10.6 The Effect of Temperature on Spontaneity

If ∆S_surr and ∆S_sys have opposite signs, what happens to ∆S_univ

depends on the temperature

• Entropy changes in the surroundings are primarily determined by heat flow 

Exothermic reaction

• Heat is transferred to the surroundings and becomes thermal energy

  • Increase in the random motions of atoms in the surroundings so entropy of the surroundings increases

∆S_surr is positive bc system lowered energy

Endothermic process

• Heat flows from the surroundings into the system

  • Random motions of the atoms in the surroundings decrease so entropy of the surroundings decrease

    • ∆S_surr is negative

Summary of Entropy Changes

• The impact of the transfer of a given quantity of energy as heat to or from the surroundings is greatest at low temperatures 

• The sign of ∆S_surr depends on the direction of the heat flow 

• The magnitude of ∆S_surr depends on the temperature

Entropy changes at constant temperature and pressure

∆S_sur = - ∆H/T

• If the reaction is exothermic, ∆H has a negative sign but since heat flows into the surroundings, ∆S_surr is positive