endothermic reaction

This is of course NOT. See this equation.
4O2(g) + 5 NH3 (g) →4NO2(g) + 6H2O(g) This is a step for the production of nitric acid when red hot platinum is used . In this equation, the no of moles of products are greater than that of no of moles of reactants. However, this reaction is extremely exothermic not endothermic. To determine whether exo or endo, you should determine the enthalpy change during the bond breaking and bond forming.
If the energy required to break the bond is greater than that of energy released after the bond is formed. The reaction is endothermic.
If the energy required to break the bond is smaller than that of energy released after the bond is formed. The reaction is exothermic.
I hope you will find my writing helpful.

我諗你指ge 係cracking, 佢係屬於endothermic reaction
而且, 佢ge molecule ge 數目的確係多左

不過, 多左 molecule 未必代表個reaction就係endothermic
因為例如 neutralization:

NaOH + HCl --> NaCl + H2O

原先得 Na+ OH- H+ 同 Cl- 四種ion
但係之後得番Na+ 同 Cl- 兩種ion 同一個H2O molecule
由之前無molecule 變到有一個 molecule
molecule 係多左
但係呢個係exthermic reaction

一般黎講,
endothermic ge chemical reaction 有 cracking 同thermal decomposition