Question

Choose the stronger acid in each of the following pairs: (a) \(\mathrm{H}_{2} \mathrm{SeO}_{3}\) or \(\mathrm{H}_{2} \mathrm{SeO}_{4}\) (b) \(\mathrm{H}_{3} \mathrm{PO}_{4}\) or \(\mathrm{H}_{3} \mathrm{~A} \mathrm{sO}_{4}\) (c) \(\mathrm{H}_{2} \mathrm{~S}\) or \(\mathrm{H}_{2} \mathrm{Te}\)

Short answer

(a) \(\text{H}_{2} \text{SeO}_{4} \) (b) \(\text{H}_{3} \text{PO}_{4} \) (c) \(\text{H}_{2} \text{Te}\)

Step-by-step solution

- Identify the Oxoacids

Recognize that \(\text{H}_{2} \text{SeO}_{3}\) and \(\text{H}_{2} \text{SeO}_{4}\) are oxoacids. Oxoacids contain hydrogen, oxygen, and another element (in this case, selenium). Compare the acid strength based on the oxidation states of selenium.

- Compare Oxidation States

In \(\text{H}_{2} \text{SeO}_{3}\), selenium is in the +4 oxidation state. In \(\text{H}_{2} \text{SeO}_{4}\), selenium is in the +6 oxidation state. The strength of an oxoacid increases with the oxidation state of the central atom.

- Determine the Stronger Acid (Part a)

Since \(\text{H}_{2} \text{SeO}_{4}\) has selenium in a higher oxidation state (+6) compared to \(\text{H}_{2} \text{SeO}_{3}\) (+4), \(\text{H}_{2} \text{SeO}_{4}\) is the stronger acid.

- Identify the Elemental Acids

Recognize that \(\text{H}_{3} \text{PO}_{4}\) and \(\text{H}_{3} \text{AsO}_{4}\) are acids of phosphorus and arsenic, respectively.

- Compare Elements in the Periodic Table

Phosphorus is above arsenic in group 15 of the periodic table. Acidity decreases as you move down a group in the periodic table.

- Determine the Stronger Acid (Part b)

Since phosphorus is above arsenic, \(\text{H}_{3} \text{PO}_{4}\) is the stronger acid compared to \(\text{H}_{3} \text{AsO}_{4}\).

- Identify the Binary Acids

Recognize that \(\text{H}_{2} \text{S}\) and \(\text{H}_{2} \text{Te}\) are binary acids, consisting of hydrogen and one other element (sulfur or tellurium).

- Compare Atomic Size

Tellurium is below sulfur in group 16 of the periodic table, meaning tellurium has a larger atomic radius. The strength of binary acids increases with the size of the non-hydrogen element.

- Determine the Stronger Acid (Part c)

Since tellurium has a larger atomic radius than sulfur, \(\text{H}_{2} \text{Te}\) is the stronger acid compared to \(\text{H}_{2} \text{S}\).

Key concepts

oxoacids

Oxoacids are acids that contain hydrogen, oxygen, and another element, typically a non-metal. In oxoacids, the acidic hydrogen is bonded to an oxygen atom, which in turn is bonded to a central atom. An example of oxoacids from the exercise is \(\text{H}_{2} \text{SeO}_{3}\) (selenious acid) and \(\text{H}_{2} \text{SeO}_{4}\) (selenic acid). The strength of an oxoacid depends on two main factors:

• Oxidation state of the central atom: The higher the oxidation state of the central atom, the stronger the oxoacid. In the given problem, \(\text{H}_{2} \text{SeO}_{4}\) is stronger than \(\text{H}_{2} \text{SeO}_{3}\) because selenium's oxidation state is +6 in \(\text{H}_{2} \text{SeO}_{4}\) compared to +4 in \(\text{H}_{2} \text{SeO}_{3}\).
• Electronegativity of the central atom: A more electronegative central atom can draw electron density away from the oxygens, making the O-H bonds more polar and releasing H⁺ ions more easily, thus increasing acid strength.

binary acids

Binary acids consist of hydrogen and one other element. Examples from the exercise are \(\text{H}_{2} \text{S}\) (hydrosulfuric acid) and \(\text{H}_{2} \text{Te}\) (hydrotelluric acid). The strength of binary acids can be understood by considering:

• Atomic size: In binary acids, as you move down a group in the periodic table, the size of the central atom increases. This makes it easier for the hydrogen atoms to dissociate, making the acid stronger. That's why \(\text{H}_{2} \text{Te}\) is stronger than \(\text{H}_{2} \text{S}\) because tellurium is larger than sulfur.
• Bond strength: Larger atoms form weaker bonds with hydrogen, which makes it easier for the acid to donate protons (H⁺ ions). Thus, \(\text{H}_{2} \text{Te}\) has weaker H-Te bonds compared to H-S bonds in \(\text{H}_{2} \text{S}\), resulting in stronger acidity.

oxidation states

Oxidation state or oxidation number is a concept that describes the degree of oxidation of an atom in a compound. The oxidation state of an atom can influence the strength of an acid containing that atom. For oxoacids, as seen in the exercise:

• Higher oxidation state, stronger acid: In the case of oxoacids such as \(\text{H}_{2} \text{SeO}_{3}\) and \(\text{H}_{2} \text{SeO}_{4}\), the oxidation state of selenium is +4 in \(\text{H}_{2} \text{SeO}_{3}\) and +6 in \(\text{H}_{2} \text{SeO}_{4}\). The higher the oxidation state, the stronger the acid, because higher oxidation states more strongly pull electron density from the O-H bonds, making it easier to release protons (H⁺).
• Periodic table trends: For elements in the same group having different oxidation states, the element with the higher oxidation state usually forms a stronger acid.

periodic table trends

The periodic table provides key trends that help predict the strength of acids. Key periodic trends related to acid strength include:

• Group trends: Moving down a group in the periodic table, atomic size increases. For binary acids, this increased size leads to weaker bonds with hydrogen and thus stronger acidity. For instance, \(\text{H}_{2} \text{Te}\) is stronger than \(\text{H}_{2} \text{S}\).
• Electronegativity and oxidation states: Moving across a period, elements generally become more electronegative and can exhibit higher oxidation states, both leading to stronger oxoacids. For instance, \(\text{H}_{3} \text{PO}_{4}\) is stronger than \(\text{H}_{3} \text{AsO}_{4}\) because phosphorus is smaller and more electronegative.
• Acidity and position: In the same group, acidity increases downwards. In the same period, oxoacids' strength generally increases with higher electronegativity and oxidation states of the central atom.