Definitions: Acids & bases

Core Concept

Acids are substances that increase the concentration of hydrogen ions ($\text{H}^+$) or donate protons, while bases increase the concentration of hydroxide ions ($\text{OH}^−$) or accept protons. These definitions are captured in the Arrhenius model (focused on aqueous solutions), the Brønsted-Lowry model (proton transfer), and the Lewis model (electron pair exchange).

  • Acids donate protons ($\text{H}^+$); bases accept protons.

  • Strong acids and bases completely dissociate, while weak ones only partially dissociate.

  • Neutralization reactions result in water and salt.

  • Understanding acid and base strength is essential for predicting reaction behavior.

Test Yourself

Assorted Multiple Choice
A constant current is passed through an electrolytic cell for 45.0 minutes, delivering a total charge of 8,100 Coulombs. How many moles of electrons were transferred during this process? (Faraday's constant = 96,485 C/mol e⁻)

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Definitions of Acids and Bases

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Definitions

Venn diagram showing the relationships among Arrhenius, Brønsted-Lowry, and Lewis acids and bases, with Arrhenius being the innermost circle, surrounded by Brønsted-Lowry in green, and Lewis in blue.

Arrhenius Definition:

  • Acid: A substance that increases $\text{H}^+$ (or $\text{H}_3\text{O}^+$) concentration in aqueous solution.

  • Base: A substance that increases $\text{OH}^−$ concentration in aqueous solution.

  • Example:

    • Acid: $\text{HCl} \rightarrow \text{H}^+ + \text{Cl}^-$

    • Base: $\text{NaOH} \rightarrow \text{Na}^+ + \text{OH}^-$

Brønsted-Lowry Definition:

  • Acid: A proton donor.

  • Base: A proton acceptor.

  • Example:

    • Acid: $\text{HCl} + \text{H}_2\text{O} \rightarrow \text{Cl}^- + \text{H}_3\text{O}^+$

    • Base: $\text{NH}_3 + \text{H}_2\text{O} \rightarrow \text{NH}_4^+ + \text{OH}^-$

Lewis Definition:

  • Acid: An electron-pair acceptor.

  • Base: An electron-pair donor.

  • Example:

    • Acid: $\text{BF}_3$

    • Base: $\text{NH}_3$

      $\text{BF}_3 + \text{NH}_3 \rightarrow \text{F}_3\text{B-NH}_3$

Properties of Acids and Bases

Acids

  • Taste: Sour (e.g., lemon juice, vinegar).

  • Reaction with Metals: React with active metals to produce hydrogen gas.

    • Example: $\text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2$

  • pH: Less than 7.

  • Indicators:

    • Turn blue litmus paper red.

    • Methyl orange: Red in acidic solutions.

  • Examples:

    • Strong acids: $\text{HCl}, \text{H}_2\text{SO}_4, \text{HNO}_3$

    • Weak acids: $\text{CH}_3\text{COOH}$ (acetic acid), $\text{H}_2\text{CO}_3$

Bases

  • Taste: Bitter and slippery (e.g., baking soda, soap).

  • Reaction with Acids: Neutralize acids to form water and a salt.

    • Example: $\text{NaOH} + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O}$

  • pH: Greater than 7.

  • Indicators:

    • Turn red litmus paper blue.

    • Phenolphthalein: Pink in basic solutions.

  • Examples:

    • Strong bases: $\text{NaOH}, \text{KOH}$

    • Weak bases: $\text{NH}_3, \text{C}_5\text{H}_5\text{N}$

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