<p>In a flask colourless&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>N</mi><mn>2</mn></msub><msub><mi>O</mi><mn>4</mn></msub></math>&nbsp;is in equilibrium with brown coloured&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>N</mi><msub><mi>O</mi><mn>2</mn></msub><mo>.</mo><mo>&#160;</mo></math>&nbsp;At equilibrium when the flask is heated at&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML" class="wrs_chemistry"><msup><mn>100</mn><mo>&#176;</mo></msup></math>C, the brown colour deepens and on cooling it becomes less coloured. The change in enthalpy,&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML" class="wrs_chemistry"><mo>&#8710;</mo><mi mathvariant="normal">H</mi></math>&nbsp;for this system is:</p>