For a reaction \( \ce{aA + bB -> cC + dD} \), the equation \( -\frac{d[R]}{dt} = k[A]^x[B]^y \) is known as the
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → differential rate equation
The equation gives the rate as a derivative, −d[R]/dt, the change in concentration over an infinitely small time, in terms of the concentrations of the reactants. Because it is written with this derivative, it is called the differential rate equation; k in it is the rate constant.
NCERT (2026-27 edition):
3.2.2 Rate Expression and Rate Constant – “…Above equation can also be written as”
\( \displaystyle \text{Rate} = k[\text{A}]^x[\text{B}]^y \quad (3.4\text{a}) \qquad -\frac{d[\text{R}]}{dt} = k[\text{A}]^x[\text{B}]^y \quad (3.4\text{b}) \)
“This form of equation (3.4 b) is known as differential rate equation, where k is a proportionality constant called rate constant.”