Radioactive materials decay by a first-order process. The rate constant for the decay of an isotope whose half-life is 7.69 years is​

k = 0.0901 yrs⁻¹

Explanation:

1st order decay is defined by the equation A = A₀e^-k·t. If A = A₀/2 and t represents the 1st half-life of the isotope then ...  

t(1/2) = ln2/k = 0.693/k => k = 0.693/t(1/2) = 0.693/7.69yrs = 0.0901 yrs⁻¹

answer: enthalpy

explanation:

for a exothermic reaction, the energy is released i.e. the energy of the products is lower than the energy of the reactants.

= enthaply

= energy of products

= energy of reactants

as , thus comes out to be negative. negative value of enthalpy shows that the reaction is exothermic.