# How do you calculate entropy change?

Mar 17, 2014

This example problem demonstrates how to calculate the change in entropy of a system's and surroundings following a chemical reaction at constant temperature and pressure.

Calculate the entropy of the surroundings for the following reaction.

a.) C2H8(g) + 5 O2(g) → 3 CO2(g) + 4H2O(g)
ΔH = -2045 kJ, the reaction takes place at 2${5}^{o} C$

This reaction is an exothermic reaction. The negative sign of enthalpy change ( -2045 kJ) shows that the system has lost 2045 kJ of energy to surroundings.

2${5}^{o} C$ = 2${5}^{o} C$ + 273.15 = 298.15 K

ΔSsurr = ΔH /T = + 2045 kJ ( heat gained by surroundings) / 298.15 K

ΔSsurr = 6.86 kJ / K

Good example with generic equation here: http://chemwiki.ucdavis.edu/Physical_Chemistry/Thermodynamics/State_Functions/Entropy/Calculating_Entropy_Changes