Feasibility: Reaction can only occur if products have lower
overall energy than reactants, then feasible or spontaneous.
Gibbs’ Equation: The overall change during chemical reaction
called free energy change- ∆G. Enthalpy change refers to heat
transfer between chemical system and surroundings.
Negative∆G: For a reaction to be feasible and take place, ∆G free energy must be negative.
Positive = does not take place.
Calculating:
– Also convert Celsius to Kelvin by adding 273.
– The units for ∆ S must be changed from J K-1 mol-1 to kJ K-1 mol-1
by dividing 1000 to match∆ H and ∆G.
– Remember brackets after subtraction – (T∆ S¿
Balance:
– Feasibility is usually supported when ∆ H is negative and ∆ S is positive. Also possible if
both positive.
– ∆ H usually much larger magnitude than∆ S so ∆G usually more dependent on∆ H.
– As temperature increases, T∆ S becomes more negative than ∆ H so reaction may become
not/ less feasible because ∆Gbecomes positive/ increases- only if both H and S negative.
Entropy and Enthalpy: As part of these questions may be asked to calculate ∆ Hand∆ S, so sum
of products- sum of reactants if given table of values, remember to multiply by balancing
number OR could use energy cycle diagram.
Calculate Minimum Temperature: ∆G = 0 at minimum temperature for reaction to take place.
T = ∆ H/∆ S. Then convert K to o
C.
How can Endothermic Reactions Take Place at Room Temp: Some ionic compounds dissolve in
water at room temperature in endothermic process e.g. enthalpy change of solution. This is
because (disorder must increase so) ∆S positive. ∆G must be negative so T∆S must be more
negative than ∆H. Since ∆H is positive as endodermic.
Why can a reaction with a negative entropy change take place spontaneously- ∆H is more
negative than T∆S.
Why is this reaction not feasible at any temperature- ∆H is positive, T∆S negative so ∆G will
always be positive.
If unsure just sub numbers into equation and play around with it.
Limitations: Many reactions have negative ∆G but reaction does not take place. This is because
of large activation energy so slow rate of reaction. Might take place eventually or add catalyst.
So ∆G tells us thermodynamic feasibility, but not kinetics/ rate of reaction.