why benzoic acid and acetanilide recrystallize from water? And why cinnamic acid and naphtalene dont?

Question

Is there any criterion to determine the ability for the recrystallization formation?

Answer 1

Because benzoic acid and acetanilide are insoluble or moderately soluble in water at room temperature but soluble in water at high temperature (hot water).

Cinnamic acid and naphtalene are both can be soluble in cold water, hence there's no need to crystallizes them anymore.

Since that, water can be considered as a good solvent for recrystallization for benzoic acid and acetanilide.

Good Luck

Answer 2

You need to set up an ICE chart (I = initial, C = change, E = equilibrium) for a weak acid. HB = cinnamic acid On the top of the chart, you will have HB --> H+ + B- The initial amount of HB is 1.6*10^-3 The change for HB is -x, while the change for H+ and B- is +x The equilibrium amount for HB is 1.6*10^-3 - x The equilibrium amount for H+ is x. Ka = [H+]^2/[HB] So Ka = x^2/(1.6*10^-3 - x) Because x is such a small number, it can be removed from the denominator without causing much disturbance. So you are left with Ka = x^2/1.6*10^-3 After plugging in Ka and solving for x, you get: x = 2.43*10^-4 = [H+] -log ([H+]) = pH pH = 3.61

Answer 3

The solute has to be much more soluble in hot water than cold water. This is more likely to happen for a big molecule that can still hydrogen bond with water.