Q.State law of Mass action. OR What is active mass?

Showing the 12 most recent of 27 on this concept.

1. CBSE 2026Set ANNUAL1 markMCQ
   Q.Rate constant depends on :

   (a) Temperature

   (b) Time

   (c) Initial concentration

   (d) None of the above

   ›Reveal solutionSolution

   The rate constant k of a reaction depends only on temperature (and the nature of the reactants/catalyst); it is independent of the initial concentration of reactants and of time.

   By definition, the rate law is Rate $= k[A]^m[B]^n$. Here k is a proportionality constant unique to a given reaction at a given temperature — it does NOT change if you alter the starting concentrations, and it does not vary with elapsed time during the reaction. What it DOES depend on is temperature, as captured by the Arrhenius equation:

   $k = A e^{-E_a/RT}$

   …

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2. CBSE 2026Set ANNUAL1 markMCQ
   Q.For a chemical reaction with rise in temperature by 10°C the rate constant is nearly:

   (a) No change

   (b) Tripled

   (c) Doubled

   (d) Ten time increases

   ›Reveal solutionSolution

   A 10 C temperature rise roughly doubles the rate constant.

   As an empirical rule, for many reactions the rate (and rate constant) becomes about two to three times larger for every 10 C rise in temperature. This ratio, k(T+10)/k(T), is called the temperature coefficient and is nearly 2. The reason (from the Arrhenius equation) …

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3. CBSE 2025Set 56/5/11 markMCQ
   Q.The role of a catalyst is to change : (A) equilibrium constant (B) enthalpy of reaction (C) Gibbs energy of reaction (D) activation energy of reaction

   ›Reveal solutionSolution

   A catalyst provides an alternative reaction pathway with lower activation energy, speeding up both forward and reverse reactions equally without altering thermodynamic properties. The answer is (D).

   Why catalysts work: the energy barrier concept

   Every chemical reaction requires reactant molecules to climb an energy hill before they can transform into products. This hill—the activation energy $E_a$—represents the minimum energy needed to break old bonds and form new ones. Think of it as a mountain pass: molecules must reach the summit before they can descend into the product valley.

   A catalyst doesn't push molecules harder or change where they start and end. Instead, it carves a tunnel through the mountain—a new reaction pathway with a lower summit. More molecules now have enough thermal energy to cross, so the reaction speeds up dramatically.

   Crucially, the catalyst lowers the barrier in both directions by the same amount. The thermodynamic landscape—the relative energies of reactants and products—remains untouched.

   What a catalyst does NOT change

   Before examining what changes, let's eliminate what stays constant:

   1. Equilibrium constant $K$

      The equilibrium constant depends only on the standard Gibbs energy change: $\Delta G^\circ = -RT \ln K$. Since a catalyst doesn't alter the stability difference between reactants and products, $K$ remains unchanged. The system reaches equilibrium faster, but the final composition is identical.

   2. Enthalpy of reaction $\Delta H$

      This is the heat absorbed or released, determined by the difference in bond energies between products and reactants. A catalyst doesn't strengthen or weaken these bonds—it merely provides an alternative route between them.

   3. Gibbs energy of reaction $\Delta G$

      The spontaneity and equilibrium position depend on $\Delta G = \Delta H - T\Delta S$, both state functions. The catalyst affects how fast the system reaches its natural destination, not where that destination lies.

   Watch out

   A common misconception is that catalysts "shift equilibrium." They don't. Both forward and reverse rates increase proportionally, so the ratio $k_f/k_r = K$ stays constant.

   What a catalyst DOES change

   4. Activation energy $E_a$ This is the catalyst's sole thermodynamic target. By stabilizing the transition state through temporary bond formation (in homogeneous catalysis) or by adsorbing reactants in favorable orientations (in heterogeneous catalysis), the catalyst lowers the energy barrier. …

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4. CBSE 2025Set ANNUAL1 markMCQ
   Q.Which of the following does not affect the rate of reaction?

   (a) delta H

   (b) Temperature

   (c) Concentration

   (d) Catalyst

   ›Reveal solutionSolution

   Rate of reaction depends on how fast reactants are converted to products (a kinetic quantity), while ΔH is a thermodynamic quantity describing the energy difference between reactants and products — the two are independent.

   • Temperature: raises rate strongly (more molecules cross the activation-energy barrier; Arrhenius equation k = A·e^(−Ea/RT)).
   • Concentration: higher concentration of reactants increases collision frequency, increasing rate (rate law dependence).
   • Catalyst: provides an alternate pathway with lower activation energy, increasing rate without being consumed. …

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5. CBSE 2025Set ANNUAL1 mark
   Q.Assertion [A] : A catalyst increases the rate of reaction without itself undergoing any permanent chemical change. Reason [R] : A catalyst does not alter Gibbs energy, ΔG of a reaction.

   ›Reveal solutionSolution

   Both statements are individually true, but the reason a catalyst speeds up a reaction is that it LOWERS THE ACTIVATION ENERGY by providing an alternative pathway — not because ΔG is unchanged. ΔG being unaffected explains why equilibrium is unshifted, not why the rate increases.

   [A]: A catalyst does increase reaction rate without itself being permanently consumed (it is regenerated at the end of the catalytic cycle) — TRUE.

   [R]: A catalyst indeed does not alter the standard Gibbs energy change (ΔG) of a reaction, since ΔG is a thermodynamic (state-function) quantity depending only on the initial and final states, not on the pathway — TRUE.

   …

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6. CBSE 2025Set ANNUAL1 markMCQ
   Q.What happens to the value of ΔG for a reaction when a catalyst is added to it ?

   (a) It increases

   (b) It decreases

   (c) It remains unchanged

   (d) It is undefined on addition of catalyst

   ›Reveal solutionSolution

   A catalyst is a thermodynamic bystander — it changes the rate of a reaction, not its free energy change.

   ΔG (Gibbs free energy change) is a state function that depends only on the initial and final states (reactants and products) of a reaction, not on the pathway taken to reach the products. A catalyst works by providing an alternative reaction pathway with a lower activation energy, $E_a$, which speeds up both the forward and reverse reactions equally, but it does not alter the energies of the reactants or products themselves. Hence the equilibrium position and ΔG (and …

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7. CBSE 2024Set 56/2/11 markMCQ
   Q.The addition of catalyst during a chemical reaction alters which of the following quantities of the reaction ? (A) Enthalpy (B) Activation energy (C) Entropy (D) Internal energy

   ›Reveal solutionSolution

   A catalyst provides an alternative reaction pathway with a lower activation energy barrier, speeding up both forward and reverse reactions without changing the thermodynamic properties of reactants or products. The answer is (B) Activation energy.

   Why catalysts work: the energy landscape

   Chemical reactions occur when reactant molecules collide with enough energy to break existing bonds and form new ones. The minimum energy required for this transformation is the activation energy $E_a$. Think of it as an energy hill that molecules must climb before they can slide down into the product valley.

   A catalyst doesn't push molecules harder or change where they start and end. Instead, it offers a different route over a lower hill—a new mechanism with intermediate steps that require less energy to traverse. The reactants and products remain chemically identical, so their inherent energy content (enthalpy, internal energy) and disorder (entropy) are unchanged.

   Examining each quantity

   Let's see what a catalyst does and doesn't alter:

   1. Activation energy $E_a$

      This is precisely what a catalyst reduces. By stabilizing the transition state or forming intermediate complexes, the catalyst lowers the energy barrier. For example, enzymes in biological systems can reduce $E_a$ by factors of $10^6$ or more, which is why reactions that would take years occur in milliseconds in living cells.

   2. Enthalpy $\Delta H$

      Enthalpy change is the difference in heat content between products and reactants:

   $\Delta H = H_{\text{products}} - H_{\text{reactants}}$

   Since the catalyst doesn't change the chemical identity of reactants or products, their bond energies and hence their enthalpies remain the same. The reaction is still exothermic or endothermic by the same amount.

   3. Entropy $\Delta S$

      Entropy measures the disorder or number of accessible microstates. The initial and final states of the system are identical with or without a catalyst, so $\Delta S$ for the overall reaction is unchanged. The catalyst may create transient intermediate states, but these don't affect the entropy difference between reactants and products.

   4. Internal energy $\Delta U$

      Internal energy is related to enthalpy by $\Delta H = \Delta U + \Delta(PV)$. For the same reason enthalpy is unaffected, internal energy change is also constant. The thermodynamic state functions depend only on the initial and final states, not the path taken. …

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8. CBSE 2023Set ANNUAL1 mark
   Q.Write two factors that affect the rate of reaction.

   ›Reveal solutionSolution

   The rate of a chemical reaction increases with an increase in the concentration of reactants and with an increase in temperature.

   • Concentration of reactants: According to the law of mass action, the rate of reaction is directly proportional to the concentration(s) of the reacting species (raised to their respective orders). Increasing concentration increases the frequency of effective collisions between reactant molecules, increasing the rate. …

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9. CBSE 2023Set ANNUAL1 mark
   Q.When the added substance reduces the rate of reaction, then it is called ________ in place of catalyst.

   ›Reveal solutionSolution

   A substance that slows down (retards) a reaction, opposite in effect to a catalyst, is called an inhibitor or negative catalyst.

   A catalyst speeds up a reaction without itself being consumed, by providing an alternative pathway of lower activation energy. When an added substance instead REDUCES the rate of reaction, it is called an inhibitor (or neg …

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10. CBSE 2023Set ANNUAL1 mark
    Q.Write the name of enzyme used in decomposition of urea into ammonia and carbondioxide.

    ›Reveal solutionSolution

    The enzyme urease specifically catalyses the breakdown of urea into ammonia and carbon dioxide, a classic textbook example of enzyme catalysis.

    Urease is a biological (enzyme) catalyst that speeds up the hydrolysis reaction: NH2-CO-NH2 + H2O --urease--> 2NH3 + CO2. This is one of the standard examples use …

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11. CBSE 2022Set ANNUAL1 mark
    Q.State law of Mass action. OR What is active mass?

    ›Reveal solutionSolution

    The Law of Mass Action states that reaction rate is proportional to the product of the reactants' active masses (molar concentrations), each raised to the power of its stoichiometric coefficient.

    Statement (Guldberg and Waage, 1864): At a constant temperature, the rate of a chemical reaction at any given instant is directly proportional to the product of the 'active masses' (i.e. molar concentrations, expressed in mol/L) of the reacting species, each concentration term raised to a power equal to that species' stoichiometric coefficient in the balanced chemical equation.

    Mathematical form: For a general reaction

    $aA + bB \longrightarrow \text{products}$

    the law of mass action gives:

    $\text{Rate} \propto [A]^a[B]^b$

    This relationship is the historical basis both for writing equilibrium constant expressions (at equilibrium, forward rate = backward rate) and, in its kinetic form, for rate laws of elementary reactions.

    …

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12. CBSE 2020Set ANNUAL1 mark
    Q.Read the given passage and answer the following questions: A substance which alters the rate of chemical reaction without itself undergoing any change in mass and chemical composition at the end of reaction is called catalyst. It may be noted that a catalyst which increase the speed of a reaction are called positive catalyst and catalyst which decrease the speed of reaction are called negative catalyst. The promoters are substances that enhance the activity of catalyst and poisons which decrease the activity of catalyst. What is catalyst?

    ›Reveal solutionSolution

    A catalyst provides an alternate reaction pathway of lower activation energy, speeding up the reaction, while being chemically unchanged at the end.

    A catalyst is a substance that changes the rate of a chemical reaction without itself undergoing any permanent change in mass and chemical composition at the end of the reaction. It works by adsorbing reactant molecules or forming a short-lived intermediate, opening up an alternative reaction pathway that has a lower activation energy than the uncatalysed path. Because more reactant molecules can then cross the (lower) energy barrier at a given temperature, the reaction proceeds faster. Although the catalyst actively participates in intermediate steps of …

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