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# Thermodynamics: Calculating equilibrium constants?

## Thermodynamics: Calculating equilibrium constants? - Biochemistry Forum

### Thermodynamics: Calculating equilibrium constants? - Discuss and post questions regarding the study of Biochemistry. If you need homework help this is the place to ask!

#1
10-25-2010, 04:54 PM
Pipette Filler

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Thermodynamics: Calculating equilibrium constants?

Hey all, I am currently studying Marine Biology and have recently started an optional biochemical statistics module, in the hope that it would give me better insight into statistical analysis. Unfortunatley I didn't expenct to be handed a Thermodynamics worksheet on the first lecture! I've been working through the worksheet but I can't understand how to go about solving the question below. Any help will be much appreciated.

5) The following two transformations occur in glycolysis:

gloucose + Pi ---> gloucose-6-P + H20 ( ∆D° = +17.1 kL/mol)
PEP + H20 ---> pyruvate + Pi ( ∆D° = -55.2 kL/mol)

In glycolysis, both reactions are coupled to either ATP snthesis or hydrolysis:

ATP + H20 ---> ADP + Pi ( ∆D° = -30.5 kJ/mol)

Write down reactions for the coupled reactions and calculate their equilibrium constants at 298 K.
#2
10-29-2010, 12:32 PM
Pipette Filler

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Re: Thermodynamics: Calculating equilibrium constants?

The first reaction is ATP + glucose --> glucose-6-Pi + ADP. The deltaG (I assume your deltaD is in fact deltaG?) for this reaction would be -13.5 kJ/mol (again, I assume your notation was a mistake? kilolitres per mole would be unusual to say the least...). This was simply calculated as the sum of the deltaGs of the two reactions, as they both proceed in the same direction. Now you use the formula deltaG = RTlnK to calculate the equilibrium constants. R is the gas constant, T is absolute temperature and lnK is the natural logarithm of the equilibrium constant.
Use the same process for the other reaction, but note that here you are producing ATP from PEP and ADP. The deltaG is therefore -24.7 kJ/mol. You sum up -55.2 kJ/mol for the PEP reaction and +30.5 kJ/mol for the ATP reaction, because you are going in the reverse direction. Hope this helps.

 Tags calculating , constants , equilibrium , thermodynamics

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