4.15 Solutions Manual Clarification

[Samantha Low 3D]

For Textbook problem 4.15, the Solutions Manual uses -4.18 J*g^-1*°C^-1 to plug into q=mc(T_f-T_i) to find T_f. Why do they use -4.18 instead of of +4.18 that we usually use?

Here's the problem for reference:

Hydrochloric acid oxidizes zinc metal in a reaction that produces hydrogen gas and chloride ions. A piece of zinc metal of mass 8.5 g is dropped into an apparatus containing 800.0 mL of 0.500 M HCl (aq). If the initial temperature of the hydrochloric acid solution is 25 °C, what is the final temperature of this solution? Assume that the density and molar heat capacity of the hydrochloric acid solution are the same as those of water and that all the heat is used to raise the temperature of the solution.

[Carolina 3E]

It's negative because they used the relationship qrxn = -qsol, so
-20000 = -(4.184)(800)deltaT.
You can also use -qrxn = qsol
-(-20000) = (4.184)(800)deltaT.