Problem 107 - Introductory Chemistry Atoms First

Question

Ethanol, \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\), and hydride ion, \(\mathrm{H}^{-}\), react to produce \(\mathrm{H}_{2}\) gas and the \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{O}^{-}\) anion. Arrhenius could not tell you what is going on, but Bronsted and Lowry would have no trouble. How would they explain this reaction?

Step-by-Step Solution

Answer

According to the Bronsted-Lowry theory, the reaction between ethanol (\(\mathrm{C}_{2}\mathrm{H}_{5}\mathrm{OH}\)) and hydride ion (\(\mathrm{H}^{-}\)) can be explained as a proton transfer reaction. Ethanol acts as an acid and donates a proton to the hydride ion, which acts as a base. This results in the formation of the ethoxide ion (\(\mathrm{C}_{2}\mathrm{H}_{5}\mathrm{O}^{-}\)) and hydrogen gas (\(\mathrm{H}_{2}\)) as the conjugate base and conjugate acid, respectively.

Step 1: Write the balanced chemical equation

We first need to write down the balanced chemical equation for the reaction between ethanol and hydride ion: \(\mathrm{C}_{2}\mathrm{H}_{5}\mathrm{OH} + \mathrm{H}^{-} \rightarrow \mathrm{H}_{2} + \mathrm{C}_{2}\mathrm{H}_{5}\mathrm{O}^{-}\)

Step 2: Identify the Bronsted-Lowry acid and base

According to the Bronsted-Lowry theory, an acid donates a proton (H+) and a base accepts a proton. In this reaction, ethanol (\(\mathrm{C}_{2}\mathrm{H}_{5}\mathrm{OH}\)) donates a proton to the hydride ion (\(\mathrm{H}^{-}\)), forming molecular hydrogen (\(\mathrm{H}_{2}\)) and the ethoxide ion (\(\mathrm{C}_{2}\mathrm{H}_{5}\mathrm{O}^{-}\)). So, ethanol acts as the Bronsted-Lowry acid, and the hydride ion acts as the Bronsted-Lowry base.

Step 3: Identify the conjugate acid-base pairs

A conjugate acid is the species formed when a base accepts a proton, and a conjugate base is the species formed when an acid donates a proton. In this reaction, the Bronsted-Lowry acid (ethanol) donates a proton to the Bronsted-Lowry base (hydride ion) and forms its conjugate base (ethoxide ion). The Bronsted-Lowry base (hydride ion) accepts a proton from the acid (ethanol) and forms its conjugate acid (hydrogen gas). Therefore, the conjugate acid-base pairs are: - Ethanol (\(\mathrm{C}_{2}\mathrm{H}_{5}\mathrm{OH}\)) and its conjugate base, Ethoxide ion (\(\mathrm{C}_{2}\mathrm{H}_{5}\mathrm{O}^{-}\)) - Hydride ion (\(\mathrm{H}^{-}\)) and its conjugate acid, Hydrogen gas (\(\mathrm{H}_{2}\))

Step 4: Explain the reaction according to Bronsted and Lowry

According to the Bronsted-Lowry theory, the reaction between ethanol and hydride ion can be explained as a proton transfer reaction. Ethanol, acting as an acid, donates a proton to the hydride ion, which is acting as a base. As a result, the ethoxide ion and hydrogen gas are formed as the conjugate base and conjugate acid, respectively.

Key Concepts

Proton Transfer Reaction

In the world of chemistry, a proton transfer reaction is a fundamental concept. It describes the process where a proton is transferred from one molecule to another. In this scenario, we look at ethanol (\( \mathrm{C}_2\mathrm{H}_5\mathrm{OH} \)) reacting with a hydride ion (\( \mathrm{H}^- \)).
This reaction leads to the formation of hydrogen gas (\( \mathrm{H}_2 \)) and an ethoxide ion (\( \mathrm{C}_2\mathrm{H}_5\mathrm{O}^- \)). Quite simply, the proton from ethanol is donated to the hydride ion.
This kind of proton transfer is essential in explaining how certain reactions occur, especially where traditional Arrhenius concepts fall short.

• Proton transfer is a central mechanism in many acid-base reactions.
• Helps classify the roles of participants in a reaction as acids or bases.

Conjugate Acid-Base Pairs

In a chemical reaction, the concept of conjugate acid-base pairs is crucial. After a proton transfer occurs, species transform into their conjugate forms.
For the reaction between ethanol and hydride ion, two pairs are identified:

• Ethanol (\( \mathrm{C}_2\mathrm{H}_5\mathrm{OH} \)) becomes its conjugate base, the ethoxide ion (\( \mathrm{C}_2\mathrm{H}_5\mathrm{O}^- \)).
• The hydride ion (\( \mathrm{H}^- \)) becomes its conjugate acid, hydrogen gas (\( \mathrm{H}_2 \)).

Understanding these pairs aids in predicting the direction and products of reactions. Conjugate pairs emerge when molecules donate and accept protons, giving each participant a new identity.

Ethanol as a Bronsted-Lowry Acid

To understand ethanol's role, we need to explore it as a Bronsted-Lowry acid. According to this theory, an acid is defined by its ability to donate a proton.
In this reaction, ethanol (\( \mathrm{C}_2\mathrm{H}_5\mathrm{OH} \)) acts as the proton donor, or acid. It gives away its proton to the hydride ion, which functions as the base.
Once it loses a proton, ethanol is transformed into its conjugate base, the ethoxide ion.

• Ethanol's acidic behavior is explained by its ability to donate a proton.
• This concept helps classify substances based on observed reactivity.

Hydride Ion as a Bronsted-Lowry Base

The hydride ion (\( \mathrm{H}^- \)) plays a critical role as a Bronsted-Lowry base. In this theory, a base is characterized by its proton accepting ability.
During the reaction, the hydride ion takes a proton from ethanol. This acceptance changes the hydride ion into its conjugate acid, hydrogen gas (\( \mathrm{H}_2 \)).
This showcases its behavior as a base, a vital lesson in understanding how diverse ions and molecules can function within Bronsted-Lowry parameters.

• The hydride ion's role demonstrates how negative ions can act as bases through proton acceptance.
• This concept broadens our understanding of acid-base interactions beyond traditional definitions.