Sign of ΔHrxn

• When thermal energy flows out of the reaction and into the surroundings (as in an exothermic reaction), then ΔHrxn is negative.
• The enthalpy of reaction for the combustion of CH4, the main component in natural gas, is as follows:

CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(g) ΔHrxn = –802.3 kJ

• This reaction is exothermic and therefore has a negative enthalpy of reaction.
• The magnitude of ΔHrxn tells us that 802.3 kJ of heat are emitted when 1 mol CH4 reacts with 2 mol O2.