exothermic or endothermic chemical reactions

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 1atm. The cylinder is also submerged in a large insulated water bath. (See sketch at right.)

The temperature of the water bath is monitored, and it is determined from this data that 

213.kJ of heat flows into the system during the reaction. The position of the piston is also monitored, and it is determined from this data that the system does 340.kJ of work on the piston during the reaction.

Is the reaction exothermic or endothermic?

Does the temperature of the water bath go up or down?

Does the piston move in or out?

Does the reaction absorb or release energy?

How much energy does the reaction absorb or release? Be sure your answer has the correct number of significant digits.

Answer 1

Is the reaction exothermic or endothermic?

Since heat flows INTO the system, the reaction is ENDOTHERMIC

Does the temperature of the water bath go up or down?

Since the reaction is endothermic, the temperature of the bath goes down

Does the piston move in or out?

Since the system is doing work (340 kJ) on the piston, the piston will move out.

Does the reaction absorb or release energy?

Since more heat is used to push the piston than the heat added, the reaction releases energy.

How much energy does the reaction absorb or release? Be sure your answer has the correct number of significant digits.

∆U = q + w

∆U = 213 kJ - 340 kJ = -127 kJ = heat released (3 sig.figs.)