caliometry question

Question

In a coffee-cup calorimeter, 60.0 g of H2O and 40.0 mL of HCl are mixed. The HCl had an initial temperature of 49.5 oC and the water was originally at 24.0 oC. After the reaction, the temperature of both substances is 33.1 oC. Was the reaction exothermic or endothermic?

Answer 1

To start, one notes that 60g of water is about 60 mL of water.

Then, one approach is to compute the loss of thermal energy of the water (due to cooling) and the gain in thermal energy of the HCl (due to warming).

For the water the loss is C 60 ( 49.5 -33.1) = 984 C where C is the heat capacity of water (J/mL/degC)

For the HCl the gain is C' 40 (33.1 - 24.) = 364 C' where C' is the heat capacity of HCl (J/mL/diegC)

If the HCl is not too concentrated, C' is not much different than C

Consequently, the system has a loss of thermal energy approximately equal to (984 - 364) C = 620 C

This energy had to go somewhere. The conclusion is that the reaction is endothermic. The 620 C was the energy supplied to make the reaction go.

This conclusion is a bit odd, because dilution of an acid is normally exothermic.