Exothermic and Endothermic Reactions

The sign of ΔH tells us whether heat is released (negative ΔH) or absorbed (positive ΔH). We say that a reaction which releases heat is exothermic and one which absorbs heat is endothermic. Thus in the above reaction, 565.98 kJ is released when 2 mol of gaseous CO2 at 1 atm, 25°C, is formed in the exothermic reaction of 2 mol of CO (g) and 1 mol of O2(g), both at 1 atm, 25°C.