Chemical reaction rates generally _________.

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Correct — option (3), rates are greater at the beginning of a reaction and decrease with time. The reason is the single most important idea in chemical kinetics: the rate of a reaction depends on the concentrations of the reactants, and those concentrations are highest at the moment the reaction begins and fall steadily as the reactants are consumed. Collision theory puts the same thing physically. A reaction occurs when reactant particles collide with enough energy and in the right orientation, so the rate is proportional to how often such effective collisions happen, and that in turn depends on how crowded the reaction mixture is with reactant particles. At the start the vessel is full of reactant and collisions between reactant molecules are frequent; as the reaction proceeds the reactant molecules are progressively replaced by product molecules, effective collisions become rarer, and the reaction slows. Plot the amount of product against time for a typical reaction and the curve is steep at first and flattens as it approaches completion — a shape every student has met in the laboratory when magnesium is dropped into acid, where the fizzing is vigorous at once and dies away. The mathematics agrees. For a first-order reaction the rate equals a rate constant multiplied by the concentration of the reactant, so as the concentration falls exponentially so does the rate, and the time taken for the concentration to halve is the same at every stage. For a second-order reaction the fall is steeper still. It is worth being precise about which quantity is changing here. The rate constant is not falling: it depends only on temperature and on the presence of a catalyst, not on how much reactant is left. What falls is the rate itself, because the concentration term in the rate law is shrinking. The word 'generally' in the stem is deliberate, because the pattern is a general one and not a universal law. A zero-order reaction, such as a photochemical reaction limited by the light falling on it or an enzyme-catalysed reaction in which the enzyme is saturated, proceeds at a constant rate until the reactant is nearly exhausted. In an autocatalytic reaction, where one of the products catalyses the reaction, the rate actually rises for a time before falling away. And a reaction that generates heat faster than the heat can escape may accelerate, because temperature raises the rate constant, which is what happens in a runaway or explosive reaction. These are real exceptions, but they are the special cases; the general behaviour is the one the keyed option describes.

Why the other options are wrong — (a) A constant rate throughout a reaction is the signature of a zero-order reaction, in which the rate does not depend on the concentration of the reactant at all. Such reactions exist — a photochemical reaction whose rate is fixed by the intensity of the light falling on the mixture, or an enzyme-catalysed reaction in which every molecule of enzyme is already occupied so that adding more substrate changes nothing — but they are the exception rather than the rule, and even they cease to be zero-order when the reactant is nearly used up. The option probably attracts candidates who are thinking of the rate constant rather than the rate. The rate constant genuinely does stay the same throughout a reaction at a fixed temperature; the rate does not, because the rate is the constant multiplied by a concentration term that keeps shrinking.  (b) This describes the opposite of the usual pattern, and it is the behaviour of a particular and uncommon class of reactions. In an autocatalytic reaction one of the products acts as a catalyst for the reaction itself, so the reaction starts slowly, gathers pace as the catalysing product accumulates, and only then slows as the reactants run out — the oxidation of oxalic acid by acidified potassium permanganate is the standard laboratory example, and the induction period at the start is exactly this effect. A reaction that heats itself up can behave similarly, since a rise in temperature raises the rate constant sharply. But neither case is the general one, and the stem asks what reaction rates generally do. Choosing this option means generalising from a special case, which is the error the word 'generally' in the stem is testing for.  (d) This is the counsel of despair, and the stem has already ruled it out with the word 'generally'. A generalisation can indeed be made, and the whole subject of chemical kinetics rests on it: rate depends on concentration, concentration falls as reactants are consumed, and therefore rate falls with time. The existence of exceptions — zero-order reactions, autocatalytic reactions, thermal runaways — does not prevent a general statement, it merely qualifies it, which is why the examiner has written 'generally' rather than 'always'. Options of this shape are worth recognising as a type. When one choice denies that any pattern exists while the others state definite patterns, the denial is usually a refuge for the candidate who cannot decide between the definite options, and a stem hedged with 'generally' or 'usually' is a signal that the examiner expects the broad pattern to be identified rather than the exceptions to be catalogued.

Concept: Chemical kinetics is the study of how fast reactions go and of what governs their speed. The rate of a reaction is the change in the concentration of a reactant or a product in unit time. It is determined experimentally and expressed in a rate law, which relates the rate to the concentrations of the reactants raised to powers called the orders of reaction; the order must be found by experiment and is not read off the balanced equation. In a first-order reaction the rate is proportional to the concentration of one reactant, so both the concentration and the rate decay exponentially with time and the half-life is constant. In a zero-order reaction the rate is independent of concentration and stays constant until the reactant is nearly gone. The factors that alter the rate are well defined: the nature of the reactants; their concentration, or for gases their pressure; the surface area of a solid reactant, which is why powdered solids react faster than lumps; temperature, a rise of ten degrees Celsius commonly increasing the rate substantially because a larger fraction of molecules then possess the activation energy; light, in photochemical reactions; and the presence of a catalyst, which provides an alternative pathway of lower activation energy and is itself recovered unchanged. Collision theory explains all of this by requiring that reacting particles collide with at least the activation energy and in a suitable orientation. Catalysts are distinguished as homogeneous or heterogeneous, and enzymes are the biological catalysts, highly specific and sensitive to temperature and pH.

State PCS 2021 Q69 — Science & Technology PYQ with Answer

State PCS 2021 Q69 — Science & Technology PYQ with Answer