3.6: Acid-Base Reactions

Learning Objectives 

• Define the Arhennius acid/base and Bronsted-Lowry acid/base and differentiate between them 

• Recognize amphiprotic substances 

• 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 Bronstec-Lowry definitions. 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 the hydronium and the hydroxide ions.

H₂O(l) + H₂O(l) --> H₃O⁺(aq) + OH^-(aq)

This is a very weak reaction and in neutral water (pH=7) the concentration of hydronium (H₃O⁺) and hydroxide (OH^-) ions both have an equal concentration of 10 ^-7 moles/liter (one mole is 6.022x10²³ molecules of water). That of the undissociated water is 55.56moles/liter. This means that about one molecule dissociates for every 5.5x10⁸ water molecules that remain in tact.  

Arrhenius Acid

An Arrhenius Acid is any compound that increase hydronium concentration to that of neutral water (which also means the hydronium ion concentration is higher than the hydroxide)

HCl(aq) + H₂O(l) --> H₃O⁺(aq) + Cl^-(aq)

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

HCl(aq) + --> H⁺(aq) + Cl^-(aq)

Arrhenius Base

An Arrhenius Base increase the hydroxide concentration to that of neutral water (which also means the hydroxide concentration is higher than the hydronium).

NaOH(aq) --> Na⁺(aq) + OH^-(aq)

or

NH₃(aq) + H₂O(l) --> NH₄⁺(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.

 

The Arrhenius definitions of acids and bases is still widely used, but two different chemists, Bronsted and Lowry, independently formulated the idea that acids/base definition are based on the transfer of protons.  

Bronsted-Lowry Acid

A Bronsted-Lowry Acid is proton donor. Note the HCl gave a proton to the water molecule in the above example. 

 

Bronstead-Lowry Base

A Bronsted-Lowry Base is a proton acceptor. Note the ammonia above accepted a proton from the water.

The Bronsted-Lowry  definition is a more general definition and the one we will use in this class since it does not necessarily need to take place in water.
For example: 

NH₃ + HCl → NH₄⁺  + Cl^- 

In this case, we cannot use the Arrhenius definition to identify the acid or base because there is no water, and so the hydronium ion is not formed. But, we can use the Bronsted-Lowry definition and identify the proton transfer. It looks like NH₃ is accepting a proton, so it is a base, whereas HCl is donating a protons, so it is an acid. 

In addition, some compounds can both function as acids and bases, depending on the other compound present.
For example, 

H₂O + HCl → H₃O⁺  + Cl^- 

In this case above,  water is the proton acceptor and acts as a Bronsted-Lowry Base.

Now, we will replace HCl with NH₃: 

H₂O + NH₃ → OH^-  + NH₄⁺

In the case above, water is a proton donor and acts as a Bronsted-Lowry Acid. 

Such compounds are called amphiprotic, and can behave as both acids and bases depending on the neighboring species. 

 

Exercise \(\PageIndex{1}\)

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

A) H₃PO₄ 

B) CaCl₂ 

C) SO₄^-2

D) HSO₄^-

Answer

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

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

C) Bronsted-Lowry base - is an anion, so most likely will accept a postive proton 

D) Both (amphiprotic) - has a H⁺ to donate, but also a negative anion and can potentially accept a proton. What direction this takes depends on the other species involved in the reaction. 

Neutralization Reactions

There are 4 types of neutralization reactions

Strong Acids and Strong Bases
Strong Acids and Weak Bases
Weak Acids and Strong Bases
Weak Acids and Weak Bases

You 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.

1. Stong Acid + Strong Base

HCl(aq) + NaOH(aq) ==> NaCl(aq) + H₂O(l)

Complete Ionic Equation - both acid and base are strong, meaning they completely dissociate into ions. 
H⁺ (aq) + Cl^- (aq)+ Na⁺ (aq) + OH^- (aq) ==> Na⁺ (aq) + Cl^- (aq) + H₂O(l)

Net Ionic Equation
H⁺ (aq) + OH^- (aq) ==> H₂O(l)

Strong Acid and Strong Base

HCl(aq) + NaOH(aq) ==> ?

1:35 minutes
_{digital corrigendum: "all" salts of sodium (not chloride) are soluble}

2. Strong Acid + Weak Base

HCl(aq) + NH₃ (aq) ==> NH₄Cl(aq)

Complete Ionic Equation - since NH₃ is a weak base, it does not dissociate into ions, where as HCl, a strong acid, does. 
H⁺ (aq) + Cl^- (aq)+ NH₃ (aq) ==> NH₄⁺ (aq) + Cl^- (aq)

Net Ionic Equation
H⁺ (aq) +NH₃ (aq) ==> NH₄⁺ (aq)

Strong Acid and Weak Base

HCl(aq) + NH₃ (aq) ==> ?

1:06 minutes

3. Weak Acid + Strong Base

HF(aq) + NaOH(aq) ==> NaF(aq) + H₂O(l)

Complete Ionic Equation - since HF is a weak acid, it does not dissociate appreciably, whereas, NaOH, a strong base, does. 
HF(aq) + Na⁺ (aq) + OH^- (aq) ==> Na⁺ (aq) + F^- (aq) + H₂O(l)

Net Ionic Equation
HF(aq) + OH^- (aq) ==> F^- (aq) + H₂O(l)

Weak Acid and Strong Base

HF(aq) + NaOH(aq) ==> ?

1:20 minutes

Weak Acid + Weak Base

HF(aq) + NH₃ (aq) ==> NH₄F(aq)

Complete Ionic Equation- both acids and base are weak, so they do not dissociate appreciably. However, the product that they form NH₄F is an ionic compound that is soluble in water, so it does dissociate into ions. 
HF(aq) + NH₃ (aq) ==> NH₄⁺ (aq) + F^- (aq)

Net Ionic Equation
HF(aq) + NH₃ (aq) ==> NH₄⁺ (aq) + F^- (aq)

Weak Acid and Weak Base Reaction

This video walks students through the following problem:

HF(aq) + NH₃ (aq) ==> ?

0:58 min

You can also have the reaction of an acid with a salt, which you can treat as a double displacement reaction, where the "H" takes the role of a cation (realize it is not a cation, but in predicting products you can treat it like one).

Acid and Salt

H₂SO₄ +.Na₂CH₃CO₂ ==> ?

3:43 minutes

Exercise \(\PageIndex{2}\)

Write the Net Ionic Equations for the following

a. AgCH₃CO₂ (aq) + HCl(aq) -->

b. H₂SO₄(aq) + Pb(NO₃)₂ (aq) -->

c. H₂SO₃(aq) + AgNO₃ -->

Answer

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)

Practice

 

DOWNLOAD PRACTICE PROBLEM SET

DOWNLOAD PRACTICE PROBLEM SET KEY

 

Contributors

• Bob Belford (UALR) and November Palmer (UALR)
• Modified by Ronia Kattoum (UA of Little Rock)