(i) How many sodium ions and chloride ions are present in a unit cell of sodium chloride ?
(ii) What is the co-ordination number of sodium and chloride ions in sodium chloride crystals ?
Give a reason for each of the following :
(i) Specific conductance decreases with dilution whereas equivalent conductance increases with dilution.
(ii) Lead is precipitated as PbS while zinc is not precipitated when H2S gas is passed through an acidic solution of lead nitrate and zinc nitrate.
(i) Specific conductance is the conductivity of all ions present in 1 cm3 of a solution. When the solution is diluted the number of ions present in 1cm3 of solution decrease and as a result specific conductance decreases. Equivalent conductance is the conducting power of all ions furnished by one gram equivalent of an electrolyte present in a definite volume of solution with dilution number of ions increase and equivalent conductance also increases.
(ii) Solubility product of PbS is lesser than that of ZnS. Precipitation occurs only when ionic product is more than solubility product. Number of sulphide ions produced by dissociation of H2S are lesser in acidic medium due to common ion effect. As a result only PbS is precipitated while ZnS is not precipitated.
(i) Enthalpy change and entropy change of a chemical reaction at 25°C are 177.0 KJ and 160.4 JK-1 respectively. Calculate free energy change of the reaction and predict whether the reaction is spontaneous or nonspontaneous.
(ii) 2 moles of an ideal gas at 27°C expand reversibly from 2 litres to 20 litres. Find the entropy change for the reaction.
(iii) State the second law of thermodynamics in terms of entropy of the universe.