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Look at the E°(M²⁺/M) values for the first transition series and one number stands out. Titanium to zinc are all negative, except copper at +0.34 V. Then there are smaller puzzles: manganese is more negative than chromium, and zinc is far more negative than copper. NCERT asks you to explain the copper value in an in-text question. This post shows how to answer all of these from three energy terms, with a worked estimate using NCERT's own data.
Where is this in the syllabus?
The topic is section 4.3.5 of the NCERT Class 12 unit The d- and f-Block Elements (reprint 2026-27), with the data in Table 4.4. The JEE (Advanced) 2026 brochure names "standard electrode potentials" in its d-block syllabus. The JEE (Main) 2026 syllabus and the NEET (UG) 2026 syllabus do not name d-block electrode potentials in their d-block unit; they list ionisation enthalpy and oxidation states there, and standard electrode potentials in electrochemistry. So you meet this through NCERT for all three exams. Check the current year's documents.
What decides E°(M²⁺/M)?
Getting from M(s) to M²⁺(aq) takes three steps:
Atomise the solid metal: ΔaH, positive.
Remove two electrons: ΔiH₁ + ΔiH₂, positive.
Hydrate the ion: ΔhydH, negative.
Add them and you get the net enthalpy cost. A bigger cost means the metal is harder to oxidise, so its E°(M²⁺/M) is less negative. Here are the sums, in kJ/mol, worked out from NCERT Table 4.4:
Ti: 469 + 656 + 1309 − 1866 = 568; E° = −1.63 V
V: 515 + 650 + 1414 − 1895 = 684; E° = −1.18 V
Cr: 398 + 653 + 1592 − 1925 = 718; E° = −0.90 V
Mn: 279 + 717 + 1509 − 1862 = 643; E° = −1.18 V
Fe: 418 + 762 + 1561 − 1998 = 743; E° = −0.44 V
Co: 427 + 758 + 1644 − 2079 = 750; E° = −0.28 V
Ni: 431 + 736 + 1752 − 2121 = 798; E° = −0.25 V
Cu: 339 + 745 + 1958 − 2121 = 921; E° = +0.34 V
Zn: 130 + 906 + 1734 − 2059 = 711; E° = −0.76 V
These sums leave out entropy, so they explain the ranking rather than predict each value.
Why is copper's E° positive?
Copper's net cost, 921 kJ/mol, is the largest in the list. Its second ionisation enthalpy, 1958 kJ/mol, is the highest in the series. Its hydration enthalpy, −2121 kJ/mol, is actually the joint most negative, so hydration is not the weak point. NCERT puts it this way: the high energy needed to turn Cu(s) into Cu²⁺(aq) "is not balanced by its hydration enthalpy".
The result is chemical. NCERT notes that copper cannot liberate H₂ from acids; only oxidising acids such as nitric acid and hot concentrated sulphuric acid attack it.
Worked example: how far apart are zinc and copper?
Zinc's net cost is 711 kJ/mol and copper's is 921 kJ/mol, a gap of 210 kJ/mol. Two electrons move, and the NCERT unit Electrochemistry uses F = 96487 C/mol with ΔG° = −nFE°, so the estimated gap in E° is:
ΔE ≈ 210 000 ÷ (2 × 96487) ≈ 1.09 V
The observed gap is 0.34 − (−0.76) = 1.10 V. Try iron against copper the same way: 921 − 743 = 178 kJ/mol, giving about 0.92 V, while the observed gap is 0.34 − (−0.44) = 0.78 V. So the method gets the direction right every time but the size only roughly. Use it to explain, not to calculate E°.
Why are Mn, Ni and Zn off the trend?
NCERT says the general move towards less negative E° across the series follows the rising sum of the first and second ionisation enthalpies, but Mn, Ni and Zn are more negative than that trend expects. It links Mn to the stable half-filled d⁵ configuration of Mn²⁺, Zn to the filled d¹⁰ of Zn²⁺, and Ni to its highly negative hydration enthalpy.
The table shows where the numbers come from. Manganese and zinc have the two lowest atomisation enthalpies in the series, 279 and 130 kJ/mol, which pull their net costs (643 and 711) below those of their neighbours. Nickel's −2121 kJ/mol of hydration is what keeps it close to cobalt despite its higher ionisation enthalpies.
Which 3d metals release hydrogen from dilute acid?
For M + 2H⁺ → M²⁺ + H₂, E°cell = 0 − E°(M²⁺/M). Zinc gives +0.76 V, so ΔG° = −2 × 96487 × 0.76 ≈ −146.7 kJ/mol: feasible. Copper gives −0.34 V and ΔG° ≈ +65.6 kJ/mol: not feasible.
NCERT adds a warning in section 4.3.8: every first-series metal except copper is oxidised by 1 M H⁺ on paper, but the rate is sometimes slow, and titanium and vanadium are in practice passive to dilute non-oxidising acids at room temperature. E° tells you what can happen, not how fast. Our post on predicting reactions from the electrochemical series covers that limit in more detail.
Which mistakes cost marks?
Mixing up E°(M²⁺/M) with E°(M³⁺/M²⁺). Questions on Mn³⁺ as an oxidising agent use the second series, covered in our post on oxidation states of transition metals.
Blaming copper's low hydration enthalpy, as above.
Reading "more negative" as "smaller in size". −1.18 V is the stronger reducing tendency.
Expecting a smooth trend and forgetting the three exceptions.
If ionisation enthalpy or hydration itself is the weak link, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, each with its own lesson and its place in the map.
A practice routine
From Table 4.4, recompute the nine net costs without looking at this post.
Rank them and compare the ranking with the E° values. Circle every pair that disagrees.
Answer NCERT in-text question 4.4 in three sentences, naming one number.
Repeat the Zn–Cu estimate for Ni and Cu, and compare with the observed gap.
Tonight, write the three-step cycle for copper from memory and add up its net cost.
