Question

In the reaction S ␣ P, if one mole of product (P) has the same free energy as one mole of substrate (S) , what is the value for the K of this reaction? What is the value of the G ? If you started with a 1M solution of S, would it spontaneously react to give any P? What would the concentrations of S and P be when the reaction had reached equilibrium?

.Consider the reaction: Glucose P ␣ glucose 6-phosphate H O i2 G 3kcal/mol What is the equilibrium constant, K eq, for this reaction? (Note: The concentration of water is ignored.) Does the positive G in the above reaction mean that the reaction can never go sponta- neously from left to right?

Consider the reaction: Glutamate ammonia ␣ glutamine H2O G 3.4 kcal /mol If the concentration of ammonia (NH3 ) is 10 mM, what is the ratio of glutamate/glutamine required for the reaction to proceed spontaneously from left to right at 25°C?

Answer 1

1)
The reaction (I can't use equilibrium arrows in this platform, sorry!):

If they both have G⁰, the change in G⁰ for the reaction is zero. Then, the relationship with K tells us that:

So:

K has a value of 1.

If we start with a 1M concentration of S, the reaction would be spontaneous, since:

Since [P]/[S] is less than 1, the ln is negative and the whole expression is negative, yielding a negative (spontaneous) delta G.

When the reaction raches equilibrium (delta G = 0), the concentrations would be:

So Q = 1, which means that [S] = [P].

2)

The same expression we used (I¿m assuming a temperature of 25°C)

It doeasn't mean it can't go spontaneously from left to right since, at conditions other than standard conditions, the expression:

Tells us that given a combination of concentrations that yields a R.T.ln(Q) more negative than 21552 J/mol, delta G is negative and the reaction, in those conditions, is spontaneous.

3. The reaction is:

Delta G is given by:

For the reaction to be in equilibrium, we need delta G = 0:

So, the ratio can be calculated as:

Any ratio lower than 3.2x10^-5 will result in a spontaneous reaction.