Chem lecture 9

Acid-Base Chemistry Overview

1. Definitions

  • Arrhenius Definitions:

    • Acid: Increases hydrogen ions (H+) in water.

    • Base: Increases hydroxide ions (OH-) in water.

  • Brønsted–Lowry Definitions:

    • Acid: Proton donor.

    • Base: Proton acceptor.

2. Water's Role in Acid-Base Chemistry

  • Water acts as a proton acceptor (base) as well as a proton donor (acid).

  • This dual behavior qualifies water as amphiprotic.

3. Brønsted–Lowry Acids and Bases

  • Acidic H-atoms:

    • Acidic protons (H+) are attached to electronegative atoms.

  • Bases:

    • Can be neutral (B:) or negatively charged (B:-).

    • If neutral, proton addition results in a positive charge.

4. Conjugate Acid-Base Pairs

  • Conjugate pairs differ by one proton (H+).

  • Acid reactions yield their conjugate bases and vice versa.

5. Identifying Acids and Bases

  • Examples:

    • PO4^3– = Brønsted-Lowry base (no protons to donate).

    • HClO4 = Brønsted-Lowry acid (can donate H+).

    • CN– = Brønsted-Lowry base (no protons to donate).

6. Relative Strengths of Acids and Bases

  • Strong acids yield weak conjugate bases.

  • Strong bases yield weak conjugate acids.

  • The strength of acids and bases inversely correlates:

    • A strong acid has a weak conjugate base.

    • A weak acid has a strong conjugate base.

7. Equilibrium in Proton-Transfer Reactions

  • The equilibrium favors reactions involving stronger acids and bases, forming weaker conjugates.

8. Autoionization of Water

  • Water is amphoteric, undergoing autoionization.

  • At 25 °C, the ion product constant (Kw) is 1.0 x 10^-14 (Kw = [H3O+][OH−]).

  • Neutral solution: [H+] = [OH–]; Acidic solution: [H+] > [OH–]; Basic solution: [H+] < [OH–].

9. pH Scale

  • pH = –log[H+].

  • Neutral pH = 7.00; Acidic pH < 7.00; Basic pH > 7.00.

10. Other p-Scale Measurements

  • pOH = –log[OH−]

  • Relationship: pH + pOH = 14.00 (at 25 °C).

11. Measuring pH

  • pH Meters: Offer precise measurement.

  • Indicators: Quick but less accurate; change color in different pH conditions.

12. Exercises and Applications

  • Practice identifying acid-base reactions, calculate pH and concentrations, and determine positions of equilibrium reactions in various examples.

Practice Exercises

  • Reflect on the Bride-Lowry theory and practice predicting conjugate acids and bases based on given reactions. Analyze the strength of acids and bases in problem-solving scenarios.