Now consider the following set of reactions: Reaction A=2NO2 --- 2NO + O2, H=109 kJ/mol Reaction B=N2O4 --- 2NO + O2, H=172 kJ/mol The equations given in the problem introduction can be added together to give the following reaction: overall: N2O4 --- 2NO2 However, one of them must be reversed. Which one: Reaction A= 2NO2 --- 2NO + O2 or Reaction B= N2O4 --- 2NO + O2