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Chemistry · Unit 9

Electrochemistry: MDCAT Notes and MCQs

Redox reactions

  • Oxidation: loss of electrons / increase in oxidation number (also gain of O, loss of H). Reduction: gain of electrons / decrease in oxidation number. Mnemonic: OIL RIG.
  • Oxidation and reduction always occur together. Oxidizing agent is itself reduced; reducing agent is itself oxidized.

Oxidation number rules

  • Free element = 0; monoatomic ion = its charge; O = −2 (peroxides −1, OF2 +2); H = +1 (metal hydrides −1); F always −1; sum in neutral compound = 0, in ion = its charge.
  • Examples: Mn in KMnO4 = +7; Cr in K2Cr2O7 = +6; S in H2SO4 = +6; N in HNO3 = +5; O in H2O2 = −1.

Oxidation-number change method (balancing)

  1. Assign O.N. to every atom; find atoms whose O.N. changes.
  2. Find increase and decrease per atom; multiply so total increase = total decrease.
  3. Balance remaining atoms (O with H2O, H with H+, etc.).

Electrodes

  • Anode: electrode where oxidation occurs. Cathode: where reduction occurs (true for both cell types). In a galvanic cell anode is −ve; in an electrolytic cell anode is +ve.
  • Electrode potential: potential difference set up between a metal and its ions in solution.
  • Standard hydrogen electrode (SHE): platinum foil coated with platinum black, dipped in 1 M H+ (HCl), H2 gas bubbled at 1 atm, 25 °C; its potential is arbitrarily taken as 0.00 V.
  • Standard electrode potential (E°): potential of an electrode in 1 M solution of its ions at 25 °C (gases at 1 atm), measured against SHE.
  • Electrochemical series: more negative E° (reduction) → stronger reducing agent (Li, K, Na, Zn); more positive E° → stronger oxidizing agent (F2, Au, Cu2+). Zn E° = −0.76 V; Cu E° = +0.34 V; Daniell cell E° = 1.10 V. E°cell = E°cathode − E°anode.
PMDC learning outcomes for this unit
  • Redox & Electrodes: 9.1 Give the characteristics of a redox reaction.
  • Redox & Electrodes: 9.2 Define oxidation and reduction in terms of a change in oxidation number.
  • Redox & Electrodes: 9.3 Use the oxidation number change method to identify atoms being oxidized or reduced in redox reactions.
  • Redox & Electrodes: 9.4 Define cathode, anode, electrode potential and S.H.E.
  • Redox & Electrodes: 9.5 Define the standard electrode potential of an electrode.

Electrochemistry MCQs with answers

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Q1
Oxidation is defined as:
AGain of electrons
BGain of hydrogen
CDecrease in oxidation number
DIncrease in oxidation number
Show answer
Answer: D (Increase in oxidation number). Oxidation = loss of electrons = increase in oxidation number.
Q2
The oxidation number of Mn in KMnO₄ is:
A+2
B+6
C+7
D+4
Show answer
Answer: C (+7). +1 + Mn + 4(−2) = 0 → Mn = +7.
Q3
The oxidation number of Cr in K₂Cr₂O₇ is:
A+7
B+12
C+3
D+6
Show answer
Answer: D (+6). 2(+1) + 2Cr + 7(−2) = 0 → Cr = +6.
Q4
The oxidation number of oxygen in H₂O₂ is:
A0
B+1
C−1
D−2
Show answer
Answer: C (−1). In peroxides oxygen is −1.
Q5
In any electrochemical cell, reduction takes place at the:
AAnode
BSalt bridge
CCathode
DBoth electrodes
Show answer
Answer: C (Cathode). Cathode = reduction; anode = oxidation.
Q6
The standard electrode potential of SHE is taken as:
A+0.34 V
B−0.76 V
C1.00 V
D0.00 V
Show answer
Answer: D (0.00 V). SHE is the reference with E° = 0.00 V by convention.
Q7
In the SHE, the concentration of H⁺ ions is:
A10 M
B1 M
C2 M
D0.1 M
Show answer
Answer: B (1 M). SHE uses 1 M H⁺, H2 at 1 atm and 25 °C.
Q8
In Zn + Cu²⁺ → Zn²⁺ + Cu, the oxidizing agent is:
ACu
BZn²⁺
CZn
DCu²⁺
Show answer
Answer: D (Cu²⁺). Cu²⁺ gains electrons (is reduced), so it is the oxidizing agent.
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