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The tube filled with NO2 will produce a white solid, while the the tube with the mixture of NO and NO2 will produce a blue liquid.
The white solid is N2O4. The blue liquid is N2O3. The products are different because of various factors, including the available starting materials.
Based on the observation that N2O4 is not produced in the tube containing both NO and NO2, it can be inferred that N2O3 is a more favorable product than N2O4 under these conditions. In fact, the production of N2O3 from NO and NO2has an overall enthalpy change of only 39.71 kJ/mol, while the production of the dimer (from the Greek, "two parts") N2O4from two NO2 molecules has an enthalpy change of 57.20 kJ/mol. The more negative an enthalpy of a reaction is, the easier it is to perform the reaction (thermodynamically). In addition to this, any small quantities of N2O4 and N2O2 produced would be almost invisible (N2O4 is white and N2O2 is colorless).
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