Question

The dominant species obtained when is dissolved in \(\mathrm{H}_{2} \mathrm{O}\) at \(\mathrm{pH}=2, \mathrm{pH}=7\) and at \(\mathrm{pH}=12\)

Short answer

For ammonia, the dominant species in solution at different pH levels are: 1. \(pH = 2\) (acidic condition): \(NH_{4}^{+}\) 2. \(pH = 7\) (neutral condition): Both \(NH_{3}\)and \(NH_{4}^{+}\) 3. \(pH = 12\) (basic condition): \(NH_{3}\) As the pH level increases from acidic to alkaline, the dominant species of ammonia in water will shift from \(NH_{4}^{+}\) to \(NH_{3}\).

Step-by-step solution

Identify the Reaction of Ammonia with Water

Ammonia in water forms the ammonium cation (\(NH_{4}^{+}\)) and hydroxide ion (\(OH^{-}\)) as follows: \[ NH_{3} + H_{2}O \rightleftharpoons NH_{4}^{+} + OH^{-} \]

Analyze the Reaction at Different pH Levels

(a) pH = 2 (Acidic condition)

At lower pH levels (acidic conditions), the concentration of \(H^{+}\) ions is high. These \(H^{+}\) ions will react with \(NH_{3}\) to form \(NH_{4}^{+}\). Hence, the dominant species in solution would be \(NH_{4}^{+}\).

(b) pH = 7 (Neutral condition)

At neutral pH, equal concentrations of \(NH_{3}\)and \(NH_{4}^{+}\) will be in the solution. Therefore, the dominant species will be both \(NH_{3}\)and \(NH_{4}^{+}\).

(c) pH = 12 (Basic condition)

In basic conditions (higher pH levels), the concentration of \(OH^{-}\) ions is high. These ions will decrease the equilibrium concentration of \(NH_{4}^{+}\) due to reaction of \(NH_{4}^{+}\) with \(OH^{-}\) ions to form \(NH_{3}\) and \(H_{2}O\), making \(NH_{3}\) as the dominant species in the solution. In summary, the dominant species of ammonia in water will shift from \(NH_{4}^{+}\) to \(NH_{3}\) as the pH level increases from acidic to alkaline.

Key concepts

pH Levels

The pH scale is a measurement of how acidic or basic a solution is. It ranges from 0 to 14, where 7 is considered neutral. Solutions with a pH less than 7 are acidic, and those with a pH greater than 7 are basic or alkaline. The lower the pH, the higher the concentration of hydrogen ions (H^{+}), leading to an acidic environment.
In the case of ammonia in water, pH levels play a crucial role in determining the dominant chemical species present. For instance, at \(\text{pH} = 2\), which is highly acidic, a lot of H^{+} ions are present. They interact with ammonia (NH_{3}), converting it into the ammonium ion (NH_{4}^{+}).

• pH = 2 (Acidic): Dominant species is NH_{4}^{+}
• pH = 7 (Neutral): Equal amounts of NH_{3} and NH_{4}^{+}
• pH = 12 (Basic): Dominant species is NH_{3}

Understanding pH levels helps us predict reactions and equilibrium states in various chemical environments.

Acid-Base Reactions

Acid-base reactions involve the transfer of protons (H^{+} ions) between reactants. They are foundational to chemistry and control reaction dynamics. Ammonia (NH_{3}) is a weak base. In water, it acts as a Lewis base, receiving hydrogen ions from the water molecules, forming NH_{4}^{+}.
The general equilibrium reaction in water can be represented as:\[NH_{3} + H_{2}O \rightleftharpoons NH_{4}^{+} + OH^{-}\]
In acidic conditions (low pH), more H^{+} ions shift the equilibrium to favor NH_{4}^{+} (ammonium ion) formation. Conversely, in basic conditions (high pH), higher amounts of OH^{-} ions push the reaction towards forming free ammonia (NH_{3}) and water.

• Low pH: NH_{3} converts to NH_{4}^{+}
• High pH: NH_{4}^{+} tends to convert back to NH_{3}

These reactions illustrate how equilibrium can shift based on environmental conditions like pH.

Equilibrium Concentrations

Chemical equilibrium is reached in a reversible reaction when the rate of the forward reaction equals the rate of the reverse reaction. At this point, the concentrations of reactants and products remain constant over time. For the ammonia-water system, the equilibrium depends heavily on pH levels.
At a low pH, the equilibrium shifts toward forming more NH_{4}^{+} due to the excess H^{+} ions pushing the reaction forward. At a high pH, increased OH^{-} ions pull the equilibrium back, forming more NH_{3} .

• Equilibrium in acidic conditions: Favor formation of NH_{4}^{+}
• Equilibrium in alkaline conditions: Favor formation of NH_{3}
• Neutral equilibrium: Approximately equal concentrations of NH_{3} and NH_{4}^{+}

Understanding these shifts helps chemists predict and control the outcomes of chemical reactions in different environments.