Combustion of gaseous ethene, C₂H₄(g), is carried out inside a cylinder fitted with a frictionless piston that can move against a constant external pressure of 3.00 atm. The reaction at 298 K is
C₂H₄(g) + 3 O₂(g) → 2 CO₂(g) + 2 H₂O(g) ΔH298=−1325kJ mol−1 for ethene.
Initially the cylinder contains 1.00 mol of C₂H₄(g) and 3.00 mol of O₂(g) occupying 25.0 L at 298 K. After the reaction goes to completion, the gaseous products (2.00 mol CO₂ and 2.00 mol H₂O) are also at 298 K. Assume ideal-gas behavior and ignore any condensed products.
(a) Calculate the pressure–volume work, w, associated with the reaction. Report your answer in kJ and include the sign.
(b) Determine the quantity of heat, q, exchanged between the reacting system and the surroundings. State its value in kJ with the appropriate sign.
(c) Using your answers to parts (a) and (b), calculate the change in internal energy, ΔU, for the reaction as carried out in the cylinder. Give the value in kJ and include the sign.
(d) Indicate whether the internal energy of the system increases or decreases and relate this change to the classification of the reaction as endothermic or exothermic.