VCE ACID BASE

Chapter Overview

Acids and bases play significant roles in household products, industries, agriculture, biological and environmental systems. This chapter covers various theories of acids and bases, their reactions in water, and emerging implications such as ocean acidity.

Key Knowledge Areas

  • Brønsted–Lowry Theory

    • Definition: Defines acids as proton donors and bases as proton acceptors.

    • Polyprotic Acids: Acids that can donate more than one proton (e.g., sulfuric acid, phosphoric acid).

    • Amphiprotic Species: Substances capable of acting as either acids or bases, like water and bicarbonate ions.

  • Writing Balanced Ionic Equations: Importance of illustrating acid-base reactions accurately, indicating all ionic species involved.

  • Ionic Product of Water: Understanding how the product of hydroxide and hydronium ion concentrations (Kw = [H3O+][OH-] = 1.0 x 10^-14 at 25°C) is crucial for calculating pH and assessing acidity or basicity.

  • pH Scale & Calculations: Methods to calculate pH, including the formula pH = -log10[H3O+], and the significance of strong/weak and concentrated/dilute classifications of acids and bases.

  • Reactions with Acids:

    • With Metals: Release hydrogen gas and form a salt (e.g., 2HCl + Zn → ZnCl2 + H2).

    • With Carbonates: Produce carbon dioxide, water, and a salt (e.g., HCl + NaHCO3 → NaCl + H2O + CO2).

    • With Hydroxides: Neutralization leading to salt and water (e.g., H2SO4 + 2NaOH → Na2SO4 + 2H2O).

  • Environmental Implications: Focus on ocean acidification due to increased atmospheric CO2 levels, affecting marine ecosystems like coral reefs.

Common Acids and Their Uses

  • Hydrochloric Acid (HCl):

    • Used in the stomach for protein digestion and widely for cleaning purposes.

  • Sulfuric Acid (H2SO4):

    • Key in manufacturing fertilizers and used in car batteries.

  • Nitric Acid (HNO3):

    • Applied in producing fertilizers and explosives.

  • Ethanoic Acid (CH3COOH):

    • Found in vinegar and used as a preservative.

  • Carbonic Acid (H2CO3):

    • Present in carbonated beverages, adds acidity and flavor.

  • Phosphoric Acid (H3PO4):

    • Common in soft drinks and used for fertilizers.

  • Citric Acid (C6H8O7):

    • Natural acid found in citrus fruits, used for flavoring and preservation.

  • Ascorbic Acid (C6H8O6):

    • Also known as Vitamin C, essential for dietary health.

Common Bases and Their Uses

  • Sodium Hydroxide (NaOH):

    • Commonly found in cleaning agents and utilized in soap making.

  • Ammonia (NH3):

    • Used extensively in household cleaners and fertilizers.

  • Calcium Hydroxide (Ca(OH)2):

    • Employed in cement production and soil pH adjustment.

  • Magnesium Hydroxide (Mg(OH)2):

    • Commonly used as an antacid for heartburn relief.

  • Sodium Carbonate (Na2CO3):

    • Utilized in washing powders and glass production due to its alkaline properties.

Properties of Acids and Bases

  • Acids:

    • Characteristics include turning litmus paper red, having a sour taste, being corrosive, conducting electricity, and generally exhibiting low pH values.

  • Bases:

    • Turn litmus paper blue, possess a bitter taste, feel slippery to touch, conduct electricity, and typically have high pH values.

Historical Developments in Acid-Base Theories

  • Robert Boyle:

    • Observed acid properties via taste and through indicator reactions.

  • Antoine Lavoisier:

    • Proposed oxygen as essential in understanding acidity, although faced challenges from exceptions.

  • Humphrey Davy:

    • Suggested hydrogen is the key factor determining acidity.

  • Svante Arrhenius:

    • Defined acids as H+ producers and bases as OH- producers, providing foundational definitions.

Brønsted-Lowry Theory Summary

  • Proton Donor: Acid (e.g., HCl)

  • Proton Acceptor: Base (e.g., H2O)

  • Conjugate Acid-Base Pairs: Created through proton transfer during reactions.

Strengths of Acids and Bases

  • Strong Acids:

    • Fully ionize in solution, such as hydrochloric acid (HCl) and sulfuric acid (H2SO4).

  • Weak Acids:

    • Partially ionize, like acetic acid (CH3COOH).

  • Importance of knowing the strength of acids/bases in terms of their ability to donate/accept protons (H+).

Calculating pH and Ion Concentration

  • pH is calculated using the formula:[ pH = -\log_{10}[H3O^+] ]

  • The relationship between hydronium and hydroxide ions, defined through the ionic product of water, impacts calculations and reactions significantly.

Reactions of Acids and Bases

Acid-Base Neutralization

  • General Form:[ \text{Acid + Base} \rightarrow \text{Salt + Water} ]

    • Example: H2SO4 + 2NaOH → Na2SO4 + 2H2O

Reaction of Acids with Carbonates

  • Produces carbon dioxide, salt, and water.

    • Example: HCl + NaHCO3 → NaCl + H2O + CO2

Reaction of Acids with Reactive Metals

  • Produces salt and hydrogen gas.

    • Example: 2HCl + Zn → ZnCl2 + H2

Ocean Acidity and Environmental Impact

  • Ocean acidity has been increasing due to CO2 absorption from the atmosphere, significantly affecting marine ecosystems, including coral reefs and other marine life.

Dilution of Acids and Bases

  • The concentration and pH change during dilution can be predicted using the formula: c1V1 = c2V2, which is essential for understanding how concentrations affect acid and base properties.

Key Terms

  • Acid

  • Base

  • Amphiprotic

  • Conjugate Pair

  • Strong Acid/Base

  • Weak Acid/Base

  • pH

  • Ionic Product of Water

  • Dilution

  • Neutralization

  • Self-Ionisation

Review Questions

  1. Discuss the role of bicarbonate ions as amphiprotic substances.

  2. Compare/contrast strong acids vs weak acids in terms of their dissociation in water and conductivity.

  3. Calculate pH given certain concentrations of H3O+. Discuss the behaviors of strong/weak acids upon dilution.