The given reaction is the decarboxylation of sodium propanoate, which is a sodium salt of a carboxylic acid, to form an alkane, specifically ethane [1, 2]. This process involves heating the sodium salt with soda lime [1, 2]. Soda lime is a mixture of sodium hydroxide ($NaOH$) and calcium oxide ($CaO$) [2]. Therefore, the missing reagent in the given equation is $CaO$.