Question

Ammonia \(\left(\mathrm{NH}_{3}\right)\) can act as a base in water. (a) Suppose methane \(\left(\mathrm{CH}_{4}\right)\) could also act as a base in water. Write a chemical equation to show this. (b) In fact, methane cannot act as a base. Why not? (Hint: One of the products you drew violates an important rule for carbon.) (c) By comparing \(\mathrm{NH}_{3}\) to \(\mathrm{CH}_{4}\), what can you say must exist on a species for it to serve as a Bronsted-Lowry base?

Short answer

(a) If methane were to act as a base in water, the chemical equation would be: \(CH_4 + H_2O \rightarrow CH_5^+ + OH^-\). (b) However, methane cannot act as a base because the methonium ion (CH5+) violates the octet rule for carbon, making it highly unstable. (c) Comparing NH3 and CH4, the necessary property for a species to serve as a Bronsted-Lowry base is the presence of lone pair electrons that can be donated to form a bond with a hydrogen ion (H+).

Step-by-step solution

1. Write a chemical equation for methane acting as a base

To write a chemical equation showing methane acting as a base in water, we need to show CH4 accepting a hydrogen ion (H+) from water. This results in the formation of a Methonium ion (CH5+) and Hydroxide ion (OH-): \[ CH_4 + H_2O \rightarrow CH_5^+ + OH^- \]

2. Explain why methane cannot act as a base

Methane (CH4) cannot act as a base because the product, Methonium ion (CH5+) is highly unstable and violates the octet rule for carbon. The octet rule states that elements prefer to have a stable electron configuration with eight electrons in their valence shell. In CH5+, the carbon atom has 10 electrons in its valence shell which is beyond the octet rule, making it an unstable and improbable species to form.

3. Identify the necessary property for a species to serve as a Bronsted-Lowry base

Comparing NH3 (which can act as a base) to CH4, the key difference is the presence of lone pair electrons on the Nitrogen atom in NH3. When Ammonia (NH3) acts as a base, it donates these lone pair electrons to form a bond with a hydrogen ion (H+). For a species to serve as a Bronsted-Lowry base, it must have lone pair electrons that can be donated to form a bond with a hydrogen ion (H+).

Key concepts

Lone Pair Electrons

Lone pair electrons are pairs of electrons that are not involved in bonding within a molecule. They are also known as non-bonding pairs. For a molecule to act as a Bronsted-Lowry base, it needs these lone pair electrons. The reason is simple: these electrons can be shared with a hydrogen ion (H+), enabling the molecule to accept a proton and act as a base.

In the case of ammonia (\(\text{NH}_3\)), the nitrogen atom has one lone pair of electrons. This lone pair allows ammonia to accept a hydrogen ion from another molecule, such as water, and hence function as a base. Unlike ammonia, methane (\(\text{CH}_4\)) lacks lone pair electrons. Methane has four hydrogen atoms bonded to carbon, and no room is left for any lone pairs. Thus, it cannot function as a Bronsted-Lowry base.

- **Key Points to Remember:**
- Lone pairs are vital for base activity in Bronsted-Lowry theory.
- They allow the molecule to donate electrons and bond with hydrogen ions.
- Molecules lacking lone pair electrons, like methane, can't usually act as bases.

Octet Rule

The octet rule is a chemical rule of thumb that helps explain the stability of molecules. According to the octet rule, atoms tend to bond in such a way that each atom has eight electrons in its valence shell, achieving a noble gas configuration.

For carbon, this means forming four covalent bonds, adding up to eight electrons in its outer shell. When methane (\(\text{CH}_4\)) reacts to form a methonium ion (\(\text{CH}_5^+\)), the carbon atom exceeds this limit with ten electrons around it. This breaches the octet rule, making methonium ion highly unstable. As a result, methane cannot reasonably act as a base because doing so would involve forming an unstable product.

- **Why the Octet Rule Matters:**
- It dictates the stability of resulting ions and molecules.
- Molecules like methane that would violate the octet rule cannot act as stable bases.
- Observing the octet rule helps prevent forming highly unstable, improbable species.

Chemical Equation for Bases

Writing chemical equations for bases involves outlining the process by which bases accept hydrogen ions. Let's consider a typical base like ammonia (\(\text{NH}_3\)). When it reacts with water, a chemical equation can be written as:\[\text{NH}_3 + \text{H}_2\text{O} \rightarrow \text{NH}_4^+ + \text{OH}^-\]Here, ammonia serves its role as a Bronsted-Lowry base by accepting a hydrogen ion (H+) from water, which leads to the formation of the ammonium ion (\(\text{NH}_4^+\)) and hydroxide ion (\(\text{OH}^-\)).

This chemical interaction demonstrates how a base interacts to form products less stable in terms of the hydrogen ion. This is how a base like ammonia functions within a chemical equation.

- **Essentials for Chemical Equations of Bases:**
- Bases accept hydrogen ions, showing them in chemical equations.
- Products include the base now protonated and a negative ion, often hydroxide.
- Proper equations help in understanding the functional chemistry of bases.