# 3.5: Acid-Base Reactions

Learning Objectives

• Define the Arhennius acid/base and Bronsted-Lowry acid/base and differentiate between them
• Recognize if an acid or base is strong or weak.
• Identify the acid and base in chemical reaction
• Write molecular, complete ionic, and net ionic equations for acid-base neutralization reactions

## Acids and Bases

What are acids and bases?

We will cover two definitions, the Arrhenius and Bronsted-Lowry definitions.

### Arrhenius Acid/Base

Arrhenius Acid - Increases Hydronium ion to that of neutral water

Arrhenius Base - Increases hydroxide ion to that of neutral water.

This definition is based on the behavior of acids and bases when they are added to water. First we need to look at the autoionization of water. When two
water molecules bump into each other a proton can be transferred from one to the other forming hydronium and hydroxide ions (eq. 3.5.1).

$H_2O(l) + H_2O(l) \rightarrow H_3O^+(aq) + OH^-(aq)$

This is a very weak reaction and in neutral water (pH=7) the concentration of hydronium and hydroxide ions both equal 10 -7 moles/liter. The concentration of pure water is is 55.56moles/liter (eq. 3.5.2)

$\frac{1000g \; H_2O}{L}\left ( \frac{mol \; H_2O }{18.02g}\right )=55.6mol/l$

This means that only one out of every 5.5x108 molecules of water is ionized (eq.3.5.3), which is such a small number that pure water is a nonelectrolyte.

$\frac{55.6 \frac{mol \; H_2O}{l}}{10^{-7}\frac{molH_3O^+}{l}}=5.56x10^8\frac{mole \; H_2O}{mole \; H_3O^+}$

#### Arrhenius Acid

These are compounds that donate a proton to water.

$HC(aq) + H_2O(l) \rightarrow H_3O^+(aq)+Cl^-(aq)$

This is often written with the waters omitted, that is H+(aq) represents hydronium ion and not a proton.

$HCl(aq) \rightarrow H^+(aq) + Cl^-(aq)$

#### Arrhenius Base

These are compounds that increase the hydroxide concentration. They can be soluble ionic compounds that have hydroxide ions, or compounds that remove a proton from water forming hydroxide, and the later can be molecules.

Sodium hydroxide is an ionic compound that is a strong base (you can it is soluble because of solubility rule IA)

$NaOH(aq) \rightarrow Na^+(aq)+OH^-(aq)$

Ammonia is a molecule that is a weak base, and it removes a proton from water forming ammonium and hydroxide. But it does not react much, so we use a two way arrow.

$NH_3(aq) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq)$

In the later case the ammonia took a proton from the water and this reaction is similar to the autoinoization of water, except that it is an ammonia and not another water molecule that acquires the second proton.

NOTE: Stong Acids and Bases are strong electrolytes

### Bronsted-Lowry Acid/Base

The Bronsted-Lowry definition is broader than the Arrhenius. If you look at the Arrhenius Acid, it donates a proton to water, so the Bronstead expands that to donating a proton to anything, not just water. Likewise, the Arrhenius accepts a water from water, and the Bronstead expands that to accepting a Proton from anything.

Bronstead-Lowry Acid: A Proton Donor. Note the HCl gave a proton to the water molecule in the above example

Bronstead-Lowry Base: A Proton Acceptor. Note the ammonia above accepted a proton from the water.

Exercise $$\PageIndex{8}$$

Identify the following as a Bronsted-Lowry acid, base, both, or neither

1. H3PO4
2. CaCl2
3. SO3-2
4. NaOH

Bronsted-Lowry acid - has 3 protons (H+) to donate

neither - ionic compound that is charge balanced. Neither gives nor accepts protons

Bronsted-Lowry base - is an anion and so can accept a positive proton (in fact two of them).

Bronsted-Lowry base - the hydroxide ion accepts a proton and so functions as a bronstead base

## Neutralization Reactions

A reaction between an acid and a base is called a neutralization reaction, and these can be considered to be a type of displacement reaction, where the proton of the acid is being displaced as it is given to another species. You may have heard that a neutralization reaction is when an acid and a base react to form a salt plus water. That is not always true, but they do always form a salt, as the hydrogen is being "displaced" from the acid in the formation of the salt.

Neutralization reactions always give off heat, that is they are "exothermic" (a concept we will study in Chapter 5), but when a neutralization reactions forms salt and a soluble salt, the only observation may be the heat evolved.

There are 4 types of neutralization reactions, depending on whether the acid and base are strong or weak.

1. Strong Acids and Strong Bases
2. Strong Acids and Weak Bases
3. Weak Acids and Strong Bases
4. Weak Acids and Weak Bases

To identify which type reaction is going on, you need to know if the acid or base is strong. need to know the following strong acids and strong bases. There are others, and if a problem calls it a strong acid, treat it as a strong acid. Strong acids were covered in section 3.4.1.2.1 Acids and strong bases are covered by the solubility rules Ia and IIIa of section 3.4.2.1. Table $$\PageIndex{1}$$ reviews them for you.

 Strong Acids Strong Bases HCl, HBr, HI HClO3, HClO4 HNO3 H2SO4 others as specified Soluble alkali metal hydroxides (solubility rule 1a) LiOH, NaOH, KOH, RbOH, CsOH Soluble heavier alkaline earth hydroxides (solubility rule 3a) Sr(OH)2, Ba(OH)2​ ​​​​​​ Ca(OH)2 should probably be considered as a strong base even though we are not calling it a soluble salt in rule 3a. A saturated solution has is 1.8 g/L) and thus strong, and we will cover these "border cases" in gen chem 2 when we get to solubility constants.

You need to know the general (molecular), total (complete) and net ionic equations for acid base neutralization reactions

### Strong Acid and Strong Base

The result is a salt plus water; the anion of the acid and the cation of the base are spectator ions. Example: HCl and NaOH

General Molecular Equation:

$HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l)$

Total Ionic Equation
$H^+(aq) + Cl^-(aq)+ Na^+(aq) + OH^-(aq) \rightarrow Na^+(aq) + Cl^-(aq) + H_2O(l)$

Net Ionic Equation

$H^+(aq) + OH^-(aq) \rightarrow H_2O(l)$

### Strong Acid and Weak Base

The cation of the strong acid is a spectator ion. Example: HCl & NH3.

General Equation: $HCl(aq) + NH_3(aq) \rightarrow NH_4Cl(aq)$

Total Ionic Equation: $H^+(aq) + Cl^-(aq)+ NH_3(aq) \rightarrow NH_4^+(aq) + Cl^-(aq)$

Net Ionic Equation: $H^+(aq) + NH_3(aq)\rightarrow NH_4^+(aq)$

### Weak Acid and Strong Base

In the reaction of a Weak Acid and a Strong base the cation of the base is a spectator ion. Example: HF + NaOH.

General Equation

$HF(aq)+NaOH(aq) \rightarrow NaF(aq) +H_2O$

Total Ionic Equation
$HF(aq) + Na^+(aq) + OH^-(aq) \rightarrow Na^+(aq) + F^-(aq) + H_2O(l)$

Net Ionic Equation

$HF(aq)+ OH^-(aq) \rightarrow F^-(aq) + H_2O(l)$

### Weak Acid and Weak Base

In the reaction of a weak acid and a weak base there is no spectator ion. Since there are no spectator ions, the total ionic and the net ionic are exactly the same. Example HF and NH3.

General Equation

$HF(aq) + NH_3(aq) \rightarrow NH_4F(aq)$

Total Ionic Equation
$HF(aq) + NH_3 (aq) \rightarrow NH_4^+(aq) + F^-(aq)$

Net Ionic Equation
$HF(aq) + NH_3 (aq) \rightarrow NH_4^+(aq) + F^-(aq)$

## Acids and Salt

Acids also react with ionic compounds, in fact many ways of leaching metals from ionic ores involves reactions with acids. These can be treated as a double displacement reaction where the hydrogen is being displaced by the cation. Realize hydrogen is not a cation, and the acid is a molecule, but just as we name it along nomenclature rules that use its salt (section 2.7.4.2), we can also treat it the way we would a cation, in predicting how it reacts. Example: H2SO4 + NaCH3CO2

General Equation:

$H_2SO_4(aq) + 2NaCH_3CO_2(aq) \rightarrow Na_2SO_4(aq) + 2HCH_3CO_2(aq)$

Total Ionic:

$2H^+(aq) +SO_4^{-2}(aq) + 2Na^+(aq)+2CH_3CO_2^-(aq) \rightarrow 2Na^+(aq)+ SO_4^{-2}(aq) + 2HCH_3CO_2(aq)$

Net Ionic:

$2H^+(aq) +2CH_3CO_2^-(aq) \rightarrow 2HCH_3CO_2(aq)$

Notice how in this reaction a covalent bond was formed and we made the molecule acetic acid.

Exercise $$\PageIndex{2}$$

Predict and Write Net Ionic Equations for the following

A) AgCH3CO2 (aq) + HCl(aq) -->

B) H2SO4(aq) + Pb(NO3)2 (aq) -->

C) H2SO3(aq) + AgNO3 -->

A) Ag+(aq)+ CH3CO2-(aq) + H+(aq)+ Cl-(aq) ® AgCl(s) + HCH3CO2 (aq)

B) Pb+2(aq) + SO4-2(aq) ® Pb SO4(s)

C) H2SO3(ag) + 2Ag+(aq) ® Ag2SO3(s) + 2H+(aq)

## Acid Reactions Evolving Gases

We have seen that acid-base reactions can produce gasses. Many of these can also produce gases, in fact, the acids of oxyanions can decompose into a nonmetal oxide and water upon heating.

### Heating OxyAcids

Oxyacids are acids of polyatomic ions that are oxyanions. These can decompose under heat into a nonmetal oxide and water.

$\underbrace{H_2SO_4(aq)}_{oxyacid} \overset{heat}{\rightarrow}\underbrace{SO_3(g)}_{\text{nonmetal oxide}} + H_2O(l)$

$\underbrace{H_2SO_3(aq)}_{oxyacid} \overset{heat}{\rightarrow}\underbrace{SO_2(g)}_{\text{nonmetal oxide}} + H_2O(l)$

$\underbrace{H_2CO_3(aq)}_{oxyacid} \overset{heat}{\rightarrow}\underbrace{CO_2(g)}_{\text{nonmetal oxide}} + H_2O(l)$

Another name for the nometal oxide is the acid anhydride, as you have removed a water from the oxyacid, therefor it is the acid without the water, the acid anhydride.

### Strong Acids and Salts of Oxyanions

These are actually two reactions, first a double displacement reaction, followed by a decomposition reaction. In the first step, the hydrogen of the strong acid displaces the cation of the salt containing a oxyanion, forming the oxyacid, which then decomposes. Consider the reaction of HCl and CaCO3 (Calcium carbonate, but known as Calcite to geologists)

In the first step, we do a double displacement type of reaction forming carbonic acid. Note, when we say HCl(aq), we actually have a solution that is mostly water, and we are adding it to a solid rock, and so the products are also dissolved into the water (watch video $$\PageIndex{6}$$ as geologists use this reaction to identify a rock vein.

$2HCl(aq) + CaCO_3(s) \rightarrow H_2CO_3(aq) + CaCl_2(aq)$

In the second step the carbonic acid decomposes into carbon dioxide and water

$H_2CO_3(aq) \rightarrow CO_2(g) + H_2O(aq)$