Answer:
- <u><em>-662 kJ/mol</em></u>
Explanation:
The <em>standard heat of formation </em>is the enthalpy change for the formation of a compound from the elements at their standard states.
<em>PCl₅ </em>is formed by P and Cl, whose standard states are solid and diatomic gas.
You just must manipulate the two given equations to obtain an overall equation from P(s) and Cl₂(g).
Based of Hess' law, the enthalpy change of the overall equation is the algebraic sum of the individual equations.
If you multiply the first equation by 2 and reverse it you get:
Write the second equation:
Add them:
After computing:
Notice that the last calculation results in the standard enthalpy of formation for two moles of PCl₅(s). Thus, to obtain the enthalpy of formation per mol you must divide by 2:
Round to 3 signficant figures: -<em>662kJ/mol</em>