BPSC 69th CCE Prelims General Studies · Question 29 of 144

Correct — D, None of the above. All three named species push a reaction in the same direction, and it is the wrong direction for this stem: every one of them makes a reaction go faster. A catalyst is defined as a substance that increases the rate of a chemical reaction while itself remaining chemically unchanged in mass and composition at the end of it. It achieves that by offering the reactants an alternative path over a lower activation-energy barrier, so that at the same temperature a much larger fraction of collisions has enough energy to react. The Arrhenius equation, k = A e^(−Ea/RT), shows how powerful that is: because the activation energy sits in an exponent, shaving even a modest amount off Ea multiplies the rate constant many times over. Note also what a catalyst does not do — it lowers the barrier by the same amount in both directions, so it speeds up the forward and the reverse reaction equally, reaches equilibrium sooner and leaves the equilibrium constant and the final yield exactly where they were. The words 'homogeneous' and 'heterogeneous' describe only where the catalyst is, not what it does. A homogeneous catalyst is in the same phase as the reactants, as nitric oxide gas is in the lead chamber process for sulphuric acid, or as sulphuric acid is in the hydrolysis of sucrose. A heterogeneous catalyst is in a different phase and works by adsorbing the reactants on its surface — finely divided iron in the Haber process, vanadium pentoxide in the contact process, finely divided nickel in the hardening of vegetable oil into vanaspati. Neither prefix can reverse the sign of the effect. The third option is faster still to dispose of: a catalytic promoter is not even a catalyst. It is a substance with little or no catalytic activity of its own that raises the activity of a catalyst when added in small quantity, as molybdenum does for iron in the Haber process — so it accelerates the acceleration. The species the question is really pointing at is missing from the list. Substances that slow a reaction down are called inhibitors or negative catalysts, and in industry a substance that destroys a catalyst's activity is a catalyst poison.

• (a)Catalytic promoter — The likeliest wrong choice, because 'promoter' is the least familiar of the three terms and an unfamiliar word invites a guess. Its meaning is the opposite of what the stem wants. A promoter has little or no catalytic power by itself but increases the efficiency of a catalyst when added in small amounts — molybdenum added to the finely divided iron of the Haber process for ammonia is the standard example, with alumina and potassium oxide serving alongside it as structural and electronic promoters. The term for the substance that does the reverse, cutting a catalyst's activity, is a catalyst poison: arsenic ruins the platinum catalyst of the contact process and carbon monoxide poisons iron.
• (b)Homogeneous catalyst — Correctly described but pointing the wrong way. 'Homogeneous' says only that the catalyst occupies the same phase as the reactants — the whole mixture is one uniform phase — and says nothing at all about whether the rate rises or falls. The classical examples all accelerate: nitric oxide gas catalysing the oxidation of sulphur dioxide to sulphur trioxide in the lead chamber process, hydrogen ions from a mineral acid catalysing the hydrolysis of an ester or of sucrose, and enzymes in solution acting as biological catalysts inside the cell. The prefix is a statement about phase, not about direction.
• (c)Heterogeneous catalyst — Also real and also an accelerator. Here the catalyst is in a different phase from the reactants, almost always a solid working on gases or liquids, and the reaction proceeds on its surface after the reactants are adsorbed there — which is why such catalysts are used finely divided or spread on a porous support, to maximise surface area. It underpins most of heavy chemical industry: iron in the Haber process, vanadium pentoxide in the contact process, a platinum–rhodium gauze in the Ostwald process for nitric acid, and nickel in the hydrogenation of oils. Every one of those exists because it makes an otherwise sluggish reaction commercially fast.

Chemical kinetics studies how fast reactions go and what can be done to change that. Before molecules can rearrange into products they must climb an energy hill, and the height of that hill is the activation energy; only collisions that are both energetic enough and correctly oriented get over it. Anything that increases the fraction of successful collisions increases the rate — raising the temperature, raising the concentration or pressure, grinding a solid finer to expose more surface, or adding a catalyst. A catalyst is the subtlest of these because it does not push the molecules harder; it digs a lower pass through the hill, supplying an alternative mechanism with a smaller activation energy, and emerges unchanged at the end. The classification that follows is a two-part one. By direction: a positive catalyst speeds a reaction up, while a negative catalyst, more usually called an inhibitor, slows it down by interfering with the mechanism, typically by mopping up the free radicals that carry a chain reaction. By phase: a homogeneous catalyst shares the phase of the reactants, while a heterogeneous catalyst does not and acts by adsorption on its own surface. Two further terms complete the vocabulary — a promoter enhances a catalyst's activity, and a poison destroys it.

Work through the stem by asking a single question of each option: does this thing raise the rate or lower it? A catalyst raises it by definition, so options (b) and (c) are settled before the phase prefixes are even read — homogeneous and heterogeneous tell you where the catalyst sits, never which way it acts, and no adjective of place can reverse a definition. A promoter improves a catalyst, so option (a) raises the rate too, and raises it twice over. With every listed choice pointing the same way and the stem asking for the opposite, 'None of the above' stops being a hedge and becomes the answer. The class the question quietly omits is worth naming because it is what a well-prepared candidate is groping for: inhibitors, or negative catalysts. Glycerol slows the decomposition of hydrogen peroxide; a little ethanol added to chloroform prevents its slow air oxidation to the poisonous gas phosgene; tetraethyllead suppresses the pre-flame radical chain that makes a petrol engine knock; and the antioxidants BHA and BHT retard the autoxidation that turns fats rancid. One honest caveat: a few advanced texts define a catalyst as a substance that alters the rate, positive or negative, and on that definition a negative catalyst could be homogeneous or heterogeneous. But that reading would make (b) and (c) equally defensible and the question defective, whereas the school definition used across Indian textbooks — a catalyst increases the rate — leaves exactly one workable answer, and that is (d).

• A catalyst increases a reaction rate by providing an alternative path of lower activation energy, and by the Arrhenius equation k = A e^(−Ea/RT) a lower Ea raises the rate constant exponentially. It lowers the forward and reverse barriers equally, so it shortens the time taken to reach equilibrium without changing the equilibrium constant or the final yield.
• Homogeneous catalysis has the catalyst in the same phase as the reactants: nitric oxide gas in the lead chamber process for sulphuric acid, and H⁺ from a mineral acid in the hydrolysis of esters and of sucrose.
• Heterogeneous or contact catalysis has the catalyst in a different phase, working by adsorption on its surface: finely divided iron in the Haber process for ammonia, vanadium pentoxide in the contact process for sulphur trioxide, platinum–rhodium gauze in the Ostwald process for nitric acid, and finely divided nickel in the hydrogenation of vegetable oil to vanaspati.
• A promoter has little catalytic activity of its own but raises a catalyst's activity — molybdenum with iron in the Haber process. Its opposite is a catalyst poison, which destroys activity: arsenic poisons the platinum of the contact process, and carbon monoxide poisons iron catalysts.
• The species that genuinely slow reactions are inhibitors or negative catalysts. Glycerol retards the decomposition of hydrogen peroxide; ethanol added to chloroform stops its air oxidation to phosgene; tetraethyllead scavenges the peroxide radicals of the pre-flame chain and so prevents engine knock; and the antioxidants BHA and BHT slow the autoxidation of fats and oils.

• Reading 'homogeneous' or 'heterogeneous' as though the prefix said something about the direction of the effect; both are statements about phase, and a catalyst accelerates whichever phase it is in
• Confusing a promoter, which increases a catalyst's activity, with a poison or inhibitor, which reduces it — the two words sit a line apart in the textbook and are routinely swapped
• Believing a catalyst improves the yield of a reversible reaction; it lowers both barriers equally, so it only gets you to the same equilibrium faster

BPSC asks catalysis as vocabulary — three technical terms in a row and a stem that inverts the direction of the effect, so the item is decided by knowing what each word means rather than by any chemistry. The 'None of the above' option is doing real work here, which is unusual and worth noting, since on most of this paper it is filler. UPSC almost never uses the term catalyst as the subject of a stem; it embeds catalysis inside an applied context instead — living organisms acting as catalysts in microbial fuel cells (2011), dietary antioxidants neutralising free radicals (2011), the catalytic converter listed among features of a modern car (2000) — so the BPSC candidate needs the definitions and the UPSC candidate needs to recognise them in disguise.

• practice — not a real PYQ

  A substance which decreases the rate of a chemical reaction is known as

  • (a)a promoter
  • (b)a negative catalyst or inhibitor
  • (c)an autocatalyst
  • (d)a heterogeneous catalyst

  Answer(b) a negative catalyst or inhibitor — glycerol added to hydrogen peroxide and ethanol added to chloroform are the standard textbook examples. A promoter raises a catalyst's activity, an autocatalyst is a product of the reaction that then catalyses it, and a heterogeneous catalyst is simply one in a different phase from the reactants — all three accelerate.

• practice — not a real PYQ

  In the Haber process for the manufacture of ammonia, molybdenum is added to the finely divided iron catalyst. Molybdenum here acts as

  • (a)a catalyst poison
  • (b)a promoter
  • (c)an inhibitor
  • (d)a homogeneous catalyst

  Answer(b) a promoter — it has little catalytic activity of its own but raises the activity of the iron catalyst, which is the definition of a promoter. A poison and an inhibitor would both reduce the rate, and molybdenum is a solid working on gaseous reactants, so nothing about the system is homogeneous.