Why Do We Square Molarities in Free Energy Calculations for HCl Dilution?

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SUMMARY

The discussion focuses on calculating the change in Gibbs free energy (ΔG) for the dilution of aqueous HCl from 0.89 M to 0.253 M at 25°C. The key equation used is ΔG = RTln Q, where R is the gas constant (8.314472 J/K/mol) and T is the temperature in Kelvin (298.15 K). The concept of equimolarity is crucial, as HCl completely dissociates in water, leading to equal concentrations of H+ and Cl-. Therefore, both concentrations are squared in the calculation of the reaction quotient (Q), resulting in the formula ΔG = RTln((0.253^2)/(0.89^2)).

PREREQUISITES
  • Understanding of Gibbs free energy and its significance in chemical reactions
  • Familiarity with the concept of molarity and dilution calculations
  • Knowledge of strong acids and their dissociation in aqueous solutions
  • Basic proficiency in logarithmic functions and their application in thermodynamics
NEXT STEPS
  • Study the principles of Gibbs free energy and its applications in chemical thermodynamics
  • Learn about the dissociation of strong acids in water, focusing on HCl
  • Explore the calculation of reaction quotients (Q) in chemical equilibria
  • Investigate the implications of concentration changes on free energy in chemical reactions
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Chemistry students, chemical engineers, and anyone involved in thermodynamic calculations related to acid-base reactions.

salman213
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Calculate ΔG for the dilution of aqueous HCl from 0.89 M to 0.253 M at 25°C.

THis was a question I had to do some time earlier for one of my assignments. I got some help from a classmate and he said THE FOLLOWING:

"H+ and Cl- are equimolar so you have to put them to the
power of two...

so the equation reads
Deltag = RTln Q
delta g = 8.314472J/K/mol*298.15K * 1kJ/1000J *ln (second
molarity^2/first molarity^2)"





WHEN he says EQUIMOLAR how do i find this out like i have no idea how he figured that out. If i did this question i would just do products over reactions to find the value of Q.

But he says put them to power of 2. WHY?? Can someone explain?
 
Chemistry news on Phys.org
HCl is a strong acid and therefore completely disassociates in water. Therefore, whatever the molarity of HCl = molarity of H+ = molarity of Cl-

Hence equimolar
 
Last edited:

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