Sign of ΔHrxn

• When thermal energy flows into the reaction and out of the surroundings (as in an endothermic reaction), then ΔHrxn is positive.
• The enthalpy of reaction for the reaction between nitrogen and oxygen gas to form nitrogen monoxide is as follows:

N2(g) + O2(g)  2 NO(g) ΔHrxn = +182.6 kJ

• This reaction is endothermic and therefore has a positive enthalpy of reaction.
• The magnitude of ΔHrxn tells us that 182.6 kJ of heat are absorbed from the surroundings when 1 mol N2 reacts with 1 mol O2.