MPSC 2021 Prelims Q69: Chemical reaction rates generally

MPSC 2021 Prelims Paper-I · Question 69 of 100

State Services Preliminary Examination 2021, held 23 January 2022. Transcribed from the commission's own booklet (संच A); answer as per the official MPSC final answer key.

Chemical reaction rates generally _________.

(1)are constant throughout a reaction
(2)are slow at the beginning and increase with time
(3)are greater at the beginning of a reaction and decrease with time
(4)no such generalization can be made

Answer

(3)

Why

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 others are wrong

(1)are constant throughout a reaction — 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.
(2)are slow at the beginning and increase with time — 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.
(4)no such generalization can be made — 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.

Kinetics reaches MPSC papers as conceptual questions rather than as calculations: what happens to the rate if concentration is doubled, why a powdered solid reacts faster than a lump, why food keeps longer in a refrigerator, what a catalyst does and does not do. This question is typical in that it can be answered by reasoning from a single principle without any recall of formulae, and typical too in the way the wrong options are drawn from genuine but special cases — a zero-order reaction for the constant rate, an autocatalytic reaction for the rate that increases with time. That is a fair test of understanding: a candidate who has learnt the subject as a list of special behaviours will find two options defensible, while one who has grasped that rate follows concentration will not hesitate. The hedge word in the stem is the other thing to notice. 'Generally' invites the broad pattern; had the stem said 'always', the option denying any generalisation would have become far more attractive. Reading such qualifiers carefully is a durable examination skill, because the Commission uses them deliberately in science and in polity alike.

Key facts

The rate of a chemical reaction generally falls as the reaction proceeds, because the rate depends on the concentration of the reactants and that concentration is highest at the start and decreases as the reactants are consumed.
Collision theory expresses the same idea physically: reaction occurs when particles collide with sufficient energy and correct orientation, and such effective collisions become less frequent as reactant particles are replaced by product particles.
The rate constant does not change during a reaction at fixed temperature — it depends only on temperature and on the presence of a catalyst — so it is the rate, not the constant, that decreases with time.
Exceptions exist: a zero-order reaction proceeds at a constant rate independent of concentration, an autocatalytic reaction speeds up as its own product accumulates, and a self-heating reaction can accelerate because a higher temperature raises the rate constant.
The factors that govern reaction rate are the nature of the reactants, concentration or pressure, surface area of a solid, temperature, light in photochemical reactions, and the presence of a catalyst, which lowers the activation energy and is recovered unchanged.

Every wrong choice here names a real class of reaction — the stem's word 'generally' is what separates the rule from its exceptions

(1) are constant throughout a reaction→THE SIGNATURE OF A ZERO-ORDER REACTION, in which the rate does not depend on the concentration of the reactant at all: a photochemical reaction whose pace 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. Such reactions exist, and even they stop being zero-order once the reactant is nearly exhausted. The choice also catches the candidate who is thinking of the rate CONSTANT, which genuinely does stay the same at a fixed temperature, rather than of the rate

(2) are slow at the beginning and increase with time→THE BEHAVIOUR OF AN AUTOCATALYTIC REACTION, in which one of the products acts as a catalyst for the reaction itself, so it starts slowly, gathers pace as the catalysing product accumulates, and slows only when the reactants run out — the oxidation of oxalic acid by acidified potassium permanganate is the standard laboratory example and its induction period is exactly this effect. A reaction generating heat faster than the heat can escape behaves the same way, since a rise in temperature raises the rate constant. Real, and special

(3) are greater at the beginning of a reaction and decrease with time→THE GENERAL CASE, and the single most important idea in chemical kinetics: the rate depends on the concentrations of the reactants, and those are highest at the moment the reaction begins and fall steadily as the reactants are consumed. Collision theory puts it physically — reaction needs particles colliding with enough energy and in the right orientation, and effective collisions grow rarer as reactant molecules are replaced by product molecules. Drop magnesium into acid and the fizzing is vigorous at once and dies away. This is option (3)

(4) no such generalization can be made→RULED OUT BY THE STEM'S OWN WORD, 'generally'. A generalisation can be made, and the whole subject of kinetics rests on it; the exceptions in the two rows above qualify the statement rather than prevent it, which is precisely why the examiner wrote 'generally' and not 'always'. Recognise the shape as a type: when one choice denies that any pattern exists while the others state definite patterns, the denial is a refuge for the candidate who cannot decide between them

Be precise about which quantity is falling. The rate constant is not: it depends only on temperature and on the presence of a catalyst, never on how much reactant is left. What falls is the rate itself, because the concentration term in the rate law keeps shrinking — for a first-order reaction the rate is the constant multiplied by the concentration of one reactant, so both decay together and the time taken for the concentration to halve is the same at every stage, and for a second-order reaction the fall is steeper still. Two of the four printed choices are true of a named and genuine class of reaction, which makes this a fair test of understanding rather than of recall: a candidate who has learnt the subject as a list of special behaviours will find them defensible, while one who has grasped that rate follows concentration will not hesitate. Hedge words in a stem are worth reading for their own sake, since the Commission uses them deliberately in science and in polity alike.

Study next

Rate of reaction, rate law and order of reaction, and why order must be determined experimentally rather than read from the balanced equationCollision theory and activation energy, and the effect of temperature on the fraction of molecules able to reactCatalysis — homogeneous, heterogeneous and enzymatic — and the difference between a catalyst, a promoter and an inhibitorChemical equilibrium and Le Chatelier's principle, and how the approach to equilibrium differs from the completion of a reactionEveryday applications of kinetics — refrigeration and food spoilage, rusting and its prevention, and combustion

Common traps

Confusing the rate of a reaction, which falls as reactants are used up, with the rate constant, which stays fixed at a given temperature
Generalising from an autocatalytic or a self-heating reaction, in which the rate rises for a time, to reactions at large
Treating a zero-order reaction as the norm, when a constant rate is the exception and requires a special limiting factor such as light intensity or enzyme saturation
Missing hedge words such as 'generally' in a stem, which signal that the broad pattern is wanted rather than a catalogue of exceptions

MPSC asks kinetics in a conceptual register: the effect of temperature, concentration, surface area or a catalyst on the speed of a reaction, the definition of activation energy, what a catalyst does to it, and everyday illustrations such as why milk sours faster in summer or why iron filings burn more readily than an iron nail. Questions of the present shape, which test whether a candidate can reason from concentration to rate, appear regularly and can be answered without any numerical work. The Commission also links the topic to biology through enzymes, which are catalysts with an optimum temperature and pH, and to the environment through the catalytic converter and through reaction rates in the atmosphere. A candidate who holds the six factors affecting rate, the definition of a catalyst, and the principle that rate follows the concentration of the reactants will find nearly every question in this family straightforward.

Related PYQs

No directly related past PYQ was found.

Practice

practice — not a real PYQ
A catalyst increases the rate of a chemical reaction because it
- (a)increases the energy of the reactant molecules
- (b)provides an alternative pathway with a lower activation energy
- (c)raises the temperature of the reaction mixture
- (d)is consumed in the reaction and forms additional products
Answer(b) It provides an alternative pathway with a lower activation energy — a catalyst offers a route from reactants to products over a lower energy barrier, so a larger fraction of the colliding molecules possess enough energy to react and the reaction proceeds faster. It does not add energy to the molecules, does not heat the mixture, and is recovered chemically unchanged at the end, which is why a small quantity suffices. A catalyst also speeds the forward and the reverse reactions equally, so it brings a reversible reaction to equilibrium sooner without altering the position of that equilibrium.
practice — not a real PYQ
Powdered calcium carbonate reacts with dilute hydrochloric acid much faster than the same mass of marble chips. The reason is that
- (a)the powder has a higher concentration than the chips
- (b)the powder has a much larger surface area exposed to the acid
- (c)the powder has a lower activation energy than the chips
- (d)the powder is chemically different from the chips
Answer(b) The powder has a much larger surface area exposed to the acid — a reaction between a solid and a liquid can only take place where the two are in contact, so grinding the solid into a powder multiplies the number of particles available to collide with acid molecules at any instant and the rate rises correspondingly. Both forms are the same substance, calcium carbonate, with the same activation energy; concentration is a property of the acid solution rather than of the solid; and it is surface area alone that has changed.