Question

Complete the following table: $$ \begin{array}{|l|l|} \hline \text { Acid } & \text { Conjugate Base } \\ \hline \mathrm{H}_{2} \mathrm{O} & \\ \hline & \mathrm{CN}^{-} \\ \hline \mathrm{HNO}_{2} & \\ \hline & \mathrm{H}_{2} \mathrm{PO}_{4}^{-} \\ \hline \end{array} $$

Short answer

H2O - OH-, HCN - CN-, HNO2 - NO2-, H3PO4 - H2PO4-

Step-by-step solution

Identify the Conjugate Base of H2O

The conjugate base of water (H2O) is obtained by removing one hydrogen ion (H+). This results in hydroxide ion (OH-). Hence, the conjugate base of H2O is OH-.

Identify the Acid for CN-

The conjugate acid of CN- is obtained by adding one hydrogen ion (H+) to it. This results in hydrogen cyanide (HCN). Hence, the acid that forms CN- as a conjugate base is HCN.

Identify the Conjugate Base of HNO2

The conjugate base of nitrous acid (HNO2) is obtained by removing one hydrogen ion (H+). This results in nitrite ion (NO2-). Hence, the conjugate base of HNO2 is NO2-.

Identify the Acid for H2PO4-

The conjugate acid of H2PO4- is obtained by adding one hydrogen ion (H+) to it. This results in phosphoric acid (H3PO4). Hence, the acid that forms H2PO4- as a conjugate base is H3PO4.

Complete the Table

Fill in the missing entries using the information obtained in the previous steps. The completed table is: | Acid | Conjugate Base ||--------|-----------------|| H2O | OH- || HCN | CN- || HNO2 | NO2- || H3PO4 | H2PO4- |

Key concepts

Conjugate Acid-Base Pairs

In acid-base chemistry, acids and bases exist in pairs known as conjugate acid-base pairs. These pairs are related by the loss or gain of a hydrogen ion (H+). When an acid donates a hydrogen ion, it becomes its conjugate base. Conversely, when a base accepts a hydrogen ion, it becomes its conjugate acid.

For example:

• Water (H₂O) acts as an acid when it donates a hydrogen ion to form its conjugate base, hydroxide ion (OH⁻).
• Cyanide ion (CN⁻) acts as a base when it accepts a hydrogen ion to form its conjugate acid, hydrogen cyanide (HCN).

Understanding these pairs is essential for predicting the outcome of acid-base reactions.

Hydrogen Ion (H+)

The hydrogen ion, often represented as H⁺, plays a crucial role in acid-base reactions. It is essentially a proton, and its presence or absence determines whether a molecule behaves as an acid or a base.

When an acid dissociates in water, it releases hydrogen ions into the solution. These H⁺ ions can then be accepted by a base, which explains why acids are described as proton donors and bases as proton acceptors.

For instance:

• H₂O loses an H⁺ to become OH⁻, showing its role as an acid.
• CN⁻ gains an H⁺ to become HCN, showing its role as a base.

This simple transfer of H⁺ is the basis for defining the properties of acids and bases.

Acid Dissociation

Acid dissociation refers to the process by which an acid releases a hydrogen ion (H⁺) when it dissolves in water. This is often represented by the equation:

HA <=> H⁺ + A⁻

Here, HA represents the acid and A⁻ is its conjugate base.

For example:

When HNO₂ (nitrous acid) dissociates, it releases an H⁺, forming its conjugate base, NO₂⁻ (nitrite ion).

Similarly, water (H₂O) can dissociate to release an H⁺, forming OH⁻ (hydroxide ion).

Acid dissociation is a fundamental concept for understanding acid strength and the behavior of acids in different environments.