Question

Calculate the standard entropy of formation (in J mol–1 K–1) of NH3(g) from the standard entropies given below.

N2(g) H2(g) NH3(g) (respectively matches below)

S°/ (J mol–1K–1) 191.6 130.7 192.5



-99.4

-139.4

+139.4

+192.5

-168.1



The correct answer is -99.4 but I cannot seem to figure out why.

Answer 1

The standard entropy of formation of NH3(g) is -99.4 J mol⁻¹ K⁻¹.

Step-by-step explanation:

The standard entropy of formation of NH3(g) can be calculated using the following equation:

S° (NH3) = 2S° (NH3) - [S° (N2) + 3S° (H2)]

Using the given values:

S° (N2) = 191.6 J mol⁻¹ K⁻¹

S° (H2) = 130.7 J mol⁻¹ K⁻¹

S° (NH3) = 192.5 J mol⁻¹ K⁻¹

Plugging in the values, we get:

S° (NH3) = 2(192.5 J mol⁻¹ K⁻¹) - [191.6 J mol⁻¹ K⁻¹ + 3(130.7 J mol⁻¹ K⁻¹]

Solving the equation gives us a value of -99.4 J mol⁻¹ K⁻¹ for the standard entropy of formation of NH3(g).