What is the rate constant of a first-order reaction that takes 143 seconds for the reactant concentration to drop to half of its initial value?

k = 4.85 x 10⁻³ / s

Explanation:

For a first order reaction we have two very important equations we need to memorize:

ln (N/N₀) = - kt

and,

0.693 / k = t₁/₂

where k is the rate constant and t i is the halflife which can be derived from the first equation.

0.693 / k = t₁/₂ ⇒ k = 0.693 / t₁/₂

k = 0.693 / 143 s = 4.85 x 10⁻³ / s