Spontaneity

1. Spontaneity IB Chemistry HL

2. In this part of Energetics we learn: • Predict whether a reaction or process will be spontaneous by using the sign of ∆Go • Calculate ∆Go for a reaction using the equation, ∆Go = ∆Ho - T∆Soand by using values of the standard free energy change of formation, ∆Go (f) • Predict the effect of a change in temperature on the spontaneity of a reaction using standard entropy and enthalpy changes and the equation, ∆Go = ∆Ho - T∆So

3. Spontaneity and Entropy • When changes takes place, Entropy of the Universe will be affected. • Nature supports changes to occur itself. • If a change occurs itself, it is termed as spontaneous. • When a change occurs, we need to consider the system and its surrounding. • Entropy change occurs to the system and the surrounding.

4. All real processes occur spontaneously in the direction that increases the entropy of the universe. • For a spontaneous process the sum of entropy changes must be positive. • ∆Suniv = ∆Ssys + ∆Ssurr > O

5. Spontaneity • The final state is more probable than the initial state. • Final entropy of the universe is greater than the initial entropy of the universe. • ∆S = entropy of the system. • Heat is gained or lost to the surroundings. • Exothermic reactions increase the entropy of the surroundings, but by what magnitude?

6. Magnitude of ∆So • Magnitude of ∆So is - ∆Ho/T, where T is absolute temperature. • Spontaneous change if: • Final state has a lower enthalpy than the initial state (∆Hois negative)-Exothermic AND • Final state is more disordered than the initial state (∆So is positive)

7. Gibb’s Free Energy change, ∆G • Gibb’s Free Energy is the energy associated with a chemical reaction that can be used to do work. • It is a thermodynamic property that was defined in 1876 by Josiah Willard Gibbs to predict whether a process will occur spontaneously at constant temperature and pressure. 

8. The relationship between Gibb’s Free energy, Enthalpy change, Entropy change and temperature is given by: • ∆G = ∆Ho - T∆So ∆Ho= standard enthalpy change ∆So = standard entropy change of the system T = temperature in Kelvin

9. ∆G is NEGATIVE when ∆Suniverse is positive and the change is spontaneous. • Whether a reaction actually occurs = spontaneous • The influence of the ∆So is always dominant at high temperatures where ∆Hodoes not vary much. • Don’t forget, ∆G is equal to the amount of energy from that system that is available to do useful work.

10. ∆G = ∆Ho - T∆So

11. Calculations using ∆G = ∆Ho - T∆So Find G and predict the reaction is spontaneous or not

12. Answer This reaction is Non-spontaneous at this temperature, what temperature would we need so that it would be spontaneous, assuming ΔS and ΔH are independent of temp?

13. Calculations using ΔGfo(standard temp and pressure)

14. THE END