Properties of acids and bases Knowledge-Based Study

Study notes

1. Overview of Acids and Bases

   Acids and bases are two fundamental classes of chemical compounds that play vital roles in chemistry, industry, and daily life. Their distinct chemical behaviors arise from how they interact with water and other chemical species.

   Historically, chemists classified these substances based on their observable macroscopic properties:

   • Acids: Substances that typically taste sour, conduct electric current when dissolved in water, and react characteristically with certain metals and indicators. Everyday examples include citric acid in citrus fruits and acetic acid in vinegar.
   • Bases: Substances that typically taste bitter, feel slippery or soapy to the touch, and conduct electricity in aqueous solutions. Common household bases include sodium hydroxide in drain cleaners and sodium hydrogen carbonate (baking soda).

   Conductivity in Aqueous Solutions

   A central property of both acids and bases is that their aqueous solutions act as electrolytes. An electrolyte is a substance that produces mobile ions when dissolved in water, allowing the solution to conduct an electrical current.

   • Acids: In aqueous solution, acids ionize to yield hydrogen ions, written as $H^+$ or hydronium ions, $H_3O^+$. For example: $$HCl(aq) + H_2O(l) ightarrow H_3O^+(aq) + Cl^-(aq)$$
   • Bases: Soluble bases (often called alkalis) dissociate or react with water to release hydroxide ions, $OH^-$. For example: $$NaOH(s) \xrightarrow{H_2O} Na^+(aq) + OH^-(aq)$$

   The presence of these freely moving ions is responsible for electrical conductivity.

   Action on Acid-Base Indicators

   Indicators are substances that change color depending on the concentration of hydrogen ions present in a solution.

   • Litmus: Acidic solutions turn blue litmus paper red. Basic solutions turn red litmus paper blue.
   • Phenolphthalein: Remains colorless in acidic and neutral solutions, but turns distinctly pink or magenta in basic solutions.
   • Bromothymol Blue: Typically yellow in acidic solutions, green at neutral conditions, and blue in basic solutions.

   The pH scale quantitatively reflects this behavior:

   • Solutions with $\text{pH} < 7$ are acidic (high $H^+$ concentration).
   • Solutions with $\text{pH} = 7$ at 25 °C are neutral.
   • Solutions with $\text{pH} > 7$ are basic (low $H^+$ concentration, high $OH^-$ concentration).

   Characteristic Reactions of Acids

   Acids participate in several characteristic chemical reactions:

   1. Reaction with Active Metals

   Acids react with active metals such as magnesium, zinc, and iron to liberate hydrogen gas, $H_2$, forming a salt in a single-displacement reaction:

   $$Mg(s) + 2HCl(aq) ightarrow MgCl_2(aq) + H_2(g)$$

   2. Reaction with Carbonates and Hydrogen Carbonates

   When an acid reacts with a metal carbonate or hydrogen carbonate, carbon dioxide gas, water, and a salt are formed:

   $$CaCO_3(s) + 2HCl(aq) ightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)$$

   This reaction produces observable effervescence due to the rapid release of $CO_2$ bubbles.

   Neutralization Reactions

   When an acid reacts with a base, they undergo a neutralization reaction. The hydrogen ions from the acid combine with the hydroxide ions from the base to produce water, while the remaining ions form an ionic salt:

   $$Acid + Base ightarrow Salt + Water$$

   A classic example is the reaction between hydrochloric acid and sodium hydroxide:

   $$HCl(aq) + NaOH(aq) ightarrow NaCl(aq) + H_2O(l)$$

   Summary Comparison

   • Acids: Sour taste, pH below 7, turn blue litmus red, liberate $H_2$ with active metals, release $CO_2$ with carbonates.
   • Bases: Bitter taste, slippery touch, pH above 7, turn red litmus blue, neutralize acids to form water and salt.