Question

The standard enthalpy of formation, ΔH°f, of HI(g) is +26 kJ mol-1. Which of the following is the approximate mass of HI(g) that must decompose into H2(g) and I2(s) to release 500. kJ of energy?

Answer 1

Answer: The mass of HI that must decompose to release the given amount of energy is 2459.72 g.

Explanation:

We are given:

$\Delta H^o_f_{HI}=+26kJ/mol$

So, for decomposition of hydrogen iodide, the enthalpy will be -26 kJ/mol

The chemical equation for the decomposition of HI into hydrogen and iodine follows:

$HI(g)\rightarrow \frac{1}{2}H_2(g)+\frac{1}{2}I_2(g);\Delta H^o=-26kJ/mol$

By Stoichiometry of the reaction:

26 kJ of energy is released when 1 mole of hydrogen iodide is decomposed.

So, 500 kJ of energy will be released when = $\frac{1}{26}\times 500=19.23mol$ of hydrogen iodide will be decomposed.

To calculate the mass of hydrogen iodide, we use the equation:

$\text{Number of moles}=\frac{\text{Given mass}}{\text{Molar mass}}$

Moles of HI = 19.23 mol

Molar mass of HI = 127.911 g/mol

Putting values in above equation, we get:

$19.23mol=\frac{\text{Mass of HI}}{127.911g/mol}\\\\\text{Mass of HI}=2459.72g$

Hence, the mass of HI that must decompose to release the given amount of energy is 2459.72 g.