Problem points$).$ Interfacial reactions.

Fused silica $($amorphous $Si{O}_2)$ is dissolved by hydrofluoric acid, following the reaction:

$Si{O}_{2(s)}+6HF(l)harr{H}_{2}Si{F}_{6(l)}+2H_2{O}_{(l)}$

You are attempting to determine whether the limiting reaction for this reaction is HF diffusion in aqueous

solution or interfacial reaction$($s$)$ at the $Si{O}_2$ surface.

$($a$)$ Sketch and contrast the expected $HF$ concentration profiles as a function of the distance to the

solid$/$liquid interface for the diffusion$-$controlled and the reaction$-$controlled cases.

$($b$)$ Dissolution rates are measured at various temperatures, and an activation energy is determined.

To what quantities would you compare this activation energy to determine whether the dissolution is

diffusion controlled?

$($c$)$ Additional experiments are then performed imposing forced convection of the HF in contact with

$Si{O}_2.$ The dissolution rate of $Si{O}_2$ is found to be nearly identical to that measured under the static

conditions used in $($b$).$ From this observation, which mechanism, diffusion or interface reaction, is most

likely controlling $Si{O}_2$ dissolution?