Question

Determine ΔS°rxn and ΔG°rxn at 298 K for each of the following reactions, and determine if it is spontaneous under these conditions.
a. Ca( s ) + Cl2 ( g ) produces CaCl2 ( s )

b. 2C2H6 ( g ) + 7O2 ( g ) produces 4CO2 ( g ) + 6H2O( l )

c. SrCl2 ( s ) produces Sr (2+) ( aq ) + 2Cl (1–) ( aq )

Answer 1

Solution.

The values of ΔS°rxn and ΔG°rxn can be obtained by using the following equations:

a)

b) The calculations are similar:

c) The calculations are quite similar as well:

ΔH_f^o(Sr²⁺) = −545.8 kJ·mol⁻¹

ΔH_f^o(Cl^-) = −167.1 kJ·mol⁻¹
ΔH_f^o(SrCl₂) = -828.9 kJ·mol⁻¹
ΔH_rxn^o = -545.8+2(-167.1)-(-828.9) = -51.2 kJ;

S^o(Sr²⁺) = −32.6 J·mol⁻¹·K^-1

S^o(Cl^-) = 56.5 J·mol⁻¹·K^-1
S^o(SrCl₂) = 114.9 J·mol⁻¹·K^-1
ΔS_rxn^o = -32.6+2(56.5)-114.9 = -34.5 J·K^-1

ΔGrxno  = ΔHrxno - TΔSrxno  = -51200-298(-34.5) = -40.9 kJ.