Chemistry, HSSC-KPK First Year. This chapter is split into 30 topics and has
731 practice questions. The topics covered are listed below, followed by
10 free sample questions from the chapter.
Topics in this chapter
Rate constant calculations and theoretical rate determination27 questions
Catalysis: types and characteristics29 questions
Factors affecting rate: nature, concentration and particle size28 questions
Factors affecting rate: temperature, catalyst and light28 questions
Rate law, order of reaction and rate constant28 questions
Collision theory of reaction rate29 questions
Activation energy and transition state29 questions
Chemical kinetics and rate of reaction basics29 questions
Second and third order reactions and units of rate constant29 questions
Rates of appearance and disappearance from the balanced equation25 questions
Average rate from concentration-time data25 questions
Order of reaction from rate data of one reactant22 questions
Rate law and rate constant from two-reactant experimental data24 questions
Effect of changing concentrations on the rate for a given rate law22 questions
Units of the rate constant and order from units22 questions
Calculating the rate from the rate law: first and second order24 questions
Third order, zero order and concentration from the rate law23 questions
Molecularity of elementary reactions23 questions
Reaction mechanisms and the rate determining step: reasoning22 questions
Potential energy diagrams: activation energy and enthalpy change from energy values27 questions
Catalyst and the energy profile: numerical and conceptual21 questions
Collision theory: reasoning and application scenarios21 questions
Temperature, molecular energies and Figure 9.322 questions
Nature of reactants, concentration and surface area: scenarios21 questions
Classifying catalysts and catalysed reactions21 questions
Assertion-reason questions on chemical kinetics26 questions
Exercise questions and the hydrogen peroxide project21 questions
Rate law, order and rate constant: statement analysis22 questions
Zero order and fractional order reactions21 questions
Photochemical reactions, warm packs and applications of kinetics20 questions
Sample questions: Chemical Kinetics
10 sample questions with the correct answer and an explanation. Try to answer each one before reading the explanation.
Q1. According to the first assumption of collision theory, for a chemical reaction to occur, particles of the reactants must:
A. Remain stationary at all times
B. Collide with each other, leading to rearrangement of atoms and formation of new substances(correct answer)
C. Avoid each other entirely
D. Only react at absolute zero
Answer: B.The first postulate of collision theory states that for a chemical reaction to occur, particles (atoms, molecules etc.) of the reactants must collide with each other; in the resulting collisions, atoms are re-arranged, bonds are broken and formed, leading to the production of new substances (products).
Q2. According to the explanation of the third assumption, the combination of two hydrogen atoms to form an H2 molecule requires:
A. A very specific orientation, unlike most bimolecular reactions
B. No specific orientation, unlike a bimolecular reaction such as CO + NO2(correct answer)
C. The presence of a catalyst always
D. Extremely high pressure
Answer: B.The combination of two H atoms to form an H2 molecule requires no specific orientation, since a single atom has no directional preference; however, for a bimolecular reaction such as CO + NO2 → CO2 + NO, the reacting molecules must be favourably oriented with respect to one another to react upon collision.
Q3. According to the exercise descriptive question, comparing a catalyzed and un-catalyzed reaction on a potential energy diagram, the key visual difference would be:
A. The catalyzed pathway shows the reactants and products at completely different energy levels than in the uncatalyzed pathway
B. The catalyzed pathway shows a lower activation energy 'hill' between reactants and products, while the reactant and product energy levels themselves remain the same(correct answer)
C. The catalyzed pathway has a higher activation energy hill
D. There is no visual difference at all
Answer: B.On a potential energy diagram, a catalyzed reaction shows a lower activation energy 'hill' (Ecat) between reactants and products compared to the uncatalyzed pathway (Ea), while the overall energy levels of the reactants and products (and thus ΔH) remain unchanged, since a catalyst does not alter the thermodynamics of the reaction.
Q4. For an exothermic reaction, the activation energy of the forward reaction compared to the activation energy of the reverse reaction is:
A. Unrelated
B. Always larger than the reverse reaction's activation energy
C. Always smaller than the reverse reaction's activation energy(correct answer)
D. Exactly equal
Answer: C.For an exothermic reaction, since the products are at a lower energy level, the activation energy for the forward reaction (reactants to transition state) is smaller than the activation energy for the reverse reaction (products to transition state, which must also overcome ΔH in addition to the forward Ea).
Q5. Why does a catalyst not appear as a separate hump in the standard potential energy diagram for an uncatalyzed reaction, but rather changes the shape of the diagram entirely?
A. A catalyst only works below the melting point of water
B. A catalyst provides an alternative reaction pathway with its own (lower) activation energy and possibly its own activated complex, effectively replacing the original energy barrier with a new, lower one(correct answer)
C. A catalyst removes the need for any activation energy
D. A catalyst has no effect on the potential energy diagram at all
Answer: B.A catalyst works by providing an entirely alternative reaction pathway, with its own lower activation energy and possibly a different activated complex, effectively replacing (not simply lowering a single fixed barrier of) the original uncatalyzed pathway shown in the potential energy diagram.
Q6. A chemist observes that increasing the concentration of a gas reactant tenfold only increases the reaction rate by a small amount, far less than expected from simple collision frequency arguments. What might collision theory suggest as an explanation?
A. This observation is impossible according to collision theory
B. The reaction likely has a very restrictive orientation requirement or the rate-determining step does not involve that reactant directly(correct answer)
C. The reactant must be a catalyst
D. The temperature must have decreased
Answer: B.If increasing concentration doesn't proportionally increase rate as much as expected, it may suggest the rate-determining step doesn't directly involve that reactant (its order with respect to that reactant may be less than expected), or that most collisions fail the orientation requirement, consistent with the collision theory framework where only 'effective' collisions matter.
Q7. Considering both the effective and ineffective collision scenarios shown in Figure 9.6, what general principle about molecular geometry does this illustrate for reactions between polyatomic molecules?
A. Molecular shape is completely irrelevant to reaction outcomes
B. All polyatomic molecules react identically regardless of shape
C. Molecular shape and the relative approach angle between reacting molecules can determine whether a collision leads to a successful reaction(correct answer)
D. Only spherical molecules can react with each other
Answer: C.Figure 9.6 illustrates that molecular shape and the specific approach angle/orientation between reacting molecules can determine whether a collision is effective (leading to reaction) or ineffective (no reaction), a key insight for understanding reactions between polyatomic molecules like CO and NO2.
Q8. A student wants to speed up a slow reaction between two solid metal oxides without changing the reactants themselves. Based on the factors discussed in this chapter, which combination of approaches would be most effective?
A. Store the solids in the dark at a lower temperature
B. Grind the solids into fine powder (increase surface area) and raise the temperature(correct answer)
C. Do nothing, since solid-solid reactions cannot be sped up
D. Compress the solids into a single large block and lower the temperature
Answer: B.Grinding the solids into fine powder increases the surface area available for contact (addressing the particle-size factor), and raising the temperature increases both collision frequency and the fraction of molecules with sufficient activation energy — both are established methods from the chapter for speeding up a reaction.
Q9. Comparing the effect of a catalyst to the effect of increased temperature on reaction rate, both methods work by ultimately:
A. Decreasing the number of reactant molecules
B. Making more molecules capable of successfully reacting, though through different underlying mechanisms (lower barrier vs. more energetic molecules)(correct answer)
C. Having completely identical effects on the reaction's thermodynamics
D. Making the reaction reverse direction
Answer: B.Both a catalyst (by lowering the activation energy barrier) and increased temperature (by giving more molecules sufficient energy to overcome the original barrier) ultimately increase the fraction of molecules capable of successfully reacting, though through different underlying mechanisms.
Q10. How does collision theory explain why reactions between large, complex molecules are often slower than reactions between small, simple molecules, even with comparable activation energies?
A. Large, complex molecules have more restrictive orientation requirements for a successful (effective) collision, reducing the fraction of collisions that lead to reaction(correct answer)
B. Large molecules never collide with anything
C. Complexity has no bearing on collision theory whatsoever
D. Large molecules always have lower activation energy, making this comparison meaningless
Answer: A.Collision theory explains that larger, more complex molecules often have more restrictive orientation requirements for an effective collision (since only specific parts of the molecule may need to align for the reaction to proceed), which reduces the overall fraction of collisions that successfully lead to reaction.