A mixture of gaseous reactants is put into a cylinder, where a chemical reaction

Question

A mixture of gaseous reactants is put into a cylinder, where a chemical reaction turns them into gaseous products. The cylinder has a piston that moves in or out, as necessary, to keep a constant pressure on the mixture of 1 atm. The cylinder is also submerged in a large insulated water bath. (See sketch at right.) From previous experiments, this chemical reaction is known to absorb 307. kJ of energy. The temperature of the water bath is monitored, and it is determined from this data that 392. kJ of heat flows out of the system during the reaction.

Explanation / Answer

(i) Since the heat flows out of water bath that means the q is negative and it represents an exothermic reaction.

(ii) Heat flows out from the system to the surroundings (water bath and veseel) and hence the water bath temperature goes up.

(iii) Since the work, w is positive that means the work done on the system and hence the piston will move in.

(iv) W is positive it means the work is done on the gas system.

(v)

delta E = 307 kJ

q = -392 kJ

from first law of thermodynamics,

delta E = q + w

delta E - q = w

307 kJ - (-392 kJ) = w

307 kJ + 392 kJ = w

699 kJ = w

So, 699 kJ of work is done on the gas mixture.

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