Question

The two gases${\mathbf{\text{BF}}}_{\mathbf{3}}\mathbf{\left(}\mathit{g}\mathbf{\right)}$and${\mathbf{\text{BCl}}}_{\mathbf{3}}\mathbf{\left(}\mathit{g}\mathbf{\right)}$are mixed in equal molar amounts. All$\mathit{B}\mathbf{-}\mathit{F}$ bonds have about the same bond enthalpy, as do all$\mathbf{\text{B}}\mathbf{-}\mathbf{\text{Cl}}$bonds. Explain why the mixture tends to react to form${\mathbf{\text{BF}}}_{\mathbf{2}}\mathbf{\text{Cl}}\mathbf{\left(}\mathbf{\text{g}}\mathbf{\right)}$and${\mathbf{\text{BCl}}}_{\mathbf{2}}\mathbf{}\mathbf{\text{F}}\mathbf{\left(}\mathit{g}\mathbf{\right)}$.

Short answer

The change in enthalpy does not make much difference, the driving force has to be the tendency to maximize the randomness. So, that is why the mixture tends to react to form ${\text{BF}}_2\text{Cl\hspace{0.17em}}\left(\text{g}\right)$and ${\text{BCl}}_2\text{F}\left(g\right)$

Step-by-step solution

Given information

The two gases ${\text{BF}}_3\left(g\right)$and $2{\text{BCl}}_3\left(g\right)$are mixed in equal molar amount. There is little difference between the enthalpy of $\mathrm{B}-F$bond and that of $\text{B}-\text{Cl}$bond.

Concept of driving force of reactions

Enthalpy and entropy are the two primary thermodynamic factors that drive chemical reactions when the change in enthalpy is negative and the change in entropy is positive. If these conditions are met, the reaction is said to as spontaneous.

Theory of spontaneous reactions

Theexhibitedresponseisunprompted.

Therearetwomaincausesofspontaneousreactions:

1.Toachievealowerenergystate(decreasingenthalpy)

2.Toachievethehighestlevelofrandomness(increasingentropy)

The urge to maximise randomness must be the driving factor because a change in enthalpy does not make much of a difference.

Inotherwords,theprimarydrivingforcebehindthisreactionisthepropensityto increaseentropy.