[hvard78]

10.0 of substance x is dissolved in 100.0 g of water in a calorimeter with a heat capacity of 127 J/C. The temperature changes from 20.0 C to 3.1 C. Is the salvation process exothermic or endothermic? How much thermal energy per gram of substance X is released (absorbed) by the process?


I know this reaction is exothermic because the change in temperature (T_f-T_i) is negative.

so the equation is (tell me if I'm wrong, thank you!)
q_calorimeter + q_water + q_process= 0
so I know q_water= ms :delta:T= 100.0g(4.184 J/gC)(3.1-20.0)

But...now I am stuck. Can someone please help me? Thanks a lot!

[opti384]

I know this reaction is exothermic because the change in temperature (Tf-Ti) is negative

Are you sure?

[hvard78]

If the temperature went from a higher temperature to a lower temperature, isn't the change in temperature negative?

[opti384]

This reaction will be an endothermic reaction because it absorbed heat from the surrounding and therefore the temperature decreased.

[KyleH]

to elaborate on opti's answer:  The surroundings (water and whatever else you had in your total Q equation) had a negative change in energy indicating that it's an exothermic reaction like initially thought.  However, something had to of 'taken' (absorbed) that energy from the water.  This would have ended up being the substance, making the reaction actually endothermic.

Remember that:                        Qsurroundings = -Qsystem

In other words, when one things loses energy (Q), then another must have gained the energy.  Since you solved for Q and got a negative answer for the surroundings, that means that the systems actually ended up absorbing that energy. 

I hope this helped