MPSC 2024 Prelims Paper-I · Question 65 of 100

Correct — option (3), 'Only (a) and (c)'. The four printed statements form two opposed pairs, one pair about concentration and one about temperature, and in each pair exactly one member is true, so the answer must name one from each. Statement (a), that the rate is proportional to the concentration of the reactants, is the true member of the first pair. Collision theory gives the reason: a reaction proceeds only when reactant particles collide with sufficient energy and in the right orientation, and packing more particles into the same volume raises the number of collisions per second, so the rate rises. This is the content of the law of mass action, and it is expressed quantitatively in the rate law, rate = k[A]^x[B]^y, in which the exponents are the orders with respect to each reactant and are found by experiment. Statement (c), that the rate increases on increasing the temperature, is the true member of the second pair, and the reason is different from and stronger than the first. Raising the temperature raises the average kinetic energy of the particles, so collisions become both more frequent and, far more importantly, more energetic: the fraction of molecules possessing at least the activation energy rises sharply, since it depends exponentially on temperature through the Arrhenius relation. That is why a modest rise in temperature can produce a large rise in rate, and why chemistry textbooks quote the rough rule that many reactions roughly double their rate for a rise of about ten degrees. Statements (b) and (d) simply invert these two, and both are false. One point of precision is worth adding without disturbing the answer: the word 'proportional' in statement (a) should be read in the loose textbook sense, because strictly the rate depends on concentration raised to the order of the reaction, and a zero-order reaction has a rate that does not change with concentration at all.

• (1)Only (b) — Statement (b) says the rate decreases when the concentration of reactants is increased, which reverses the actual dependence. More reactant in the same volume means more collisions per unit time and therefore a faster reaction, which is why chemists dilute a mixture to slow a reaction and concentrate it to speed one up. This option also fails for a second reason: it names only one statement when two of the four printed statements are true, so even a candidate unsure about the concentration effect should notice that the temperature pair has not been dealt with at all.
• (2)Only (d) — Statement (d) says the rate decreases on increasing the temperature, the opposite of the truth for the ordinary run of reactions. Heating increases both the frequency and the energy of collisions, and the number of molecules crossing the activation-energy barrier rises steeply with temperature. The everyday evidence is familiar — food is cooked by heating and preserved by refrigeration, precisely because chemical and biochemical change runs faster when hot and slower when cold. As with the previous option, naming a single statement also leaves the concentration pair unresolved.
• (4)Only (b) and (d) — This is the exact inverse of the correct answer: it takes the false member of each pair. It is the option a candidate selects after reading the question as though it asked for the incorrect statements, or after confusing the rate of a reaction with the equilibrium yield of a reversible one — for an exothermic reaction the equilibrium yield does fall as temperature rises, but the rate at which equilibrium is approached still increases. Keeping the two ideas apart, how fast a reaction goes and how far it goes, is the distinction this option is designed to probe.

Chemical kinetics asks how fast a reaction proceeds, a separate question from thermodynamics, which asks whether and how far it will go. The rate is the change in concentration of a reactant or product per unit time, and for a general reaction it is expressed by a rate law of the form rate = k[A]^x[B]^y, where k is the rate constant and the exponents give the order of the reaction with respect to each reactant. Collision theory supplies the picture behind the algebra: molecules must collide, with at least the activation energy and in a suitable orientation, for a reaction to occur. Every factor that changes the rate does so through one of those requirements. Concentration and, for gases, pressure change the frequency of collisions. Temperature changes both the frequency and, decisively, the fraction of collisions that are energetic enough, which is why the rate constant itself depends on temperature through the Arrhenius equation. Surface area matters for heterogeneous reactions because a powdered solid presents far more contact area than a lump. A catalyst provides an alternative path of lower activation energy, raising the rate without being consumed and without shifting the position of equilibrium. The nature of the reactants matters too: ionic reactions in solution are nearly instantaneous, while reactions requiring covalent bonds to be broken are slow.

MPSC's chemistry questions on kinetics are conceptual rather than computational — no rate constants are to be calculated — and they are usually set as a list of statements in which the true and the false members of an opposed pair are both printed. That structure is itself the clue: when a question prints 'increases on increasing X' and 'decreases on increasing X' as separate statements, exactly one of them is intended to be true, so the candidate's task reduces to fixing the direction of each effect. Here there are two such pairs and therefore two true statements, which immediately rules out any option naming only one. The underlying facts are the ones from school chemistry — concentration, temperature, surface area, catalyst and the nature of the reactants — and they are worth holding with the collision-theory reason attached to each, because the reason is what lets a candidate answer an unfamiliar variant.

• Increasing the concentration of reactants increases the rate of reaction by increasing the frequency of collisions between reactant particles.
• The rate law, rate = k[A]^x[B]^y, expresses this dependence, and the exponents — the orders of the reaction — are determined experimentally, not from the balanced equation.
• Increasing the temperature increases the rate, chiefly because the fraction of molecules possessing at least the activation energy rises steeply with temperature, as described by the Arrhenius equation.
• A catalyst increases the rate by providing an alternative path of lower activation energy; it is not consumed and does not shift the position of equilibrium.
• Rate and extent are different questions: kinetics governs how fast a reaction goes, thermodynamics and equilibrium govern how far.

• Confusing the rate of a reaction with its equilibrium yield; heating an exothermic reaction speeds it up while reducing the equilibrium yield
• Choosing an option that names only one statement when the question prints two opposed pairs, each of which must be settled
• Assuming the exponents in a rate law can be read off the coefficients of the balanced equation; they are experimental quantities
• Believing a catalyst shifts the equilibrium; it changes only the speed at which equilibrium is reached

Kinetics appears in MPSC science sections as statement-based recall of the factors that change a reaction's rate, sometimes wrapped in an everyday example such as food spoiling faster in summer or a powdered solid dissolving faster than a lump. The Commission's habit of printing a statement and its exact negation in the same list is worth exploiting, because it tells the candidate how many true statements to expect. Companion questions ask what a catalyst does, why a reaction has an activation energy, and how equilibrium differs from rate, so the two chapters are best revised together.

No directly related past PYQ was found.

• practice — not a real PYQ

  The addition of a catalyst to a reaction mixture increases the rate of the reaction mainly because the catalyst :

  • (a)Increases the concentration of the reactants
  • (b)Provides an alternative path of lower activation energy
  • (c)Raises the temperature of the mixture
  • (d)Shifts the position of equilibrium towards the products

  Answer(b) Provides an alternative path of lower activation energy — with a lower barrier, a larger fraction of collisions is energetic enough to react, so the rate rises. A catalyst is not consumed, does not change the concentrations and does not shift the position of equilibrium; it only shortens the time taken to reach it.

• practice — not a real PYQ

  Food kept in a refrigerator spoils more slowly than food kept at room temperature chiefly because a lower temperature :

  • (a)Increases the activation energy of the reactions involved
  • (b)Reduces the fraction of molecules having the activation energy, and so slows the reactions
  • (c)Removes the reactants from the food
  • (d)Changes the order of the reactions involved

  Answer(b) Reduces the fraction of molecules having the activation energy, and so slows the reactions — cooling lowers the average kinetic energy of the particles, so fewer collisions are energetic enough to react and both the chemical and microbial changes that spoil food proceed more slowly. The activation energy itself is a property of the reaction path and is not altered by cooling.