For a first-order reaction, the half-life is constant. It depends only on the ra

Question

For a first-order reaction, the half-life is constant. It depends only on the rate constant k and not on the reactant concentration. It is expressed as t1/2=0.693k For a second-order reaction, the half-life depends on the rate constant and the concentration of the reactant and so is expressed as t1/2=1k[A]0

Part A

A certain first-order reaction (Aproducts) has a rate constant of 4.20×103 s1 at 45 C. How many minutes does it take for the concentration of the reactant, [A], to drop to 6.25% of the original concentration?

Express your answer with the appropriate units.

Part B

A certain second-order reaction (Bproducts) has a rate constant of 1.35×103M1s1 at 27 C and an initial half-life of 244 s . What is the concentration of the reactant B after one half-life?

Express your answer with the appropriate units.

Explanation / Answer

a) For first order reaction –ln (CA/CAO)= kt

Where K= rate constant = 4.2*10-3/s and CA/CAO= 6.25/100CAO/CAO

CAO= initial concentration and CA = concentration at time t

-ln (6.25/100)= 4.2*10-3*t

2.77= 4.2*10-3t and t= 660 seconds

b)

For second order reacton half life = 1/KCAO

CAO= initial conentration and K= rate constant =1.35*10-3 /M.S

244= 1/(1.35*10-3CAO)

CAO= 1/(244*1.35*10-3)=0.3294 M

Concentratino at half life =0.3294/2= 0.1647M

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