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 4.50×103 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 2.00×103M1s1 at 27 C and an initial half-life of 214 s . What is the concentration of the reactant B after one half-life?

Express your answer with the appropriate units.

Explanation / Answer

1. k= 2.303/tlog a/a-x

(4.50×10^(3)) = (2.303/t)log(100/6.25)

t = 616.24 sec

2.    t1/2= 1/k[A]0

214 = 1/(2.00*10^(3)*a)

a = INITIAL CONCENTRATION = 2.336 M

K = 1/t[1/a-x*1/a]

2.00*10^(3) =(1/214)* ((1/a-x)-(1/2.336))

a-x = concentration of B after first one half life = 1.17 M

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