Chapter 7.2: Acid Strength of Bronsted-Lowry Acids

Chapter 7: Acid Strength of Bronsted-Lowry Acids

Overview of Key Concepts

  • Definition and nature of acids in Bronsted-Lowry theory.

  • Identification of acidic hydrogens.

  • Trends in acid strengths, focusing on:

    • Binary acids

    • Oxoacids

    • Carboxylic acids

Identifying Acidic Hydrogens on Bronsted-Lowry Acids

  • Definition of Acid:

    • An acid is defined as a hydrogen ion (H+) or proton donor.

  • Definition of Acidic Hydrogen:

    • An acidic hydrogen is specifically defined as a hydrogen atom that can be easily donated to a base.

    • Such hydrogens are typically found bonded to electronegative atoms, usually oxygen (O), chlorine (Cl), bromine (Br), or iodine (I).

    • Examples of acids with prominent acidic hydrogens:

    • HNO₃ (Nitric acid)

    • H₂SO₄ (Sulfuric acid)

    • HCl (Hydrochloric acid)

    • H₂O (Water)

Characteristics of Acidic Hydrogens

  • Hydrogen Bonds:

    • Acidic hydrogens are generally bonded to electronegative atoms, enhancing their propensity to be donated.

  • Electronegativity:

    • The more electronegative the atom attached to hydrogen, the more acidic the hydrogen, due to increased bond polarity.

Understanding Acid Strength under Bronsted-Lowry Theory

  • General Concept:

    • Acid Strength: A measurable concept indicating the ability of an acid to donate its proton (H+).

    • Stronger Acids: These easily give up their protons.

    • Weaker Acids: These retain their protons more firmly than stronger acids.

Factors Influencing Proton Donation

  • Key Conditions for Easy Proton Donation:

    • The bond between hydrogen and the other atom (H-X bond) must be easily broken.

    • The conjugate base produced (X-) must be stable and not readily revert to form HX.

Relative Acid Strength among Binary Acids

  • Definition of Binary Acids:

    • Composed solely of hydrogen (H) and one other atom, represented generally as HX.

  • **Trends in Acid Strength:

    • Across a Period:** Acid strength increases as you move across a periodic table row.

    • Example Trend: CH₄ < NH₃ < H₂O < HF.

    • The acid strength of binary acids correlates with the electronegativities of the bonded atom (X).

    • More electronegative atoms lead to a more polar H-X bond, yielding stronger acids.

  • Strength of HF:

    • Inspect why HF emerges as the strongest acid.

    • The polarity of the F-H bond increases due to electronegativity; it leads to a large partial positive charge on the acidic H, which is more attractive to bases.

    • The F- conjugate base is very stable due to fluorine’s high electronegativity.

    • Consequently, a weak bond is formed that helps to weaken the H-F bond, culminating in HF being a strong acid.

Rationale for Trends in Acid Strength Across a Period

  • Electronegativity:

    • A higher electronegativity difference between hydrogen and atom X leads to increased bond polarity.

    • A more polar bond results in a higher partial positive charge on the acidic hydrogen, strengthening the acid's ability to donate H+.

  • Stability of Conjugate Base:

    • The capacity of the conjugate base to accommodate or disperse a negative charge leads to increased stability.

    • More electronegative atoms can stabilize negative charges better, leading to stronger acids.

Acid Strength Trends Down a Group

  • General Trend:

    • Acid strength increases down the group: HF < HCl < HBr < HI.

  • Explanation:

    • While HF contains the most electronegative atom, it is not the strongest acid due to bond strength considerations.

    • Bond Strength:

    • Bond strength decreases as you move down a group. The corresponding larger size of atoms leads to diminished overlap of bonding orbitals.

    • Conclusion:

      • Weaker H-X bonds result in higher acid strength as they break more easily to release H+.

Characteristics of Oxoacids and Carboxylic Acids

  • Definition of Oxoacids:

    • Compounds containing hydrogen, oxygen, and at least one other element with the structure X-O-H.

    • Each oxoacid has at least one acidic hydrogen attached to an O atom.

    • Important Note: Bond strength in oxoacids is not a primary determinant of acid strength.

  • Carboxylic Acids:

    • This class of acids features a carbon atom double-bonded to an oxygen atom (C=O) and single-bonded to a hydroxyl group (–OH).

Trends in Acid Strength of Oxoacids and Carboxylic Acids

  • Same Central Atom:

    • Oxoacid strength increases with an increasing number of oxygen atoms attached to the central atom.

  • Same Number of O Atoms:

    • Oxoacid strength increases with the increasing electronegativity of the central atom.

  • Influence of Electronegative Atoms:

    • More electronegative atoms help distribute the negative charge on the conjugate base through inductive effects.

    • More electronegative atoms or additional oxygen atoms lead to greater stability of the conjugate base, thus resulting in stronger acids.

Additional Notes on Acid Strength

  • Delocalization of Charge:

    • The presence of greater numbers of oxygen atoms leading to more delocalization of negative charge results in stronger acid strength.

Final Notes

  • Understanding Acid Behavior:

    • Mastery of acid strength concepts is critical for predicting acid behavior in various chemical reactions.

    • Interactive worksheets may assist in solidifying concepts learned in this chapter.