Brønsted–Lowry Theory

Introduction to Brønsted–Lowry Theory

Contextual Overview

• Understanding acid-base chemistry is essential for comprehending various chemical processes.
• The Brønsted-Lowry Theory expands the scope of the Arrhenius Theory by explaining reactions beyond aqueous environments.
• Key Advancement: Protons (H$^+$) serve as the fundamental criteria for classifying acids and bases.

Basic Concepts of Brønsted-Lowry Theory

• Acid (Proton Donor): Entities that donate protons (H$^+$). For instance, HCl donates a proton to water, thereby forming H$_3$O$^+$.
• Base (Proton Acceptor): Entities that accept protons (H$^+$). For example, NH$_3$ accepts a proton from water to form NH$_4^+$.

Comparison to Arrhenius Theory

• The Arrhenius model confines reactions to aqueous environments, considering only H$^+$ or OH$^-$.
• In contrast, the Brønsted-Lowry model is applicable in diverse media, enhancing our grasp of chemical behaviour.

Example Reactions

• Example Reaction 1:

  • $\text{NH}_3 + \text{H}_2\text{O} \rightleftharpoons \text{NH}_4^+ + \text{OH}^-$
  • Analysis: NH$_3$, being a base, accepts a proton. Conversely, H$_2$O, acting as an acid, donates a proton.

• Example Reaction 2:

  • $\text{HCl} + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ + \text{Cl}^-$
  • Analysis: HCl donates a proton while H$_2$O accepts a proton.

Historical Context

• Transition from Arrhenius to Brønsted-Lowry: It was vital to enable the explanation of more complex reactions beyond simple aqueous systems.
• Contributions by Brønsted and Lowry:
  • Their work underpins contemporary understanding of acids and bases.
• Importance: Integral to modern chemistry education and relevant to varied chemical contexts.

Objective

• Aim: Equip students with the skills to measure pH and understand its practical applications, such as:
  • Examples: Assessing water quality, ensuring food safety.

Materials and Techniques

• Materials:

  • pH Metres: Highly precise.
  • Litmus Paper & Universal Indicator: Economical and educational.
  • Household Substances: Easily accessible and pertinent.

• Techniques:

  • pH Metres: Provide accurate measurements.
  • Litmus Paper/Indicators: Indicate changes visually.

Safety and Procedure

Safety Considerations

Using pH Metres

1. Calibration: Utilise buffer solutions to calibrate before use.
2. Sample Preparation: Ensure the use of clean equipment.
3. Measurement: Insert the electrode, allow to stabilise, then record observations.

Using Litmus Paper and Universal Indicators

1. Dipping: Briefly submerge and let dry.
2. Comparison: Observe colour changes against a reference chart.

Concept of pOH

• Definition – "pOH": Represents hydroxide ion concentration, paralleling pH for hydrogen ions.

• pH and pOH Relationship In aqueous solutions:

  $\text{pH} + \text{pOH} = 14$

Formulae for [H$^+$] and [OH$^-$]

Definitions

• Hydrogen Ion Concentration \textcolor{green}{\textbf{[H^+]}}: Indicates solution acidity.
• Hydroxide Ion Concentration \textcolor{green}{\textbf{[OH^-]}}: Indicates solution basicity.

Calculation

• pH and [H$^+$]: $\text{pH} = -\log_{10}[\text{H}^+]$
• Computing [H$^+$]: $[\text{H}^+] = 10^{-\text{pH}}$

Practical Implications

• Healthcare: pH balance monitoring is crucial in fields such as human physiology (e.g., blood pH).
• Environment: Helps interpret consequences of acid rain.

Addressing Misconceptions

Strength vs Concentration

Strength: Refers to the degree of ionisation.

Concentration: Denotes the amount of a substance present.

• Common Misconception: Assuming that a diluted strong acid is less potent than a concentrated weak acid.

Acid	Ionisation	Concentration
Strong Acid (dilute)	High	Low
Weak Acid (concentrated)	Low	High

Misunderstanding Logarithmic Scale

• Compression: The pH scale compresses vast ion concentration ranges.
• Mathematical Representation: $\text{pH} = -\log_{10}[H^+]$.

Graphical Illustration:

• A change from $10^{-3}$ to $10^{-2}$ elevates the pH from 3 to 2, indicating a tenfold increase in concentration.

Role of Water

• Active Influence: Water influences ion equilibrium via auto-ionisation.
• Impact: Significantly affects ion concentration levels.