Electrochemistry Calculations
E°red (cathode) = +0.80 V
E°red (anode) = −0.34 V.
Find E°cell.
E°cell = E°cathode − E°anode
E°cell = 0.80 − (−0.34)
E°cell = 0.80 + 0.34 = 1.14 V
What is 1.14 V?
This is the loss of electrons.
What is oxidation?
The unit used to measure cell potential.
What are volts?
This variable in ΔG = −nFE represents the number of these transferred in the reaction.
What are moles of electrons?
This process is the oxidation of metals.
What is corrosion?
How much charge flows when 2.0 moles of electrons are transferred?
(F = 96,485 C/mol e⁻)
q = nF
q = (2.0 mol e⁻)(96,485 C/mol e⁻)
q = 192,970 C
What is 192,970 C?
The electrode where reduction occurs.
What is the cathode?
This electrode is assigned a potential of 0.00 V under standard conditions.
What is the standard hydrogen electrode?
If ΔG° is −150 kJ, this can be said about the sign of E°cell.
What is it is positive?
Corrosion of iron requires the presence of this substance to allow electron flow between regions.
What is water?
Calculate ΔG° for n = 2 and E°cell = +1.10 V.
ΔG° = −nFE°
ΔG° = −(2)(96,485)(1.10)
ΔG° = −212,267 J ≈ −212 kJ
What is −212 kJ?
If electrons are flowing into an electrode, this process is happening there.
What is reduction?
When reversing a half-reaction, this happens to the sign of its potential.
What is it changes sign?
At 25°C, if Q > 1, this happens to E compared to E°.
What is E is less than E°?
In electroplating, increasing the current will have this effect on the rate of plating.
What is it increases the rate?
A current of 2.0 A runs for 300 s. How many moles of electrons are transferred?
Step 1: Q = It
Q = (2.0 A)(300 s) = 600 C
Step 2: Convert to moles
mol e⁻ = 600 / 96,485 = 0.00622 mol e⁻
What is 0.00622 mol e⁻?
This device converts chemical energy into electrical energy.
What is a galvanic (voltaic) cell?
For a galvanic cell to be spontaneous, this must be true of E°cell.
What is it must be positive?
A cell has n = 3 and E° = 0.59 V. Calculate log(K).
log(K) = nE° / 0.0592
log(K) = (3 × 0.59) / 0.0592
log(K) = 1.77 / 0.0592 ≈ 29.9
What is 29.9?
A metal pipe is protected from corrosion by attaching a piece of magnesium metal to it. As the system operates, this happens to the magnesium.
What is it is oxidized (corrodes)?
Calculate E°cell for the reaction:
Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s)
E°(Cu²⁺/Cu) = +0.34 V
E°(Zn²⁺/Zn) = −0.76 V
Cu²⁺ → Cu (reduction → cathode)
Zn → Zn²⁺ (oxidation → anode)
Step 2: Use standard reduction potentials
E°cell = E°cathode − E°anode
E°cell = 0.34 − (−0.76)
E°cell = 0.34 + 0.76
E°cell = 1.10 V
This component allows ions to flow and maintain charge balance between half-cells.
What is a salt bridge?
A half-reaction has E°red = −0.25 V. It is reversed and multiplied by 2 to balance electrons in a redox equation. What is its new potential?
What is +0.25 V?
A cell has E° = 1.20 V, n = 2, and Q = 100. Find E at 25°C.
log(100) = 2
E = E° − (0.0592/n) log Q
E = 1.20 − (0.0592/2)(2)
E = 1.20 − (0.0592)
E = 1.14 V
What is 1.14 V?
A current of 1.50 A runs through a solution of Ag⁺ for 965 seconds. How many grams of Ag are produced?
(Ag⁺ + e⁻ → Ag, molar mass = 107.87 g/mol)
What is 1.62 g Ag?
Q = It
Q = 1.50 × 965 = 1447.5 C
mol e⁻
= 1447.5 / 96485 = 0.0150 mol e⁻
mole ratio (1:1)
mol Ag = 0.0150 mol
grams
g Ag = 0.0150 × 107.87 = 1.62 g