Ch5: Conductivity

Conductivity & The Concept of Electrolytes - Electrical conductivity of a solution is governed by the presence (or absence) of mobile ions in aqueous phase. - Key operational definition used in lab:- If a dissolved substance permits current to flow (e.g. a light-bulb in a simple conductivity tester glows), the solution is labelled electrolyte. - If no current flows, the solution is a nonelectrolyte. - Example demonstrations- \text{NaCl}{ (s)} \xrightarrow{\text{H}2\text{O}} \text{Na}^+{ (aq)} + \text{Cl}^-{ (aq)}

• Produces two freely moving, oppositely charged ions → bright bulb → electrolyte. - Glucose ("sugar") crystals disperse into individual neutral molecules:
\text{C}6\text{H}{12}\text{O}6\ (s) \to \text{C}6\text{H}{12}\text{O}6\ (aq)
• Molecules remain intact, no ions generated → bulb stays dark → nonelectrolyte. - Practical / real-world link: Electrolyte labels on sports drinks exploit the same chemistry—ions such as \text{Na}^+, \text{K}^+, \text{Cl}^- restore conductivity (and physiological ion balance). ### Degrees of Electrolyte Strength - Chemists refine the binary electrolyte/nonelectrolyte idea with three categories:1. Strong electrolytes - Dissociation (or ionization) in water is essentially 100%. - Typical members: all soluble ionic salts (e.g. \text{NaCl}, \text{KNO}_3), many strong acids/bases. 1. Weak electrolytes - Only partial dissociation; light glow is faint. - Classic example: weak acids such as acetic acid.
\text{HA} \rightleftharpoons \text{H}^+ + \text{A}^- with \alpha \approx 0.01\text{–}0.02 (1–2 %). 1. Nonelectrolytes - \alpha = 0; no detectable ions → no conductivity (e.g. glucose, ethanol, urea). - Importance during titration & analytical chemistry:- Choice of indicator electrode depends on whether solution behaves as strong or weak electrolyte. ### Bond Type as Predictor of Electrolytic Behavior - Empirical rule extracted by instructor:- Ionic compounds → when soluble, break into constituent ions → usually strong electrolytes. - Covalent (molecular) compounds → dissolve as neutral molecules → usually nonelectrolytes (exceptions: acids, bases that ionize). - Conductivity test therefore functions as a quick diagnostic for underlying bond type.- Bright light → likely an ionic salt present. - No light → likely purely molecular substance in water. ### Review: Covalent vs. Ionic Bonding Mechanics - Animation 1 (O–H):- Electrons are shared between O and H. - Characteristic of covalent bond. - Fundamental reason atoms bond:- Drive toward a noble-gas electron configuration (the octet rule). - For most main-group atoms: achieve 8 valence electrons. - Hydrogen is special case: aims for duplet (2 electrons in first shell). - Animation 2 (Na vs Cl mailed in as thought experiment):1. If Na and Cl tried to share a pair (covalent), Na would have only 2 outer electrons while Cl would reach 8; Na remains far from octet → unfavourable. 2. Observed path: total transfer of the 3s electron from Na to Cl.
\text{Na} \to \text{Na}^+ + e^-
\text{Cl} + e^- \to \text{Cl}^-
• New electron configurations: [\text{Ne}] for \text{Na}^+, [\text{Ar}] for \text{Cl}^- → both achieve octet. 3. Electrostatic attraction between oppositely charged ions constitutes the ionic bond. ### Conceptual Clarification: "Bond" ≠ Physical Object - A bond is energy, not matter.- Cannot be "seen" directly even with advanced microscopes; depictions (lines, wedges, dashes) are symbolic. - Pedagogical caution:- Drawing thick lines in Lewis or structural formulas may mislead students into imagining tangible sticks. - Always interpret diagrams as energy relationships & probability distributions, not literal connections. ### Ethical & Pedagogical Implications - Precise language (e.g. calling something a weak electrolyte rather than non-electrolyte) prevents misconception. - Highlighting the intangible nature of bonding counters common student errors and nurtures