Question

(a) How does the strength of an acid vary with the polarity and strength of the \(\mathrm{H}-\mathrm{X}\) bond? (b) How does the acidity of the binary acid of an element vary as a function of the electronegativity of the element? How does this relate to the position of the element in the periodic table?

Short answer

The strength of an acid increases with the polarity of the H-X bond but decreases with the strength of the H-X bond. The acidity of a binary acid generally increases with the electronegativity of the element X and follows an increasing trend across a period and down a group in the periodic table.

Step-by-step solution

Part (a) - Strength of an acid and H-X bond properties

To understand how the strength of an acid varies with the polarity and strength of the H-X bond, let's first recall what an acid is and how its strength is determined. An acid is a substance that donates a proton (H+) to a solution. The strength of an acid is determined by its ability to donate this proton. A strong acid is one that easily donates a proton, while a weak acid has a lower tendency to do so. Now let's consider the H-X bond. Here, X is a generic placeholder for any element bonded to hydrogen. The polarity of the bond relates to the difference in electronegativity between hydrogen and X. A more polar H-X bond implies a larger electronegativity difference. The bond strength is also important, as it is the energy required to break the bond. A stronger H-X bond is harder to break and requires more energy, whereas a weaker bond is easier to break and requires less energy. Now, to break down the relationship: 1. Polarity of H-X bond: A more polar bond means that X is more electronegative and has a stronger attraction to the proton (H+). As a result, it is easier for the bond to break, and the proton can be donated more easily, resulting in a stronger acid. 2. Strength of H-X bond: A stronger H-X bond means more energy is required to break it, making it more difficult for the proton to be donated. Therefore, a stronger bond results in a weaker acid. In summary, the strength of an acid increases with the polarity of the H-X bond, but decreases with the strength of the H-X bond.

Part (b) - Acidity of binary acid, electronegativity, and periodic table

Now let's analyze the relationship between the acidity of a binary acid and the electronegativity of the element (X). As we discussed in Part (a), a more polar H-X bond (higher electronegativity difference) means that the proton (H+) can be donated more easily, resulting in a stronger acid. Therefore, the acidity of a binary acid generally increases with the electronegativity of the element X. Now let's relate this to the periodic table. Moving from left to right across a period, the electronegativity increases. Thus the acidity of binary acids generally increases across a period. Conversely, moving downwards in a group, the electronegativity decreases. However, the bond strength also decreases (atoms have larger radii and bonds are longer), making it easier for the proton to be donated. Therefore, the acidity of binary acids generally increases as well when moving down a group. In summary, the acidity of a binary acid increases with the electronegativity of the element X, and generally follows an increasing trend across a period and down a group in the periodic table.

Key concepts

H-X Bond Polarity

The polarity of the H-X bond plays a crucial role in determining the strength of an acid. Bond polarity arises when there is a difference in electronegativity between the hydrogen (H) and another atom (X) it is bonded to. As the electronegativity difference increases, the bond becomes more polar. This is important for acids, as a more polar H-X bond means that the hydrogen atom (H) is more positively charged, while the X atom becomes negatively charged.

This increased charge separation makes it easier for the hydrogen ion (H+), also known as a proton, to dissociate from the molecule, enhancing the acidic strength. In essence, the more polar the bond, the stronger the acid can be. By understanding this concept, one realizes why a chlorine-hydrogen (H-Cl) bond, which is highly polar due to chlorine's high electronegativity, results in a strong acid like hydrochloric acid.

Bond Strength

Bond strength refers to the energy required to break a bond between two atoms. For acids, the strength of the H-X bond is a critical factor in determining how easily the proton (H+) can be released.

If the H-X bond is strong, it requires a lot of energy to break, meaning the acid does not readily dissociate to release protons. Thus, a strong bond generally indicates a weaker acid. Conversely, a weak H-X bond means the bond can break more easily, allowing the acid to release protons more freely, resulting in a stronger acid.

This relationship can be observed in acids like hydrogen fluoride (HF), where the bond strength is high, making HF a weak acid despite its high bond polarity. Therefore, both bond polarity and bond strength must be considered together to fully understand acid strength.

Electronegativity

Electronegativity is a measure of how strongly an atom can attract and hold onto electrons. In the context of the H-X bond, it indicates how strongly the element X is pulling on the shared electrons with hydrogen (H).

A higher electronegativity for X means it has a greater ability to attract electrons, resulting in a more polar bond. This makes the H-X bond more likely to dissociate, releasing the hydrogen ion (H+) into the solution, which in turn increases the acidity.

• Highly electronegative elements, such as fluorine, often form stronger acids because the hydrogen is more easily released as a proton.
• Lesser electronegative elements, like iodine in HI, still form strong acids due to their larger atomic size, leading to weaker H-X bonds.

Periodic Table Trends

When discussing acid strength and electronegativity, periodic table trends offer insightful patterns. Moving across the periodic table from left to right, electronegativity increases because atoms have more protons, creating a stronger pull on the shared electrons.

Consequently, acids formed with elements towards the right side of the periodic table are often stronger, as these elements tend to form more polar bonds with hydrogen, facilitating easier proton release. Conversely, when moving down a group in the periodic table, atomic radii increase.

• As atoms get larger moving down a group, the bond length increases, which typically decreases bond strength.
• This makes it easier for the H-X bond to split, enhancing acid strength, despite the decrease in electronegativity.

These trends help predict the behavior of various acids, showing why elements such as chlorine and iodine contribute to the formation of strong acids like HCl and HI, respectively.