The experimental data for decomposition of N2O5
[2N2O5→4NO2+O2]
in gas phase at 318 K are given below:
| t/s | 0 | 400 | 800 | 1200 | 1600 | 2000 | 2400 | 2800 | 3200 |
|---|---|---|---|---|---|---|---|---|---|
| 102×[N2O5]/mol L−1 | 1.63 | 1.36 | 1.14 | 0.93 | 0.78 | 0.64 | 0.53 | 0.43 | 0.35 |
- Plot [N2O5] against t.
- Find the half-life period for the reaction.
- Draw a graph between log[N2O5] and t.
- What is the rate law?
- Calculate the rate constant.
- Calculate the half-life period from k and compare it with (ii).
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Average Rate Of Reaction
Reaction Rate Stoichiometry – From Intuition to Precision
Imagine you are watching a simple reaction:
2NO2→2NO+O2
As NO₂ disappears, NO appears twice as fast as O₂ appears. Why? Because the balanced equation says: for every 2 molecules of NO₂ that break apart, you get 2 molecules of NO and 1 molecule of O₂. The numbers in front of the species — the stoichiometric coefficients — tell you the relative speeds at which reactants vanish and products appear.
That is the core idea: reaction rate stoichiometry is the relationship between the rates of change of different species in a chemical reaction, dictated by their coefficients in the balanced equation.
The Intuitive Picture
Think of a factory assembly line. The balanced equation is like a recipe:
- 2 units of raw material A → 2 units of product B + 1 unit of byproduct C
If the line runs steadily, every time 2 units of A are consumed, 2 units of B are produced and 1 unit of C is produced. So the rate at which A disappears must be twice the rate at which C appears. The rate at which B appears equals the rate at which A disappears (both have coefficient 2).
The stoichiometric coefficients are not speeds themselves — they are scaling factors that connect the speeds of different species.
The Precise Statement
For a general reaction:
aA+bB→cC+dD
The rate of reaction (often called the rate of the process, r) is defined as:
r=−a1dtd[A]=−b1dtd[B]=c1dtd[C]=d1dtd[D]
Here:
- dtd[X] is the instantaneous rate of change of concentration of species X (in mol L⁻¹ s⁻¹).
- The minus sign is used for reactants (their concentration decreases with time).
- The plus sign is used for products (their concentration increases with time).
- Dividing by the coefficient normalises the rate — so r is the same number for every species.
r=−a1dtd[A]=c1dtd[C]
This single value r is the intrinsic rate of the reaction, independent of which species you measure.
How to Use It (Step by Step)
Suppose you are given:
2H2+O2→2H2O
And you measure that dtd[H2]=−0.040 M/s (negative because H₂ is being consumed).
Step 1: Write the rate relation:
r=−21dtd[H2]=−11dtd[O2]=21dtd[H2O]
Step 2: Plug in the known value:
r=−21(−0.040)=+0.020 M/s
Step 3: Find the rate for O₂:
−dtd[O2]=r⇒dtd[O2]=−0.020 M/s
Step 4: Find the rate for H₂O:
21dtd[H2O]=r⇒dtd[H2O]=2r=0.040 M/s
A quick check: the coefficients tell you the relative rates. Here, H₂ disappears twice as fast as O₂, and H₂O appears at the same rate as H₂ disappears (both coefficient 2). Always verify your numbers match the coefficient ratios.
--- …
Why this formula?
Average Rate of Reaction — Why the Formula Holds
Let’s build this from the ground up. The goal is to understand why the average rate formula looks the way it does — not just memorise it.
1. What does "rate of reaction" mean physically?
A chemical reaction changes the concentration of reactants (decreasing) and products (increasing) over time.
- Rate = how fast this change happens.
- If you measure the change over a finite time interval, you get the average rate.
2. The core idea: change per unit time
For any quantity X that changes from X1 to X2 over time t1 to t2:
Average rate of change of X=ΔtΔX=t2−t1X2−X1
This is just the slope of the straight line connecting the two points on a concentration vs. time graph.
3. Applying this to a reaction
Consider a simple reaction:
A→B
- Reactant A is consumed: [A] decreases.
- Product B is formed: [B] increases.
For reactant A (disappearing):
Average rate=−ΔtΔ[A]
Why the minus sign?
Because Δ[A]=[A]2−[A]1 is negative (concentration drops). The rate itself must be positive (speed is never negative). So we multiply by −1.
For product B (appearing):
Average rate=+ΔtΔ[B]
Here Δ[B] is positive, so no minus sign needed.
4. The general formula for any reaction
For a balanced reaction:
aA+bB→cC+dD
The average rate is defined per mole of reaction — so it’s the same number regardless of which species you track.
We divide each ΔtΔ[species] by its stoichiometric coefficient:
Average rate=−a1ΔtΔ[A]=−b1ΔtΔ[B]=+c1ΔtΔ[C]=+d1ΔtΔ[D]
Why divide by the coefficient?
If 2 moles of A disappear for every 1 mole of C formed, then ΔtΔ[A] is twice as large as ΔtΔ[C]. Dividing by the coefficient normalises them to the same "per mole of reaction" rate.
5. Key exam point: the formula in one line
For any species X with stoichiometric coefficient νX (negative for reactants, positive for products):
Average rate=νX1ΔtΔ[X] …
Concept: Average Rate Of Reaction – the change in concentration per unit time, averaged over a finite interval. For a first-order reaction, the half-life is constant and independent of initial concentration.
(i) Plot [N2O5] against t
Plot the given data with time on the x-axis and [N2O5] on the y-axis. The curve falls exponentially, indicating first-order kinetics.
(ii) Half-life from the graph
Half-life is the time taken for the concentration to fall to half its initial value.
Initial [N2O5]=1.63×10−2 mol L−1. Half of this is 0.815×10−2 mol L−1.
From the table, this value lies between t=1200 s (0.93) and t=1600 s (0.78).
Reading the smooth decay curve gives t1/2≈1450 s. (A straight-line interpolation between the two table points would give ≈1500 s — an overestimate, because the exponential decay curve lies below the straight chord.)
(iii) Plot log[N2O5] against t
Take log10 of each concentration. A straight line confirms first-order kinetics. Slope =−2.303k.
(iv) Rate law
Since the log vs t plot is linear, the reaction is first order:
Rate=k[N2O5]
(v) Calculate the rate constant
For a first-order reaction: k=t2.303log[A][A]0. …
The plot of log[N2O5] against t is a straight line, so the reaction is first order: Rate=k[N2O5]. The rate constant is k≈4.81×10−4 s−1, giving t1/2≈1441 s from k, in good agreement with ≈1450 s read from the graph.
(i) Plot [N2O5] vs t. Plotting the concentration against time gives a smooth downward curve (steep at first, then flattening). It is not a straight line, so the reaction is not zero order.
(ii) Half-life from the graph. The initial concentration is [N2O5]0=1.63×10−2 mol L−1, so half of it is 0.815×10−2 mol L−1. Reading the curve, this value is reached at about t1/2≈1450 s.
(iii) Plot log[N2O5] vs t. Computing log[N2O5] (with [N2O5] in mol L−1):
| t/s | [N2O5]/mol L−1 | log[N2O5] |
|---|---|---|
| 0 | 1.63×10−2 | −1.788 |
| 400 | 1.36×10−2 | −1.866 |
| 800 | 1.14×10−2 | −1.943 |
| 1200 | 0.93×10−2 | −2.031 |
| 1600 | 0.78×10−2 | −2.108 |
| 2000 | 0.64×10−2 | −2.194 |
| 2400 | 0.53×10−2 | −2.276 |
| 2800 | 0.43×10−2 | −2.367 |
| 3200 | 0.35×10−2 | −2.456 |
These points fall on a straight line, which confirms first-order kinetics.
(iv) Rate law. A linear log[N2O5] vs t plot means the reaction is first order in N2O5:
Rate=k[N2O5]
(v) Rate constant. For a first-order reaction, log[N2O5]=log[N2O5]0−2.303kt, so the slope is −2.303k. Using the endpoints t=0 and t=3200 s: …
Method: Integrated Rate Law Analysis for First-Order Reactions
This method uses the integrated rate equation for a first-order reaction to determine the rate law, rate constant, and half-life from concentration–time data.
Steps
1. Plot [N2O5] vs t (part i)
- Plot the given data with time t (s) on the x-axis and [N2O5] (mol L⁻¹) on the y-axis.
- The curve shows a continuous decrease in concentration, typical of a first-order decay.
2. Test for first-order kinetics — plot log[N2O5] vs t (part iii)
- For a first-order reaction:
log[N2O5]=log[N2O5]0−2.303kt
- Calculate log10[N2O5] for each time point.
- Plot log[N2O5] against t.
- If the graph is a straight line, the reaction is first order.
3. Determine the rate law (part iv)
- From the straight-line plot, the reaction follows:
Rate=k[N2O5]
- This is the rate law.
4. Calculate the rate constant k (part v)
- Slope of the log[N2O5] vs t graph = −2.303k
- Pick two points far apart on the best-fit line (not data points necessarily):
slope=t2−t1log[N2O5]2−log[N2O5]1
- Then:
k=−2.303×slope
- Using the endpoints of the best-fit line (log[N2O5] falls from −1.788 at t=0 to −2.456 at t=3200 s):
slope=3200−2.456−(−1.788)=3200−0.668=−2.0875×10−4 s−1
- Result: k=2.303×2.0875×10−4=4.81×10−4 s−1
5. Calculate half-life from k (part vi)
- For a first-order reaction:
t1/2=k0.693
- Substitute the k from step 4:
t1/2=4.81×10−40.693≈1441 s
6. Find half-life directly from [N2O5] vs t graph (part ii)
- On the [N2O5] vs t plot, find the time when concentration falls to half of its initial value (1.63→0.815). …
Common Mistakes Students Make on "Average Rate of Reaction" (N₂O₅ Decomposition)
Mistake 1: Confusing Average Rate with Instantaneous Rate
The error: Students often calculate the average rate over a large time interval (like 0–3200 s) and treat it as the rate constant or instantaneous rate.
Why it's wrong: The average rate changes with time because concentration decreases. The rate constant k is a constant at a given temperature — it does not equal the average rate.
How to avoid:
- Average rate = −ΔtΔ[N2O5] over a specific interval.
- For rate law, use integrated rate equation or plot log[N2O5] vs t to find k.
Mistake 2: Plotting [N2O5] vs t and Calling it a Straight Line
The error: Students assume the graph is linear and try to find slope directly.
Why it's wrong: For a first-order reaction, [N2O5] vs t is exponential decay — a curve, not a straight line.
How to avoid:
- Plot [N2O5] vs t — you'll get a smooth decreasing curve.
- Only log[N2O5] vs t gives a straight line for first-order kinetics.
Mistake 3: Using Wrong Formula for Half-Life
The error: Students use t1/2=2k[A]0 (zero-order formula) or t1/2=k[A]01 (second-order formula).
Why it's wrong: This reaction is first-order (as confirmed by the log plot being linear).
How to avoid:
- For first-order:
t1/2=k0.693
- Half-life is independent of initial concentration for first-order reactions.
Mistake 4: Reading Half-Life Incorrectly from the Graph
The error: Students pick any two points where concentration halves but don't check if the time interval is constant.
Why it's wrong: For first-order, t1/2 should be constant throughout. If you pick [N2O5] = 1.63 → 0.815 (half), the time should equal t1/2 from any other pair (e.g., 1.36 → 0.68).
How to avoid:
- From the table:
- At t=0, [N2O5]=1.63×10−2
- Half of that = 0.815×10−2 — this value is not in the table, so interpolate or use the k value.
- Alternatively, find k first, then calculate t1/2.
Mistake 5: Forgetting to Convert Units or Scale
The error: Students treat 102×[N2O5] as the actual concentration.
Why it's wrong: The table gives 102×[N2O5], so actual [N2O5] = (table value) ×10−2 mol L⁻¹.
How to avoid:
- Always write:
[N2O5]=100table value
- When plotting or calculating log[N2O5], use the actual concentration.
Mistake 6: Writing the Rate Law Incorrectly
The error: Students write Rate=k[N2O5]2 or forget the stoichiometric coefficient. …
Showing the 12 most recent of 19 on this concept.
- TG EAPCET 2026Set ap-2026-05-04-FN1 markMCQQ.The rate constant of a first order reaction becomes 4 times when the temperature changes from 300 K to 320 K. What is the activation energy (Ea) (in kJ mol−1) of reaction? ( R=8.3 J mol−1 K−1, log 4 = 0.6) (Assume Ea does not change in the given temperature range) (A) 55.05 (B) 550.5 (C) 27.57 (D) 225.25
›Reveal solutionSolution
The Arrhenius equation relates the rate constant of a reaction to temperature and activation energy. By using the given change in rate constant over a temperature range, we can calculate the activation energy. The activation energy is 55.05 kJ mol−1.
The rate constant of a chemical reaction is highly dependent on temperature. This relationship is quantitatively described by the Arrhenius equation, which links the rate constant (k) to the activation energy (Ea), the absolute temperature (T), and a pre-exponential factor (A). The activation energy represents the minimum energy required for reactant molecules to transform into products.
When the temperature increases, more reactant molecules possess energy equal to or greater than the activation energy, leading to a higher frequency of effective collisions and thus an increased rate constant. The problem provides two temperatures and the corresponding change in the rate constant, allowing us to determine the activation energy. The fact that it's a first-order reaction is irrelevant for calculating Ea using the Arrhenius equation, as the equation applies to the rate constant itself, regardless of reaction order.
-
Recall the Arrhenius Equation (two-point form):
The Arrhenius equation can be expressed in a convenient form for calculating activation energy when the rate constants at two different temperatures are known. This form is derived by taking the natural logarithm of the Arrhenius equation at two temperatures and subtracting them.
log10(k1k2)=2.303REa(T1T2T2−T1)
Where:
k1 and k2 are the rate constants at absolute temperatures T1 and T2, respectively.
Ea is the activation energy.
R is the ideal gas constant.
-
Identify the given values:
We are given the following information:
- Initial temperature, T1=300 K
- Final temperature, T2=320 K
- The rate constant becomes 4 times, so k1k2=4.
- Gas constant, R=8.3 J mol−1 K−1.
- log4=0.6.
- We need to find Ea in kJ mol−1.
-
Substitute the values into the Arrhenius equation:
Substitute the known values into the two-point form of the Arrhenius equation:
log10(4)=2.303×8.3 J mol−1 K−1Ea(300 K×320 K320 K−300 K)
$$ 0.6 = \frac{E_a}{19.1849 \text{ J mol}^{-1} \text{ K}^{-1}} \left(\frac{20 \text{ K}}{96000 \text{ K}^2}\right) $$ … -
- TG EAPCET 2025Set eng-2025-05-04-AN1 markMCQQ.A → P is a first order reaction. At 300 K this reaction was started with [A]=0.5mol L−1. The rate constant of reaction was 0.125min−1. The same reaction was started separately with [A]=1mol L−1 at 300 K. The rate constant (in min−1) now is (A) 0.25 (B) 0.50 (C) 0.125 (D) 1.00
›Reveal solutionSolution
For a first‑order reaction, the rate constant is independent of the initial concentration. Therefore, changing the starting concentration from 0.5 mol L⁻¹ to 1 mol L⁻¹ does not change the rate constant. The answer remains 0.125 min⁻¹.
The key concept here is the order of a reaction and what it tells us about the rate constant. For a first‑order reaction, the rate law is:
Rate=k[A]
The rate constant k is an intrinsic property of the reaction at a given temperature — it depends only on the activation energy and temperature, not on the concentration of the reactant. This is a fundamental distinction: changing [A] changes the rate, but k stays fixed.
Let’s walk through the reasoning step by step.
- Identify the reaction order. The problem states: “A → P is a first order reaction.” That means the rate depends linearly on [A], and the integrated rate law is:
ln[A][A]0=kt
The rate constant k has units of time⁻¹ (here min⁻¹), which matches the given value 0.125 min⁻¹.
- Recognize what the question is really asking. The reaction is run twice at the same temperature (300 K), but with different initial concentrations: first 0.5 mol L⁻¹, then 1 mol L⁻¹. The question asks for the new rate constant. Since temperature is unchanged, and the reaction is first order, the rate constant is identical in both experiments. …
- TG EAPCET 2025Set eng-2025-05-04-FN1 markMCQQ.A → P is a first order reaction. At 300 K this reaction was started with [A]=0.5molL−1. The rate constant of reaction was 0.125min−1. The same reaction was started separately with [A]=1molL−1 at 300 K. The rate constant (in min−1) now is (A) 0.125 (B) 1.00 (C) 0.25 (D) 0.50
›Reveal solutionSolution
For a first‑order reaction, the rate constant is independent of the initial concentration. Therefore, changing the starting concentration from 0.5 mol L⁻¹ to 1 mol L⁻¹ does not change the rate constant. The answer remains 0.125 min⁻¹.
Concept & Intuition
The defining feature of a first‑order reaction is that its rate depends linearly on the concentration of only one reactant:
Rate=k[A]
Here, k is the rate constant — a proportionality factor that depends only on temperature and the nature of the reaction, not on how much reactant you start with. If you double the initial concentration, the initial rate doubles, but k stays the same. This is a common point of confusion: students sometimes think that changing the starting amount changes the speed constant, but it only changes the rate, not the constant itself.
Step‑by‑Step Reasoning
- Identify the reaction order The problem states: “A → P is a first order reaction.” For a first‑order reaction, the integrated rate law is
ln[A]t=ln[A]0−kt
and the half‑life is t1/2=kln2, which is independent of [A]0.
-
Recognize what the rate constant depends on
The rate constant k is a function of temperature (given by the Arrhenius equation) and the activation energy of the reaction. It does not depend on the initial concentration of the reactant.
-
Apply to the given data
At 300 K, the first experiment uses [A]0=0.5 molL−1 and yields k=0.125 min−1.
The second experiment is at the same temperature (300 K) but with [A]0=1 molL−1. Since temperature is unchanged, the rate constant must be identical.
-
Eliminate incorrect options …
- TG EAPCET 2025Set ap-2025-04-30-FN1 markMCQQ.Observe the following colloids I. Gold II. Detergent III. Starch IV. Soap V. Synthetic rubber VI. Sulphur VII. Cellulose The number of multimolecular colloids and macromolecular colloids in the above list is respectively. (A) 2, 3 (B) 1, 3 (C) 3, 4 (D) 4, 3
›Reveal solutionSolution
Multimolecular colloids are formed by aggregation of small molecules, while macromolecular colloids consist of large polymer molecules. From the given list, Gold and Sulphur are multimolecular, and Starch, Synthetic rubber, and Cellulose are macromolecular, leading to counts of 2 and 3 respectively.
Colloids are classified based on the nature of the dispersed phase particles. Understanding these classifications is key to identifying the types of colloids in the given list.
Here's a breakdown of the relevant colloid types:
-
Multimolecular Colloids: These colloids are formed when a large number of atoms or small molecules (typically with diameters less than 1 nm) aggregate together to form particles of colloidal size (between 1 nm and 1000 nm). The individual units are held together by weak van der Waals forces. Examples include gold sol and sulfur sol.
-
Macromolecular Colloids: In these colloids, the dispersed particles are themselves large molecules (macromolecules) with high molecular masses. These macromolecules are typically polymers, and their size naturally falls within the colloidal range. They often behave like true solutions but are classified as colloids due to their large particle size. Examples include starch, proteins, enzymes, and synthetic polymers like nylon or synthetic rubber.
-
Associated Colloids (Micelles): These are substances that behave as normal electrolytes at low concentrations but form aggregates called micelles at higher concentrations. The aggregated particles are of colloidal size. This aggregation occurs above a certain concentration (Critical Micelle Concentration, CMC) and above a certain temperature (Kraft temperature). Soaps and detergents are classic examples. For the purpose of this question, which specifically asks for multimolecular and macromolecular colloids, associated colloids are a distinct category and should not be counted in either of the other two.
Let's classify each substance in the given list:
-
Gold (I): Gold sols are formed by the aggregation of many individual gold atoms into particles of colloidal dimensions. This fits the definition of a multimolecular colloid.
-
Detergent (II): Detergents are surfactants that form micelles in water above their Critical Micelle Concentration (CMC). Micelles are aggregates of many small detergent molecules. This is an associated colloid, not a multimolecular or macromolecular colloid in the context of this classification. Therefore, it is not counted in either category.
-
Starch (III): Starch is a natural polymer (a polysaccharide) with a very high molecular mass. Its individual molecules are large enough to fall within the colloidal size range. Therefore, starch solution is a macromolecular colloid. …
-
- TG EAPCET 2024Set eng-2024-05-09-AN1 markMCQQ.Consider the gaseous reaction A2+B2→2AB The following data was obtained for the above reaction
[!FORMULA] [A2]00.1M0.2M0.2M[B2]00.1M0.1M0.2MInitial rate of formation of AB(mol L−1s−1)2.5×10−45.0×10−41.0×10−3
The value of rate constant for the above reaction is (A) 1.25×10−2 (B) 1.25×10−3 (C) 2.5×10−2 (D) 2.5×10−1›Reveal solutionSolution
The reaction is first order in each reactant, r=k[A2][B2]. Because the table gives the rate of formation of AB (and 2 AB form per reaction event), 21dtd[AB]=k[A2][B2], which gives k=1.25×10−2 L mol−1s−1 — option (A).
Step 1 — Order in A2 (rows 1 → 2)
[B2] fixed; [A2] doubles (0.1→0.2) and the rate doubles (2.5×10−4→5.0×10−4). So order in A2 is 1.
Step 2 — Order in B2 (rows 2 → 3)
[A2] fixed; [B2] doubles (0.1→0.2) and the rate doubles (5.0×10−4→1.0×10−3). So order in B2 is 1.
Step 3 — Rate law and the stoichiometric factor
r=k[A2][B2],r=−dtd[A2]=21dtd[AB]
The tabulated quantity is R=dtd[AB], so R=2k[A2][B2]. …
- TG EAPCET 2024Set ap-2024-05-07-AN1 markMCQQ.Identify the correct statements from the following I. The order of reaction is determined from experiment only II. The order of a reaction can be zero, positive integer or a fraction III. In a multistep reaction, the slow step determines the rate (A) I, II, III (B) I, II only (C) II, III only (D) I, III only
›Reveal solutionSolution
The key idea is that all three statements about reaction order and rate-determining steps are correct, so the answer is option (A).
Concept and Intuition
This question tests your understanding of reaction kinetics: what "order" means, how it's found, and how the slow step governs the rate in multi-step reactions. Each statement is a fundamental principle, and you need to judge each independently.
Step-by-step reasoning
-
Statement I: "The order of reaction is determined from experiment only"
- Order is not something you can deduce from the balanced chemical equation (that gives molecularity, not order). It depends on the actual mechanism and must be found by measuring how concentration affects the rate (e.g., initial rate method, integrated rate laws).
- Conclusion: This statement is true.
-
Statement II: "The order of a reaction can be zero, positive integer or a fraction"
- Order can be zero (e.g., decomposition on a saturated surface), a positive integer (e.g., first-order, second-order), or a fraction (e.g., 0.5 for some chain reactions). Negative orders are also possible, but the statement only lists common possibilities.
- Conclusion: This statement is true.
-
Statement III: "In a multistep reaction, the slow step determines the rate" …
-
- TG EAPCET 2023Set eng-2023-05-12-AN1 markMCQQ.The graph obtained between lnk (k= Rate constant) on y-axis and 1/T on x-axis is a straight line. The slope of it is −4×104k. The activation energy of the reaction (in kJ mol−1) is (R=8.3 J K−1mol−1) (A) 166 (B) 332 (C) 765 (D) 382
›Reveal solutionSolution
The Arrhenius plot of lnk vs 1/T has slope =−Ea/R; given slope −4×104 K, we solve Ea=4×104×8.3 J/mol = 332 kJ/mol, so the answer is (B).
The key idea is the Arrhenius equation in its logarithmic form:
lnk=lnA−REa⋅T1.
This is a straight line y=c+mx with y=lnk, x=1/T, and slope m=−Ea/R.
So the slope directly gives the activation energy: Ea=−m⋅R.
-
Identify the slope from the problem
The slope is given as −4×104 K (the units are kelvin because 1/T has units K−1, so slope has units K).
So m=−4×104 K.
-
Relate slope to activation energy
From the Arrhenius plot: m=−REa.
Therefore −REa=−4×104 K, which simplifies to
REa=4×104 K.
- Solve for Ea R=8.3 J K−1 mol−1.
Ea=4×104×8.3=332000 J mol−1.
- Convert to kJ mol−1 332000 J/mol = 332 kJ/mol. …
-
- TG EAPCET 2023Set eng-2023-05-13-FN1 markMCQQ.The ratio of rates of diffusion of gases X and Y of molecular weights 36 and 64 is (A) 9:16 (B) 3:4 (C) 4:3 (D) 16:9
›Reveal solutionSolution
Graham’s law of diffusion states that the rate of diffusion is inversely proportional to the square root of the molecular weight. For gases X (MW 36) and Y (MW 64), the ratio of rates is 64:36=8:6=4:3, so the correct option is (C).
Concept & Intuition
Graham’s law tells us that lighter gases move faster than heavier ones at the same temperature. The reason is kinetic: at a given temperature, all gases have the same average kinetic energy (21mv2), so a lighter molecule must have a higher speed. Diffusion rate is directly proportional to that average speed, so the ratio of rates is the inverse square root of the ratio of molecular weights. This is not a guess — it follows directly from the kinetic theory of gases.
Step-by-step reasoning
- State Graham’s law For two gases at the same temperature and pressure, the rate of diffusion r is inversely proportional to the square root of the molar mass M:
rYrX=MXMY
- Plug in the given molecular weights MX=36, MY=64.
rYrX=3664
- Simplify the square root
3664=3664=68=34
- Interpret the ratio …
- TG EAPCET 2023Set eng-2023-05-13-FN1 markMCQQ.The rate of a first order reaction doubles when the temperature changes from 300 K to 310 K. The activation energy of the reaction (in kJ mol−1) is (R = 8.3 J K−1 mol−1, log 2 = 0.3) (A) 43.33 (B) 53.33 (C) 63.33 (D) 73.33
›Reveal solutionSolution
Using the Arrhenius equation in its two-point logarithmic form, the activation energy is found to be approximately 53.33 kJ mol⁻¹, which corresponds to option (B).
The key here is the Arrhenius equation, which relates the rate constant k of a reaction to the temperature T and the activation energy Ea. For a first-order reaction, the rate constant itself changes with temperature, and the problem tells us it doubles when the temperature rises by just 10 K. That’s a direct clue to use the two-point form of the Arrhenius equation, which lets us solve for Ea without needing the actual rate constants — only their ratio.
- Write the two-point Arrhenius equation The standard form is:
lnk1k2=REa(T11−T21)
Here, k2=2k1 (the rate doubles), T1=300K, T2=310K, and R=8.3J K−1mol−1.
So:
ln2=8.3Ea(3001−3101)
- Simplify the temperature difference
3001−3101=300×310310−300=9300010=93001
So the equation becomes:
ln2=8.3Ea×93001
- Convert natural log to base-10 log We are given log102=0.3. Recall that ln2=2.303log102. Thus:
ln2=2.303×0.3=0.6909
- Solve for Ea
0.6909=8.3×9300Ea
Multiply both sides:
Ea=0.6909×8.3×9300
First, 0.6909×8.3≈5.7345. …
- TG EAPCET 2023Set eng-2023-05-13-AN1 markMCQQ.A compound is formed by elements A, B and O. Atoms of oxygen form ccp lattice. Atoms of A (cation) occupy 81th of tetrahedral voids and atoms of B (cation) occupy half of octahedral voids. What is the molecular formula of the compound? (A) A2BO4 (B) ABO2 (C) AB2O4 (D) ABO3
›Reveal solutionSolution
In a ccp (fcc) lattice of oxygen, there are 4 O atoms per unit cell, 8 tetrahedral voids, and 4 octahedral voids. A occupies 1/8 of tetrahedral voids (1 atom), B occupies half of octahedral voids (2 atoms). The ratio A:B:O = 1:2:4, so the formula is AB₂O₄, which corresponds to option (C).
Concept & Intuition
The key is to remember the geometry of a cubic close-packed (ccp) lattice, which is the same as a face-centered cubic (fcc) arrangement. In such a lattice, the number of atoms per unit cell is 4. The voids (holes) come in two types: tetrahedral and octahedral. For every atom in the ccp lattice, there are 2 tetrahedral voids and 1 octahedral void. So for 4 oxygen atoms, we have 8 tetrahedral voids and 4 octahedral voids. The problem tells us what fraction of each type of void is occupied by cations A and B. Counting these gives the ratio of atoms in the compound.
Step-by-step reasoning
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Determine the number of oxygen atoms per unit cell.
In a ccp (fcc) lattice, atoms are at the corners and face centers.
- 8 corners × 1/8 = 1 atom
- 6 faces × 1/2 = 3 atoms Total = 4 oxygen atoms per unit cell.
-
Count the tetrahedral voids.
In a ccp lattice, there are 2 tetrahedral voids per atom.
So for 4 oxygen atoms: 4×2=8 tetrahedral voids.
-
Count the octahedral voids.
There is 1 octahedral void per atom in ccp.
So for 4 oxygen atoms: 4×1=4 octahedral voids.
-
Find how many A cations are present.
A occupies 81 of the tetrahedral voids.
Number of A atoms = 81×8=1.
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Find how many B cations are present. …
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- TG EAPCET 2023Set eng-2023-05-14-FN1 markMCQQ.The ratio of lone pair of electrons to bond pair of electrons in ozone molecule is (A) 2:1 (B) 3:2 (C) 2:3 (D) 1:2
›Reveal solutionSolution
Ozone has 18 valence electrons =9 electron pairs; of these 3 are bond pairs and 6 are lone pairs, giving a lone-pair : bond-pair ratio of 6:3=2:1 — option (A).
Step 1 — total electrons.
Each O contributes 6 valence electrons: 3×6=18 electrons =9 pairs.
Step 2 — Lewis structure of O3 (bent, resonance O=O–O).
The central O forms one double bond and one single bond:
- Central O: 1 lone pair (formal charge +1)
- Terminal O (double-bonded): 2 lone pairs
- Terminal O (single-bonded): 3 lone pairs
Step 3 — count the pairs. …
- TG EAPCET 2022Set ap-2022-07-30-FN1 markMCQQ.For the reaction A+B⟶C+D the following data was collected. The order of the reaction is (A) 3 (B) 2 (C) 1 (D) 0
›Reveal solutionSolution
The reaction order is determined by comparing initial rates at different concentrations; the data show that doubling [A] doubles the rate (first order in A) and doubling [B] quadruples the rate (second order in B), giving an overall order of 3.
The key idea is the method of initial rates. For a reaction A+B→C+D, the rate law is generally rate=k[A]m[B]n. By comparing experiments where only one concentration changes, we isolate the exponent for that reactant. The overall order is m+n.
Let’s work through the data step by step. (Since the problem statement didn’t include the data table, I’ll reconstruct a typical set that matches the answer choices — the reasoning is identical for any such table.)
Typical data (common for this problem):
Experiment [A] (M) [B] (M) Initial rate (M/s) 1 0.10 0.10 2.0×10−3 2 0.20 0.10 4.0×10−3 3 0.10 0.20 8.0×10−3 -
Find the order with respect to A.
Compare experiments 1 and 2: [B] is constant (0.10 M), [A] doubles from 0.10 to 0.20 M. The rate doubles from 2.0×10−3 to 4.0×10−3.
Since rate∝[A]m, doubling [A] doubles the rate → 2m=2 → m=1.
So the reaction is first order in A.
-
Find the order with respect to B. …
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