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Lesson Explainer: Lewis Acids and Bases Chemistry

In this explainer, we will learn how to explain what Lewis acids and bases are, along with their characteristic properties, and identify them in chemical reactions.

Chemists look for patterns in chemical reactions. They put chemicals in groups and compare them with one another. Two of these groups are acids and bases.

The descriptions of “acid” and “base” have changed over time, and multiple descriptions have existed at once. Today, a few of these descriptions are still in use, although some substances are considered acidic or basic in one system, but not in others.

The simplest description of an acid could be a substance that tastes sour, while the simplest description of a base could be a substance that reacts with an acid.

In 1887, Svante Arrhenius described acids and bases on the basis of whether they produce hydrogen ions (H+) or hydroxide ions (OH) when added to water: HA()H()+A()BOH()B()+OH()aqaqaqaqaqaqArrheniusacidArrheniusbase++

Hydrogen ions and hydroxide ions react to make water. This accounts for the fact that, generally, Arrhenius bases will react with Arrhenius acids in a consistent way.

Reaction: Arrhenius Acids and Bases

Arrheniusacid+Arrheniusbasesalt+water

Another way Arrhenius bases are described is as “substances that when added to water raise the hydroxide ion concentration.” This would apply to substances that do not contain hydroxide ions but raise the hydroxide ion concentration of water by reacting with it, such as ammonia (NH3). The distinctions at this level show that descriptions and definitions can vary a little.

In 1923, Johannes Brønsted and Thomas Lowry extended the description of bases, including substances that do not yield hydroxide ions in solution.

A Brønsted–Lowry base is any substance that can accept a proton. For instance, ammonia (NH3) can react with hydrogen ions (H+) in water, forming the ammonium ion (NH+4). Generally speaking, Arrhenius acids are also Brønsted–Lowry acids, since they have a hydrogen ion they can donate. However, it is important to realize that Arrhenius acids are those chemicals that can donate hydrogen ions to water specifically.

There are also some substances that will act as acids in some circumstances and as bases in others; these are known as amphoteric substances.

Another description of acids and bases was invented by Gilbert Lewis in 1923. He described a Lewis acid as any substance that can accept a lone pair of electrons, forming a bond. He also described a Lewis base as any substance that has a lone pair of electrons that can be donated to form a bond.

Definition: Lewis Acid

A Lewis acid is a substance that can accept a lone pair of electrons, forming a bond.

Definition: Lewis Base

A Lewis base is a substance that can donate a lone pair of electrons, forming a bond.

In general, Brønsted–Lowry bases are Lewis bases and vice versa. However, while all Brønsted–Lowry acids are Lewis acids, not all Lewis acids are Brønsted–Lowry acids.

Example 1: Identifying the Best Description of Brønsted–Lowry and Lewis Bases

Which of the following best describes the difference between a Brønsted–Lowry base and a Lewis base?

  1. A Lewis base is a proton donor, while a Brønsted–Lowry base is a species that can accept an electron pair or more.
  2. A Lewis base is a proton acceptor, while a Brønsted–Lowry base is a species that can donate an electron pair or more.
  3. A Brønsted–Lowry base is a proton donor, while a Lewis base is a species that can accept an electron pair or more.
  4. A Brønsted–Lowry base is a proton acceptor, while a Lewis base is a species that can donate an electron pair or more.
  5. A Brønsted–Lowry base is an OH ion acceptor, while a Lewis base is a species that can donate an electron pair or more.

Answer

Before answering the question, we can lay out the answers so that they are easy to compare.

A Lewis base is A Brønsted–Lowry base is
Aa proton donora species that can accept an electron pair or more
Ba proton acceptora species that can donate an electron pair or more
Ca species that can accept an electron pair or morea proton donor
Da species that can donate an electron pair or morea proton acceptor
Ea species that can donate an electron pair or morean OH ion acceptor

From this, we can see a few different suggested ways of describing bases: in terms of OH ions, protons (H+ ions), or lone pairs.

The Brønsted–Lowry description of an acid is “a proton donor,” such as hydrochloric acid. Meanwhile, a Brønsted–Lowry base is “a proton acceptor,” such as sodium hydroxide: HCl+NaOHHO+NaCl2

The Lewis description of an acid is “a lone-pair acceptor.” Meanwhile, a Lewis base is “a lone-pair donor.”

A Lewis acid will accept a lone pair from a Lewis base, forming a bond.

Answer A suggests a Lewis base is a proton donor; this is incorrect. The donation of a proton will not involve the donation of a lone pair, so this is an incorrect description of a Lewis base.

Answer B suggests a Lewis base is a proton acceptor. The acceptance of a proton will involve a donation of a lone pair of electrons. However, there are other ways that a Lewis base can behave. A Lewis base can donate a lone pair to things other than protons. At best, this is a poor description.

Answer C suggests a Lewis base can accept an electron pair, but the opposite is true.

Answers D and E have the correct description of a Lewis base (an electron pair donor), so we can look at the descriptions for the Brønsted–Lowry base for each.

Answer D suggests that a Brønsted–Lowry base is a proton acceptor, which is correct.

Answer E suggests that a Brønsted–Lowry base is a hydroxide ion acceptor, which is not correct.

Therefore, the answer is D. The best description of the set correctly describes a Lewis base as a species that can donate an electron pair (or more), and a Brønsted–Lowry base as a proton acceptor.

To see how the definitions of acids and bases can be applied, consider hydrogen chloride (HCl).

Is HCl this type of acid or base?/×Why?
AcidArrheniusHCl has hydrogen ions that it can donate to water.
Brønsted–LowryAll Arrhenius acids are also Brønsted–Lowry acids since Arrhenius acids can donate protons.
LewisAll Brønsted–Lowry acids are Lewis acids since hydrogen ions can accept a lone pair to form a bond.
BaseArrhenius×HCl does not have any hydroxide ions, so it cannot dissociate in water to give OH ions.
Brønsted–Lowry×HCl is not a Brønsted–Lowry base because it does not readily accept more protons.
Lewis×HCl is not a Lewis base because it does not readily donate a lone pair to form a bond.

Hydrogen chloride can be described by all three descriptions of acids.

However, now consider borane (BH3). This is the Lewis structure of borane:

HHHB

The boron atom in the middle has two vacancies in its valence shell. It will readily accept a lone pair from another substance.

This is the full table for BH3.

Is BH3 this type of acid or base?/×Why?
AcidArrhenius×BH3 does not readily donate its hydrogens as hydrogen ions.
Brønsted–Lowry×BH3 does not readily donate its hydrogens as hydrogen ions.
LewisBH3 will readily accept a lone pair from a Lewis base, forming a bond.
BaseArrhenius×BH3 does not have any hydroxide ions, so it cannot dissociate in water to give OH ions.
Brønsted–Lowry×BH3 does not readily accept more protons.
Lewis×BH3 does not have a lone pair to donate.

The table above shows that borane can be described as a Lewis acid, but not an Arrhenius nor Brønsted–Lowry acid or base.

For sodium hydroxide (NaOH), the following table can be constructed.

Is NaOH this type of acid or base?/×Why?
AcidArrhenius×NaOH does not readily donate its hydrogen as a hydrogen ion.
Brønsted–Lowry×NaOH does not readily donate its hydrogen as a hydrogen ion.
Lewis×NaOH does not readily accept a lone pair from a Lewis base.
BaseArrheniusNaOH dissociates in water, releasing OH ions.
Brønsted–LowryNaOH readily accepts H+ ions.
LewisNaOH readily donates a lone pair to form a bond.

We can see how NaOH behaves as both a Brønsted–Lowry base and a Lewis base in a single diagram. The oxygen of the hydroxide ion of sodium hydroxide can donate one of its lone pairs to form a bond with a hydrogen ion:

OH+H

A fourth, and final, example is ammonia (NH3).

Is NH3 this type of acid or base?/×Why?
AcidArrhenius×NH3 does not readily donate its hydrogens as hydrogen ions.
Brønsted–Lowry×NH3 does not readily donate its hydrogens as hydrogen ions.
Lewis×NH3 does not readily accept a lone pair from a Lewis base.
BaseArrhenius×NH3 does not dissociate in water to produce OH ions but does react with water, producing ammonium hydroxide, which is an Arrhenius base.
Brønsted–LowryNH3 readily accepts H+ ions.
LewisNH3 readily donates a lone pair to form a bond.

Here, ammonia can definitely be described as a Brønsted–Lowry and Lewis base. However, its description as an Arrhenius base is not as definite and shows why other descriptions were needed.

The advantage of Lewis’s description is that the set of Lewis acids and bases is bigger than the set of substances included by Arrhenius, or Brønsted and Lowry, and Lewis acids and Lewis bases react in predictable ways.

Lewis’s description allows us to easily compare a greater number of chemical reactions. If we see an area of one molecule that is electron deficient and another area that is electron rich with a reactive lone pair, we can guess how they might interact, meaning that we can predict where bonds are likely to form. In practice, a little more information than this is needed, but it is a great starting point.

Example 2: Using acid–base Conventions to Identify Electron Pair Donors

Fill in the blank: A species that can donate an electron pair is known as .

  1. a Lewis base
  2. a Brønsted–Lowry base
  3. a Brønsted–Lowry acid
  4. a Lewis acid
  5. an Arrhenius base

Answer

A Lewis base is a substance that can donate a lone pair of electrons to another substance, forming a bond. Conversely, a Lewis acid is a substance that can accept a lone pair of electrons. Consequently, it appears as if answer A is the correct answer and answer D is incorrect, but we will check the others to be sure.

Answers B and C refer to Brønsted–Lowry acids and bases. These types of acids and bases are defined by the loss or gain of protons and not lone pairs of electrons, so answers B and C are incorrect. Answer E seeks to define the species in terms of Arrhenius acid–base definitions. However, these definitions are related to hydrogen and hydroxide ions and not lone pairs of electrons, and so answer E is also incorrect.

Taken together, as we first thought, the correct answer is A, a Lewis base.

When a Lewis acid reacts with a Lewis base, they form what is called a Lewis acid–base adduct. The chemical equation below shows a Lewis base (ammonia) reacting with a Lewis acid (borane):

+HHHHHHHHHNHHHBNB

The lone pair from the nitrogen of ammonia can form a bond with the boron of borane. The result, HNBH33, is an example of a Lewis acid–base adduct.

The adduct can be written showing the coordinate covalent bond arrow:

NHHHBHHH

At the center of the BH3 molecule is a boron atom that can accept electrons, and the ammonia molecule has a reactive lone pair on the nitrogen. We can predict that the nitrogen will form a bond with the boron.

Definition: Coordinate Covalent Bond

A coordinate covalent bond is a covalent bond where the pair of electrons forming the bond are donated from one atom only

One of the useful features about Lewis acid and base theory is the strong relationship it has with Lewis structures. Lewis structures allow us to highlight lone pairs as well as electron-deficient areas, for instance, when an atom or ion does not have a full octet.

However, not all lone pairs are reactive. The reasons for this are complex, but some common Lewis bases are

  • halides (F, Cl, Br, I),
  • oxide (O2),
  • hydroxide (OH),
  • water (HO2),
  • cyanide (CN),
  • cyano groups (CN),
  • amines (RN3),
  • ammonia (NH3).

We tend to see more reactive lone pairs on more electronegative elements.

Examples of some electron-deficient species that will act as Lewis acids are

  • boranes (RB3, BH3),
  • aluminum trihalides (e.g., AlCl3),
  • various transition metal ions.

In many cases, knowing how to draw Lewis structures and identify lone pairs and incomplete octets should make it clear where bonds would form.

Example 3: Identifying the Species That Is Not a Lewis Acid in a Set of Chemical Formulas

Which of the following species is not a Lewis acid?

  1. H+
  2. Mg2+
  3. NH3
  4. AlCl3
  5. BF3

Answer

A Lewis acid is a substance that can accept a lone pair of electrons, forming a bond. If a species is electron deficient, this is a good sign that it could be a Lewis acid.

A hydrogen ion (H+) can accept a lone pair from a water molecule, forming the hydronium ions (HO3+).

A magnesium ion (Mg2+) can dissolve in water, and multiple water molecules donate a lone pair to form a bond with it. In these circumstances, we call the water molecules ligands.

Ammonia has a lone pair, suggesting it is electron rich rather than electron deficient. The nitrogen and hydrogen atoms have full valence shells and would not accept a lone pair from a Lewis base.

Meanwhile, aluminum chloride and trifluoroborane both have a group 13 element at their center. Atoms of group 13 elements have only 3 valence electrons. The maximum number of simple covalent bonds they can form is therefore 3, giving them 6 electrons in their valence shell and making them 2 short of an octet. They can be even more stable if they form a bond with a Lewis base.

The only species of the set that does not behave as a Lewis acid is ammonia (NH3).

It is important that we can identify Lewis acids and bases in reactions. For example, this is the reaction of trifluoroborane (BF3) and a fluoride ion (F): BF+FBF34

If we draw the Lewis structures, we can see where the lone pair that forms the bond comes from:

+FFFBFFFBFF

The fluoride ion with its reactive lone pair forms a bond with the electron-deficient boron. This can also be drawn with a coordinate covalent bond, although in reality the negative charge is distributed evenly in BF4:

FBFFF

The fluoride ion is donating a lone pair to form a bond—it is acting as a Lewis base.

The trifluoroborane is accepting a lone pair, forming a bond—it is acting as a Lewis acid.

There are some substances that are Lewis acids under some circumstances and Lewis bases under others. These are known as Lewis amphoteric substances.

When hydrogen chloride dissolves in water, a water molecule reacts with the hydrogen ion in HCl, forming the hydronium ion and the chloride ion. We often treat the hydronium ion (HO3+) like H+. However, whenever we see H+, we are really dealing with a Lewis acid–base adduct. One of the lone pairs on the oxygen in water is donated to form a bond with H+. In this example, water is acting as a Lewis base, donating a lone pair:

++H+OHHOHHH

We see a different behavior from water in the formation of ammonium hydroxide when ammonia is added to water. In this case, it is the lone pair on the nitrogen that forms a bond with the H+ ion. What we form is the ammonium ion (NH+4) and the hydroxide ion (OH):

++NHHHOHHNHHHHOH

In this example, the water is the Lewis acid as one of the hydrogens accepts an electron pair from ammonia. Water is therefore amphoteric; it can be a Lewis acid or, in other circumstances, a Lewis base.

Key Points

  • Arrhenius, Brønsted and Lowry, and Lewis described acids and bases slightly differently.
  • Lewis’s description covers the biggest range of substances.
  • A Lewis acid is a substance that can accept a lone pair of electrons, forming a bond.
  • A Lewis base is a substance that can donate a pair of electrons, forming a bond.
  • Lewis acids and bases react to form Lewis acid–base adducts.
  • Lewis structures can be used to work out what is a Lewis base and what is a Lewis acid in a particular reaction.
  • An amphoteric substance will behave as an acid in some circumstances and as a base in others.

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