Question

1. What is the reason why activites are often called effective concentrations? Explain why there are differences between activities and concentrations.

2. Methane dissociates into H2(g) and C(s, graphite).

- Calculate the equilibrium constant K at 298 K.

- Calculate K at 50°C (assuming that ΔrH^Θ is temperature independent.

- Calculate α (degree of dissociation of methane) at 25°C and a total pressure of 0.010 bar.

3. What is the value of ΔrG for the reaction

NO2(g) ⇔ N2O4(g) when

Q = 0.1

Q = 1.0

Q = 10.0

Q= 100.0

Estimate (by interpolation) the value of K from the values you calculated.

Compare with the value obtained from the thermodynamic definition of the equilibrium constant.

Answer 1

1. In chemical thermodynamics, activity (symbol a) is a measure of the “effective concentration” of a species in a mixture, in the sense that the species' chemical potential depends on the activity of a real solution in the same way that it would depend on concentration for an ideal solution.Concentration can be related to activity using the activity coefficient γ, where [a] = γ (c) Until now we have assumed that activity, a, is equal to concentration, c, by setting γ = 1 when dealing with dilute aqueous solutions. As Ion-ion and ion-H2O interactions (hydration shell) cause number of ions available to react chemically ("free" ions) to be less than the number present.



2. CH4(g )= C (s ) + H2 (g )

k = (pH2)². (1)/ (pCH4)

We have seen that
and
Thus we can write
or	Therefore, if ΔrH^Θ is independent of T, then the value of K becomes independent of K, thus K remains the same. Degree of dissociation (DOD) Degree of dissociation is the fraction of a mole of the reactant that underwent dissociation. It is represented by α. Number of moles dissociated It is defined as the product of the initial concentration of the reactant and the degree of dissociation Now suppose you have a reaction like this A⟶B+C The initial state of A is always the concentration of A (should be given in the question) while initial moles of B and C are zero (if anything else is not specified). The final state of A is always defined as (number of moles initially present) - (Number of moles dissociated) while for B and C it isjust ( number of moles of A dissociated) Writing our equation again, A--------------> B + C Initial moles a 0 0 Final moles a - a*(DOD) a*(DOD) a*(DOD)