Chapter 10: Acids, Bases, and Salts

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Chapter Overview & SLOs

What are acids and bases? Acids and bases are chemical substances that exhibit characteristic properties. Understanding their definitions is fundamental to chemistry.

What is the Arrhenius theory of acids and bases? Proposed by Svante Arrhenius:

• Arrhenius acid: A substance that dissociates in water to produce hydrogen ions ($H^+$). Example: $$HCl \rightarrow H^+ + Cl^-$$ $$HNO_3 \rightarrow H^+ + NO_3^-$$
• Arrhenius base: A substance that dissociates in water to produce hydroxide ions ($OH^-$). Example: $$NaOH \rightarrow Na^+ + OH^-$$ $$KOH \rightarrow K^+ + OH^-$$

What is the Bronsted-Lowry theory of acids and bases? Proposed by Johannes Bronsted and Thomas Lowry:

• Bronsted-Lowry acid: A proton ($H^+$) donor. Example: $HCl$ donates a proton to water.
• Bronsted-Lowry base: A proton ($H^+$) acceptor. Example: $NH_3$ accepts a proton to form $NH_4^+$.
• This theory is broader than Arrhenius as it does not require the presence of water.

What are amphoteric substances? Amphoteric substances can act as either an acid or a base depending on the reaction.

• Water as an amphoteric substance:
  • Acts as a base (proton acceptor) when reacting with $HCl$: $$HCl + H_2O \rightarrow H_3O^+ + Cl^-$$
  • Acts as an acid (proton donor) when reacting with $NH_3$: $$NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^-$$

What is the auto-ionization of water? Water molecules react with each other to form ions:

• $$2H_2O \rightleftharpoons H_3O^+ + OH^-$$
• At $25^\circ C$, $[H_3O^+] = [OH^-] = 1.0 \times 10^{-7}$ M
• Ionic product of water: $$K_w = [H_3O^+][OH^-] = 1.0 \times 10^{-14}\text{ at }25^\circ C$$

What is the pH scale? The pH scale measures the acidity or basicity of a solution:

• $$pH = -\log[H_3O^+]$$$$Acidic solution: [H^+] > [OH^-]\text{, }pH < 7$$$$\text{Neutral solution: }[H^+] = [OH^-]\text{, }pH = 7$$$$\text{Basic (alkaline) solution: }[H^+] < [OH^-]\text{, }pH > 7$$

What are pH indicators? pH indicators are substances that change color depending on the pH of the solution.

• Phenolphthalein: Colorless in acidic solution, pink in basic solution (turns pink around pH 8.2-10).
• Methyl red: Red in acidic solution (pH < 4.4), yellow in basic solution (pH > 6.2).
• Litmus paper: Red in acidic solution, blue in basic solution, purple in neutral.
• Universal indicator: Mixture that shows a range of colors across the pH scale.

What are neutralization reactions? A neutralization reaction is the reaction between an acid and a base to produce a salt and water.

• General equation: $$\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water}$$
• Example: $$HNO_3 + NH_4OH \rightarrow NH_4NO_3 + H_2O$$
• Example: $$HCl + NaOH \rightarrow NaCl + H_2O$$

What are acid-metal reactions? Reactive metals react with acids to produce a salt and hydrogen gas.

• General equation: $$\text{Acid} + \text{Metal} \rightarrow \text{Salt} + H_2(g)$$
• Example: $$2HCl + Mg \rightarrow MgCl_2 + H_2 \uparrow$$

What is a weak base? A weak base only partially ionizes in water to produce hydroxide ions.

• Example: Ammonia ($NH_3$) is a weak base: $$NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^-$$
• It establishes an equilibrium (shown by $\rightleftharpoons$) because only a small fraction of ammonia molecules accept protons.

Comparison of Arrhenius and Bronsted-Lowry theories:

| Property | Arrhenius Theory | Bronsted-Lowry Theory | |----------|-----------------|----------------------| | Acid definition | Produces $H^+$ in water | Proton donor | | Base definition | Produces $OH^-$ in water | Proton acceptor | | Requires water? | Yes | No | | Scope | Limited (aqueous only) | Broader (includes non-aqueous) | | Example acid | $HCl$, $HNO_3$ | $HCl$, $NH_4^+$ | | Example base | $NaOH$, $KOH$ | $OH^-$, $NH_3$ |

These notes are strictly aligned with the Student Learning Outcomes (SLOs) for the FBISE 2026 annual examination.

• How do we classify acids and bases using different theories? Identify Arrhenius acids like $HNO_3$ that dissociate in water to produce $H^+$ ions, Arrhenius bases like $NaOH$ that produce $OH^-$ ions, and Bronsted-Lowry species like ammonia ($NH_3$) that act as proton acceptors (bases) and $HCl$ that act as proton donors (acids).
• How do we explain the self-ionization (auto-ionization) of water? Write the balanced equation $2H_2O \rightleftharpoons H_3O^+ + OH^-$ and identify which molecule acts as the proton donor (acid) and which as the proton acceptor (base) in this reaction, understanding that water is amphoteric.
• How do we determine the products of neutralization and acid-metal reactions? Formulate balanced equations for the reactions between various acids and bases (neutralization: acid + base → salt + water) and between acids and reactive metals (acid + metal → salt + hydrogen gas), and identify the resulting salts such as $NH_4NO_3$, $NaCl$, or $MgCl_2$.
• How do we measure acidity and basicity using the pH scale and indicators? Apply knowledge of the pH scale to classify substances as acidic ($pH < 7$, $[H^+] > [OH^-]$), neutral ($pH = 7$, $[H^+] = [OH^-]$), or basic ($pH > 7$, $[H^+] < [OH^-]$), and use indicators like phenolphthalein (pink in base), methyl red (red in acid), and litmus paper to visually determine the nature of a solution.

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