Negative deviation from Raoult’s law is observed in non-ideal solution whose vapour pressure is lower than that predicted by Raoult’s law.A mixture of phenol and aniline exhibit this type of deviation. Sign of ΔmixH for negative deviation is positive.
Explanation: When a solution does not obey Raoult’s law then it is said to be a non-ideal solution. When the vapor pressure of such a solution is low than that predicted by Raoult’s law then it is said to exhibit negative deviation from Raoult’s law. Examples are - mixture of phenol and aniline; mixture of chloroform and acetone. Take the example of mixture of chloroform and acetone, the attraction between chloroform and acetone is more than that between chloroform and chloroform or acetone and acetone molecules. The vapor pressure decreases as it is difficult for thses molecules to escape. Hence, negative deviation from Raoult’s law. ΔmixH is the enthalpy of mixing of pure components to form a solution. As in this case there is bond formed between two different molecule, energy is released and hence the sign is negative.
Azeotropic mixtures are binary mixtures which boil at a constant temperature and distil over without any change in composition at any given pressure. Maximum boiling azeotrope is formed by negative deviation from Raoult’s law. Mixture of 68% nitric acid and 32% water exhibit this type of azeotrope. Their boiling point is 393.5 K.
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