Simulation · Chemistry · Class 12
Can you find a rate law from initial rates alone?
From the lesson Rate law and rate constant in Chemical Kinetics. Change the values and watch what happens.
Can you find a rate law from initial rates alone?Chemistry · Class 12
The idea behind it
NCERT §3.2; §3.2.1; §3.2.2
- 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.