BENJAMIN BANKS - Microsoft Word - Topics 8.1 - 8.3

8.1 Introduction to Acids and Bases

Key Concepts

• The concentrations of hydronium ion ([H3O+]) and hydroxide ion ([OH–]) are reported as pH and pOH, respectively.

  • pH formula: pH = –log[H3O+]

  • pOH formula: pOH = –log[OH–]

• There is an interchangeability between the terms "hydrogen ion" and "hydronium ion." While H+(aq) is commonly used, H3O+(aq) is preferred, particularly in the context of the AP Exam.

• Water autoionizes with an equilibrium constant represented as Kw:

  • Kw = [H3O+][OH–] = 1.0 × 10–14 at 25°C.

• In pure water, it is noted that pH = pOH, defining pure water as a neutral solution. At 25°C, pKw is 14.0, leading to pH and pOH both equating to 7.0.

  • Relationship: pKw = 14 = pH + pOH at 25°C.

• The value of Kw varies with temperature, indicating that the pH of neutral pure water will not always equal 7.0 outside of 25°C.

4.8 Introduction to Acid-Base Reactions (Review)

Brønsted-Lowry Definitions

• A Brønsted-Lowry acid is defined as a proton donor, while a Brønsted-Lowry base serves as a proton acceptor.

• Water plays a crucial role in many acid-base reactions due to its molecular structure, allowing it to both donate and accept protons from dissolved species.

Examples of Acid-Base Reactions

1. Reaction: HCN(aq) + H2O(l) ⇌ CN–(aq) + H3O+(aq)

   • Acid (Reactant): HCN

   • Conjugate Base (Product): CN–

   • Base (Reactant): H2O

   • Conjugate Acid (Product): H3O+

2. Reaction: CH3NH2(aq) + H2O(l) ⇌ CH3NH3+(aq) + OH–(aq)

   • Acid (Reactant): CH3NH3+

   • Conjugate Base (Product): CH3NH2

   • Base (Reactant): H2O

   • Conjugate Acid (Product): H3O+

Autoionization of Water

• In pure water, a molecule of H2O functions as an acid, donating a H+ ion, while another accepts it, portraying the dynamics of autoionization.

• Autoionization reaction: 2 H2O(l) ⇌ H3O+(aq) + OH–(aq)

• The equilibrium constant Kw remains consistent at 25°C: Kw = [H3O+][OH–] = 1.0 × 10–14.

Understanding pH, pOH, and Kw

Logarithmic Nature of pH and pOH

• The letter "p" in formulas (pH, pOH, pKw) implies taking the negative logarithm of a concentration.

  • Concentrations of H+ or OH– have units of mol/L (M), while pH and pOH are dimensionless.

• To calculate values using [H3O+] and [OH–]:

  • If [H3O+] is known, pH can be calculated as: pH = –log[H3O+]

  • To find [H3O+] from pH: [H3O+] = 10–pH

  • If [OH–] is known, pOH can be calculated using: pOH = –log[OH–]

  • And to find [OH–] from pOH: [OH–] = 10–pOH

Key Relationships at 25°C

• Kw = [H3O+][OH–] = 1.0 × 10–14

• pKw = pH + pOH = 14

Significant Digits in pH Calculations

• The significance of digits in pH is determined by the number of significant figures in [H3O+]. The following examples illustrate how changes in concentration affect pH accuracy:

  • [H3O+] = 1 × 10–3 M → pH = 3.0

  • [H3O+] = 1.2 × 10–3 M → pH = 2.92

  • [H3O+] = 1.22 × 10–3 M → pH = 2.914

• Students need to fill in tables for [H3O+], pH, [OH–], pOH while considering significant digits. Elements in the table also help reinforce the concept that:

  • As [H3O+] increases, pH decreases.

  • As [OH–] increases, pH increases.

Effects of Temperature on Kw

Autoionization of Water and Temperature

• The reaction is endothermic, where the equilibrium constant Kw changes with temperature.

• The following values encapsulate changes in Kw with temperature increases:

  • Temperature (°C) | Kw | pKw | pH | pOH

    • 0°C → 1.14 × 10–15; pKw = 14.943

    • 10°C → 2.93 × 10–15; pKw = 14.533

    • 20°C → 6.81 × 10–15; pKw = 14.167

    • 25°C → 1.01 × 10–14; pKw = 13.996

    • 30°C → 1.47 × 10–14; pKw = 13.833

    • 40°C → 2.92 × 10–14; pKw = 13.535

    • 50°C → 5.48 × 10–14; pKw = 13.261

True or False Statements

• Analyze statements regarding pure H2O:

  • A sample of pure H2O cannot have a pH of 7.0 at all temperatures.

  • [H+] equals [OH–] in pure H2O at all temperatures.

  • A sample of pure H2O is defined as neutral if pH = pOH.

  • If the pH is less than 7, the temperature must be less than 25°C.

  • H2O ionizes more at 50°C than at 25°C.

  • As temperature rises, pH of pure H2O declines.

  • If the pH of pure H2O is 6.8, pOH will be 7.2.