JEE Main 2022 · 25 June · morning · Numerical answer · +4 / −0
For a given chemical reaction
γ₁A + γ₂B → γ₃C + γ₄D
Concentration of C changes from 10 mmol dm⁻³ to 20 mmol dm⁻³ in 10 seconds. Rate of appearance of D is 1.5 times the rate of disappearance of B which is twice the rate of disappearance A. The rate of appearance of D has been experimentally determined to be 9 mmol dm⁻³ s⁻¹. Therefore, the rate of reaction is _____________ mmol dm⁻³ s⁻¹. (Nearest Integer)
Answer and solution
Answer 1
Solution
Chemistry ma’am · 4 steps
The four rates give you the coefficients, then divide any one rate by its own coefficient.
- Rate of C. C rises by 20 - 10 = 10 mmol dm⁻³ in 10 s. So C appears at 1 mmol dm⁻³ s⁻¹.
- Other rates. D appears at 9. D is 1.5 times B, so B disappears at 9 / 1.5 = 6. B is twice A, so A disappears at 6 / 2 = 3. All are in mmol dm⁻³ s⁻¹.
- Coefficients. In a balanced equation, each species changes at a rate proportional to its coefficient. The rates 3, 6, 1 and 9 give γ₁ : γ₂ : γ₃ : γ₄ = 3 : 6 : 1 : 9, so the equation is 3A + 6B → C + 9D.
r = 1/3(3) = 1/6(6) = 1/1(1) = 1/9(9) = 1
- So. Dividing each rate by its own coefficient gives the same value. The rate of reaction is 1 mmol dm⁻³ s⁻¹.
The common slip: 9. Reporting the rate of appearance of D gives 9. The rate of reaction is that rate divided by D's coefficient, 9, which is 1.
The idea to keep. The rate of reaction is any species' rate of change divided by its coefficient. When the coefficients are unknown, the ratio of the individual rates gives them.
Answer from the NTA final answer key. Solution by PrepWiser.