Why must a solution of naoh be standardized using a primary standard, such as khp?

Which of the following reactions is endothermic? h2(g) + ½ o2(g) h2o(g), h = -57.82 kcal ½n2(g) + o2(g) + 8.1 kcal no2(g) ½ n2(g) + 3/2 h2(g) nh3(g) + 11.0 kcal c(diamond) + o2(g) co2, h = -94.50 kcal