Consider the following statements :
Rate of a chemical reaction is ________.
(a) proportional to the concentration of reactants.
(b) decreases on increasing the concentration of reactants.
(c) increas

Question

Consider the following statements :
Rate of a chemical reaction is ________.
(a) proportional to the concentration of reactants.
(b) decreases on increasing the concentration of reactants.
(c) increases on increasing the temperature.
(d) decreases on increasing the temperature.
Which of the statement given above is/are correct ?

UnlockIAS · Accepted Answer

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.

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

Concept: 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.

State PCS 2024 Q65 — Science & Technology PYQ with Answer

State PCS 2024 Q65 — Science & Technology PYQ with Answer