Practice: Decomposition of a certain substance Y at 45°C was found to be zero order. What is the half-life of substance Y if it took 15.5 minutes to decompose 67% of this substance? [Y]0 = 0.25 M.
Half-Life is the time it takes for half of a reactant to decay in a certain time period.
Understanding Half-Life
Concept #1: Zero-Order Half-Life
Example #1: The reverse Haber reaction: 2 NH4 (g) → 3 H2 (g) + N2 (g), has a rate constant of 1.45 x 10-6 M•s-1 at 25ºC. Calculate the half-life for the reaction where [NH4]0 = 2.47 x 10-2 mol/L.
Concept #2: First-Order Half-Life
Example #2: Rate constant for the following reaction was found to be 2.3 x 10-3 s-1 at 35ºC:
2 NO2 → N2O4
If the initial concentration of NO2 was 1.4 x 10-1, what is the half-life on this reaction?
Practice: Radioactive plutonium-239 (t1/2 = 2.41 × 105 yr) is used in nuclear reactors and atomic bombs. If there are 5.70 × 102 g of plutonium isotope in a small atomic bomb, how long will it take for the substance to decay to 3.00 × 102 g?
Concept #3: Second-Order Half-Life
Example #3: The half-life of a certain reaction with 2nd order was found to be 0.45 seconds. What was the initial concentration of a reactant if the slope of the straight line for this reaction is 3.5 x 10-3?
Practice: Use the data below to determine the half-life of decomposition of NOCl reaction which follows 2nd order kinetics.
