Question

Which of the following is a stronger base: \(\mathrm{NH}_{3}\) or \(\mathrm{PH}_{3}\) ? (Hint: The \(\mathrm{N}-\mathrm{H}\) bond is stronger than the \(\mathrm{P}-\mathrm{H}\) bond. \()\)

Short answer

PH3 is a stronger base than NH3.

Step-by-step solution

Identify the Bond Strength According to the Hint

NH3 has a stronger N-H bond, while PH3 has a weaker P-H bond, as indicated in the hint provided.

Relation of Bond Strength with Basicity

We need to understand that a stronger base is one that can easily accept a proton (H+). A molecule with weaker bonds more readily accepts a proton, making it a stronger base. This is because a weaker bond can be broken more easily, allowing the proton to become associated with the molecule, forming a new bond.

Comparison of the Bases

In this instance, PH3 has weaker bonds; therefore, it is more capable of accepting a proton and acts as the stronger base when compared to NH3.

Key concepts

Base Strength

Base strength refers to the ability of a substance to accept protons (H+ ions). In the context of acid-base chemistry, a base is considered strong if it can readily and effectively take on protons from its surroundings.
Factors influencing base strength include the stability of the conjugate acid formed after proton acceptance and the availability of lone pairs of electrons that can form new bonds with protons.
Stronger bases tend to have:

• Greater availability of lone electron pairs.
• Lower electronegativity of the atom with the lone pair, which makes it easier to share or donate electrons.
• Weaker bonds that can readily be broken to accommodate new protons.

When comparing \(\mathrm{NH}_{3}\) and \(\mathrm{PH}_{3}\), the base strength of \(\mathrm{PH}_{3}\) is greater due to its weaker bond which allows it to accept protons more readily. This highlights the direct relationship between bond strength, discussed below, and base strength. The weaker the existing bonds, the stronger the base tends to be.

Bond Strength

Bond strength is an essential concept in determining the reactivity of molecules in acid-base reactions. Stronger bonds are more stable and harder to break, while weaker bonds are more flexible and easier to break, allowing for the formation of new chemical species.
The strength of a bond can influence a molecule's ability to react and accept protons, directly impacting its basicity.

• A strong N-H bond, as seen in \(\mathrm{NH}_{3}\), is more resistant to breaking.
• In contrast, a weaker P-H bond, present in \(\mathrm{PH}_{3}\), makes the molecule more reactive and willing to participate in forming new bonds with protons.

The hint in the original exercise points out this difference, indicating that \(\mathrm{PH}_{3}\) will more easily accept protons due to its weaker bond strength. This conclusion is consistent with the chemical principle that weaker bonds generally translate to increased reactivity in terms of proton acceptance.

Proton Acceptance

Proton acceptance is a fundamental concept in acid-base chemistry, referring to the ability of a base to incorporate a proton into its structure. This process is central to understanding what makes certain substances stronger bases compared to others.
The efficiency of proton acceptance is dependent on several factors:

• The availability of lone pairs on the atom bonded to the hydrogen.
• The existing bond strength that may be broken upon accepting a proton.
• The overall thermodynamic stability provided once a new bond is formed.

In the original exercise comparing \(\mathrm{NH}_{3}\) and \(\mathrm{PH}_{3}\), \(\mathrm{PH}_{3}\) is identified as a stronger base due to its ability to more readily accept protons. This is closely tied to the weaker bond strength between P-H as compared to N-H, affording \(\mathrm{PH}_{3}\) a greater proton acceptance capacity and thus a stronger basic character in this comparison.