Introduction to Acid-Base Chemistry: Definitions, Reactions, and Stoichiometry

Introduction to Acid-Base Chemistry

This study guide provides an overview of acid-base chemistry, including key definitions, types of acids and bases, representative reactions, and stoichiometric calculations. Understanding these concepts is fundamental for mastering general chemistry.

Acid-Base Definitions

Arrhenius Definition

• Acid: A substance that increases the concentration of H+ ions in aqueous solution.

• Base: A substance that increases the concentration of OH- ions in aqueous solution.

Brønsted-Lowry Definition

• Acid: A proton (H+) donor.

• Base: A proton (H+) acceptor.

Example: In the reaction $\mathrm{HCl} + \mathrm{H_2O} \rightarrow \mathrm{H_3O^+} + \mathrm{Cl^-}$, HCl acts as a Brønsted-Lowry acid and water as a base.

Acid-Base Reactions in Water

Acid-base reactions in aqueous solution involve the transfer of protons between reactants. The nature of the acid and base determines the products formed.

• Example 1: $\mathrm{HNO_3(aq)} \rightarrow$ (products not specified; typically forms $\mathrm{H_3O^+}$ and $\mathrm{NO_3^-}$)

• Example 2: $\mathrm{HCl(aq)} + \mathrm{H_2O(l)} \rightarrow \mathrm{H_3O^+} + \mathrm{Cl^-}$

Proton Transfer and Electronegativity

H+ can be transferred between atoms of moderate to high electronegativity. The ability of a bond to donate a proton depends on the polarity and the electronegativity difference between the atoms involved.

Bond	Electronegativity (X)	Notes
C–H	2.6 (C), 2.2 (H)	Slightly polar; carbons do not donate H+ (relevant in organic chemistry)
N–H	3.0 (N), 2.2 (H)	Polar bond; neutral nitrogens do not readily donate H+
O–H	3.5 (O), 2.2 (H)	Polar bond; many acids feature an O–H bond
X–H (X = halogen)	Varies	Great H+ donors; $\Delta \chi$ = 1.0, 0.8, 0.5 for HCl, HBr, HI, respectively

Acids of Polyatomic Ions

Many acids are derived from polyatomic ions, often by adding one or more protons to the anion.

• Acetic acid: $\mathrm{CH_3CO_2H}$

• Chlorous acid: $\mathrm{HClO_2}$

• Nitrous acid: $\mathrm{HNO_2}$

• Phosphoric acid: $\mathrm{H_3PO_4}$

Example: $\mathrm{H_2CO_3(aq)} + \mathrm{H_2O(l)} \leftrightarrow$ (products not specified; typically forms $\mathrm{HCO_3^-}$ and $\mathrm{H_3O^+}$)

Strong Acids and Bases

Strong acids and bases dissociate completely in aqueous solution, making them important in stoichiometric calculations and titrations.

• Strong acids: HCl, HBr, HI, HNO3, H2SO4, HClO3, HClO4

• Strong bases: LiOH, NaOH, KOH, RbOH, Ca(OH)2, Sr(OH)2, Ba(OH)2

Example: $\mathrm{NaOH(aq)} \rightarrow \mathrm{Na^+} + \mathrm{OH^-}$ (complete dissociation)

Reactions of Oxides with Water

Nonmetal Oxide + Water = Acidic Solution

• $\mathrm{2 NO_2(g) + H_2O(l) \rightarrow 2 HNO_3(aq) + NO(g)}$

• $\mathrm{CO_2(g) + H_2O(l) \rightarrow H_2CO_3(aq)}$

Nonmetal oxides react with water to form acids, contributing to phenomena such as acid rain.

Group 1 & 2 Metal Oxides + Water = Basic Solution

• $\mathrm{Na_2O(s) + H_2O(l) \rightarrow 2 Na^+(aq) + 2 OH^-(aq)}$

• $\mathrm{CaO(s) + H_2O(l) \rightarrow Ca^{2+}(aq) + 2 OH^-(aq)}$

Metal oxides from Groups 1 and 2 react with water to form basic solutions.

Acid-Base Neutralization Reactions

Neutralization reactions occur when an acid and a base react to form water and a salt.

• $\mathrm{NaOH(aq) + HCl(aq) \rightarrow H_2O(l) + NaCl(aq)}$

• $\mathrm{2 KOH(aq) + H_2SO_4(aq) \rightarrow 2 H_2O(l) + K_2SO_4(aq)}$

General Equation: $\mathrm{Acid + Base \rightarrow Water + Salt}$

Stoichiometry of Acid-Base Reactions

Stoichiometry allows us to calculate the quantities of reactants and products in acid-base reactions, especially for titrations and neutralizations.

• Key formula: $M_1V_1 = M_2V_2$ (for monoprotic acid-base reactions)

• For polyprotic acids or bases, account for the number of ionizable protons or hydroxide ions.

Example Problem 1: What volume of 0.22 M KOH(aq) is required to completely neutralize 75 mL of 1.5 M HBr(aq)?

• Write the balanced equation: $\mathrm{KOH + HBr \rightarrow KBr + H_2O}$

• Calculate moles of HBr: $0.075\,L \times 1.5\,mol/L = 0.1125\,mol$

• Since the reaction is 1:1, moles of KOH needed = 0.1125 mol

• Volume of KOH: $V = \frac{0.1125\,mol}{0.22\,mol/L} = 0.511\,L = 511\,mL$

Example Problem 2: What mass of calcium hydroxide must be added to neutralize 150 mL of 3.0 M HNO3(aq)?

• Balanced equation: $\mathrm{Ca(OH)_2 + 2 HNO_3 \rightarrow Ca(NO_3)_2 + 2 H_2O}$

• Moles of HNO3: $0.150\,L \times 3.0\,mol/L = 0.45\,mol$

• Moles of Ca(OH)2 needed: $0.45\,mol \div 2 = 0.225\,mol$

• Mass: $0.225\,mol \times 74.09\,g/mol = 16.67\,g$

Additional info: The above calculations are based on standard stoichiometric methods for acid-base titrations.