# A student conducting a calorimetry investigation determines a negative #DeltaH#. What does the negative value indicate about the reaction?

##### 2 Answers

We don't know. Not unless it is specified what kind of

- If
#q_(sol n)# is**negative**, i.e. the heat flow and enthalpy for the solution was**exothermic**,#DeltaH_(rxn) > 0# , so the reaction was**endothermic**and heat was*released*from the solution (#DeltaT < 0# , solution gets colder). - If
#q_(sol n)# is**positive**, i.e. the heat flow and enthalpy for the solution was**endothermic**,#DeltaH_(rxn) < 0# , so the reaction was**exothermic**and heat was*absorbed*into the solution (#DeltaT > 0# , solution heats up).

A **calorimeter** experiment in general is when one measures the change in temperature of a system due to a chemical process occurring in (usually) a solution.

I assume you mean a *constant-pressure* scenario, i.e. a coffee-cup calorimeter open to the atmosphere, as heat flow **ONLY** at constant pressure.

Upon determining the change in temperature, and knowing the specific heat capacity of the solvent, one could then calculate the heat involved in the process (with respect to the solution):

#q_(sol n) = mC_PDeltaT# ,where:

#m# is themassof the solution in#"g"# . One can measure the initial solution volume (before the process occurs) and assume a density equal to that of the pure solvent, whatever it may be. That allows for the determination of this mass.#C_P# is thespecific heat capacityat CONSTANT PRESSURE. For water at ordinary temperatures, it is about#"4.184 J/g"^@ "C"# .#DeltaT = T_2 - T_1# is thechange in temperaturein#""^@ "C"# (because#C_P# used#""^@ "C"# ), sign included.

Note that ** solution**, specifically, i.e. it is the heat transferred between the system (the solution) and its surroundings (the object in the solution and everything else).

If you wish to know the molar enthalpy of ** reaction**,

#q_(rxn) + q_(sol n) = 0#

Thus,

#color(blue)(DeltabarH_(rxn) = -q_(sol n)/(n_"object"))# where

#n_"object"# is the mols of whatever you put into the solution, whether it was a solute, or the limiting reactant of a chemical reaction.

Thus...

- If
#q_(sol n)# is**negative**, i.e. the heat flow and enthalpy for the solution was**exothermic**,#DeltaH_(rxn) > 0# , so the reaction was**endothermic**and heat was*released*from the solution (#DeltaT < 0# , solution gets colder). - If
#q_(sol n)# is**positive**, i.e. the heat flow and enthalpy for the solution was**endothermic**,#DeltaH_(rxn) < 0# , so the reaction was**exothermic**and heat was*absorbed*into the solution (#DeltaT > 0# , solution heats up).

That the reaction was

#### Explanation:

By convention we report an enthalpy change that RELEASES energy as negative. To give an actual example, we know that when we burn methane we produce heat, which we use to cook our breakfast or warm our homes........i.e.

We can certainly measure and report the