Which pure substance will not conduct electricity? — Electrochemistry Chemistry Question

💡 Solution & Explanation

Step 1 - Define the Criteria for Electrical Conductivity For any substance to conduct electricity, it must contain mobile charge carriers that can migrate under the influence of an external electric field. These charge carriers can be classified into two types: 1. **Free, mobile ions:** Responsible for electrolytic (ionic) conduction. This occurs in molten ionic compounds or aqueous solutions of electrolytes. 2. **Free, mobile electrons:** Responsible for metallic (electronic) conduction. This occurs in solid or liquid metals and certain non-metals like graphite. If a pure substance lacks both free ions and mobile electrons, it behaves as an electrical insulator. Step 2 - Analyze Option (A): Molten \ce{NaCl} Sodium chloride ($\ce{NaCl}$) is a highly ionic compound. In its solid state, the sodium cations ($\ce{Na^+}$) and chloride anions ($\ce{Cl^-}$) are locked in a rigid three-dimensional crystal lattice, rendering solid $\ce{NaCl}$ a non-conductor. However, when heated above its melting point to form molten $\ce{NaCl}$, the high thermal energy overcomes the lattice energy: $$\ce{NaCl(s) ->[\Delta] Na^+(l) + Cl^-(l)}$$ In this molten state, the $\ce{Na^+}$ and $\ce{Cl^-}$ ions are highly mobile and free to migrate toward the electrodes when an electric potential is applied. Therefore, molten $\ce{NaCl}$ is a strong electrolytic conductor. Step 3 - Analyze Option (B): Molten \ce{KOH} Potassium hydroxide ($\ce{KOH}$) is an ionic solid consisting of potassium cations ($\ce{K^+}$) and hydroxide anions ($\ce{OH^-}$). Like sodium chloride, solid potassium hydroxide does not conduct electricity due to the immobility of its ions in the crystal lattice. Upon heating to its molten state: $$\ce{KOH(s) ->[\Delta] K^+(l) + OH^-(l)}$$ The electrostatic attractions are disrupted, liberating the $\ce{K^+}$ and $\ce{OH^-}$ ions to move freely through the liquid medium. Consequently, molten $\ce{KOH}$ is an active electrolytic conductor. Step 4 - Analyze Option (C): Liquefied \ce{HCl} Hydrogen chloride ($\ce{HCl}$) is a polar covalent molecular compound. In its pure, gaseous state, it consists of discrete diatomic molecules ($\ce{H-Cl}$) held together internally by a strong polar covalent bond, and externally by weak dipole-dipole intermolecular attractions. When gaseous $\ce{HCl}$ is cooled and compressed into a liquid (liquefied $\ce{HCl}$): * It remains as discrete, neutral covalent molecules ($\ce{HCl}$). * Because there is no polar solvent (such as water) present to coordinate with the proton and stabilize the resulting ions, no ionization or dissociation takes place: $$\ce{HCl(l) \not\to H^+(l) + Cl^-(l)}$$ Since pure liquefied $\ce{HCl}$ contains only neutral molecules and entirely lacks free ions or mobile electrons, it cannot conduct electricity. *(Note: This is drastically different from an aqueous solution of hydrochloric acid, $\ce{HCl(aq)}$, where water molecules actively react with $\ce{HCl}$ to form highly mobile hydronium and chloride ions, resulting in a highly conductive solution).* Step 5 - Analyze Option (D): Liquid \ce{Hg} Mercury ($\ce{Hg}$) is a transition metal that exists in the liquid state at room temperature ($298\text{ K}$). Like all metals, mercury is held together by metallic bonding, which is described by the electron-sea model as a regular array of metal cations (kernels) surrounded by a sea of highly mobile, delocalized valence electrons. When an electric potential is applied, these free electrons drift easily through the liquid metal, allowing liquid mercury to conduct electric current with very high efficiency. Step 6 - Conclusion Among the given options, only liquefied $\ce{HCl}$ consists of neutral covalent molecules with no free ions or mobile electrons, meaning it will not conduct electricity. $$\text{Correct Option: } \boxed{\text{C}}$$