Chemistry TYS: speed of reaction

anpanman SG

Came across this question. It comes in the form of a few experiments but I'll just shorten the question and go straight to the point.

When 25cm3 of 1.5mol/dm3 of H2SO4 is added to excess powdered CaCO3, the initial rate of reaction was fast. But reactio stopped suddenly after only a small amount of gas ahd been produced.

1. Explain why the reaction stops suddenly.

Explanation: CaSO4 is only very slightly soluble in water.

I do not get what the answer is trying is say. May I know how does the solubility of CaSO4 have got to do with reaction suddenly stopping. If I did not remember wrongly, in class, I remember that there's another valid reason which my teacher had brought up for mention. Can't really remember what, but I'm open to any reasonable suggestions. :DDD

Next,

Why is nickel more likely to be an effective catalyst than magnesium? (Is Ni more reactive than Mg by the way?)

I would appreciate any form of assistance! Thank you!

Oh by the way, I think there's this question in TYS which provides a wrong answer. Would draw out the diagram and let you guys take a look.

anpanman SG

Anyway, I might be wrong (referring to the questio I refer to previously) After thinking it through, I find the TYS answer logically. HAHA.

lavastar SG

I think because CaSO4 is only slightly soluble in water, it will form a white precipitate in the solution itself. This will form a protective layer that prevents the ions from coming into contact, hence stopping the reaction.

xstryker SG

forms a protective layer around the calcium carbonate*

dkcx SG

Next,

Why is nickel more likely to be an effective catalyst than magnesium? (Is Ni more reactive than Mg by the way?)

Normally you will not choose a reactive metal for a catalyst since for it to be a catalyst, it has to be unchanged before and after the reaction.

Mg is a very reactive metal and in the reactivity series studied in sec sch, its 4th from the top if i didn't recall wrongly.

Ni is much less reactive and thus there is lower chance of it reacting with any of your components in the reaction.

UltimaOnline SG

As others above have already said, the insoluble ppt coats the carbonate solid, preventing further reaction between the acid and the carbonate.

>>> Why is nickel more likely to be an effective catalyst than magnesium? (Is Ni more reactive than Mg by the way?) <<<

'O' Level answer :

Because nickel is a transition metal and many transition metals can function as a catalyst.

'A' Level answer :

Transition metals and their compounds function as catalysts either because of their capacity for variable oxidation states (hence allowing them to readily transfer electrons; this provides an alternative pathway with lower activation energy (as compared to the uncatalyzed pathway), hence increasing the rate of reaction) which is usually the modality of homogenous catalysts; or because of their capacity to adsorb/adsorp other substances (specifically, the reactants) on to their surface, and activate them in the process (transition metals have partially filled d-orbitals and hence can have different number of bonds, and can bond to a wide variety of ions and molecules; and so the catalyst can form temporary bonds with the reactant molecules, which weakens the remaning bonds within the reactant molecules (eg. from double to single bonds), eventually causing them to dissociate into highly reactive atoms, and/or by holding the reactant molecules in closer proximity with each other on the catalyst surface as compared to in free gaseous state, all of which lowers activation energy as well as increasing surface area concentration of reactants available for reaction), which is usually the modality of heterogenous catalysts.

A case of the former (homogenous catalyst; partially filled d-orbitals with capacity for variable oxidation states) would be the redox reaction (catalyzed by Fe²⁺/Fe³⁺ ions) between peroxydisulphate(VI) ions S₂O₈^2-, and iodide ions I-; to generate sulfate(VI) ions SO₄^2- and iodine molecules I₂.

Overall Redox : 2I- + S2O8 2- --> I2 + 2SO4 2-

Step 1 : S2O8 2- + 2e- <---> 2SO4 2-

Reduction potential = +2.01V

Step 2 : Fe3+ + e- <---> Fe2+

Reduction potential = +0.77V

Step 3 : I2 + 2e- <---> 2I-

Reduction potential = +0.54V

The uncatalyzed reaction (steps 1 and 3) is energetically feasible (according to redox potential values); however, the reaction is slow because the negatively charged S2O8 2- and I- anions repel each other.

In the presence of Fe2+/Fe3+ ions, a alternative pathway (that is also energetically feasible, but in addition) with much lower activation energy (since the reactants are oppositely charged ions), and with a much faster rate of reaction, can now occur :

S2O8 2- + 2Fe2+ --> 2SO4 2- + 2Fe3+

2Fe3+ + 2I- --> I2 + 2Fe2+

Two examples of the latter (heterogenous catalyst) would be nickel acting as a catalyst for the hydrogenation reaction between ethene and hydrogen in the presence of a nickel catalyst; as well as iron acting a catalyst for the synthesis of ammonia in the Haber process.

For heterogenous solid metal catalysts in the hydrogenation reaction of alkenes, the mechanism is as follows :

A similar mechanism occurs for the Haber process :

First the H2 and N2 molecules form temporary bonds with the surface of the iron catalyst (ie. adsorbtion/adsorption); this interaction with the catalyst weakens the covalent bonds within the molecules and eventually causing the H2 and N2 molecules to dissociate into atomic hydrogen and nitrogen; which are highly reactive, and rapidly combine with each other to form NH3 molecules, which then leave the surface of the iron catalyst.

As to whether nickel or magnesium is more reactive, check out my redox potentials :

Notice that the standard oxidation potential of nickel is +0.25V, while the standard oxidation potential of magnesium is +2.38V. Hence magnesium is significantly more reactive than nickel.