Question

9.Given the equation and 𝐻 values(see standard enthalpy of formation f of various 𝑓

∆𝐻°

compounds table given in the discussion ) What is the heat of reaction at 298 K for the reaction:

C(s) + 2 H2(g) CH4(g)

a. -74.8 kJ c. 278 kJ

b. -27.6 kJ d. 296 kJ

10. Knowing the enthalpy of the reaction in number 9, the reaction is:

a. endothermic c. either a or b

b. exothermic d. cannot be determined

Answer 1

C(s) + 2 H₂(g) = CH₄(g)

From the literature data:

$\Delta$ H_f⁰(C) = 0 kJ/mol;

$\Delta$ H_f⁰(O₂) = 0 kJ/mol;

$\Delta$ H_f⁰(CH₄) = -74.8 kJ/mol;

The heat of reaction ($\Delta$ H_r⁰) = $\Sigma$ H_f⁰(product) - $\Sigma$ H_f⁰(reactants)

$\Delta$ H_r⁰ = $\Delta$ H_f⁰(CH₄) - ($\Delta$ H_f⁰(C) + 2 * $\Delta$ H_f⁰(O₂)) = -74.9 - (0 + 2 * 0) = -74.8 kJ/mol.

Since $\Delta$ H_r⁰ = < 0, then this reaction is exothermic.

Answer:

1. a (-74.8 kJ/mol);
2. b (exothermic).