Acids and bases are key players in chemistry, affecting everything from your stomach to your shampoo. They have distinct properties, like taste and touch, and can neutralize each other in reactions that produce water and salt.
Different theories explain acids and bases, from Arrhenius to Lewis. These theories help us understand their strength, dissociation, and how they interact with other substances. Knowing the difference between strong and weak acids and bases is crucial for predicting reactions.
Properties of Acids and Bases
Characteristics of Acids
- Have a sour taste (lemons, vinegar)
- React with metals to produce hydrogen gas (HCl + Zn → ZnCl2 + H2)
- Change the color of acid-base indicators such as litmus paper to red
- Common examples include hydrochloric acid (HCl), sulfuric acid (H2SO4), and acetic acid (CH3COOH)
Characteristics of Bases
- Have a bitter taste (baking soda, soap)
- Feel slippery to the touch due to the formation of soluble hydroxides
- Change the color of acid-base indicators such as litmus paper to blue
- Common examples include sodium hydroxide (NaOH), potassium hydroxide (KOH), and ammonia (NH3)
Neutralization Reactions
- Acids and bases neutralize each other when mixed in the appropriate molar ratios
- Neutralization produces water and a salt (NaOH + HCl → NaCl + H2O)
- The heat of neutralization is the enthalpy change associated with the formation of water from H+ and OH- ions
Acid-Base Theories
Arrhenius Theory
- Defines acids as substances that dissociate in water to produce hydrogen ions (H+)
- Defines bases as substances that dissociate in water to produce hydroxide ions (OH-)
- Limited to aqueous solutions and does not account for substances without H+ or OH-
Brønsted-Lowry Theory
- Defines acids as proton (H+) donors and bases as proton acceptors
- Extends the definition of acids and bases to include substances that do not contain hydroxide ions or hydrogen ions
- Introduces the concept of conjugate acid-base pairs (NH3 + H2O ⇌ NH4+ + OH-)
Lewis Theory
- Defines acids as electron pair acceptors and bases as electron pair donors
- Further broadens the definition of acids and bases to include substances that do not involve proton transfer
- Explains the basicity of compounds like ammonia (NH3) and the acidity of metal ions like Fe3+
Acid and Base Strength
Relationship between Strength and Dissociation
- The strength of an acid or base is related to its degree of dissociation in aqueous solution
- Strong acids and bases dissociate completely in water, while weak acids and bases only partially dissociate
- The dissociation constant (Ka for acids and Kb for bases) quantifies the strength of an acid or base
Dissociation Constants
- A higher Ka or Kb value indicates a stronger acid or base, respectively
- The relationship between Ka and Kb is defined by the equation: $K_w = K_a \times K_b$, where Kw is the ionization constant of water ($1.0 \times 10^{-14}$ at 25°C)
- For monoprotic acids, the acid dissociation constant is represented as: $K_a = \frac{[H^+][A^-]}{[HA]}$
Factors Affecting Acid and Base Strength
- The strength of an acid depends on the polarity and strength of the H-A bond
- The strength of a base depends on its ability to accept protons and the stability of the conjugate acid formed
- Inductive effects, resonance, and the size of the atom bonded to H+ or OH- can influence acid and base strength
Strong vs Weak Acids and Bases
Properties of Strong Acids and Bases
- Dissociate completely in aqueous solution (HCl → H+ + Cl-)
- Have very high electrical conductivity due to the presence of free ions
- React rapidly with other substances due to the high concentration of H+ or OH-
- Examples of strong acids include HCl, H2SO4, and HNO3, while strong bases include NaOH and KOH
Properties of Weak Acids and Bases
- Only partially dissociate in aqueous solution (CH3COOH ⇌ CH3COO- + H+)
- Have lower electrical conductivity compared to strong acids and bases
- React more slowly with other substances due to the lower concentration of H+ or OH-
- Examples of weak acids include acetic acid (CH3COOH) and phosphoric acid (H3PO4), while weak bases include ammonia (NH3) and methylamine (CH3NH2)
Experimental Determination of Acid and Base Strength
- The strength of an acid or base can be determined experimentally by measuring its electrical conductivity
- Reaction rates with other substances can also indicate the strength of an acid or base
- Dissociation constants (Ka or Kb) can be calculated from pH and concentration data obtained through titrations or spectrophotometric methods