AS/A-Level Chemistry- Gibbs Free Energy

Gibbs free energy change(ΔG) 

is the balance between entropy and enthalpy changes which determines the feasibility/spontaneity of a reaction, the relationship being the more negative ΔG, the more feasible the reaction. For the reaction to take place only thing is required is the activation energy. 

ΔG =ΔH − TΔS

change in free energy (kJmol^-1) = change in enthalpy (kJmol^-1) - temperature(K) x change in entropy (kJK^-1mol^-1)

(can express the formula as y=mx+c) ( T = ∆H/∆S )

ΔG can predict the direction of the chemical reaction under two conditions:  

1. constant temperature
2. constant pressure

The sign of ΔG indicates the direction of a chemical reaction and determine if a reaction is spontaneous or not.  

• ΔG<0: reaction is spontaneous in the direction written (i.e., the reaciton is exergonic) 
• ΔG=0: the system is at equilibrium and there is no net change either in forward or reverse direction. 
• ΔG>0: reaction is not spontaneous and the process proceeds spontaneously in the reserve direction. To drive such a reaction, we need to have input of free energy (i.e., the reaction is endergonic)

Some questions in A-Level Chemistry:

1. State the equation for the reaction between C and Cl₂ to produce CO and TiCl₄ and given that the reaction is exothermic, explain why the reaction will occur at any temperature.

• 2C(s)+ 2Cl₂(g)⇨2CO(g)+ TiCl₄(g)
• Enthalpy change ΔH is negative, ΔH < 0
• Increase in number of gas species, 2 moles of gas to 3 moles of gas
• Entropy change ΔS is positive, ΔS > 0, increase in disorder
• Free-energy change ΔG is negative at all temperatures, reaction feasible at all temperatures, ΔG < 0

2. State the equation for the reaction between NO and O₂ to produce NO₂ and given that the reaction is exothermic, explain why the reaction will not occur in an internal combustion engine.

• 2NO + O₂⇨2NO₂
• Enthalpy change ΔH is negative, ΔH < 0
• Decrease in number of species, 3 moles of species to 2 moles of species
• Entropy change ΔS is negative, ΔS < 0
• Free-energy change ΔG is positive at high temperatures, internal combustion engine is at high temperatures above the critical temperature, reaction not feasible at high temperatures, ΔG > 0

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Drafted by Eunice Wong (Chemistry)

References:

• https://www.google.com/url?sa=i&url=https%3A%2F%2Fsurfguppy.com%2Fthermodynamics%2Funderstanding-gibbs-free-energy%2F&psig=AOvVaw3G-vBMsmH2XIjCAikT0Wrv&ust=1625059191083000&source=images&cd=vfe&ved=0CAsQjhxqFwoTCICH5bL3vPECFQAAAAAdAAAAABA6
• https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Thermodynamics/Energies_and_Potentials/Free_Energy/Gibbs_(Free)_Energy
• https://bio.libretexts.org/Courses/University_of_California_Davis/BIS_2A%3A_Introductory_Biology_(Igo)/Winter_2018_Igo_Readings/W2018_Bis2A_Lecture10_reading