Entropy Change of the Surroundings
The entropy change experienced by the surroundings when they absorb or release heat from a system.
Formula / equation
ΔS_surr = −ΔH_sys / T
Unit
J/K
In depth
Because the surroundings are effectively infinite, they absorb heat reversibly at constant temperature, so their entropy change is simply heat divided by temperature. The colder the surroundings, the greater the entropy gain per joule, which is why exothermic reactions become more strongly favoured at low temperature.
Examples in the real world
An exothermic reaction releasing 100 kJ at 298 K raises the surroundings' entropy by 336 J/K.