Which of the following slows down the reaction rate?

Question

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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.

Why the other options are wrong — (a) 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) 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) 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.

Concept: 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.

State PCS 2023 Q29 — Science & Technology PYQ with Answer

State PCS 2023 Q29 — Science & Technology PYQ with Answer