Lesson 2 of 11 · 7 min
Rate law and rate constant
NCERT §3.2; §3.2.1; §3.2.2
The club wants to know which ingredient controls the speed: the peroxide, the iodide or both. They decide to change one at a time and time only the first minute of each run.
The lesson in notes
In short
Rate depends on the concentration of reactants (pressure for gases), temperature and catalyst.
The rate law (rate equation) expresses rate as k times concentrations raised to powers: for aA + bB → products, rate = k[A]^x[B]^y.
The powers x and y are found by experiment. They need not equal the coefficients a and b of the balanced equation.
The standard method compares initial rates. For 2NO + O₂ → 2NO₂, doubling [NO] at fixed [O₂] makes the rate four times larger and doubling [O₂] at fixed [NO] makes it twice as large, so rate = k[NO]²[O₂].
Two reactions from the book whose powers do not follow the equation: CHCl₃ + Cl₂ → CCl₄ + HCl has rate = k[CHCl₃][Cl₂]^½, and the acid hydrolysis of an ester has rate = k[ester][H₂O]⁰ in excess water.
k, the rate constant, equals the rate when every concentration in the rate law is 1 mol L⁻¹. At a given temperature it is fixed for the reaction; it changes with temperature and with a catalyst, not with concentration.
Rate falls as the reaction proceeds because concentrations fall; k stays the same throughout.