Question

For which of the following reactions is ΔH∘rxn equal to ΔH∘f of the product(s)?

Choose all that apply.
a. C(s,graphite)+O2(g)→CO2(g)
b. Li(s)+12Cl2(g)→LiCl(s)
c. Li(s)+12Cl2(l)→LiCl(s)
d. 2Li(s)+Cl2(g)→2LiCl(s)
e. CO(g)+12O2(g)→CO2(g)
f. BaCO3(s)→BaO(s)+CO2(g)

Answer 1

The correct answers are A, B, and D. The ΔHrxn is equal to the ΔHf when the ΔHf of the reactants is zero, and this can only occur when the reactants are pure elements (not compounds) in their natural phase at ambient temperature.

c) Chlorine's natural phase is gaseous, but since it is a liquid in this reaction, ΔHf ≠ 0.

e) CO is a compound, not a pure element.

f) BaCO3 is a compound, not a pure element.

Answer 2

The following reactions is ΔH∘rxn equal to ΔH∘f of the product(s) : option a, b and d

Further explanation

The change in enthalpy in the formation of 1 mole of the elements is called enthalpy of formation

The enthalpy of formation measured in standard conditions (25 ° C, 1 atm) is called the standard enthalpy of formation (ΔHf °)

Determination of the enthalpy of formation of a reaction can be through a calorimetric experiment, based on the principle of Hess's Law, enthalpy of formation table, or from bond energy data

Delta H reaction (ΔH) is the amount of heat / heat change between the system and its environment

(ΔH) can be positive (endothermic = requires heat) or negative (exothermic = releasing heat)

The value of ° H ° can be calculated from the change in enthalpy of standard formation:

∆H ° rxn = ∑n ∆Hf ° (product) - ∑n ∆Hf ° (reactants)

ΔH∘rxn = ΔH∘f of the product(s) if ∆Hf ° (reactants)  = 0

The elements in standard conditions are not included in the enthalpy calculations because the enthalpy of those elements under the standard conditions is zero.

a.ΔH°rxn = ∆Hf ° CO₂(∆Hf °C and ∆Hf °O₂ =0)

b.ΔH°rxn = ∆Hf ° LiCl(∆Hf °Li and ∆Hf °Cl₂=0)

c.ΔH°rxn = ∆Hf ° LiCl- 12∆Hf° Cl₂

d. ΔH°rxn = 2∆Hf °LiCl (∆Hf °Li and ∆Hf °Cl₂=0)

e. ΔH°rxn = ∆Hf °CO₂ - ∆Hf °CO

f. ΔH°rxn = ∆Hf °CO₂ + ∆Hf °BaO - ∆Hf ° BaCO₃

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Keywords: Delta H reaction (ΔH), enthalpy of standard formation