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) = -65.9 kcal/mol; DeltaHf of compound ZY(g) = -26.6 kcal/mol; and DeltaHf of compound XZYYX(l) = -106.4 kcal/mol. Given this information, what is DeltaHrxn in kcal/mol to the nearest tenths? (How nice of the aliens to use familiar terms!)

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

Explanation / Answer

Given that;

DeltaHf of compound X2Y(l) = -65.9 kcal/mol;

DeltaHf of compound ZY(g) = -26.6 kcal/mol; and

DeltaHf of compound XZYYX(l) = -106.4 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 (-106.4 kcal/mol ) – DeltaH (-65.9 kcal/mol-26.6 kcal/mol )

=-106.4 kcal/mol + 92.5 kcal/mol

= -13.9 kcal/mol

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