Acids and Bases

The Bronsted-Lowry concept expands the acid-base theory to include solvents beyond water.

1. Acid: A substance that can donate a proton ($H^+$) to another substance.
   • Example: $HCl \rightarrow H^+ + Cl^-$
2. Base: A substance that can accept a proton ($H^+$) from another substance.
   • Example: $NH_3 + H^+ \rightarrow NH_4^+$

Conjugate Acid-Base Pairs

For every acid, there is a corresponding base, called a conjugate base, and for every base, there is a corresponding acid, called a conjugate acid. Examples of conjugate acid-base pairs include:

$HA + H^+ \rightarrow A^-$

For example, $HCl$ is an acid, and $Cl^-$ is its conjugate base. Similarly, $NH_3$ is a base, and $NH_4^+$ is its conjugate acid.

Examples of Bronsted-Lowry Acids and Bases

1. Monoprotic Acid: An acid that donates only one proton (e.g., $HCl$, $HNO_3$).
2. Diprotic Acid: An acid that donates two protons (e.g., $H_2SO_4$).
3. Polyprotic Acid: An acid that donates more than one proton (e.g., $H_3PO_4$).
4. Monoprotic Base: A base that accepts only one proton (e.g., $NH_3$).
5. Polyprotic Base: A base that accepts more than one proton (e.g., $CO_3^{2-}$).

Strength of Acids and Bases

According to the Bronsted-Lowry concept, the stronger the acid, the weaker its conjugate base, and the stronger the base, the weaker its conjugate acid.

Example: $CH_3COOH$ is a weak acid, but its conjugate base $CH_3COO^-$ is a strong base.

Weakness of the Bronsted-Lowry Concept

The Bronsted-Lowry concept does not account for acids and bases that do not contain protons, such as $AlCl_3$, $BF_3$, or $SO_3$.

The Lewis concept broadens the definition of acids and bases by focusing on the transfer of electron pairs instead of protons.

1. Lewis Acid: A substance that can accept a pair of electrons (electron pair acceptor).
   • Example: $BF_3$, $AlCl_3$
2. Lewis Base: A substance that can donate a pair of electrons (electron pair donor).
   • Example: $NH_3$, $OH^-$

Example of a Lewis Acid-Base Reaction

When ammonia ($NH_3$) donates an electron pair to boron trifluoride ($BF_3$), a Lewis acid-base reaction occurs, forming a complex:

$NH_3 + BF_3 \rightarrow NH_3BF_3$

Advantages of the Lewis Concept

1. Applies to any solvent, not just water.
2. Includes reactions where no proton transfer occurs, such as reactions involving $AlCl_3$ and $BF_3$.

Weakness of the Lewis Concept

The Lewis concept is primarily applicable in special cases and may not be as widely applicable to all acid-base reactions as the Bronsted-Lowry theory.

Question 1: Conjugate Acid-Base Pair

i) Define conjugate acid-base pair.

ii) Define conjugate base.

Question 2: Conjugate Bases and Conjugate Acids

a) Write the formula and give the name of the conjugate base for each of the following acids:

i) $HCl$

ii) $HNO_3$

b) Write the name and formula of the conjugate acids for each of the following bases:

i) $NH_3$

ii) $Br^-$

iii) $HS^-$

Answer to Example Problem

Answer a) For $HCl$, the conjugate base is $Cl^-$. For $HNO_3$, the conjugate base is $NO_3^-$.

Answer b) The conjugate acids are $NH_4^+$, $HBr$, and $H_2S$ for $NH_3$, $Br^-$, and $HS^-$, respectively.