Generally all the reaction that take place on their on accord are exothermic by the second rule of thermodynamics. For example acid base neutralisation 1equvalance of acid when treated with 1eq of base (both strong) give a neutral salt and water and the heat of reaction is nearly equal to-52.6J. the heat of reaction will be different when the acid and base are not strong. (And yeah -ve value of heat of reaction means the reaction is exothermic)