Catalytic properties — examples (V₂O₅, Fe, Ni, Pt): Question bank

Symbols and terms used on this page (define before you use)

Before the traps, here is every symbol and term this page leans on, in plain words. Nothing below is used before it is anchored here.

Definition The five quantities and two mechanism words

$E_{a}$ — activation energy: the height of the "energy hill" reactants must climb to become products. Higher hill ⇒ slower reaction. A catalyst carves a lower path over the same hill (see figure below). More in Activation energy and reaction kinetics.
$Δ H$ — enthalpy change: the height difference between reactant valley and product valley (heat released or absorbed). It is fixed by the molecules themselves.
$Δ G$ — Gibbs free energy change: whether the reaction is "downhill overall" (spontaneous, $Δ G < 0$ ) or "uphill overall" ( $Δ G > 0$ ). Also fixed by the molecules, not by the path.
$K_{e q}$ — equilibrium constant: the ratio of products to reactants once the reaction settles. It is set purely by $Δ G$ , so a catalyst can never change it.
$θ$ — surface coverage (Greek "theta"): the fraction of a solid catalyst's surface sites that are occupied by adsorbed molecules. $θ = 0$ means bare surface, $θ = 1$ means completely covered. It is a number between $0$ and $1$ .

Figure — Catalytic properties — examples (V₂O₅, Fe, Ni, Pt)

Look at the two hills above: same start valley, same end valley (so same $Δ H$ , same $Δ G$ , same $K_{e q}$ ), but the dashed catalysed path has a shorter peak (lower $E_{a}$ ). That single picture kills half the traps on this page.

The volcano plot above is Sabatier's "Goldilocks" rule made visible: rate on the vertical axis, binding strength on the horizontal. Too weak (left, $θ \to 0$ ) — nothing sticks. Too strong (right, $θ \to 1$ ) — nothing leaves. The peak is where Ni, Pt, Pd sit.

Finally, the orbital-overlap sketch: the metal's outward d-orbital (blue) overlaps the H–H bond (yellow), draining electron density and stretching the bond until it snaps into two surface-bound H atoms. That is chemisorption doing its job.

True or false — justify

A catalyst makes an otherwise non-spontaneous (

Δ G > 0

) reaction go.

False — a catalyst only lowers

E_{a}

(the barrier), never

Δ G

; if the reaction is thermodynamically uphill it stays uphill, no matter how fast the pathway.

A catalyst is consumed and must be topped up as the reaction runs.

False — by definition it is regenerated unchanged; it cycles (e.g.

V^{+ 5} ⇌ V^{+ 4}

) so the same atoms serve again and again.

A catalyst speeds up the forward reaction more than the reverse.

False — it lowers the same barrier (

E_{a}

) for both directions equally, so forward and reverse rates rise by the same factor and

K_{e q}

is untouched.

Adding a catalyst increases the yield of product at equilibrium.

False — it changes only the time to reach equilibrium, not the equilibrium position; yield depends on

Δ G

, which the catalyst never alters.

V₂O₅ catalyses the Contact process by physically adsorbing SO₂ on its surface.

False — the accepted route is the oxidation-state relay (V gives O to SO₂ then regrabs O₂); it is a homogeneous-style oxygen-transfer / redox mechanism, not mere sticking.

Iron in the Haber process works by the same mechanism as V₂O₅.

False — Fe is heterogeneous (chemisorbs N₂/H₂ on its surface, weakening N≡N), whereas V₂O₅ runs a homogeneous variable-oxidation-state cycle; different metals, different mechanisms.

A catalyst supplies the extra energy needed to break a strong bond like N≡N.

False — it supplies no energy; it provides an alternative pathway where the bond is pre-weakened by adsorption, so the same job needs a smaller

E_{a}

Molybdenum in the Haber process is itself the catalyst.

False — Mo is a promoter; it enhances Fe's activity (e.g. by improving the surface) but is not the active catalytic site, and does little on its own.

Every transition metal is a good catalyst because they all have d-electrons.

False — you also need accessible variable oxidation states AND partly filled/empty d-orbitals;

S c^{3 +}

(

d^{0}

) and

Z n^{2 +}

(

d^{10}

) have neither and catalyse poorly.

More surface area always gives a proportionally faster heterogeneous reaction.

False — rate rises with area only up to the Sabatier optimum; past that (or with poisons) binding becomes too strong, coverage

θ \to 1

, and products can't desorb.

Spot the error

"Pt is best because it binds gases as strongly as possible, so it holds the volcano-plot's right edge."

Two errors: "as strongly as possible" and "right edge" both mean over-binding (

θ \to 1

, products trapped); Pt sits at the peak of the volcano, binding moderately.

"A catalyst raises turnover frequency mainly by adding more grams of catalyst to the flask."

TOF is per active site, so adding mass adds sites (more total conversions) but leaves TOF unchanged; TOF only rises if each site itself works faster.

"A perfectly fast catalyst is automatically a perfectly good one."

Speed is not enough — selectivity matters: a fast catalyst that steers reactants to the wrong product (e.g. over-oxidation) is industrially useless despite high rate.

"A catalyst lowers ΔH of the reaction, which is why the reaction speeds up."

A catalyst changes neither ΔH nor ΔG (the valley-to-valley heights on the reaction-coordinate figure); it lowers

E_{a}

, the peak between them, leaving reactant/product energies fixed.

"Zn²⁺ is a poor catalyst because it has too few d-electrons."

Wrong direction —

Z n^{2 +}

d^{10}

, i.e. completely full; with no easily accessible variable oxidation state and no empty/partly-filled d-orbitals it can't relay electrons or chemisorb.

"Catalyst poisons like CO work by dissolving the metal away."

Poisons don't dissolve the metal; they bind too strongly to active sites (pushing local

θ \to 1

) and block them, so reactants can no longer adsorb.

"Adsorbing two gas molecules onto fixed surface sites makes the activation entropy more unfavourable."

The opposite — pre-aligning the reactants on the surface pays the ordering cost in advance, making

Δ S^{‡}

less unfavourable and helping the reaction over the hill.

Why questions

Why does splitting reactants into two easy electron-transfer steps beat one direct collision?

Each small step has a low

E_{a}

, while a direct

A + B

reaction needs both partners to rearrange simultaneously across one large barrier — two low hills are easier than one tall one.

Why does chemisorption weaken the adsorbed molecule's internal bond?

The metal's d-orbitals overlap with the molecule's orbitals (see the orbital-overlap figure) and drain electron density out of its own bond, so that bond stretches and weakens, lowering the barrier to break it.

Why is Sabatier's principle a "Goldilocks" rule?

Rate

\propto θ (1 - θ)

: if binding is too weak

θ \to 0

(nothing sticks) and if too strong

θ \to 1

(nothing leaves); maximum rate needs intermediate coverage — the peak of the volcano plot.

Why is variable oxidation state essential for V₂O₅'s action?

It lets vanadium hand an oxygen to SO₂ (getting reduced) and then reclaim it from O₂ (getting re-oxidised); without switching between

+ 5

and

+ 4

there is no relay and no catalysis.

Why can Fe lower the barrier to making NH₃ despite N≡N being 945 kJ/mol strong?

Fe chemisorbs N₂ and pulls electron density from the triple bond, weakening it enough that the split (the rate-limiting step) happens with a far smaller

E_{a}

— without the metal adding any energy.

Why are Ni, Pt and Pd the famous hydrogenation metals rather than, say, W or Au?

They sit near the peak of the volcano — strong enough to dissociate H₂ but weak enough to release the hydrogenated product; too-strong (W) or too-weak (Au) binders sit off the peak.

Why doesn't a catalyst appear in the overall balanced equation?

It is consumed in one elementary step and regenerated in another, so it cancels out; it appears only above the arrow, signalling it participates but survives.

Edge cases

What if the reaction is already at equilibrium and you add catalyst?

Nothing net happens to composition — forward and reverse rates both rise equally, so the system stays at the same equilibrium, just able to re-reach it faster if disturbed.

What happens to catalytic rate as surface coverage

θ \to 1

Rate

\propto θ (1 - θ) \to 0

: the surface is saturated, products can't desorb, active sites are blocked — effectively self-poisoning (the right edge of the volcano).

What happens as

θ \to 0

(very weak binding)?

Rate

\propto θ (1 - θ) \to 0

again: almost nothing sticks long enough to react, so a too-inert metal is as useless as a poisoned one (the left edge of the volcano).

Two catalysts have the same rate but different selectivity — which do you pick?

The more selective one — same speed but fewer unwanted by-products means less waste, easier purification, and higher usable yield of the target molecule.

Does a metal with

d^{0}

configuration (like

S c^{3 +}

) fail for the relay reason, the adsorption reason, or both?

Both —

d^{0}

has no lower oxidation state readily available for the relay and no d-electron density to chemisorb and weaken bonds, so it fails on both structural counts.

If you double the catalyst amount in a homogeneous redox cycle, does

K_{e q}

change?

No — more catalyst gives more relay sites so equilibrium is reached faster, but

K_{e q}

depends only on

Δ G

(reactant/product energies) and is unaffected.

Is a promoter useless on its own without the main catalyst?

Yes for practical purposes — a promoter (e.g. Mo) has little catalytic activity alone; it works by enhancing the primary catalyst's surface or electronic properties.

Could raising temperature substitute for a catalyst in the Haber process?

Only partially and at a cost — heat speeds the reaction but shifts this exothermic equilibrium back toward reactants (Le Chatelier), whereas a catalyst speeds it with no equilibrium penalty.

Recall One-line summary to keep

A catalyst = lower barrier, same destination. It never touches $Δ G$ , $Δ H$ or $K_{e q}$ ; it only changes how fast (lower $E_{a}$ ) and by what route. Match each metal to its correct mechanism (V₂O₅ → homogeneous redox relay; Fe/Ni/Pt → heterogeneous surface adsorption), remember the two structural must-haves (variable oxidation state and partly filled/empty d-orbitals), and judge real catalysts by TOF and selectivity, not raw speed alone.