Lesson 5: Brønsted-Lowry Concept

In this section we will consider the Brønsted-Lowry concept. This concept focuses on what an acid or base does. This includes proton-transfer reactions, the concept of conjugate pairs, and amphoterism.

The Brønsted-Lowry Concept | Proton-Transfer Reactions | Conjugate Pairs | Amphoterism

The Brønsted-Lowry Concept

Acids

With the Brønsted-Lowry concept we usually refer to a hydrogen ion as a proton. That is because a proton is all that is left when a hydrogen atom loses an electron to become an ion.

Brønsted and Lowry independently came up with the idea that an acid is an acid because it provides or donates a proton to something else. When an acid reacts, the proton is transferred from one chemical to another. As will be noted later, the chemical which accepts the proton is a base.

When an acid dissolves and dissociates in water it gives a proton to the water. Equations to represent this are shown here ( and in example 10 in your workbook). The Brønsted-Lowry view is that the acid (HCl) gives a proton to water to make two ions, one of which is H3O+. H3O+ is called hydronium ion. (By the way, a hydronium ion is sometimes called an oxonium ion, although we'll always refer to it as hydronium in this course.)	HCl + H2O H3O+ + Cl-
These equations show a different acid (H2SO4) giving a proton to water. In this case, the product HSO4- still has a proton that can be donated to another water molecule.	H2SO4 + H2O   H3O+ +  HSO4- HSO4- + H2O H3O+ + SO42-
This equation shows HCl giving a proton to a hydroxide ion (OH-) rather than water.	HCl + OH- H2O + Cl-

The first chemical in each of these equations is an acid because they are each giving a proton to something else.

Bases

Note that in order for an acid to act like an acid, there needs to be something for it to react with. There needs to be something to take the proton. There needs to be a base. A base is a proton acceptor. Compare this to the definition that an acid is a proton donor.

Bases are the opposite of acids. Bases are basic because they take or accept protons. Hydroxide ion, for example, can accept a proton to form water. Brønsted and Lowry realized that not all bases had to have a hydroxide ion. As long as something can accept a proton it is a base.	OH- + H+ H2O
So anything, hydroxide or not, that can accept a proton is a base under the Brønsted-Lowry definition. The water molecules that accept protons when HCl dissolves in water are acting as bases.	H2O + HCl H3O+ + Cl-

 

Some additional examples of Brønsted-Lowry bases are shown accepting protons in these equations (which are also shown in example 11 in your workbook). These examples do not show the acids which are providing the protons.	OH- + H+ H2O
Ammonia can accept or react with hydrogen ion to give ammonium ion NH4+.	NH3 + H+ NH4+
Carbonate ion can accept a hydrogen ion, or accept a proton, to become bicarbonate ion.	CO32- + H+ HCO3-
Also, water molecules, as mentioned before, can act as a base by accepting protons.	H2O + H+ H3O+

Hydroxide, ammonia, carbonate and water are all Brønsted-Lowry bases.

Be sure to note the distinction between ammonia and ammonium. NH₃ is ammonia and NH₄⁺ is ammonium. They sound very much the same and their formulas are very similar, but their chemical properties are quite different. They are different because one has one more proton than the other. Ammonia is a base and ammonium is an acid. We'll take up another aspect of their relationship when we consider conjugate pairs.

Some phenomena that are readily explained using the Brønsted-Lowry concept are acid-base reactions (explained as proton-transfer reactions), conjugate pair relationships, and amphoterism.

 

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Proton-Transfer Reactions

The Brønsted-Lowry definitions of acids and bases are the ones that will generally be most useful in this course, because they describe how acids and bases interact. We will talk about acid-base reactions as being proton transfer reactions, because a hydrogen ion will move from one chemical to another. When a hydrogen ion moves from one chemical to another, we say that an acid-base reaction has taken place.

Practice (Ex. 12)

Now demonstrate your awareness of what happens in acid-base or proton transfer reactions by completing and balancing equations for the reactions between the acids and bases listed below (and also shown in exercise 12 in your workbook). When you have done that, check your answers below before you continue.

HNO3 + OH-

HCl + NH3

H2CO3 + H2O

HBr + OH-

H2O + NH3

 

Answers to Exercise 12

In the first reaction, a proton is transferred from the acid HNO3 to the base OH- to make NO3- and H2O.   HNO3 + OH- NO3- + H2O

In the second, a proton is transferred form the acid HCl to the base NH3 to make Cl- and NH4+.   HCl + NH3 Cl- + NH4+

In the third, a proton is transferred from the acid H2CO3 to the base H2O to make HCO3- and H3O+. If you said that two protons are transferred from H2CO3 to two water molecules to make CO32- , that is all right at this time. We will take up that issue in a couple of weeks.   H2CO3 + H2O HCO3- + H3O+ or H2CO3 + 2 H2O CO32- + 2 H3O+

In the fourth, a proton is transferred from the acid HBr to the base OH- to make Br- and H2O.   HBr + OH- Br - + H2O

In the fifth, a proton is transferred from the acid H2O to the base NH3 to make NH4+ and OH-.   H2O + NH3 NH4+ + OH-

 

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Conjugate Pairs

A natural consequence of the Brønsted-Lowry definitions of acids and bases, perhaps a diabolical one (certainly one that's been confounding chemistry students for many years), is that we end up with a very interesting relationship between acids and bases. An acid can donate a proton. The chemical that remains after the proton is donated is a base.

Let's consider the acid HF dissociating and giving up its proton. This equation does not show what is accepting the proton so that we can concentrate on what is left from the HF. What's left is fluoride ion. Because the fluoride ion no longer has a proton, it's no longer an acid. HF is an acid, but F- is not an acid. Being without a proton it could presumably accept a proton. If it can, we would consider the fluoride ion to be a base.

HF H+ + F-

Let's expand the example. Let's look at HF donating its proton to a molecule of water, as is shown here. In this reaction, a proton is transferred from HF to H2O. That makes H2O a base in this reaction because it's accepting a proton. After it has accepted a proton, the water becomes a hydronium ion. It now has a proton, which it can donate, therefore it's an acid.

HF + H2O H3O+ + F-

 

When a base accepts a proton, it becomes an acid because it now has a proton that it can donate. And when an acid donates a proton it becomes a base, because it now has room to accept a proton.

These are what we call conjugate pairs of acids and bases.

When an acid gives up its proton, what remains is called the conjugate base of that acid. When a base accepts a proton, the resulting chemical is called the conjugate acid of that original base. HF and F^- are a conjugate acid-base pair. H₂O and H₃O⁺ are a conjugate pair, where H₃O⁺ is the acid and H₂O is the base.

Being able to recognize conjugate pairs of acids and bases becomes important dealing with comparisons between the strengths of acids and bases, buffered solutions, and the hydrolysis of various salts. Those topics will come up in later lessons.

Practice (Ex.14)

Now try your hand at identifying the conjugate acid-base pairs in the equations shown below (and also in exercise 14 in your workbook). You can check your answers below.

Answers to Exercise 14:

If you have problems with any of these, please check with an instructor to get squared away.

Identify the conjugate acid-base pairs in the following equations.

equation	first pair acid - base	second pair acid - base
H2SO4 + H2O HSO4- + H3O+	H2SO4  HSO4-	H3O+  H2O
HCl + OH- Cl- + H2O	HCl   Cl-	H2O  OH-
HCl + NH3 Cl- + NH4+	HCl   Cl-	NH4+  NH3
NH3 + H2O NH4+ + OH-	H2O  OH-	NH4+  NH3

 

 

More Practice (Ex. 15)

Next work through the questions that are listed below and also given in exercise 15. After you've done those, check your answers below. If you have any problems at all in working through them, be sure to consult with the instructor. Then continue with the lesson.

What are the conjugate bases of these acids?

 

What are the conjugate acids of these bases?

Answers to Exercise 15:

What are the conjugate bases of these acids?

original acid	conjugate base
HNO3	NO3-
H2O	OH-
H3O+	H2O
H2SO4	HSO4-
HBr	Br -
HCO3-	CO32-

 

What are the conjugate acids of these bases?

original base	conjugate acid
OH-	H2O
H2O	H3O+
HCO3-	H2CO3
SO42-	HSO4-
ClO4-	HClO4

 

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Amphoterism

Did you notice in previous examples that water was sometimes an acid and sometimes a base? The ability of some chemicals to act either as an acid or a base is called amphoterism. Whether an amphoteric chemical acts as an acid or a base depends on what other chemicals happen to be around.

If a base ( like NH3) is present, water can act as an acid and react by donating a proton to that base. In doing so, water is changed into its conjugate base, hydroxide ion.	H2O + NH3 NH4+ + OH-
If an acid (like HCl) is present, water can act as a base and react by accepting a proton from that acid. In doing so, water is changed into its conjugate acid, hydronium ion.	H2O + HCl Cl- + H3O+

 

The self-ionization of water is another example of water being able to react either as an acid or a base. The molecules in pure water continuously collide and react with one another. In that reaction, one water molecule can transfer a proton to another water molecule. One water molecule acts as an acid and the other acts as a base. The solution is neutral because equal quantities of H3O+ and OH- are made.

H2O + H2O OH- + H3O+

 

Bicarbonate ion is also amphoteric. It can act either as an acid or a base as shown in these reactions.

HCO3- + OH- CO32- + H2O HCO3- + H3O+   H2CO3 + H2O

 

Some metal hydroxides have amphoteric properties, such as lead(II) hydroxide, shown here. This insoluble compound can be made to dissolve by reacting with either acid (H+) or base (OH-).	Pb(OH)2
In this reaction Pb(OH)2 is reacting as a base and dissolving to make lead ions.	Pb(OH)2(s) + 2 H+ Pb2+(aq) + 2 H2O
In this reaction Pb(OH)2 is reacting as an acid and forming a soluble lead complex.	Pb(OH)2(s) + 2 OH- Pb(OH)42-(aq)
We will make use of this property of some metal hydroxides to help identify them later in Lesson 10.

 

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