Electrolysis Comprehensive Flashcards

Flashcards covering the concepts, definitions, mechanisms, equations, and applications of electrolysis from lecture notes.

Ionic Compound (Electrovalent Compound)

A chemical compound formed when metallic atoms donate electrons and non-metallic ions accept electrons.

Etymology of Electrolysis

Derived from two words: electro, meaning the flow of electrons, and lysis, meaning pertaining to breakdown or decomposition.

Electrolysis

The process by which a chemical compound in the fused state or in aqueous solution undergoes a chemical change on the passage of direct electric current.

Electrolytic Cell (Voltameter)

A non-conducting glass or silica vessel containing two electrodes and an electrolyte in which electrolysis is carried out.

Electrodes

The graphite or metal rods, plates, or wires immersed in an electrolyte through which electric current enters or leaves the electrolytic cell.

Cathode

The electrode connected to the negative terminal of the battery, which has an excess of electrons, attracts cations, and serves as the site of reduction.

Anode

The electrode connected to the positive terminal of the battery, which has a deficiency of electrons, attracts anions, and serves as the site of oxidation.

Metallic Conductors

Substances such as metals and alloys that conduct electricity in both solid and liquid states through the flow of electrons from negative pole to positive pole without chemical decomposition.

Electrolytic Conductors

Compounds that conduct electricity in molten or aqueous solution states through the flow of mobile ions to respective electrodes, accompanied by chemical decomposition.

Electrolyte

A chemical compound which in fused state or in aqueous solution allows electric current to pass through it, resulting in its chemical decomposition.

Strong Electrolyte

An electrolyte that undergoes complete dissociation in molten or aqueous solution, allows a large amount of electricity to flow, and contains almost exclusively free mobile ions.

Weak Electrolyte

An electrolyte that undergoes partial dissociation in molten or aqueous solution, allows a small amount of electricity to flow, and contains both ions and molecules in solution.

Non-electrolyte

A chemical compound that does not conduct electric current in fused state or aqueous solution and does not undergo chemical decomposition.

Ions

Electrically charged atoms or groups of atoms formed when a chemical compound breaks up in fused state or aqueous solution.

Cation

A positively charged ion (e.g., $\text{Na}^+$, $\text{Ca}^{2+}$, $\text{Al}^{3+}$) that migrates toward the cathode during electrolysis to gain electrons and get reduced.

Anion

A negatively charged ion (e.g., $\text{Cl}^-$, $\text{OH}^-$, $\text{SO}_4^{2-}$) that migrates toward the anode during electrolysis to lose electrons and get oxidized.

Theory of Electrolytic Dissociation

A theory put forward by Swedish chemist Svante Arrhenius stating that electrolytes dissociate into cations and anions when dissolved in water or fused.

Ionisation

The process of conversion of polar covalent molecules (which were not initially in an ionic state) into positively and negatively charged ions in aqueous solution.

Dissociation

The separation of ions that are already present in an electrovalent (ionic) compound when in a fused state or dissolved in water.

Degree of Dissociation ($\alpha$)

The fraction or extent to which an electrolyte dissociates into ions in a solvent, calculated as $\alpha = \frac{\text{Number of molecules dissociated}}{\text{Total number of molecules taken}}$.

Electrochemical Series

A vertical arrangement of metals or non-metals in order of their ease of losing or gaining electrons to form ions.

Electropositive Series

An arrangement of metals based on their ease of losing electrons; metals at the top ionise easily, whereas cations at the bottom are discharged most easily during electrolysis.

Electronegative Series

An arrangement of non-metals based on their ease of gaining electrons; anions at the top are hardest to discharge, whereas anions at the bottom are discharged most easily.

Selective Discharge of Ions

The preferential discharge of one particular cation at the cathode and one particular anion at the anode when an electrolyte contains two or more competing ions of the same charge.

Inert Electrodes

Electrodes made of materials like graphite, carbon, iron, or platinum that do not react chemically with the electrolyte or products formed during electrolysis.

Active Electrodes

Electrodes made of metals like copper, nickel, or silver that take part in the electrolytic reaction during electrolysis.

Electropositive Elements

Neutral atoms of metals and hydrogen that lose electrons to form positive ions and are liberated at the cathode during electrolysis.

Electronegative Elements

Neutral atoms of non-metals that gain electrons to form negative ions and are liberated at the anode during electrolysis.

Faraday's Law of Electrolysis

Law stating that the mass of a substance discharged at an electrode is directly proportional to the quantity of electricity passing through the electrolyte.

Type of Current for Electrolysis

Direct Current ($\text{D.C.}$), because Alternating Current ($\text{A.C.}$) causes discharge and ionisation to alternate continuously, yielding no net chemical change.

Conductance of Solid Ionic Compounds

Solid ionic compounds do not conduct electricity because their ions are fixed in positions by strong electrostatic forces and are not free to migrate.

Conductance of Fused/Molten Ionic Compounds

Upon heating, ions gain kinetic energy that overcomes electrostatic attraction, allowing free mobile ions to migrate and conduct electricity.

Silica Crucible (in Lead Bromide Electrolysis)

A vessel used for electrolysing lead bromide because silica is non-reactive, withstands high heat, and is a bad conductor of electricity.

Temperature for Lead Bromide Electrolysis

Maintained at or above $380^\circ\text{C}$, which is the melting point of lead bromide ($\text{PbBr}_2$), by continuous external heating.

Electrodes in Lead Bromide Electrolysis

Both cathode and anode are made of graphite plates, chosen because graphite is inert and unaffected by reactive bromine vapours.

Lead Bromide Electrolysis Cathode Observation

A greyish discharge of lead metal is formed at the cathode according to the reaction $\text{Pb}^{2+} + 2e^- \rightarrow \text{Pb}$, settling at the base of the crucible.

Lead Bromide Electrolysis Anode Observation

Dark reddish-brown fumes of bromine gas evolved at the anode according to $\text{Br}^- \rightarrow \text{Br} + e^-$ and $\text{Br} + \text{Br} \rightarrow \text{Br}_2$.

Hofmann's Voltameter

An electrolytic cell consisting of three interconnected glass tubes used for the electrolysis of acidulated water.

Electrolyte for Electrolysis of Water

Distilled water containing $2\%$ dilute sulphuric acid ($\text{H}_2\text{SO}_4$), added because pure water is a non-electrolyte and acid aids in dissociation.

Acidulated Water Electrolysis Cathode Reaction

$\text{H}^+ + e^- \rightarrow [\text{H}]$, followed by $2\text{H} + 2\text{H} \rightarrow 2\text{H}_2$, liberating colourless hydrogen gas that burns with a pop sound.

Acidulated Water Electrolysis Anode Reaction

$\text{OH}^- \rightarrow \text{OH} + e^-$, followed by $4\text{OH} \rightarrow 2\text{H}_2\text{O} + \text{O}_2$, liberating colourless oxygen gas that rekindles a burning splint.

Volume Ratio of Gases in Water Electrolysis

The ratio of Hydrogen gas to Oxygen gas collected during the electrolysis of water is $2 : 1$ by volume.

Electrolysis of Aqueous CuSO4 with Copper Electrodes

An electrolytic process where copper dissolves at the anode ($\text{Cu} \rightarrow \text{Cu}^{2+} + 2e^-$) and deposits at the cathode ($\text{Cu}^{2+} + 2e^- \rightarrow \text{Cu}$).

Fading of Blue Colour in CuSO4 Electrolysis

Occurs when platinum (inert) anode is used, because $\text{Cu}^{2+}$ ions are removed at the cathode without being replenished at the anode.

Unchanged Blue Colour in CuSO4 Electrolysis

Occurs when copper (active) anode is used, because for every $\text{Cu}^{2+}$ ion reduced at the cathode, one $\text{Cu}^{2+}$ ion dissolves into solution from the anode.

Spectator Ions

Ions present in an electrolyte (such as $\text{OH}^-$ and $\text{SO}_4^{2-}$ when active copper electrodes are used) that do not take part in the electrode reactions.

Electroplating

The process of depositing a thin and compact layer of a superior metal (e.g., gold, silver, nickel) over an inferior metallic article using electricity.

Electroplating Purpose

Conducted to protect metallic articles from rusting and corrosion, and to improve their appearance for decorative purposes.

Cathode Selection in Electroplating

The article to be electroplated is always made the cathode so that metallic cations migrate to it and gain electrons to deposit as neutral atoms.

Anode Selection in Electroplating

A block of the pure metal to be plated is made the anode, which continuously dissolves as ions to replace those deposited from solution.

Current Condition for Electroplating

Low direct current ($\text{D.C.}$) for a longer duration is required to ensure a smooth, uniform, thick, and firm metal coating.

Electrolyte for Silver Plating

Saturated aqueous solution of sodium argentocyanide ($\text{Na}[\text{Ag}(\text{CN})_2]$) or potassium argentocyanide ($\text{K}[\text{Ag}(\text{CN})_2]$) acidified with hydrocyanic acid ($\text{HCN}$).

Silver Plating Anode Reaction

Silver atoms from the pure silver block anode lose electrons to enter the solution as ions: $\text{Ag} \rightarrow \text{Ag}^+ + e^-$.

Silver Plating Setup Diagram

Circuit setup showing brass article cathode, pure silver anode block, and sodium argentocyanide electrolyte solution.

Why Sodium Argentocyanide is Preferred over AgNO3

Sodium argentocyanide allows smooth, slow, and uniform deposition of silver, whereas $\text{AgNO}_3$ causes rapid, uneven, and non-firm deposition.

Electrolyte for Nickel Plating

Saturated aqueous solution of nickel sulphate ($\text{NiSO}_4$) acidified with traces of dilute sulphuric acid ($\text{H}_2\text{SO}_4$).

Nickel Plating Cathode Observation

A firm yellowish-brown deposit of nickel metal ($\text{Ni}$) is formed on the article cathode according to $\text{Ni}^{2+} + 2e^- \rightarrow \text{Ni}$.

Electro-refining

The process by which impurities are removed from an impure metal using electrolysis to produce a highly pure metal.

Anode in Electro-refining of Copper

A thick block or sheet of impure copper, which continuously dissolves during electrolysis.

Cathode in Electro-refining of Copper

A thin strip of pure copper, onto which pure copper ions deposit from the solution.

Anode Mud (Slime)

Insoluble impurities present in impure copper that settle down at the bottom of the container below the anode during electro-refining.

Purity of Electro-refined Copper

The purity of copper deposited at the cathode during electro-refining reaches $99.9\%$.

Electro-metallurgy

The process of extracting highly reactive metals (e.g., $\text{K}$, $\text{Na}$, $\text{Mg}$) from their fused ores using electrolysis.

Electrolyte for Electro-metallurgy

Fused halides of reactive metals (e.g., fused $\text{KBr}$, fused $\text{NaCl}$, fused $\text{MgCl}_2$); aqueous solutions cannot be used because $\text{H}^+$ ions would discharge preferentially.

Cathode Product in Sodium Extraction

Sodium metal ($\text{Na}$) formed at the iron cathode by reduction: $\text{Na}^+ + e^- \rightarrow \text{Na}$.

Anode Product in Sodium Extraction

Chlorine gas ($\text{Cl}_2$) evolved at the graphite anode by oxidation: $\text{Cl}^- \rightarrow \text{Cl} + e^-$ and $\text{Cl} + \text{Cl} \rightarrow \text{Cl}_2$.

Arrhenius Strong Acid

An acid that dissociates completely in aqueous solution to yield $\text{H}^+$ ions (forming hydronium ions, $\text{H}_3\text{O}^+$), such as $\text{HCl}$, $\text{HNO}_3$, and $\text{H}_2\text{SO}_4$.

Conjugate Base of a Strong Acid

Under the Bronsted-Lowry concept, the conjugate base of a strong acid is a weak base (e.g., $\text{Cl}^-$ is the weak conjugate base of strong acid $\text{HCl}$).

Conjugate Base of a Weak Acid

Under the Bronsted-Lowry concept, the conjugate base of a weak acid is a strong base (e.g., $\text{CH}_3\text{COO}^-$ is the strong conjugate base of weak acid $\text{CH}_3\text{COOH}$).

Acid-Base Indicators (Phenolphthalein and Bromothymol Blue)

Weak acids that exhibit distinct different colors in their acid ($\text{HIn}$) and conjugate base ($\text{In}^-$) forms, used in titrations.