For reactions involving a solid and a liquid, increasing the solid's surface area will

• A. Increase rate.
• B. Decrease rate.
• C. No effect.
• D. Change equilibrium constant.

Answer: (A)

The key idea is that the rate of a solid–liquid reaction depends on how much surface the solid presents to the liquid. More exposed surface means more sites where the liquid molecules can collide and react, so increasing the solid’s surface area provides more opportunities for reaction per unit time and speeds up the process. Crushing a solid to make it finer, for instance, increases surface area and thus increases the rate. The equilibrium constant, however, is determined by thermodynamics at a given temperature and does not change with surface area. So while the rate goes up with greater surface area, the position of equilibrium stays the same. If diffusion away from the surface or another step becomes the slowest part, that can limit the extent to which the rate increases, but in general higher surface area raises the rate for solid–liquid reactions.