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Electrochemistry Essentials: Identifying Redox Reactions and Electron Accounting

Capítulo 3

Estimated reading time: 7 minutes

1) Redox or Not? Fast Criteria Using Oxidation-Number Changes

The most reliable way to identify a redox reaction is to check whether any element’s oxidation number changes from reactants to products. If at least one element increases oxidation number (oxidation) and at least one element decreases oxidation number (reduction), the reaction is redox. If no oxidation numbers change, it is not redox—even if the reaction looks “dramatic” (gas formation, precipitate, heat release).

Redox indicators (oxidation numbers change)

Single-displacement patterns (a metal displaces H⁺ from acid; a more reactive halogen displaces a less reactive halide).
Combustion and many reactions with O₂ (O usually becomes −2 in products; the fuel’s C/H often increase oxidation number).
Disproportionation (the same element is both oxidized and reduced in one reaction).
Reactions involving elemental forms (oxidation number 0) turning into ions or covalent compounds, or vice versa.

Usually non-redox (oxidation numbers do not change)

Double replacement / metathesis: ions swap partners; oxidation states stay the same (e.g., precipitation).
Acid–base reactions where only proton transfer occurs and oxidation numbers remain unchanged.
Complexation can be non-redox if oxidation states of the metal and ligands do not change (though some complex reactions are redox—always check oxidation numbers).

Quick non-redox check with examples

Precipitation: BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2 NaCl(aq)

Ba stays +2, Cl stays −1, Na stays +1, S stays +6, O stays −2. No changes → non-redox.

Acid–base neutralization: HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

H is +1, Cl is −1, Na is +1, O is −2 throughout. No changes → non-redox.

2) The “Electron Audit”: Bookkeeping Before Balancing

An electron audit is a structured way to extract the electron-loss and electron-gain information from a skeletal equation before you attempt full balancing (especially before adding H₂O, H⁺, or OH⁻). The goal is to identify which atoms change oxidation state and compute how many electrons are transferred per formula unit.

Step-by-step electron audit procedure

Write the skeletal reaction (unbalanced is fine).
Assign oxidation numbers to each element in reactants and products (use your oxidation-number rules; do not guess).
List only the elements whose oxidation numbers change. Ignore spectator ions and unchanged elements.
Compute the oxidation-number change per atom (e.g., +2 → +3 is a change of +1 per atom).
Multiply by the number of atoms of that element in its formula unit to get total change per formula unit.
Translate change into electrons: increase in oxidation number = electrons lost; decrease = electrons gained. The magnitude of the oxidation-number change equals the number of electrons per atom.
Check conservation of electrons: total electrons lost must equal total electrons gained after you choose appropriate coefficients. If they don’t match yet, you will later scale coefficients to make them match.

Electron audit template (fill-in table)

Element changing	Ox. # (reactant)	Ox. # (product)	Change per atom	# atoms in formula unit	Total e− lost/gained
(example)	+2	+3	+1	2	2 e− lost

Important: the electron audit does not require you to fully balance mass and charge yet. It is a diagnostic and planning tool that tells you what must happen with electrons.

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3) Guided Examples in Common Contexts

A) Metal + acid (hydrogen gas formation)

Skeletal reaction: Zn(s) + 2 HCl(aq) → ZnCl2(aq) + H2(g)

Oxidation numbers:

Zn(s): 0 → Zn in ZnCl₂: +2
H in HCl: +1 → H in H₂: 0
Cl: −1 → −1 (unchanged)

Electron audit:

Element	Reactant ox. #	Product ox. #	Change	Total e−	Meaning
Zn	0	+2	+2	2 e− lost	oxidation
H	+1	0	−1 per H	2 e− gained (because 2 H atoms form H2)	reduction

Electron statements (before balancing details):

Oxidation: Zn → Zn2+ + 2 e−
Reduction: 2 H+ + 2 e− → H2

Notice how the coefficients in the reduction statement (2 H⁺) naturally match the fact that H₂ contains two H atoms, giving 2 electrons gained total.

B) Halogen displacement (single replacement among halogens)

Skeletal reaction: Cl2(aq) + 2 Br−(aq) → 2 Cl−(aq) + Br2(aq)

Oxidation numbers:

Cl in Cl₂: 0 → Cl⁻: −1 (reduction)
Br⁻: −1 → Br in Br₂: 0 (oxidation)

Electron audit:

Element	Reactant ox. #	Product ox. #	Change per atom	# atoms	Total e−
Cl	0	−1	−1	2 (in Cl2)	2 e− gained
Br	−1	0	+1	2 (to make Br2)	2 e− lost

Electron statements:

Oxidation: 2 Br− → Br2 + 2 e−
Reduction: Cl2 + 2 e− → 2 Cl−

This is a clean example where the electron audit immediately shows a 2-electron transfer.

C) Combustion (oxidation by oxygen)

Skeletal reaction: CH4(g) + O2(g) → CO2(g) + H2O(g)

To identify redox here, focus on C and O (H often stays +1 in covalent compounds with nonmetals).

Oxidation numbers:

C in CH₄: −4 → C in CO₂: +4 (oxidation)
O in O₂: 0 → O in CO₂ and H₂O: −2 (reduction)
H: +1 → +1 (unchanged)

Electron audit (per CH4 reacting):

Element	Reactant ox. #	Product ox. #	Change	Total e−	Meaning
C	−4	+4	+8	8 e− lost	oxidation
O	0	−2	−2 per O atom	8 e− gained (requires 4 O atoms total)	reduction

Interpretation: Carbon loses 8 electrons total (its oxidation number increases by 8). Each oxygen atom gains 2 electrons when going from 0 to −2, so you need 4 oxygen atoms (i.e., 2 O₂ molecules) to accept 8 electrons. This electron audit explains why combustion equations often involve even coefficients for O₂.

D) Disproportionation (same element oxidized and reduced)

Skeletal reaction in basic solution (common household chemistry): Cl2(aq) + OH−(aq) → Cl−(aq) + ClO−(aq) + H2O(l)

Here chlorine starts in the elemental form (0) and ends in two different oxidation states.

Oxidation numbers:

Cl in Cl₂: 0
Cl in Cl⁻: −1 (reduction)
Cl in ClO⁻: +1 (oxidation; because O is −2 and the ion is −1 overall, Cl must be +1)

Electron audit (focus on chlorine only):

Chlorine pathway	Start ox. #	End ox. #	Change	e−
Cl → Cl−	0	−1	−1	1 e− gained
Cl → ClO−	0	+1	+1	1 e− lost

Key idea: In disproportionation, the same element supplies the electrons (oxidation branch) and consumes them (reduction branch). The electron audit often shows equal-magnitude loss and gain, which helps you anticipate a 1:1 split of atoms into two products (though coefficients may still be needed for O/H balance later).

4) Mini-Exercises: Identify Changes and Write Electron Statements (No Full Balancing Yet)

Directions: For each reaction, (i) identify which element(s) change oxidation number, (ii) state which is oxidized and which is reduced, and (iii) write brief electron-loss and electron-gain statements like X: a → b + ne− or Y: c + ne− → d. Do not attempt full balancing of O/H with H⁺/OH⁻ unless asked; focus only on electron bookkeeping.

Exercise 1: Metal + acid

Mg(s) + H2SO4(aq) → MgSO4(aq) + H2(g)

List oxidation-number changes: ________
Electron-loss statement (oxidation): ________
Electron-gain statement (reduction): ________

Exercise 2: Halogen displacement

Br2(aq) + 2 I−(aq) → 2 Br−(aq) + I2(aq)

Which halogen is reduced? ________
Total electrons transferred: ________
Write oxidation statement for iodide: ________

Exercise 3: Combustion

C2H5OH(l) + O2(g) → CO2(g) + H2O(l)

Oxidation number of average C in ethanol (use overall accounting): ________
Oxidation number of C in CO₂: ________
Electrons lost per ethanol molecule (total for both carbons): ________
Electrons gained per O atom from O₂: ________

Exercise 4: Disproportionation

2 H2O2(aq) → 2 H2O(l) + O2(g)

Oxidation number of O in H₂O₂: ________
Oxidation number of O in H₂O: ________
Oxidation number of O in O₂: ________
Write one electron-gain statement and one electron-loss statement for oxygen (use O atoms, not whole molecules): ________

Exercise 5: Redox or non-redox?

Classify each as redox or non-redox by checking oxidation numbers (no need to balance).

AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq) : ________
2 Na(s) + Cl2(g) → 2 NaCl(s) : ________
NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH−(aq) : ________

Self-check hints (use only if stuck)

Elemental forms have oxidation number 0.
In peroxides like H₂O₂, oxygen is −1.
In sulfate SO₄²⁻, sulfur is +6 and oxygen is −2 (unchanged in many reactions where sulfate is a spectator).

Now answer the exercise about the content:

During an electron audit of the reaction Zn(s) + 2 HCl(aq) → ZnCl2(aq) + H2(g), which statement correctly describes the electron transfer?

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You missed! Try again.

Zn goes from 0 to +2, so it loses 2 e− (oxidation). Hydrogen goes from +1 in acid to 0 in H2, so two H atoms gain 2 e− total (reduction). Electron loss must equal electron gain.