As the freezing point depression equation states,
ΔTf = kf.m.i ,
where kf depends on the solvent, m depends on the concentration and i depends on the particles, the only parameter that changes between the two scenarios presented is i, since it relates to the amount of particles resulted in the dissolution of the solute. While a covalent compound, such as the acetic acid, remains as one particle when it dissolves (i = 1), ionic compounds such as KF dissolves into more particles. Therefore, the i for KF is greater than 1, resulting in a bigger change in freezing temperature.