An acid-base reaction, also known as a neutralization reaction, is a type of chemical reaction that occurs between an acid (H+) and a base (OH-). In this reaction, the acid and base react with each other to produce a salt and water. One way to look at acid-base reactions is that the acid donates a proton (H+) to the base, which is usually negatively charged. This reaction results in the formation of a neutral compound. The general equation for an acid-base reaction is:
\[ Acid + Base \Rightarrow Salt + Water\]
For example, the reactios between hydrochloric acid (\(HCl \rightarrow H^+ + Cl^-\)) and sodium hydroxide (\(NaOH \rightarrow Na^+ + OH^-\)) can be represented as:
\[HCl + NaOH \Rightarrow NaCl + H_2O\]
In this reaction, HCl is the acid and NaOH is the base. They react to form sodium chloride (NaCl) and water (H2O).
In this article, we will learn all about acid-base reactions, what they look like, their types, and how these reactions occur.
- This article is about acid-base reactions
- We will learn the difference between the two types of acid-base reactions: Brønsted-Lowry and Lewis acid-base reactions
- We will learn about a special kind of Brønsted-Lowry acid-base reaction called a neutralization reaction
- Lastly, we will learn about complex ions and how the Lewis concept of acids and bases explains how they are formed.
Acid-base Reaction Definition
Have you ever made a baking soda volcano? You pour some vinegar into a paper-mâché volcano full of baking soda, and BAM your volcano erupts getting a red, bubbly slurry all over your kitchen table.
Fig.1A baking soda volcano is an acid-base reaction between baking soda and vinegar. Flickr
The reaction of vinegar and baking soda is a classic example of an acid-base reaction. In this example, vinegar is the acid and baking soda is the base.
Acid-base reactions come in two types: Brønsted-Lowry and Lewis acid-base reactions. These two types of reactions are based on the different definitions of an acid and a base. For both types, an acid or base can be identified by its pH.
The pH of a solution indicates its acidity. It formally means "presence of hydrogen" since the formula is:
\[p\,H=-log[H^+]\]
Since this is a negative logarithm, the smaller the pH, the greater the concentration of hydrogen. The pH scale goes from 0 to 14, where 0-6 is acidic, 7 is neutral, and 8-14 is basic.
Let's start off by covering the first type of acid-base reaction.
Brønsted-Lowry Acid-base reaction
The first type of acid-base reaction is the one that is between a Brønsted-Lowry acid and base.
A Brønsted-Lowry acid is a species that can donate a proton (H+ ion) while a Brønsted-Lowry base is a species that will accept that proton. The basic form for these acid-base reactions is:
\[HA + B \rightarrow A^- + HB\]
In the above reaction, the acid, HA, becomes the conjugate base, A-, meaning it can now act as a base. For the base, B, it becomes the conjugate acid, HB, so it now acts as an acid. Here are some other examples of this type of reaction:
\(HCO_3^- + H_2O \rightarrow H_2CO_2 + OH^-\)\(HCl + H_2O \rightarrow Cl^- + H_3O^+\)\(NH_4^+ + OH^- \rightarrow NH_3 + H_2O\)
As seen in the examples above, water is amphoteric. This means it can act as both an acid and a base. How it will act is based on the acidity of whatever species it is reacting with.
So, how can you tell whether water will act as an acid or base? We can use the acid dissociation constant (Ka) and/or the base dissociation constant (Kb) to determine the relative acidity/basicity of a species and compare them to see how a species will act. The formula for these constants respectively is:
\(K_a=\frac{[H_3O^+][A^-]}{[HA]}\)
\(K_b=\frac{[OH^-][BH]}{[B^-]}\)
For pure water, since it is a neutral species, Ka = Kb. This value (Kw) is equal to 1x10-14:
\(H_2O \rightarrow H^++OH^-\)
\(K_w=\frac{[H^+][OH^-]}{[H_2O]}=1X10^{-14}\)
Let's compare the Kw of water to the Kb of bicarbonate, HCO3-. The Kb of HCO3- is 4.7 · 10-11. Since Kb > Kw, that means that HCO3-, is more basic and therefore water will act as an acid in this reaction (as shown in the previous example above). The larger the Ka or Kb value is, the stronger that base or acid is.
Polyprotic Acids
Some acids can be classified as polyprotic acids.
A polyprotic acid has multiple protons it can donate. Once it loses a proton, it is still considered both the acid and a conjugate base. This is because it is becoming less acidic with each proton lost (and therefore more basic).
Phosphoric acid, H3PO4, is a polyprotic acid that can give up three protons:
\( \begin {align}H_3PO_4 + H_2O &\rightarrow H_2PO_4^- + H_3O^+ \\H_2PO_4^- + H_2O &\rightarrow HPO_4^{2-} + H_3O^+ \\HPO_4^{2-} + H_2O &\rightarrow PO_4^{3-} + H_3O^+ \\\end {align}\)
Note, that these types of acids won't necessarily keep donating protons until they have none left. Depending on the conditions, they may lose only 1, or even lose 2, and subsequently then gain a proton back (since it is now more basic). Acid-base Neutralization Reaction
A special type of Brønsted-Lowry acid-base reaction is neutralization.
In a neutralization reaction, a Brønsted-Lowry acid and base react to form a neutral salt and water.
| Strong Acids | Strong Bases |
| HCl (hydrochloric acid) | LiOH (lithium hydroxide) |
| HBr (hydrobromic acid) | NaOH (sodium hydroxide) |
| HI (hydroiodic acid) | KOH (potassium hydroxide) |
| HNO3 (nitric acid) | Ca(OH)2 (calcium hydroxide) |
| HClO4 (perchloric acid) | Sr(OH)2 (strontium hydroxide) |
| H2SO4 (sulfuric acid) | Ba(OH)2 (barium hydroxide) |
The other key characteristic of strong acids/bases is that they completely ionize in water, which is why they can neutralize when combined. Here are some examples of neutralization reactions:
\(HBr + NaOH \rightarrow NaBr + H_2O\)
\(HClO_4 + KOH \rightarrow KClO_4 + H_2O\)
\(H_2SO_4 + Ba(OH)_2 \rightarrow BaSO_4 + H_2O\)
Since the acid and base are completely neutralized, the pH of the solution is 7.
Lewis Acid-base Reaction
The second type of acid-base reaction is the reaction between a Lewis acid and Lewis base. The Lewis acid-base concept focuses on electron lone pairs rather than protons.
A Lewis acid-base reaction is between a Lewis acid and a Lewis base. A Lewis acid (also called an electrophile) accepts electrons from a Lewis base (also called a nucleophile). An electrophile "loves electrons" and has an empty orbital that can accommodate a lone pair of electrons from the nucleophile. The nucleophile "attacks" the positively charged electrophile and gives it that extra lone pair of electrons.
A molecular orbital is a quantum-mechanical mathematical function that describes the physical properties (discrete energy levels, wave-like nature, probability amplitude, etc.) of an electron within a molecule.
The probability amplitude of an electron in a molecule describes, mathematically, the probability of finding an electron, in a given quantum state, in a specific region of a given molecule.
A quantum state is one from a set of mathematical functions, based on the physics of quantum mechanics, that together describe all of the possible energy levels, and possible outcomes of experimental measurements, for an electron within a molecule.
Here is a breakdown between nucleophiles and electrophiles:
| Nucleophiles (Lewis Base) | Electrophiles (Lewis Acid) |
| Typically have a (-) charge or lone pair | Typically have a (+) charge or an electron-withdrawing group (pulls electron density towards it, causing a partial positive charge) |
| Donates electrons to the electrophile | Can also have a polarizable bond (In a double bond, there is a difference in polarity between the two elements) |
| When sharing electrons, it forms a new bond with the electrophile | Accept electrons from the nucleophile |
| Examples:\(OH^-\,\,CN^-\,\,O^-R\,\,RC\equiv C\)Note: R is any -CH2 group like -CH3 | Examples:\(R-Cl\,\,BF_3^+\,\,Cu^{2+}\,SO_3\,\,H_2C^{\delta +}=O^{\delta -}\)Note: O is pulling the e- density from C, so the bond is partially polarized |
While Lewis acid-base reactions also involve the donating/accepting of something like Brønsted-Lowry acid-base reactions, the key difference is that a bond is formed. The electrons being donated by the nucleophile are shared between the two species. Here are some examples of this reaction:
Fig.2-Examples of Lewis acid-base reactions. The Lewis base/nucleophile donates electrons to the Lewis acid/electrophile.
The new bond formed is highlighted in red for each compound.
One of the reasons why the electron pair in a Lewis base attacks and bonds with a Lewis acid is because this bond is lower in energy. The lone pair of electrons are in the Highest Occupied Molecular Orbital (HOMO), meaning they are in the highest energy level in that molecule. These electrons will interact with the acid's Lowest Unoccupied Molecular Orbital (LUMO) to form this bond.
Fig.3-The lone pair in the highest occupied orbital of the base interacts with the lowest unoccupied orbital of the acid to form a bond.
Electrons always want to be in as low of an energy state as possible, and bonding orbitals are lower in energy than non-bonded orbitals. This is because a bond is much more stable than a reactive lone pair.
Complex Ions/Coordination Complexes
The Lewis concept of acid and base is a more expansive theory than its counterpart. It can explain some things that the Brønsted-Lowry concept cannot: such as how coordination complexes are formed.
A coordination complex is a complex with a metal ion in the center and other smaller ions bonded to it. A Lewis base is typically the ligand (things attached to the metal), while the metal acts as a Lewis acid. A complex ion is a coordination complex that has a charge.
Fig.4-The formation of the coordination complex is an example of a Lewis acid-base reaction, with CN acting as the base and Zn acting as the acid.
CN- is acting as our Lewis base and is donating its excess electrons to Zn2+. Bonds are formed between each of the CN- and Zn2+, which creates the complex ion
Coordination complexes are typically formed with transition metals, but other metals like aluminum can also form these complexes.
Acid-base Reaction Examples
Now that we've covered the different types of acid-base reactions, let's look at some examples and see if we can identify them.
Identify the type of acid-base reaction and subtype if applicable:
\(HI + KOH \rightarrow H_2O + KI\)
\(Cu^{2+} + 4NH_3 \rightarrow [Cu(NH_3)_4]^{2+}\)
\(F^- + H_2O \rightarrow HF + OH^-\)
\(Al^{3+} + 3OH^- \rightarrow Al(OH)_3\)
1. The key piece here is that water is being formed. We see that HI is losing H+ and KOH is gaining H+, so this is a Brønsted-Lowry neutralization acid-base reaction.
2. Here, a metal is surrounded by NH3 ions. This is a coordination complex, which is formed by a Lewis acid-base reaction
3. F- is gaining H+ and H2O is losing H+ so it is a Brønsted-Lowry acid-base reaction
4. Since a bond is being formed, this is a Lewis acid-base reaction. The oxygen in the OH- ions is donating a lone pair to the aluminum (Al3+) ion, which also shows that this is a Lewis acid-base reaction
The easiest way to distinguish between a Lewis acid-base reaction and a Brønsted-Lowry acid-base reaction is whether a bond is being formed (Lewis) or if a proton (H+) is being swapped (Brønsted-Lowry).
Acid-Base Reactions - Key takeaways
- There are two types of acid-base reactions: Brønsted-Lowry acid-base and Lewis acid-base reactions
- A Brønsted-Lowry acid is a species that can donate a proton (H+ ion) while a Brønsted-Lowry base is a species that will accept that proton.
- During a Brønsted-Lowry acid-base reaction, the acid is converted into a conjugate base, and the base is converted into a conjugate acid.
- A polyprotic acid has several protons it can donate in a reaction.
- In a neutralization reaction, a Brønsted-Lowry acid and base react to form a neutral salt and water.
- A Lewis acid-base reaction is between a Lewis acid and a Lewis base. A Lewis acid (also called an electrophile) accepts electrons from a Lewis base (also called a nucleophile). An electrophile "loves electrons" and has an empty orbital for a lone pair from the nucleophile. The nucleophile "attacks" the positively charged electrophile and gives it that extra lone pair
- A coordination complex is a complex with a metal ion in the center and other smaller ions bonded to it. A Lewis base is typically the ligand (things attached to the metal), while the metal acts as a Lewis acid. A complex ion is a coordination complex that has a charge.
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