Consider the following thermochemical equations. PCl5 (s)→PCl3 (g)+Cl2 (g)2P (s)+3Cl2 (g)→2PCl3 (g)ΔH∘rxn=87.9kJmol ΔH∘rxn=−574kJmol Using this data, determine the heat of formation for PCl5.

Explanation:

The standard heat of formation is the enthalpy change for the formation of a compound from the elements at their standard states.

PCl₅ 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: -662kJ/mol

in theory, limestone is composed principally of the compound calcium carbonate - caco3, but in reality this compound is deposited in conditions involving a number of other compounds as impurities so it ends up being a mixture of many compounds.

hope this : -)

tony (lordcaos066)