Question

The change in enthalpy (\( \Delta H \)) for the reaction
\[ 2 \, P (s) + 3 \, Br_2 (l) \rightarrow 2 \, PBr_3 (l) \] is -243 kJ. In this reaction, if the amount of phosphorus consumed is 3.1 g, the change in enthalpy (rounded off to two decimal places) is \(\underline{\hspace{2cm}}\) kJ.

Hint:

To calculate the enthalpy change for a given amount of substance, use the molar relationship from the balanced equation and adjust for the amount of substance consumed.

Answer 1

Correct Answer: -12.16

The molar mass of phosphorus is 31 g/mol. Thus, the number of moles of phosphorus consumed is:
\[ \text{moles of } P = \frac{3.1}{31} = 0.1 \, \text{mol}. \] From the given reaction, \( 2 \) moles of phosphorus produce a change in enthalpy of \( -243 \, \text{kJ} \). Therefore, the enthalpy change for \( 0.1 \) mol of phosphorus is:
\[ \Delta H = \frac{-243}{2} \times 0.1 = -12.15 \, \text{kJ}. \] Thus, the change in enthalpy for the given amount of phosphorus is \( -12.15 \, \text{kJ} \).