Acid-Base Reactions

Acid-base reactions: Acids are substances that increase the concentration of H⁺ ions in solution, while bases increase the concentration of OH⁻ ions. These reactions are vital in various everyday and industrial processes and constitute a fundamental component of chemical theory.

Introduction

• Balanced chemical equations illustrate the law of conservation of mass, essential for predicting chemical reaction outcomes and ensuring safety in laboratory environments.
• Significance: Acid-base reactions are pivotal in academia and industry, particularly in fields such as pharmaceuticals and agriculture.

Definitions and Distinctions

• Acid: A substance that increases the concentration of H⁺ ions in solution. According to Arrhenius, acids release hydrogen ions (H⁺) when dissolved in water, thereby increasing acidity.

  • Example: Hydrochloric acid (HCl) facilitates digestion.

• Base: A substance that increases OH⁻ ion concentration in solution. Bases release hydroxide ions in aqueous solutions.

  • Example: Sodium hydroxide (NaOH) is commonly used in cleaning solutions.

• Brønsted-Lowry Theory:

  • Acid: Proton donor
  • Base: Proton acceptor
    • Example: Ammonia (NH₃) accepts H⁺ to form ammonium (NH₄⁺).

Exothermic Reaction: A chemical reaction that releases heat, causing an increase in the surrounding temperature.

Reactions Involving Acids and Bases

Neutralisation Reactions

• Neutralisation produces water and a salt as products.
• Example Reaction: $\text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O}$
  • Step-by-Step:
    • HCl dissociates into H⁺ and Cl⁻.
    • NaOH dissociates into Na⁺ and OH⁻.
    • H⁺ combines with OH⁻ to form water (H₂O).
    • Na⁺ and Cl⁻ form NaCl (table salt).

Acid-Metal Reactions

• Example Reaction: $\text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2$
  • Step-by-Step:
    • Zinc reacts with HCl, displacing H⁺ ions.
    • H⁺ ions combine to form H₂ gas, observable as bubbles.

Historical Development of Acid-Base Theories

• Arrhenius Model: Acids generate H⁺ ions in water; bases produce OH⁻ ions.

• Brønsted-Lowry Model: Broadens the definition of acids and bases to non-aqueous solutions.

• Lewis Theory: Focuses on electron pair donation and reception.

• Comparison Table: Differentiates Arrhenius, Brønsted-Lowry, and Lewis theories, highlighting the differences between electron and proton transfer and the roles of solvents.

Misunderstandings and Practical Usage

• Example in Practice: Catalyst use in renewable biofuel production relies on optimising acid-base reactions.

Nomenclature and Properties of Inorganic Acids and Bases

IUPAC Nomenclature

• Oxoacids: Contain oxygen. Example: H₂SO₄ - Sulphuric Acid.
• Binary Acids: Prefixed with 'hydro-', e.g., HCl - Hydrochloric Acid.
• Bases: Named by the metal followed by 'hydroxide', e.g., NaOH - Sodium Hydroxide.

pH Levels: Acids have a pH < 7; bases have a pH > 7.

• Conductivity: Strong electrolytes dissociate completely, whereas weak ones do so partially.
• Reactivity: Acid-metal interactions release hydrogen gas.

Preparation and Use of Indicators

Key Concepts

Indicators are substances that change colour in response to pH changes.

• Types of Indicators:

  • Litmus: Red in acidic solutions, blue in basic solutions.
  • Phenolphthalein: Colourless in acidic conditions, turns pink in basic solutions.

• Practical Experiments: Use natural indicators, such as red cabbage, to observe colour changes in solutions with varying pH levels.

Working with Equations and Calculations

Writing and Balancing Equations

• Steps: Determine reactants and products, confirm formulas, write, and balance the chemical equations.

Examples and Exercises

• Example: Neutralise HCl with NaOH and balance the equation. $\text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O}$ This balanced equation shows that one mole of HCl reacts with one mole of NaOH to produce one mole of NaCl and one mole of water.

• Practice: Write and balance: $\text{HCl} + \text{Na} \rightarrow \text{?}$ Solution: $2\text{HCl} + 2\text{Na} \rightarrow 2\text{NaCl} + \text{H}_2$ Here, two moles of HCl react with two moles of sodium to produce two moles of sodium chloride and one mole of hydrogen gas.

Neutralisation and Enthalpy

Thermochemistry: Neutralisation reactions are exothermic, with heat released measurable using calorimetric methods.

Hess's Law: Demonstrates that enthalpy changes are consistent regardless of the reaction pathway taken.

Titrations and Buffers

Titrations

• Used to determine the concentration of a solution with precision, observing the endpoint through a colour change.
  • Key Concepts: Titrant versus analyte, and understanding the equivalence point.

Buffers

• Help maintain pH stability, crucial in both biological and chemical systems.
• Employ the Henderson-Hasselbalch equation for effective calculations.

Overview of Practice Applications

• Industrial Use: Lime can neutralise soil acidity, and acetate is used in pharmaceutical synthesis.

• Environmental Concerns: Lime treatment helps mitigate acid rain, supporting ecosystem protection.

• Practical Exploration: Engage in simple experiments like testing household solutions with pH indicators.