Electrolysis, Brine, Electroplating, and Periodic Trends Explained

Electrolysis of Molten Sodium Chloride

Example: Molten sodium chloride:

• Cathode: 2H⁺(aq) + 2e^- → H₂(g) (hydrogen bubbles off)
• Anode: 2Cl^- → Cl₂(g) + 2e^- (chlorine gas produced)

Electrolysis of Aqueous Sodium Chloride

Aqueous solution of sodium chloride:

• At the cathode: Na⁺ and H⁺(aq) are present. H⁺(aq) is discharged because hydrogen is below sodium in the reactivity series. 2H⁺(aq) + 2e^- → H₂(g)
• At the anode: OH^- and Cl^- are present. Cl^- is discharged because it is in high concentration. 2Cl^-(aq) → Cl₂(g) + 2e^-

Positively charged ions move to the negative electrode during electrolysis. They receive electrons and are reduced.

Negatively charged ions move to the positive electrode during electrolysis. They lose electrons and are oxidized.

Brine and Electrolysis

Brine is a concentrated solution of sodium chloride or common salt.

Electrolysis of Copper

• Anode: Impure copper (dissolves, sludge forms, giving precious metals)
• Cathode: Pure copper (builds up)

Electroplating

Electroplating uses electricity to coat one metal with another to improve appearance or prevent corrosion. For example, steel car bumpers are coated with chromium.

Periodic Trends in Reactivity

Periodic trend: Down a period of metals (excluding transition metals), reactivity increases. This is because metals lose electrons more easily with more electron shells, weakening the bond between the positive nucleus and valence electrons. For non-metals, reactivity decreases down a period (opposite reason to metals).

Uses of Noble Gases

• Helium: Filling balloons and aircraft because it is lighter than air and non-flammable.
• Argon: Filling (tungsten) light bulbs to prevent the filament from reacting with oxygen (inert).
• Neon: Used in advertising signs because it glows red.

Alloys

Alloys: Improve a compound’s properties by mixing it with other substances.

Group I Reactivity

Group I elements are reactive because they need to lose only one electron to gain a stable outer shell, resulting in a strong drive to react with other elements and compounds.

Reactivity increases down Group I.

Halogen Displacement

A halogen will displace a less reactive halogen from a solution of its halide.

Reactivity decreases down Group 7.

Noble Gases

The noble gases are unreactive and monatomic because their atoms already have a stable outer electron shell.