Final answer:
In chemistry, the concept of work done by a system involves gas expansion wherein a reaction results in more moles of gas than were initially present. Reactions (a) COCl₂(g) → Co(g) + Cl₂(g), (d) Ca₃P₂(s) + 6 H₂O(l) → 3 Ca(OH)₂(s) + 2 PH₃(g), (e) 2 CH₃OH(l) + 3 O₂(g) → 2 CO₂(g) + 4 H₂O(l), and (f) I₂(s) → I₂(g), indicate work done by the system on the surroundings.
Step-by-step explanation:
In chemistry, when we discuss work being done by a system on the surroundings, we're often referring to reactions that result in the expansion of gases, because work is a product of pressure and volume change. Let's evaluate the provided reactions:
(a) COCl₂(g) → Co(g) + Cl₂(g): This reaction produces more moles of gas than are consumed, indicating expansion and work done by the system.
(b) N₂(g) → N₂(l): Here, a gas condenses into a liquid, which is a reduction in volume and thus no work is done on the surroundings.
(c) CO(g) + H₂O(g) → H₂(g) + CO₂(g): There's no change in the number of moles of gas, so no net work is done.
(d) Ca₃P₂(s) + 6 H₂O(l) → 3 Ca(OH)₂(s) + 2 PH₃(g): Starting with solids and liquids, and ending with a gas, indicates work done by the system due to gas formation.
(e) 2 CH₃OH(l) + 3 O₂(g) → 2 CO₂(g) + 4 H₂O(l): This combustion process leads to the formation of more moles of gas than are consumed, meaning work is done by the system on the surroundings.
(f) I₂(s) → I₂(g): The sublimation of solid iodine into gaseous iodine results in work done by the system because of the expansion.
From these options, reactions (a), (d), (e), and (f) involve work being done by the system on the surroundings.