Question

Gaseous BF3 and BCl3 are mixed in equal molar amounts. All B-F bonds have about the same bond enthalpy, as do all B-Cl bonds. Compare the numbers of microstates to explain why the mixture tends to react to form BF2Cl(g) and BCl2F(g

Answer 1

Solution :

$BF_3 (g) + BCl_3 (g) \rightarrow BF_2 Cl + BCl_F(g)$

Explanation 1 :

Spontaneity of the reaction is based on two factors :

-- the tendency to acquire a state of minimum energy

-- the energy of a system to acquire a maximum randomness.

Now, since there isn't much difference in the bond enthalpies of B-F and B-Cl. So, we can say the major driving factor is tendency to acquire a state of maximum randomness.

Explanation 2 :

A system containing the $\text{"chemically mixed"}$ B halides has a $\text{greater entropy}$ than a system of $BCl_3$ and $BF_3$.

It has the same number of $\text{gas phase molecules}$, but more distinguishable kinds of $\text{molecules}$, hence, more microstates and higher entropy.