Which of the following process when carried out at a constant pressure can have value of w to be positive ?

Sn + 2 F₂ --------- Sn F₄

PCl₅-------- PCl₃ + cl₂

C+ O₂-------CO₂

CaCO₃------------CaO + CO₂

Work done will be positive in the reaction,

Sn (s) + 2F₂ (g)--> SnF₄ (s)

1mol......2mol........1mol

Here, we see that the no. of moles of fluorine gas is decreased in the reaction in forming product and so the surrounding will have to work on the system which makes W , positive.