Problem 91 - Chemistry: The Central Science in SI Units, Global Edition

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 \(\mathrm{H}_{2} \mathrm{~S}\) because Te is more electronegative than \(\mathrm{S}\).

Step-by-Step Solution

Answer

(a) True. The acidity of binary acids increases from left to right in a given row of the periodic table due to increasing electronegativity of the elements. (b) False. In a series of acids that have the same central atom, acid strength increases with the electron withdrawing ability of the surrounding atoms, not the number of hydrogen atoms bonded to the central atom. (c) False. H2Te is a stronger acid than H2S, but not due to electronegativity. H2Te is a stronger acid because Te is larger and has a longer bond length with hydrogen, making it easier to lose the H+ ion.

Step 1: Statement (a)

In general, the acidity of binary acids increases from left to right in a given row of the periodic table. This statement is true. As we move across a period from left to right, the electronegativity of the elements increases. Binary acids are compounds containing hydrogen and another element. Greater electronegativity of the other element in the binary acid helps in stabilizing the anion formed after losing the hydrogen ion (H+). Therefore, binary acids become stronger acids as we move across the period.

Step 2: Statement (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. This statement is false. In a series of acids with the same central atom, the acid strength depends on the stability of the anion formed after losing the hydrogen ion (H+). Having more hydrogen atoms bonded to the central atom does not necessarily lead to increased acid strength. Instead, it is the electron-withdrawing ability of the surrounding atoms (such as electronegative halogens) that affects the acid strength. To correct the statement, we can say: In a series of acids that have the same central atom, acid strength increases with the electron withdrawing ability of the surrounding atoms.

Step 3: Statement (c)

Hydrotelluric acid (H2Te) is a stronger acid than H2S because Te is more electronegative than S. This statement is false. It is true that H2Te is a stronger acid than H2S, but the reason provided in this statement is incorrect. In fact, the electronegativity of Te is lower than that of S. As we move down the group in the periodic table, the size of the atom increases, and the bond length between hydrogen and the central atom also increases. As a result, it becomes easier for the H+ ion to be lost, and the acidity of the binary acid increases. So, H2Te is a stronger acid than H2S because Te is larger and has a longer bond length with hydrogen, making it easier to lose the H+ ion. To correct the statement, we can say: Hydrotelluric acid (H2Te) is a stronger acid than H2S because Te is larger than S and has a longer bond length with hydrogen, making it easier to lose the H+ ion.

Key Concepts

Periodic Table Trends

As we explore periodic table trends, one crucial aspect to consider is how elements behave as you move across a period. This is especially significant when discussing the acidity of binary acids. Binary acids contain hydrogen and another element.
As you go from left to right on the periodic table, the electronegativity of elements increases. Electronegativity is the ability of an atom to attract electrons.
This increase in electronegativity is tightly linked with the acid strength:

• Higher electronegativity means stronger attraction to the bonding pair of electrons.
• In binary acids, this means the bond between hydrogen and the other element becomes more polarized.
• The increased polarity facilitates the release of the hydrogen ion ( H^+ ), leading to stronger acidity.

Therefore, within the same period, as you move from left to right, you'll generally see a trend where binary acids increase in acidity.

Binary Acids

Understanding binary acids is foundational in chemistry, as they play a major role in discussions about acidity. A binary acid is composed of hydrogen and one other element. The bond between hydrogen and the other element in these compounds determines the acid's strength.
There are several factors at play:

• The element's ability to stabilize the resulting anion after losing H^+ affects the acidity.
• Stability of the anion often increases with the element's electronegativity and size.

With larger atoms, such as those lower down in the periodic table, the bonds tend to be weaker. This weaker bond makes it easier for the molecule to lose H^+ , thus enhancing acidity. This is why H₂Te is a stronger acid than H₂S—tellurium, being larger, forms a weaker bond with hydrogen.

Electronegativity

Electronegativity is a fundamental concept when discussing acid strength, particularly in binary acids. It's the measure of an atom's ability to attract and hold onto electron pairs. This property has direct implications on how acidic a molecule will be.
When an element in a binary acid is highly electronegative, two effects are observed:

• The bond between hydrogen and the other element in the acid becomes more polarized.
• This increased polarity facilitates the release of H^+ ions into solution, which is the hallmark of a strong acid.

However, when considering acids like H₂Te and H₂S, size plays a bigger role than electronegativity, showcasing that sometimes size trumps electronegativity in determining acid strength.
Understanding this balance is key to predicting and explaining acidity trends in different acids across the periodic table.

Anion Stability

Anion stability is a pivotal factor in determining acid strength. When an acid molecules release H^+ , the resulting anion's stability dictates how likely this release is to occur. The more stable the anion, the stronger the acid.
Several factors influence anion stability:

• Electronegativity of the element: Higher electronegativity generally leads to more stable anions as electrons are held more tightly.
• Atom size: Larger atoms distribute charge over a larger volume, contributing to anion stability.
• Resonance effects—where applicable—can further stabilize the anion by delocalizing charge over multiple atoms.

In acids with the same central atom, the surrounding atoms' electronegative nature can impact the acid's strength. An anion stabilized by electron-withdrawing groups leads to higher acidity, demonstrating the interconnected nature of these chemical concepts.
By understanding anion stability, you gain insights into how subtle changes in molecular structure can significantly impact acid behavior.