Problem 91 - Chemistry The Central Science In Si Units

Question

Indicate whether each of the following statements is true or false. For each statement that is false, correct the statement to make it true. (a) In general, the acidity of binary acids increases from left to right in a given row of the periodic table. (b) In a series of acids that have the same central atom, acid strength increases with the number of hydrogen atoms bonded to the central atom. (c) Hydrotelluric acid \(\left(\mathrm{H}_{2} \mathrm{Te}\right)\) is a stronger acid than \(\mathrm{H}_{2}\) S because Te is more electronegative than S.

Step-by-Step Solution

Answer

(a) True. (b) False, acid strength increases with more oxygen atoms. (c) False, \\(\mathrm{H}_{2} \mathrm{Te}\\) is stronger due to Te's size, not electronegativity.

Step 1: Analysis for Statement (a)

Statement (a) talks about the trend in acidity of binary acids across a period. As we move from left to right in a period of the periodic table, the electronegativity of the elements increases. The higher the electronegativity, the stronger the acid, because it better stabilizes the conjugate base after losing a proton. Therefore, statement (a) is true.

Step 2: Analysis for Statement (b)

Statement (b) mentions a trend regarding a series of acids with the same central atom. For such a series, acid strength actually decreases with an increasing number of hydrogen atoms and increases with more electronegative atoms or higher oxidation states. Hence, this statement is false. The correct version is: "In a series of acids that have the same central atom, acid strength increases with the number of oxygen atoms bonded to the central atom."

Step 3: Analysis for Statement (c)

For statement (c), we need to compare the acid strengths of hydrotelluric acid \(\mathrm{H}_{2} \mathrm{Te}\) and \(\mathrm{H}_{2} \mathrm{S}\). Acid strength increases with the size of the element across a group rather than electronegativity. Since tellurium (Te) is larger than sulfur (S), \(\mathrm{H}_{2} \mathrm{Te}\) is a stronger acid than \(\mathrm{H}_{2} \mathrm{S}\), but not due to electronegativity. This statement is conditionally false—correcting it: "Hydrotelluric acid \(\mathrm{H}_{2} \mathrm{Te}\) is a stronger acid than \(\mathrm{H}_{2} \mathrm{S}\) because Te is larger than S, leading to better stabilization of its conjugate base."

Key Concepts

Binary Acids

Binary acids are a type of acid consisting of two elements: hydrogen (H) and one other element.
These acids are prominent in chemistry due to their simple composition, yet they show distinct variances in strength. This variance is influenced by several factors which we'll explore further.

Key influences on binary acid strength include:

• **Electronegativity:** The ability of the other element to attract electrons makes acids stronger.
• **Atomic Size:** Larger atoms in the second position lead to stronger acids.

For example, when comparing hydrotelluric acid (\( \mathrm{H}_{2} \mathrm{Te} \)) to \(\mathrm{H}_{2}\mathrm{S}\), although tellurium is less electronegative than sulfur, the larger size of Te allows for greater acid strength. This highlights how size can sometimes outweigh the effects of electronegativity.

Electronegativity

Understanding electronegativity is vital in predicting acid strength. Electronegativity is a measure of how strongly atoms attract bonding electrons.
This characteristic is crucial in assessing the behavior of binary acids across different periods in the periodic table.

Some insights about electronegativity:

• **Period Trends:** Electronegativity increases as you move from left to right across a period, making elements more efficient at stabilizing the conjugate base of acids.
• **Impact on Acid Strength:** The more electronegative an atom, the stronger the acid, since the element can stabilize the negative charge that develops when the acid loses a proton.

Thus, in the case of binary acids, moving across a period, stronger acids are often those paired with more electronegative elements like halogens.

Periodic Table Trends

The periodic table offers valuable insights into elemental properties, including trends that affect acid strength in surprising ways. Two useful trends to understand are:

• **Across a Period:** As mentioned, moving from left to right increases electronegativity, generally increasing binary acid strength.
• **Down a Group:** Moving down a group in the periodic table, elements increase in size, often boosting the strength of the acids they form. This happens because larger size leads to more effective conjugate base stabilization.

For instance, while both hydrotelluric acid \( \mathrm{H}_{2} \mathrm{Te} \) and \(\mathrm{H}_{2}\mathrm{S}\) follow the group trend, \( \mathrm{Te} \) offers such stability due to its larger atomic size compared to \( \mathrm{S} \). Hence, acid strength may sometimes be more about the size of atoms than just electronegativity.

Conjugate Base Stability

Conjugate base stability is a fundamental factor in assessing acid strength. When an acid donates a proton, its conjugate base forms. The stability of this base influences the overall strength of the original acid.

Factors that stabilize a conjugate base include:

• **Electronegativity:** A more electronegative element holding the charge tends to stabilize the base better, resulting in a stronger acid.
• **Atomic size:** Larger atoms can disperse a charge over a larger volume, stabilizing the conjugate base.
• **Resonance:** The ability for the negative charge to resonate between atoms enhances stability.

Considering hydrotelluric acid, its conjugate base is stabilized by the relatively large size of \( \mathrm{Te} \), thus contributing to its strength as an acid.