Need help understanding why the melting range would be lowered in each question?
ID: 543365 • Letter: N
Question
Need help understanding why the melting range would be lowered in each question? Please explain. Thank you What effect will a relatively small amount of a higher melting impurity (e.g. benzoic acid, mp 122 C) have upon the melting point of E,E-dibenzalacetone (mp of pure E.E-dibenzalacetone is 111-112 C)? Answer The melting point of the contaminated E.E-dibenzalacetone would be lowered than 111- 112"C with a melting range >2°C 2 of 15 | P a 2. If a 75 mg quantity of compound A (mp. 106-108°C) was thoroughly mixed with 15 mg of compound B (mp, 10 107) will the melting range of this mixture bes 2°C or > 2°C? Answer P2°C The melting point of the contaminated A would be lowered than 106-108°C with a melting range >2C oS-168Explanation / Answer
I believe you want to know why the melting point will be lowered, right? The question says that the melting range is increased due to the presence of impurity.
It is common knowledge that the melting point of an impure solid is lower than that of the pure solid. Most solids assume a crystalline geometry where the molecules or ions have an ordered arrangement. The crystal is held together by strong intermolecular forces operating between the molecules or the ions. Hence, a large amount of energy must be expended in order to break this ordered crystal and thus, a pure solid has a high melting point.
The presence of impurities break this ordered arrangement of the molecules or ions in a crystal. Consequently, lower energy is required to break the crystal structure and thus, an impure solid has a lower melting point.
As the melting point lowers, the range over which the impure solid melts increases due to imperfections in the crystal structure introduced by the impurity.