৪৯তম বিসিএস ⎯ রসায়ন [৫৩১]

Explanation:
The rate of a chemical reaction depends primarily on:

Temperature: Increasing temperature generally increases molecular kinetic energy, leading to more frequent and energetic collisions that surpass activation energy.

Reactant Concentration: Higher concentrations lead to a greater number of collisions per unit time.

Catalyst: A catalyst provides an alternative pathway with lower activation energy, increasing reaction rate without being consumed.

Surface area: Increasing the surface area of a solid reactant generally increases the rate of a chemical reaction

So, the answer is B

Other factors like pressure affect gases, but humidity and product concentration are not universal determinants for rate laws.


Source: Any text book or relevant sites.

Explanation:
In a first-order reaction, rate = k[A], so k = (rate)/[A]. 
Rate has units of concentration/time, and concentration cancels, leaving time⁻¹. 
So, the answer is B

Other units like 
  molL⁻¹time⁻¹ stands for rate constant of zero order reaction & 
 (mol/l)⁻¹ time⁻¹ stands for rate constant of 2nd order reaction

Source: Any text book or relevant sites.

Explanation:
H₂SO₄ = 2H⁺ + SO₄^2-

​We know, 
​pH = -log[H+] = -log (2x0.05) = -log (0.1) = 1

Source: Any text book or relevant sites.

​Explanation:
k is the rate constant for a given reaction at a fixed temperature & dimension of k is M^(1-n)T^(-1). it is independent of the concentration of reactants. 
It only changes with temperature.

So, the right answer is A.

Source: Any text book or relevant sites.

Explanation:
Temperature raises collision between the molecules as well as  kinetic energy of molecules. Increasing the fraction of molecules exceeding the activation energy threshold.
So the right answer is A.

According to Arrhenius equation: k = A * exp(-Ea / (RT))
As the temperature of a reaction is increased, the rate of the reaction is increased but activation energy remains the same.

Reference: Arrhenius, 1889 paper/ Any text book

​Explanation:
The rate-determining step is the slowest step. In a multi-step mechanism, the slowest step acts as a bottleneck controlling the overall rate.
So, the right answer is C

The simplest reaction does not have a relation with the rate-determining step. the main reaction involving the major reactant & the fastest reaction can't determine rate.

Reference: Laidler & Meiser, Physical Chemistry/ Any other text book of HSC level.

​Explanation:

Powdered zinc reacts faster with acid due to greater surface area. Greater surface area exposes more reactant particles for collisions, increasing reaction rate.

So the right answer is D

The rate of a chemical reaction depends primarily on:

Temperature: Increasing temperature generally increases molecular kinetic energy, leading to more frequent and energetic collisions that surpass activation energy.

Reactant Concentration: Higher concentrations lead to a greater number of collisions per unit time.

Catalyst: A catalyst provides an alternative pathway with lower activation energy, increasing reaction rate without being consumed.

Surface area: Increasing the surface area of a solid reactant generally increases the rate of a chemical reaction


Reference: Chemistry LibreTexts, Factors Affecting Reaction Rate./ Any text book from HSC level

Explanation:
Using Henderson-Hasselbach equation:

P^OH = P_kb + log [NH₄Cl]/[NH₃]= - log (10^-5) + log 1 = 5
using P^H+P^OH = 14
pH = 9
So the right answer is D

Reference: Any text book from HSC level

​Explanation:

Reaction orders are determined by experiment. 
So the right answer is  C

Order of a reaction: The sum of the exponents of the concentration terms in the rate law equation, which expresses the relationship between reactant concentrations and reaction rate. An experimentally determined quantity, found by analyzing reaction rates at different concentrations. Can be 0, 1, 2, fractional, or even negative. Reaction orders are empirical, determined experimentally, not necessarily matching stoichiometry.

Reference: Laidler, Chemical Kinetics/Any text book from HSC level.

​Explanation:
First-order plot that’s linear is ln[A] vs time
For first-order: ln[A] = -kt + ln[A]₀, a straight line with slope -k.
So the right answer is C 

Reference: Atkins, Chemical Kinetics/Any text book from HSC level

Explanation:
We know, 
Henderson-Hasselbach equation:
P^H= P_ka+log [ CH₃COONa]/[CH₃COOH]
Now, if 0.01 mol HCl is added, it will increase mole number of CH₃COOH & decrease mole number of CH₃COONa. So, pH will decrease slightly.

So, the right answer is B

Reference: Brown, Chemistry: The Central Science.

Explanation:
Q = K means system is at equilibrium, no net change

The reaction quotient (Q) means A measure of the relative amounts of products and reactants present in a reaction at a given time. 
Q = K means system is at equilibrium, no net change
Q > K means the reaction shifts toward reactants
Q < K means the reaction shifts toward products

Reference: Atkins, Chemical Equilibrium/Any text book from HSC level

​Explanation:
The best buffer capacity occurs when pH=pKa — True. Buffer capacity is maximal when acid and base are equimolar.
So the right answer is A

Why others are false:
(b) A good buffer resists pH change to small additions;
(c) Buffer capacity does increase with total buffer concentration;
(d) pH depends on both temperature and the base/acid ratio.

Reference: Any text book from HSC level

Explanation:
C + O₂ ⇌ CO₂ has huge Kc → goes to completion
Answer: (b)

A very large K_c (>10¹⁰) means product-favored equilibrium.

Reference: Atkins, Equilibrium Constants.

Explanation:
A buffer solution resists changes in pH when small amounts of acid or base are added.  Buffer solutions are mixtures of a weak acid and its conjugate base, or a weak base and its conjugate acid.

here, CH₃COOH is a weak acid & CH₃COONa is its conjugate base. So the right answer is B

Reference: Any text book from HSC level 

Explanation:
In N₂ + 3H₂ ⇌ 2NH₃, NH₃ forms at 2/3 rate of H₂ consumption

From stoichiometry: Coefficient of NH₃ is 2 & Coefficient of H₂ is 3
So, rate(NH₃) = (2/3) × rate(H₂).

Reference: Brown, Chemistry/Any text book from HSC level

Explanation: As the physical state of NH₄HS is solid so
Total pressure = partial pressure of NH₃ + partial pressure of H₂S

​P_total = P_NH₃ + P_H₂S = 2x;
Kp = (P_NH₃) (P_H₂S) = x² = 1/4 (P_total)² 
Kp ∝ (P_total)² 
So the right answer is C


Reference: Atkins, Gas Equilibria/Any text book from HSC level

Explanation: pH of blood of human is 7.4
​


Reference: Atkins/ Any text book from HSC level

​Explanation:
Second-order half-life (t₁/₂) inversely proportional to concentration

t₁/₂ = 1 / (k[A]₀). If [A]₀ decreases 5 times, t₁/₂ increases 5 times
So, the right answer is C

Reference: Laidler/Any text book from HSC level

If the temperature is increased, the ion product of water (Kw) will also increase. 

This is because the ionization of water is an endothermic process, meaning it absorbs heat. When temperature increases, the equilibrium shifts to favor the forward reaction

Kw = [H+][OH-]
at 25⁰C Kw= 1x 10^-14

Reference: Any text book from HSC level

Explanation:
Radioactive decay is first-order.
after 3 half-lives, (1/2)³ = 1/8; or radioactive sample becomes 1/8 of its concentration 
(1/2)³ = 1/8, not 1/9.
So the right answer is B

Half life: The time it takes for the concentration of a substance to decrease by half during a chemical reaction or radioactive decay
for first-order reaction half life ∝ 1/rate constant.

Reference: Atkins/Any text book from HSC level.

​A positive catalyst does decrease the activation energy of a chemical reaction. 

Activation energy is the minimum energy required for reactants to start a reaction. Catalysts, by definition, speed up reactions, and they do this by providing an alternative reaction pathway with a lower activation energy barrier. This allows a larger proportion of reactant molecules to overcome the energy barrier and react, resulting in a faster reaction rate.

The collision rate between the reactant molecules is increased by temperature only.

Source: Any text book from HSC level

​Explanation:
It is a second-order reaction
straight line can be found by plotting 1/[A] vs t; not 1/[A]² vs t

Equation for second-order: 1/[A] = kt + 1/[A]₀.
Which satisfies y=mx+c
So the right answer is C

Reference: Atkins/Any text book from HSC level

​The reaction quotient, denoted as Q, is a measure of the relative amounts of products and reactants present in a reaction at a given time, not necessarily at equilibrium.

Qc=[CO]² / [CO₂]= 0.01²/0.01=0.01 <Kc
So, the reaction will favor the forward direction.

The reaction quotient is calculated using the same formula as the equilibrium constant (K), but it can be calculated at any point in a reaction, not just at equilibrium.

If Q < K, the reaction will favor the forward direction
f Q > K, the reaction will favor the reverse direction
If Q = K, the reaction is at equilibrium

Source: Any text book from HSC level

Explanation:
If the temperature is increased, the reaction will shift towards the reactants in exothermic reactions.
So, the right answer is D

Le chatelier principle : If a change of condition (like temperature, pressure, or concentration) is applied to a system in equilibrium, the system will shift its equilibrium position to counteract the change and establish a new equilibrium
*If you remove  products, the reaction will shift towards the products
*If the pressure on a gaseous system at equilibrium is increased, the reaction will shift towards the side with fewer gas molecules to reduce the pressure.
*If the volume on a gaseous system at equilibrium is decreased, the reaction will shift towards the side with fewer gas molecules to reduce the pressure.
So, A,B & C are false

Reference: Atkins./Any text book from HSC level

​Phosphoric acid in hydrogen peroxide decomposition is not a Positive Catalyst

Positive Catalyst: A substance that speeds up a chemical reaction without being consumed or permanently altered in the process. 
List of positive Catalyst:
Platinum in Catalytic Converters
Enzymes in Biological Systems
Iron in Haber-Bosch Process
Manganese Dioxide in Decomposition of Potassium Chlorate
Phosphoric acid in hydrogen peroxide decomposition

Source: Any text book from HSC level

The equilibrium constant is set by Kp = P_CO2
It is independent of the masses of solids as long as both solids are present, and independent of container volume.
So the right answer is B


Source: Any text book from HSC level

Only temperature changes the value of an equilibrium constant.

Changes in pressure, volume, or concentration may shift the position of equilibrium (the mixture) but do not change Kc or Kp
Van't hoff equation:
ln(K2/K1) = -ΔH/R * (1/T2 - 1/T1)


Source: Any text book from HSC level

Kp = Kc(RT)^Δn
where R is the ideal gas constant,
T is the temperature, and
Δn is the change in the number of moles of gas in the reaction.

Here, Δn=-2, So, Kp < Kc

Source: Any text book from HSC level

Explanation:
Le chatelier principle: If a change of condition (like temperature, pressure, or concentration) is applied to a system in equilibrium, the system will shift its equilibrium position to counteract the change and establish a new equilibrium.
According to principle, If you add more products, the reaction will shift towards the reactants.

So the right answer is A.

Reference: Atkins/Any text book from HSC level