Problem 109 - Introductory Chemistry Atoms First

Question

Acetic acid is a weak acid. Upon losing a proton, acetic acid yields the acetate anion. (a) Draw a dot diagram for the acetate anion. (b) Hydrogen atoms that are bound to carbon atoms are not acidic. Knowing this, is it possible for the acetate anion to serve as an acid? Explain. (c) In fact, acetate ion is a weak base. Write a chemical equilibrium with unequal-length arrows showing how acetate in water produces a weakly basic solution. (d) Is water acting as an acid or a base? Explain.

Step-by-Step Solution

Answer

(a) The dot diagram for the acetate anion is represented as \(CH_3COO^-\), with a fulfilled octet or duet for each atom. (b) The acetate anion cannot serve as an acid because it cannot donate a proton since the hydrogen atoms are bound to carbon atoms, which are not acidic. (c) The chemical equilibrium for acetate in water is: \(CH_3COO^-(aq) + H_2O(l) \rightleftharpoons CH_3COOH(aq) + OH^-(aq)\), showing the formation of hydroxide ions (\(OH^-\)). (d) In this reaction, water acts as an acid as it donates a proton to the acetate ion.

Step 1: (a) Drawing the Dot Diagram for Acetate Anion

To draw the dot diagram for the acetate anion, first, consider its chemical formula, \(CH_3COO^-\). It consists of two carbon atoms, three hydrogen atoms, and two oxygen atoms. The central carbon atom is bonded to the three hydrogen atoms and the other carbon atom. The second carbon atom is double bonded to one oxygen and single bonded to the other oxygen atom, which bears the negative charge. The acetate anion can be represented as: \[ CH_3COO^- \] Remember that all atoms should have fulfilled their octet (or duet for hydrogen) in the dot diagram representation.

Step 2: (b) Can Acetate Anion Serve as an Acid?

Since hydrogen atoms bound to carbon atoms are not acidic, the hydrogen atoms in the acetate anion are not capable of being donated as protons. Therefore, the acetate anion cannot serve as an acid because it cannot donate a proton.

Step 3: (c) Chemical Equilibrium for Acetate in Water

When acetate ion reacts with water, it forms a weak base solution. The chemical equilibrium for this reaction can be written as: \[ CH_3COO^-(aq) + H_2O(l) \rightleftharpoons CH_3COOH(aq) + OH^-(aq) \] This equation depicts the behavior of acetate ion as a weak base: it accepts a proton from water, forming hydroxide ions (\(OH^-\)) in the process.

Step 4: (d) Is Water Acting as an Acid or a Base?

In the given equilibrium reaction, water donates a proton to the acetate ion and subsequently forms hydroxide ions (\(OH^-\)). This means water is acting as an acid in this context because it donates a proton to another species (the acetate ion).

Key Concepts

Dot Diagram for Acetate Anion

To understand the acetate anion, it's essential to draw its dot diagram. The chemical formula is \(CH_3COO^-\). This formula tells us about the atoms involved:

• 2 Carbon atoms
• 3 Hydrogen atoms
• 2 Oxygen atoms

In the structure, one carbon attaches to the three hydrogen atoms and links to the other carbon. The second carbon shares a double bond with one oxygen and a single bond with another oxygen, carrying the negative charge.
This structure ensures that all atoms meet their octet (or duet, in hydrogen's case) requirement, highlighting how valence electrons are shared or transferred in the molecule.

Understanding Weak Acids

Acetic acid is classified as a weak acid. But what does this mean? Unlike strong acids, weak acids do not completely dissociate in water.
In the case of acetic acid:

• It partially releases hydrogen ions (\(H^+\)) in a solution.
• This incomplete ionization is why the solution isn't strongly acidic.

For example, when acetic acid gives away a proton, it becomes acetate. But, as noted, the acetate anion itself can't serve as an acid because it lacks hydrogen atoms that can be donated as protons.

Chemical Equilibrium in Acetate Reaction

Chemical equilibrium refers to a state where the forward and reverse reactions occur simultaneously at the same rate. In the context of acetate in water, the reaction is represented as:

• \[CH_3COO^-(aq) + H_2O(l) \rightleftharpoons CH_3COOH(aq) + OH^-(aq)\]

Here, the acetate ion reacts with water to form acetic acid and hydroxide ions. The equilibrium lies to one side more than the other due to the weak nature of the base produced. This weak base means that there are more reactants and fewer products at equilibrium, indicated by the unequal-length arrows in the equation.

Role of Water in Acid-Base Reactions

In an acid-base reaction involving acetate, water plays a crucial role. In this reaction:

• Water donates a proton (\(H^+\)) to the acetate ion.
• This interaction forms hydroxide ions (\(OH^-\)).

Thus, water acts as an acid in this scenario since it transfers a proton to the acetate ion.
This example underlines the amphoteric nature of water, meaning it can behave as an acid or a base depending on the substances it interacts with.