Acetylsalicylic acid (hc9h7o4) is a weak acid that dissociates in water in the following manner. hc9h7o4(aq) + h2o(l) equilibrium reaction arrow c9h7o4−(aq) + h3o+(aq) at a temperature of 298.15 k, the acid-dissociation constant (ka) for acetylsalicylic acid is 3.3 ✕ 10−4. (a) calculate the change in standard free energy (δg°) for this equilibrium reaction. kj/mol (b) what is the value of δg at equilibrium? δg > 0 δg = 0 δg < 0 (c) if the reaction were spontaneous in the forward direction, what type of value would you expect for δg?

(a) -20 kJ/mol

(b) ΔG = 0

(c) A negative value

Explanation:

(a) Calculate the change in standard free energy (ΔG°) for this equilibrium reaction.

We can calculate the standard Gibbs free energy using the following expression:

ΔG° = R.T. lnKc

where,

R is the ideal gas constant

T is the absolute temperature

Kc is the equilibrium constant (in this case Kc = Ka because the reaction is an acid dissociation)

If we replace this expression with the given values, then:

ΔG° = R.T. lnKc

ΔG° =

(b) What is the value of ΔG at equilibrium?

Let us consider a more general expression for equilibrium and non-equilibrium situations:

ΔG = ΔG° + R.T. lnQ

where,

ΔG is Gibbs free energy in that situation

Q is the reaction quotient

At equilibrium, ΔG = 0, Q = Kc, and we get the first expression.

(c) If the reaction were spontaneous in the forward direction, what type of value would you expect for ΔG?

A negative ΔG implies that the forward reaction is spontaneous.

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