question_answer

  What is meant by positive and negative deviations from Raoult's law and how is the sign of  related to positive and negative deviations from Raoult's law?                                                                                        [2]

Answer:

  An ideal solution consisting of two components A and B is one in which the intermediate attractions A...A, B...B and A...B are equal. For such cases                   In such cases, there is no deviation from Raoult's law If A..B attractions are smaller than the average A...A and B...B attractions then the vapour pressure of the mixture is larger than the expected value. For such cases  (endothermic) This is called positive deviation from Raoult's law.                                                                      [1] Example Ethanol and acetone form solution with positive deviation. If A...B attractions are greater than the average of A...A and B...B attraction, then vapour pressure of the mixture is less than the expected value. For such case  (exothermic) This is called negative deviation from Raoult's law. Example Acetone and chloroform mixture  [1] I Ideal II Positive deviation III Negative deviation