Problem 16 - Chemistry The Science in Context

Question

Identify the acids and bases in the following reactions: a. \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{N}(a q)+\mathrm{H}_{2} \mathrm{O}(\ell) \rightleftharpoons\left(\mathrm{CH}_{3}\right)_{3} \mathrm{NH}^{+}(a q)+\mathrm{OH}^{-}(a q)\) b. \(\mathrm{CO}_{2}(a q)+\mathrm{H}_{2} \mathrm{O}(\ell) \rightleftharpoons \mathrm{HCO}_{3}^{-}(a q)+\mathrm{H}_{3} \mathrm{O}^{+}(a q)\) c. \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{COH}(a q)+\mathrm{H}_{3} \mathrm{O}^{+}(a q) \rightleftharpoons\) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{COH}_{2}^{+}(a q)+\mathrm{H}_{2} \mathrm{O}(\ell)\)

Step-by-Step Solution

Answer

Question: Identify the acids and bases in each reaction. a. \((\mathrm{CH}_{3})_{3} \mathrm{N}(a q)+\mathrm{H}_{2} \mathrm{O}(\ell) \rightleftharpoons(\mathrm{CH}_{3})_{3} \mathrm{NH}^{+}(a q)+\mathrm{OH}^{-}(a q)\) Base: ____________, Acid: ____________ b. \(\mathrm{CO}_{2}(a q)+\mathrm{H}_{2} \mathrm{O}(\ell) \rightleftharpoons \mathrm{HCO}_{3}^{-}(a q)+\mathrm{H}_{3} \mathrm{O}^{+}(a q)\) Acid: ____________, Base: ____________ c. \((\mathrm{CH}_{3})_{3} \mathrm{COH}(a q)+\mathrm{H}_{3} \mathrm{O}^{+}(a q) \rightleftharpoons (\mathrm{CH}_{3})_{3} \mathrm{COH}_{2}^{+}(a q)+\mathrm{H}_{2} \mathrm{O}(\ell)\) Acid: ____________, Base: ____________

Step 1: In this reaction, we can see that the nitrogen in the trimethylamine (CH3)3N molecule gains a hydrogen ion, so it must be the base. On the other hand, water (H2O) loses a hydrogen ion (forming \(\mathrm{OH}^{-}\)) and is thus the acid in this reaction. #Step 2: Write the Identified Acids and Bases#

Base: \((\mathrm{CH}_{3})_{3} \mathrm{N}(a q)\), Acid: \(\mathrm{H}_{2} \mathrm{O}(\ell)\) #Reaction b# b. \(\mathrm{CO}_{2}(a q)+\mathrm{H}_{2} \mathrm{O}(\ell) \rightleftharpoons \mathrm{HCO}_{3}^{-}(a q)+\mathrm{H}_{3} \mathrm{O}^{+}(a q)\) #Step 1: Identify Proton Donors and Acceptors#

Step 2: In this reaction, we can see that water (H2O) donates a hydrogen ion to carbon dioxide (CO2), turning it into the bicarbonate anion (HCO3-). Thus, water acts as an acid, and carbon dioxide acts as a base. #Step 2: Write the Identified Acids and Bases#

Acid: \(\mathrm{H}_{2} \mathrm{O}(\ell)\), Base: \(\mathrm{CO}_{2}(a q)\) #Reaction c# c. \((\mathrm{CH}_{3})_{3} \mathrm{COH}(a q)+\mathrm{H}_{3} \mathrm{O}^{+}(a q) \rightleftharpoons (\mathrm{CH}_{3})_{3} \mathrm{COH}_{2}^{+}(a q)+\mathrm{H}_{2} \mathrm{O}(\ell)\) #Step 1: Identify Proton Donors and Acceptors#

Step 3: In this reaction, the hydronium ion (H3O+) donates a proton to the tertiary alcohol molecule (\((CH_3)_3COH\)) to form a tert-alkylammonium ion (\((CH_3)_3COH_2^+\)). Thus, the hydronium ion is an acid and the \((CH_3)_3COH\) is a base in this reaction. #Step 2: Write the Identified Acids and Bases#

Acid: \(\mathrm{H}_{3} \mathrm{O}^{+}(a q)\), Base: \((\mathrm{CH}_{3})_{3} \mathrm{COH}(a q)\)

Key Concepts

Proton Transfer

In acid-base reactions, proton transfer is a fundamental concept. It refers to the movement of protons (H⁺ ions) between molecules. This transfer is what defines a substance as an acid or a base in many chemical reactions.
For example, in the reaction of trimethylamine \(((CH_3)_3N)\) with water, nitrogen accepts a proton, making it a base, while water donates a proton, acting as an acid:

• Base: \((CH_3)_3N(aq)\)
• Acid: \(H_2O(\ell)\)

Understanding proton transfer is crucial because it reveals how substances interact at the molecular level, enabling the prediction of reaction outcomes.

Bronsted-Lowry Theory

The Bronsted-Lowry theory offers a more comprehensive view of acids and bases. It defines acids as proton donors and bases as proton acceptors.
This theory helps us analyze reactions beyond simple hydrogen ions, accommodating substances that do not fit the classical definition of acids and bases.
In the reaction between carbon dioxide \((CO_2)\) and water \((H_2O)\), water donates a proton to form a hydronium ion \((H_3O^+)\). This makes \(H_2O\) an acid in this context.

• Acid: \(H_2O(\ell)\)
• Base: \(CO_2(aq)\)

The Bronsted-Lowry theory is pivotal for understanding versatile chemical interactions involving proton exchange.

Chemical Equilibrium

Chemical equilibrium occurs when the rates of the forward and reverse reactions are equal, leading to stable concentrations of reactants and products. In acid-base reactions, equilibrium reflects the balance between proton donors and acceptors.
Consider the reaction between tertiary alcohol \(((CH_3)_3COH)\) and hydronium ion \((H_3O^+)\). This reaction reaches equilibrium when the conversion to and from \((CH_3)_3COH_2^+\) and water stabilizes:

• Acid: \(H_3O^+(aq)\)
• Base: \((CH_3)_3COH(aq)\)

At equilibrium, the forward and backward proton transfer processes occur at equal rates. This ensures that the concentrations of all ions and molecules remain steady. Understanding chemical equilibrium is crucial for predicting the behavior of reactions in various conditions.