidswas proposed independently in 1923 by

This Theory of Acids was proposed independently in 1923 by two people


Johannes Nicolaus Brønsted, a Danish chemist, and




Thomas Martin Lowry, an English chemist.

Thomas Martin Lowry, an English chemist. We’ll introduce the main points of this theory.

A hydrogen atom “H”has 1 proton and 1 electron.

Before we do that, just a quick word here about a hydrogen atom. It’s atomic number is one, so it has 1 proton.


A neutral hydrogen atom also has one electron.


It is known that over 99.98% of the hydrogen atoms on Earth contain no neutrons.

Over 99.98% of the hydrogen atoms on Earth contain no neutrons.

A hydrogen atom “H”has 1 proton and 1 electron,and no neutrons.

So we’ll state here that a hydrogen atom has no neutrons, which is nearly always the case.

Over 99.98% of the hydrogen atoms on Earth contain no neutrons.

A hydrogen atom “H”has 1 proton and 1 electron, and no neutrons.

If 1 electron is removed from an H atom, it forms an H+ ion.

If one electron is removed from a hydrogen atom, it forms an H + ion.



Here’s the equation showing that.

H H e



A neutral H atom has 1 proton and 1 electron,

H+

1 p1 e–

H H e



So removing an electron

H+

1 p1 e–

H H e

Removing 1 electron,

will leave a charge of +1, 1 proton, and zero electrons.

H+

1 p1 e–

+

H H e



Removing 1 electron, leaves 1 proton and “0” electrons.

Removing 1 electron,




So an H+ ion contains no electrons, no neutrons, and one proton

H 1 p+

H H e




Therefore, an H+ ion is the same thing as one proton.

H H e

Therefore, an H+ ion is the same as 1

proton. H 1 p+




Or we can say that H+ equals 1 proton. We use the terms “H+ ion” and “proton” interchangeably in Chemistry 12.

H+ = 1 proton

H H e

Therefore, an H+ ion is the same as 1

proton.

According to the Brønsted-Lowry Theory:

According to the Brønsted-Lowry Theory:

According to the Brønsted-Lowry Theory:An acid is any species that donates a proton (H+) to another species.

An acid is any species that donates a proton, or H+ ion to another species. Let‘s look at an example of this.

Let’s start with a molecule of Hydrogen chloride, HCl, and a molecule of water, H2O. Here are Lewis structures for these molecules.

Cl H O HH

HCl H2O


HCl is a polar molecule. There is a partial negative charge on the chlorine atom and a partial positive charge on the hydrogen atom, shown by the delta minus and delta plus.

Cl H O HH

HCl H2O

+–

The water molecule is also polar: the oxygen atom has a partial negative charge and each hydrogen atom has a partial positive charge.

Cl H O HH

HCl H2O

+–

–

++

The negative charge on the oxygen pulls the partially positive hydrogen atom (click) away from the chlorine. The hydrogen leaves its shared electron with the chlorine atom.

Cl H O HH

HCl H2O

+–

–

++

The chlorine atom has gained an electron, so it acquires a negative charge and becomes a Cl minus ion.

Cl O HH

Cl– H2O

–

++

H

The hydrogen atom lost an electron, so it acquires a positive charge, forming an H+ ion.

Cl O HH

Cl– H2O

–

++

H+ proton

Which is also called a proton.

Cl O HH

Cl– H2O

–

++

H+ proton

The proton moves to the water molecule and (click) attaches to one of its lone pairs.

Cl O HH

Cl– H2O

–

++

H+ proton

Instead of staying with the hydrogen atom, the positive charge is considered as the charge of the whole ion. So we’ll (click) move it over here.

Cl O HH

Cl–

H+

There are now 3 H atoms attached to one O, so the formula is H3O instead of H2O

O HH

H+

H3O+

Cl

Cl–

We’ll draw square brackets around the H3O because it is an ion.

H3O+

+

O HH

HCl

Cl–

And write the positive charge here in the formula.

H3O+

+

O HH

HCl

Cl–

Chemists call the H3O + ion, the hydronium ion.

H3O+

+

Hydronium ionO HH

HCl

Cl–

So we can summarize the whole process here. We started with a molecule of HCl

And we added a molecule of water.

+

Which gave us

+

An H3O +, or hydronium ion

+

plus

+ +

a Cl minus, or chloride ion.

+ +

We can now write an equation to show this process. We write HCl gas

+

(g) 2 ( ) 3 (aq) (aq)HCl H O H O Cll

Plus H2O liquid.

+

2 ( ) 3 ((g aq) ) (aq) H O H O ClHCl l

+

forms

+

2 ( ) 3 ((g aq) ) (aq) H O H O ClHCl l

+

H3O + aqueous

+

3 (aq) ((g) 2 q( a) )H H O ClCl H O l

+

Plus Cl minus aqueous

+

(g) 2 ( ) 3 (aq) (aq)HCl H O ClH Ol

+

We can also represent the process like this.

(g) 2 ( ) 3 (aq) (aq)Cl H O H O CH l l

This shows that an H+ ion, or proton is being transferred from the HCl molecule to the water molecule.


H+

So this can also be called a proton transfer.


proton transfer

H2O has gained 1 proton, so it forms an H3O+ ion

3(g) 2 ( ) (( a) qq )aHCl H O Cl H Ol

proton transfer

H2O has gained 1 proton, so it

forms an H3O+ ion

HCl has lost 1 proton, or H+ ion, so it forms a Cl– ion

(g) 2 ( ) 3 (aq) (aq)Cl H O ClH H O l

proton transfer

HCl has lost 1 proton, so it

forms a Cl– ion


Because the HCl is losing, or donating a proton, according to the Bronsted-Lowry theory, it is called an acid. To indicate this, we often call it a Bronsted-Lowry acid, or Bronsted acid for short.

2 ( ) 3 ((g aq) ) (aq)H O H O ClHCl l

proton transfer

Brønsted-LowryAcid


A base is any species that accepts a proton (H+) from another species.

According the Bronsted-Lowry theory, a base is defined as any species that gains or accepts a proton, or H+ ion, from another species.

Brønsted-Lowry

definition of a BASE

According to the Brønsted-Lowry Theory:An acid is any species that donates a proton (H+) to another species.A base is any species that accepts a proton (H+) from another species.

Because the H2O is gaining, or accepting a proton, it can be defined as a Bronsted-Lowry Base, or Bronsted base for short.

(g) 3 (2 ( aq) (aq) )HCl H O CH O l l

proton transfer

Brønsted-LowryBase


So in this reaction, HCl is an acid,

3 (aq) ((g) 2 q( a) )H H O ClOl C H l

proton transfer

BaseAcid

According to the Brønsted-Lowry Theory:An acid is any species that donates a proton (H+) to another species.A base is any species that accepts a proton (H+) from another species.

and water is a base.


proton transfer

BaseAcid

Because the reactants HCl and H2O are not ions.


proton transfer

Not ions

But the products H3O+ and Cl minus ARE ions.

3 (aq) ((g) 2 q( a) )H H O ClCl H O l

proton transfer

Not ions Ions

We can call this process the ionization of HCl.

(g) 2 ( ) 3 (aq) (aq)HCl H O H O Cll

Ionization of HCl

Here’s a question. We’re asked to write the equations for the ionization of the following acids when they are added to water, and to identify the acids and the bases on the reactant side:

3(aq) 2 ( ) 3 (aq) 3(aq)HNO H O H O NOl

Write the equations for the ionization of the following acids when they are added to water. Identify the acids and the bases on the reactant side:

(g) 2 ( ) 3 (aq) (aq)HCN H O H O CNl

We’ll start the first reaction by adding liquid water as the other reactant.

23(aq) 3 (aq) 3 )( () aqHNO H O NOH O l


The formula for this compound starts with an H, so we assume it acts as an acid.

3(aq) 2 ( ) 3 (aq) 3(aq)NO H O H OH NOl


And we’ll label it as an acid.

2 ( ) 3 (aq3 ) 3(aq( ) )aq H O H O NOHNO l


acid

So the other reactant, water, must act as a base in this case.

3 (aq) 3(3(aq 2) ( q) a ) H O NH OHNO Ol


acid

base

Now an acid is a proton donor



acid

base

proton donor

And a base is a proton acceptor.



acid

base

proton donor

proton accepto

r

So that means a proton, or H+ ion will be transferred (click) from the acid, HNO3, to the base, H2O.

3 (aq) 3(3(aq 2) ( q) a ) H O NH ONOH Ol


acid

base

proton donor

proton accepto

r

H+

So this means that water will gain an H+, which is one H and one + charge



acid

base

H+

Will gain an H+ , which is one H

and one + charge

Adding an H and one + charge to water, gives us H3O+, the hydronium ion.



acid

base

H+

The hydronium

ion

The acid HNO3, has lost one H+, which means it has lost one H and one + charge



acid

base

H+

Has lost one H+, which is one H

and one + charge

We remove one H atom from HNO3 giving us NO3, and taking away one + charge is the same as adding one negative charge, so we have NO3 minus, the nitrate ion. Both of the new ions formed are aqueous.



acid

base

H+

So it will form NO3

– , the nitrate ion

So this is the equation for the ionization of HNO3. HNO3 is the acid on the left side and water is the base on the left side.



acid

base

Ionization of HNO3

Now, we’ll see what we get if we add HCN to water.



acid

base

Because HCN starts with H, we’ll treat it as an acid.


3 (aq) ((g) 2 q( a) )H H O CNHCN O l

acid

base

acid

And H2O as a base



acid

base

acid

base

So there will be a (click) proton transfer from HCN to H2O.


acid

base

acid

base

H+

3 ((2( aq) )g) ( a) qH CH NON OC H l

The H2O will gain a proton, or H+ and form H3O +, or hydronium


3 ((2( aq) )g) ( a) qH CH NON OC H l

acid

base

acid

base

H+

And the HCN will lose a proton, or H+, and form CN minus


3 ((2( aq) )g) ( a) qH H O CNHCN O l

acid

base

acid

base

H+

So this is the equation for the ionization of HCN. HCN is the acid on the left and water is the base. A double arrow is used here because the ionization of HCN does not go to completion.



acid

base

acid

base

Ionization of HCN

In a solution of HCN, only a few molecules are ionized. You’ll be shown how you can tell whether to use a single arrow or double arrow later in the course.



acid

base

acid

base

Ionization of HCN

Amphiprotic Species

Now, lets consider this reaction. Again, the double arrow here just tells us that this reaction does not go to completion. Instead, an equilibrium exists here.

Consider the reaction:

3(g) 2 ( ) 4(aq) (aq)NH H O NH OHl

We see that the NH3 has been converted to NH4 +.

2 ( ) (3(g) 4(aq) aq)NH NH H O OHl

In doing so, it gains one H and one + charge, therefore it gains one H+, or one proton.


Because it gains a proton in this case, the NH3 is classified as a base.


base

Looking at the water, we see that is has been converted to OH minus.

2 ( ) (a3(g) q4 aq )( )NH NHH O OH l

base

OH minus has one less H and one less + than H2O, therefore it has one less H+ than H2O.

2 ( ) (a3(g) q4 aq )( )NH NHH O OH l

base One

less H+ than H2O

therefore when H2O converts to OH minus, it loses an H+, or proton.

2 ( ) (a3(g) q4 aq )( )NH NHH O OH l

base

For that reason, we identify H2O as an acid in this case.

(a2 (3(g) 4(aq q) ) )NH NH OHO H l

base

acid

Now, we’ll look at a previous reaction we had in which HNO3 reacts with water.

4(aq) (2 (3(g q) ) a )NH ON HOH H l

base

acid


acid

base

In this reaction, water acted as a base.


base

acid


acid

base

So we can see that, depending on what it’s reacting with, water can play the role of an acid or the role of a base.


base

acid


acid

base

Such a species is said to be amphiprotic.


base

acid


acid

base

Amphiprotic

An amphiprotic species is one that can act either as an acid or as a base, depending on what it is with. Water is one amphiprotic substance. There are many more, as we shall see later in this unit.


base

acid


acid

base

Amphiprotic

can act either as an acid or as a

base

Monoprotic, Diprotic, Triprotic,

and Polyprotic Acids

Consider these two acids, HCl, and HNO3.

base

acid


acid

base


Both of these are able to lose one proton only.

base

acid


acid

base


An acid that has one proton available to donate is called a monoprotic acid. So both HCl and HNO3 are monoprotic acids.

base

acid


acid

base


Monoprotic

Monoprotic

Now, consider this acid, H2SO4. It’s called

3 (aq) 4(a2 4 2 ((a ) q) )q H O HSH OH SO Ol

sulphuric acid

3 (aq) 4(a2 4 2 ((a ) q) )q H O HSH OH SO Ol

sulphuric acid

Notice it has 2 H atoms at the beginning of the formula.

3 (aq) 4(a4 2 ((aq) q) )2 H O HSH OSOH Ol

sulphuric acid

Acids that have two protons they can donate, are said to be diprotic. So H2SO4 is a diprotic acid.


sulphuric acid

Diprotic acid

However, when diprotic acids like H2SO4, are added to water, they do not lose both of their protons at once. They do it in steps, losing one proton at a time.


sulphuric acid

Diprotic acid

In the first step of the reaction of H2SO4 in water, the H2SO4 loses one proton to water.


sulphuric acid

H+

So the water is converted to H3O +, or hydronium. (end of statement)


sulphuric acid

H+

Because the H2SO4 is losing an H+, it means it’s losing one H atom and one + charge.


sulphuric acid

H+

Will lose one H and one +

charge

This gives us HSO4 with a minus charge.


sulphuric acid

H+

Will lose one H and one +

charge

HSO4 minus is called the hydrogen sulphate, or bisulphate ion.

4(aq)22 4( ( 3aq) ( )) aq H HSOOH SO H O l

sulphuric acid

H+

Called the hydrogen

sulphate or bisulphate ion

Notice that HSO4 minus has one hydrogen it can lose.

3 (aq) 4(a2 4 2 ((a ) q) )q H O SOH OS HH O l

sulphuric acid

H+

Has one H that it can

lose

So in the second step of the ionization of sulphuric acid, the HSO4 minus will react with

2 4(aq) 2 ( ) 3 (aq) 4(aq)H SO H O H O HSOl

sulphuric acid

H+

23 (aq) 4(a4(aq) ( q2 ) ) H OHH S O SOO l

Water.


sulphuric acid

H+


And donate its proton to water


sulphuric acid

H+


H+

Which would produce another hydronium ion. (end of statement)


sulphuric acid

H+


H+

The HSO4 minus ion loses a proton, so it loses one hydrogen and one positive charge.


sulphuric acid

H+


H+

Loses one H and one +

charge

So it will be left as SO4.


sulphuric acid

H+


H+


charge

And losing one positive charge will cause its charge to go down one, from negative 1 to negative 2, or 2 minus.


sulphuric acid

H+

3 (aq) 4(a4(aq 2 )() ) q2H H O SOSO H O l

hydrogen sulphate

ion

The product SO4 2minus, or sulphate ion, does not have any hydrogens to donate, so this is the last step in the ionization of sulphuric acid


sulphuric acid

H+

324(a24(a ( q)q) ( )) aqH SOH O HO S O l

hydrogen sulphate

ion

The double arrow here implies there is an equilibrium. This reaction does not go to completion.


sulphuric acid

H+


hydrogen sulphate

ion

Whereas in the first step of the ionization of sulphuric acid, there is a single arrow, which means this step goes to completion. 100% of the sulphuric acid is converted to hydronium and hydrogen sulphate ions.

3 (aq) 4(a2 4 2 ((a ) q) )q H O HSOH H S OO l

sulphuric acid

24(aq) 2 ( ) 3 (aq) 4(aq)HSO H O H O SO l

hydrogen sulphate

ion

Now we’ll look at phosphoric acid.

3 (aq) 2 4(2 (3 4 a( q) ) q)aH H O H O H P PO Ol

phosphoric acid

We see that it has 3 H’s at the front of its formula, which means it has three protons it can lose.

3 (aq) 2 4(2 (4(aq) a) q)3H H O H O H P PO Ol

phosphoric acid

For that reason, phosphoric acid is called a triprotic acid.


phosphoric acid

Triprotic acid

When it’s combined with water, it doesn’t lose all three protons at once. Just one at a time.


phosphoric acid

Triprotic acid

In the first step, one proton is transferred to a water molecule, so it produces one hydronium ion. (end of statement)


phosphoric acid

H+

because it loses one H and one positive charge,


H+


charge

the other product would have 2 H’s and a charge of negative 1.


H+


charge

H2PO4 minus is called the dihydrogen phosphate ion. We see that it still has two protons it can donate.

3 2 4(23 4 aq)(aq)(q )(a )H H O H POHO O P l

H+

called the dihydrogen

phosphate ion

In the second step, the H2PO4 minus ion loses one of its protons to water

3 4(aq) 2 ( ) 3 (aq) 2 4(aq)H PO H O H O H POl

H+

23 (aq) 4(a4( 2 (a qq) ) )2 H O HPH O POH Ol

H+

so it produces another hydronium ion


H+


H+

And since this loses one H and one positive charge,


H+


H+


charge

The other product would be HPO4 2 minus.


H+

22 4(aq)2 3 (aq)q) (( )4 a HH HPOH O O O P l

H+


charge

this product is called the monohydrogen phosphate ion. We can see that it has one proton available to donate.


H+

24(a2 4 2(a 3 (aq) q(q )) )H HPOH OPO H O l

H+

Called the monohydrogen phosphate ion

So in the last step, the HPO4 2minus donates its single proton to water, producing another hydronium ion. (end of statement)


H+

22 4(aq) 2 ( ) 3 (aq) 4(aq)H PO H O H O HPOl

H+

33 (aq) 4(a

24(a 2 q(q )) )H O PO H OP H O l

H+

And because it loses one H and one positive charge,


H+


H+

33 (aq) 4(a

24(a 2 q(q )) )H O PO H OP H O l

H+

Loses one H and one + charge

the final ion that it forms is just PO4 3 minus.


H+


H+

34(aq)2

24(aq 3) )( q) (aHH POH OO OP l

H+

Loses one H and one + charge

This is simply called the phosphate ion


H+


H+

34(aq)2

24(aq 3) )( q) (aH POH OPO H O l

H+

The phosphate ion

Polyprotic acid

Polyprotic Acid

is just a general term for any acid that can donate more than one proton.

Polyprotic Acid—just a general term for any acid that can donate more

than one proton.

Examples could be carbonic acid, which is diprotic


than one proton.e.g.) H2CO3 (carbonic acid-diprotic)

phosphoric acid, which is triprotic



H3PO4 (phosphoric acid-triprotic)

and pyrophosphoric acid, which has 4 protons it can donate.



H3PO4 (phosphoric acid-triprotic)

H4P2O7 (pyrophosphoric acid)

the brønsted-lowry theory of acids two important theories have been developed to explain the...

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