19. Redox HL

Describe the standard hydrogen electrode

Standard Hydrogen Electrode- A platinum black (unreactive) electrode pumped with H₂ and placed 1moldm^-3 H⁺ ions (usually from HCl) that catalyzes the reaction H­₂ ßà 2H⁺ + 2e^-

Define the term “standard hydrogen potential” (E^o)

Standard Electrode Potential- The potential difference (voltage) of an electrode (a.k.a. a half-cell) when it is put in a voltaic cell with the Standard Hydrogen Electrode (the SHE is used as a relative comparison)

E°- Symbol for standard electrode potential

Standard Electrodes are measured in volts at 298K, 101.3 kPa, and using 1moldm^-3_­ sol’ns

Calculate cell potentials using standard electrode potentials

Cell Potential (E­_cell­)- is the cell potential found by combining the Eº of  the two half equations

E_cell = E_reduction + (- E_oxidation)

It’s the OPPOSITE of E­_oxidation because you’re using the reverse reaction (from left à right of the series), so the sign has to flip

EXAMPLE: Find the E_cell for the reaction Sn²⁺ + 2Fe³⁺ à 2Fe²⁺ + Sn⁴⁺

Sn⁴⁺ à Sn²⁺ = + 0.154

Fe³⁺ à Fe²⁺ = + 0.771

(Sn is more negative, so that is E_oxidation)

E_cell = 0.771 + (- 0.154)

E_cell = + 0.617

Predict whether a reaction will be spontaneous using standard electrode potential values

As E_cell increases in the positive direction, the reaction increases in spontaneity

As E_cell increases in the negative direction, the reaction decreases in spontaneity.

            (E_cell < 0 = not spontaneous!)

Take the E_cell value from the previous example. Since it is + 0.617, it will be spontaneous

Predict and explain the products of electrolysis of aqueous solutions

Electrolysis of water…

At the cathode, water and H⁺ ions are reduced to form hydrogen gas.

An equation for the reduction of water is 2H₂O + 4e^- à 2 H­₂ + O₂

At the anode, water and OH^- ions are oxidized to form oxygen gas.

An equation for the oxidation of water is 2 H­₂O à O₂ + 4H⁺ + 4e^-

Electrolysis of aqueous solution…

Reactions at the cathode- Metal or H₂ will form                                

            Hydrogen gas will form unless- Cu²⁺ or Ag⁺ is present

Reactions at the anode- Non-metal or oxygen will form. Reactive conductors will degrade

            Oxygen gas will form unless- Br^- or I^- is present

Reactive metals can be used as conductors if you want to purify a metal ore into the elemental form. Metal ions will be ionized (oxidized) and reduced while impurities are removed into solution

Describe the use of electrolysis in electroplating

Electroplating- is the process by which a think layer of metal is deposited on the other electrode (used for decorating + corrosion production).

            Usually, metal ions in solution are used to electroplate the cathode (reduction).

Practice Problems:

http://alpha.chem.umb.edu/chemistry/ch115/Mridula/CHEM%20116/documents/Chapter20PracticeQuestions.pdf