2 KClO3 (s) → 2 KCl (s) + 3 O2 (g)

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Decomposition

Stoichiometry

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 KClO3          Mass: g KCl            Mass: g O2             Mass: g or Gas Volume: L Heat Released: kJ

Enthalpy of Reaction

[2ΔHf(KCl (s)) + 3ΔHf(O2 (g))] - [2ΔHf(KClO3 (s))]
[2(-435.89) + 3(0)] - [2(-391.2)] = -89.38 kJ
-89.38 kJ     (exothermic)

Entropy Change

[2ΔSf(KCl (s)) + 3ΔSf(O2 (g))] - [2ΔSf(KClO3 (s))]
[2(82.68) + 3(205.03)] - [2(142.97)] = 494.51 J/K
494.51 J/K     (increase in entropy)

Free Energy of Reaction (at 298.15 K)

From ΔGf° values:
[2ΔGf(KCl (s)) + 3ΔGf(O2 (g))] - [2ΔGf(KClO3 (s))]
[2(-408.32) + 3(0)] - [2(-289.91)] = -236.82 kJ
-236.82 kJ     (spontaneous)

From ΔG = ΔH - TΔS:
-236.82 kJ     (spontaneous)

Equilibrium Constant, K (at 298.15 K)

3.100113322e+041
This process is favorable at 25°C.

Reference(s):

Ebbing, Darrell D. General Chemistry 3rd ed.; Houghton Mifflin Company: Boston, MA, 1990; pp 74, 162, 174, 181, 222.
Zumdahl, Steven and Zumdahl, Susan A. Chemistry 9th ed.; Brooks/Cole: Belmont, CA, 2014; p 131.