Question

1. Calculate the Hrxnfor the following reaction using the provided Hf values. Describe the flow of energy in the reaction based on the value you calculate. C2H2(g)+(5/2)O2(g)->2CO2(g)+H2O(g)

	Hf(kJ/mol)
C2H2(g)	227.0
CO2(g)	-393.5
H2O(g)	-241.8
O2(g)	0

2. Why is the enthalpy change considered a state function? How does this relate to Hess’s Law?

3. For an endothermic reaction, circle the correct sign of q_cal, q_soln, and q_rxn. Justify your answer for each.

   1. q_cal: positive/negative: Why?

   2. q_soln: positive/negative: Why?

   3. q_rxn: positive/negative: Why?

4. Calorimeters are not perfect insulators. What does that mean? How do we, as scientists, account for imperfect insulation?

Answer 1

Solution:

>>>For the given reaction,

C2H2(g) + 5/2 O2(g) = 2CO2 (g) + H2O(g)

ΔHrxn = Hf (products) - Hf (reactants)

= 2 mol x Hf (CO2) + 1 mol x Hf (H2O) - 1 mol x Hf (C2H2) - 5/2 mol x Hf (O2)

= 2 mol x (-393.5) kJ/mol + 1 mol x (-241.5) kJ/mol - 1 mol x 227 kJ/mol - 5/2 mol x 0 kJ/mol

= (-787 kJ - 241.5 kJ - 227 kJ + 0 kJ)

= -1255.5 kJ

>>> Enthalpy change is considered as state function becuase it delends upon initial and final states of the system. Since it is calculated by substracting reactants enthalpies from product enthalpies.

>>> For endothermic reaction,

qcal = positive

q sol = positive

q rxn = positive

In all the cases, heat is absorbed which indicates the endothermic reaction.