15.13
酸强和分子结构
二进制酸和碱
在没有任何调平效果的情况下，含非金属 (A) 氢二进制化合物的酸强会随着 H-A 键强在元素周期表中的一组降下而增加。 对于第 17 组，酸度增加的顺序是 HF < HCL < HBr < HI。 同样，对于第 16 组，酸强的增加顺序是 H2O < H2S < H2Se < H2Te。 在元素周期表中，二进制氢化合物的酸强随着非金属原子的电负性的增加而增加，因为 H-A 键的极性增加。 因此，第二排的酸度…
盐酸是一种强酸，而氢氟酸是一种弱酸。但是什么决定了他们的强度呢？只有两种元素的二元酸的强度 由键能和键的极性决定。键能较高的酸具有较强的键，因此，将是一种较弱的酸。比较同一组中的酸，像氢氟酸等 弱酸中的键更难断裂，因此这种酸不太可能产生质子。相反，键能较低的酸的键较弱，因此，将是一种较强的酸。例如，强酸中的键，像盐酸，更容易断裂，并且比氢氟酸更容易提供质子。一个键，像盐酸中的键，当一个原子比另一个原子电负性更强时，它是极性的。一种化合物，当 附着在氢上的原子具有比氢更高的电负性时，它可以作为一种酸。原子将带部分负电荷，使氢带部分正电荷，这样氢 就可以作为质子释放。键极性较高的酸具有较弱的键，因此，将是一种较强的酸。比较同一周期内的化合物，盐酸比硫化氢强，是因为氯比硫更具电负性，因此更容易释放质子。如果氢的电负性等于或大于 另一个原子，那么这个分子就不能提供质子，因此也就不能作为一种酸。含氧酸是一种酸，其中一个 OH 附着在 比氢电负性更强的第三个原子上。含氧酸的强度取决于 电负性和附着在第三个原子上的 氧原子的数量。原子的电负性越高，极化越强，削弱了氧和氢 之间的键。如果中心原子与额外的氧原子相连，它会进一步增加氧和氢 之间的键的极性。例如，含有三个额外氧原子的 高氯酸比含有两个额外氧原子的 氯酸强。反过来，氯酸比只有一个 额外氧原子的亚氯酸 和没有额外氧原子的次氯酸强。羧酸是含有羧基的弱酸。第二个氧原子使氧-氢键 更具极性，从而使分子可以 提供一个质子。乙酸和甲酸是羧酸的例子。
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Q1: Why is hydrochloric acid stronger than hydrofluoric acid?
Hydrochloric acid is stronger because it has lower bond energy than hydrofluoric acid. A weaker H-Cl bond breaks more easily, allowing the acid to donate protons readily. In contrast, hydrofluoric acid has a stronger, harder-to-break H-F bond, making it less likely to release protons and therefore a weaker acid.
Q2: How does electronegativity affect acid strength in binary acids?
Higher electronegativity of the atom bonded to hydrogen increases bond polarity, making the hydrogen more likely to be released as a proton. The electronegative atom attracts electrons, giving hydrogen a partial positive charge. Comparing across a period, hydrochloric acid is stronger than hydrogen sulfide because chlorine is more electronegative than sulfur.
Q3: What makes oxyacids stronger when additional oxygen atoms are added?
Additional oxygen atoms bonded to the central atom increase the polarity of the oxygen-hydrogen bond, weakening it and making proton release easier. Perchloric acid with three additional oxygens is stronger than chloric acid with two, which is stronger than chlorous acid with one. This pattern demonstrates how oxygen atoms progressively enhance acid strength.
Q4: Why are carboxylic acids considered weak acids?
Carboxylic acids are weak acids because the second oxygen atom attached to the carbon increases the polarity of the oxygen-hydrogen bond, weakening it only partially. Additionally, after losing a proton, the carboxylate ion exhibits resonance, which stabilizes the negative charge by delocalizing it over several atoms, preventing complete ionization in water.
Q5: How does bond energy determine whether a binary acid is strong or weak?
An acid with higher bond energy has a stronger bond that resists breaking, making it a weaker acid because protons are less readily donated. Conversely, an acid with lower bond energy has a weaker bond that breaks easily, allowing protons to be released more readily and making it a stronger acid. Bond strength is therefore inversely related to acid strength.
Q6: What determines whether a hydrogen-containing molecule can act as an acid?
A molecule can act as an acid only when the atom bonded to hydrogen has higher electronegativity than hydrogen itself. This creates a polar bond where the atom has a partial negative charge and hydrogen has a partial positive charge, allowing hydrogen to be released as a proton. If hydrogen has equal or greater electronegativity, the molecule cannot donate protons.
Q7: How does the oxidation number of the central atom affect oxyacid strength?
Increasing the oxidation number of the central atom increases the acid's strength because it enhances the atom's attraction for electrons shared with oxygen, thereby weakening the oxygen-hydrogen bond. Sulfuric acid with sulfur oxidation number +6 is more acidic than sulfurous acid with +4. Similarly, nitric acid with nitrogen oxidation number +5 is stronger than nitrous acid with +3.