Thermochemical Stoichiometry

Question

The following thermochemical equation is for the reaction of hydrogen bromide(g) with chlorine(g) to form hydrogen chloride(g) and bromine(g).

1. 2HBr(g) + Cl₂(g)2HCl(g) + Br₂(g)    H = -81.1 kJ

When 24.1 grams of hydrogen bromide(g) react with excess chlorine(g),  _____kJ of energy are _________evolved or absorbed.

Answer 1

Convert 24.1 g HBr to moles:

24.1 g x 1 mol HBr / 80.9 g = 0.2989 moles

According to the balanced equation, 81.1 kJ of energy is released for every TWO moles of HBr.

Therefore, to find amount of energy released by 0.2989 moles of HBr, we have ....

81.1 kJ / 2 moles HBr x 0.2989 moles HBr = 24.2 kJ (3 sig. figs.)