You are visiting another planet, and the aliens there find out that you are a bi

Question

You are visiting another planet, and the aliens there find out that you are a biochemist. They want you to determine change in enthalpy for the reaction shown below, but all they provide you are the following values for the standard molar enthalpies of formation: DeltaHf of compound X2Y(l) = -71.8 kcal/mol; DeltaHf of compound ZY(g) = -23.2 kcal/mol; and DeltaHf of compound XZYYX(l) = -101.1 kcal/mol. Given this information, what is DeltaHrxn in kcal/mol to the nearest tenths? (How nice of the aliens to use familiar terms!)

Explanation / Answer

Given this information, what is DeltaHrxn in kcal/mol to the nearest tenths?

Given that

DeltaHf of compound X2Y(l) = -71.8 kcal/mol;

DeltaHf of compound ZY(g) = -23.2 kcal/mol; and

DeltaHf of compound XZYYX(l) = -101.1 kcal/mol.

Reaction: X2Y(l) + ZY(g) ---> XZYYX(l)

DeltaHrxn = DeltaH (Products) – DeltaH (reactants)

DeltaHrxn = DeltaH (XZYYX(l) ) – DeltaH [X2Y(l) + ZY(g)]

DeltaHrxn =

DeltaH (-101.1 kcal/mol ) – DeltaH (-71.8 kcal/mol-23.2 kcal/mol )

=-101.1 kcal/mol + 95.0 kcal/mol

= -6.1 kcal/mol

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