What is the enthalpy change?

Question

Red phosphorus reacts with liquid bromine in an exothermic reaction, 2P(s)+3Br2​(l)→2PBr3​(g):Δr​Ho=−246 kJ. Calculate the enthalpy change when 7.03 g of phosphorus reacts with 31.2g liquid bromine?  

Answer 1

2P(s) + 3Br₂(l) ==> 2PBr₃(g) ... ∆Hº = -246 kJ

moles of P present = 7.03 g P x 1 mol P/31.0 g = 0.227 mol P

moles of Br₂ present = 31.2 g Br₂ x 1 mol Br2/160 g = 0.195 mol Br₂

Because P and Br₂ react in a 2:3 mol ratio (balanced eq.), the Br₂ is LIMITING

∆H = 0.195 mol Br2 x -246 kJ/3 mol Br2 = -47.97 kJ = -48.0 kJ (3 sig. figs)