Consider the exothermic reaction 4nh3(g)+5o2(g)→4no(g)+6h2o(g) calculate the standard heat of reaction, or δh∘rxn, for this reaction using the given data. also consider that the standard enthalpy of the formation of elements in their pure form is considered to be zero.

The first thing you need to know about this problem is that delta total = delta Hf products - delta Hf reactants. 
Delta total = (4*NO + 6*H2O) - (5*O2 + 4*NH3) 
Delta total = [(4*91.3 kJ/mol) + (6*-241.8 kJ/mol.) - (5*0 kJ/mol.) + (4*-45.9 kJ/mol.) 
Products= 4*(365.2)+(-1450.8)=-1085.6 kJ 
Reactants=4*(-45.9) + 5*(0)= -183.6 kJ 
Delta Rxn= (-1085.6 kJ) - (-183.6 kJ) =-902.0 kJ