Comprehensive Study Guide on the Strengths, Properties, and Theories of Acids and Bases

Classification of Acids and Bases

Concentration vs. Strength: A fundamental distinction must be made between the concentration of a solution and the strength of the acid or base within it.
- It is possible for a very dilute solution of a strong acid or base to have a higher or lower $pH$ than a more concentrated solution of a weak acid or base.
- Chemists classify acids and bases as strong or weak based on the degree to which they dissociate or ionize in water.

Properties and Classification of Bases

Criteria for Classification: Bases are classified as strong or weak based on the degree to which they dissociate in water.
Strong Bases:
- Definition: These are soluble ionic hydroxides that dissociate completely ( $100\%$ ) into metal ions and hydroxide ions in aqueous solution.
- Chemical Equation Example: $NaOH_{(s)} \rightarrow Na^+_{(aq)} + OH^-_{(aq)}$
- Empirical Properties: Strong bases exhibit high electrical conductivity and a very high $pH$ .
Weak Bases:
- Definition: These are compounds that do not necessarily contain hydroxide ions in their structural formula but produce hydroxide ions through a partial reaction with water.
- Chemical Equation Example: NH_{3(g)} + H_2O_{(l)} \rightleftharpoons NH_4^+_{(aq)} + OH^-_{(aq)}
- Empirical Properties: They exhibit low electrical conductivity and a $pH$ value closer to $7$ .

Properties and Classification of Acids

Criteria for Classification: Acids are classified based on the degree to which they ionize in aqueous solution. This can be measured experimentally via conductivity and the rate of chemical reaction.
Strong Acids:
- Definition: These acids ionize completely (>99\%) in aqueous solution. The resulting properties are due to the high concentration of hydronium ions ( $H_3O^+_{(aq)}$ ).
- Chemical Equation Examples:
  - $HCl_{(g)} + H_2O_{(l)} \rightarrow H_3O^+_{(aq)} + Cl^-_{(aq)}$ (>99\%)
  - HNO_{3(aq)} + H_2O_{(l)} \rightarrow H_3O^+_{(aq)} + NO_3^-_{(aq)} (>99\%)
- Known Strong Acids List:
  - $HCl_{(aq)}$ (Hydrochloric acid)
  - $HClO_{4(aq)}$ (Perchloric acid)
  - $HNO_{3(aq)}$ (Nitric acid)
  - $H_2SO_{4(aq)}$ (Sulfuric acid)
  - $HI_{(aq)}$ (Hydroiodic acid)
  - $HBr_{(aq)}$ (Hydrobromic acid)
- Empirical Properties: High electrical conductivity, high rate of reaction with active metals, and a relatively low $pH$ .
Weak Acids:
- Definition: These acids ionize only slightly (<50\%) in aqueous solution, resulting in a low concentration of hydronium ions.
- Chemical Equation Examples:
  - $CH_3COOH_{(s)} + H_2O_{(l)} \rightleftharpoons H_3O^+_{(aq)} + CH_3COO^-_{(aq)}$ (<50\%; typically around $1\%$
  - H_3PO_{4(aq)} + H_2O_{(l)} \rightleftharpoons H_3O^+_{(aq)} + H_2PO_4^-_{(aq)} (<50\%)
- Empirical Properties: Relatively low conductivity, lower rate of reaction with active metals, and a relatively high $pH$ compared to strong acids.
- Classification Note: All acids not listed as strong are considered weak.

Empirical Comparison of Acids and Bases

Property	Strong Acids	Weak Acids	Strong Bases	Weak Bases
pH (at same c and t)	Very low (<<7)	Medium to low (<7)	Very high (>>7)	Medium to high (>7)
Conductivity	High	Low	High	Low
Reaction Rate	Fast	Slow	Fast	Slow
Modified Arrhenius Theory	Completely react with water to form $H_3O^+_{(aq)}$	Partially react with water to form $H_3O^+_{(aq)}$	Completely dissociate to form $OH^-_{(aq)}$	Partially react with water to form $OH^-_{(aq)}$

Polyprotic Substances

General Concepts:
- Not all hydrogen atoms in a compound make it an acid.
- Only hydrogens involved in very polar bonds are ionizable.
- Hydrogen ions are released when molecules with these polar bonds dissolve in water.
Polyprotic Acids:
- Monoprotic Acid: Contains only one ionizable hydrogen (proton).
  - Example: HNO_{3(aq)} + H_2O_{(l)} \rightarrow H_3O^+_{(aq)} + NO_3^-_{(aq)}
- Diprotic/Polyprotic Acid: Contains two ionizable protons.
  - Example ( $H_2SO_4$ ):
    - Step 1: H_2SO_{4(aq)} + H_2O_{(l)} \rightarrow H_3O^+_{(aq)} + HSO_4^-_{(aq)}
    - Step 2: HSO_4^-_{(aq)} + H_2O_{(l)} \rightleftharpoons H_3O^+_{(aq)} + SO_4^{2-}_{(aq)}
- Triprotic/Polyprotic Acid: Contains three ionizable protons.
  - Example ( $H_3PO_4$ ):
    - Step 1: H_3PO_{4(aq)} + H_2O_{(l)} \rightleftharpoons H_3O^+_{(aq)} + H_2PO_4^-_{(aq)}
    - Step 2: H_2PO_4^-_{(aq)} + H_2O_{(l)} \rightleftharpoons H_3O^+_{(aq)} + HPO_4^{2-}_{(aq)}
    - Step 3: HPO_4^{2-}_{(aq)} + H_2O_{(l)} \rightleftharpoons H_3O^+_{(aq)} + PO_4^{3-}_{(aq)}
Polyprotic Bases:
- Monobasic Bases: Contain only one hydroxide ion (e.g., $KOH_{(s)} \rightarrow K^+_{(aq)} + OH^-_{(aq)}$ ).
- Dibasic/Polyprotic Bases: Contain two hydroxide ions (e.g., $Ca(OH)_{2(s)} \rightarrow Ca^{2+}_{(aq)} + 2OH^-_{(aq)}$ ).
Strength Note: In general, polyprotic substances are weak, and their strength decreases with each successive reaction with water.

Comprehensive Properties and Nomenclature of Bases

General Properties of Bases:
- Taste: Bitter.
- Touch: Slippery.
- Conductivity: Act as electrolytes.
- Indicators: Change color; turn red litmus paper blue.
- $pH$ Range: Approximately $7.1$ to $14$ .
- Reactivity: React with compounds containing hydrogen or hydronium ions ( $H^+_{(aq)} / H_3O^+_{(aq)}$ ) to form water and a salt.
- Ion Production: Produce hydroxide ions ( $OH^-_{(aq)}$ ) in solution.
Nomenclature of Bases: Named by the cation followed by the name of the anion (e.g., Sodium hydroxide for $NaOH$ , Potassium hydroxide for $KOH$ ).

Comprehensive Properties and Nomenclature of Acids

General Properties of Acids:
- Taste: Sour.
- Touch: Squeaky.
- Conductivity: Act as electrolytes.
- Indicators: Change color; turn blue litmus paper red.
- $pH$ Range: Approximately $1$ to $6.9$ .
- Reactivity with Metals: React with some metals to produce hydrogen gas ( $H_{2(g)}$ ).
- Reactivity with Bases: React with compounds containing hydroxide ions ( $OH^-_{(aq)}$ ) to form water and a salt.
- Ion Production: Produce hydrogen or hydronium ions ( $H^+_{(aq)} / H_3O^+_{(aq)}$ ) in solution.
- Requirement: Acids must be in solution, denoted with the subscript $(aq)$ .
Nomenclature for Binary Acids:
- Composition: $H^+_{(aq)}$ and a non-metal.
- Rule: Use prefix "hydro-", the root name of the non-metal with the suffix "-ic", followed by the word "acid".
- Examples: $HCl$ (Hydrochloric acid), $HF$ (Hydrofluoric acid), $HBr$ (Hydrobromic acid).
Nomenclature for Oxoacids (Polyatomic Ions):
- Rule: Do not use the prefix "hydro-". Change the suffix of the polyatomic ion name.
- Suffix Conversion:
  - "-ate" ions become "-ic" acids.
  - "-ite" ions become "-ous" acids.
- Examples:
  - $H_2SO_3$ (Sulfurous acid - from Sulfite)
  - $H_2SO_4$ (Sulfuric acid - from Sulfate)
  - $HNO_2$ (Nitrous acid - from Nitrite)
  - $HNO_3$ (Nitric acid - from Nitrate)

Evolution of Acid-Base Theories

Arrhenius Theory:
- Acid: A substance that ionizes to form hydrogen ions ( $H^+_{(aq)}$ ) in solution.
- Base: A substance that dissociates to produce hydroxide ions ( $OH^-_{(aq)}$ ) in aqueous solution.
- Key Distinction:
  - Ionization: Molecular substances breaking up to form ions.
  - Dissociation: Ionic substances breaking into their component ions.
- Limitations: It fails to explain why substances like ammonia ( $NH_{3(g)}$ ) and sodium bicarbonate ( $NaHCO_{3(s)}$ ) act as bases despite lacks a hydroxide ion in their formulas.
The Hydronium Ion Concept:
- Individual $H^+_{(aq)}$ ions (isolated protons) do not exist in aqueous solutions.
- They interact with water molecules to exist as hydronium ions, $H_3O^+_{(aq)}$ .
Modified Arrhenius Theory:
- Acid: Reacts with water to produce $H_3O^+_{(aq)}$ ions.
- Base: Dissociates or reacts with water to produce $OH^-_{(aq)}$ ions.

Metallic and Non-Metallic Oxides

*   **Non-metallic oxides**: React with water to form acidic solutions.
*   **Metallic oxides**: React with water to form basic solutions.
*   These reactions typically occur in a two-step process.
*   Examples include  $CO_{2(g)}$  (forming acids) and  $BaO_{(s)}$  (forming bases).