What is the enthalpy change in kJ of a chemical reaction that raises the temperature of 250.0 mL of solution having a density of 1.25g/mL by 7.80 degree Celsius? (The specific heat of the solution is 3.74 J/g * K)

1 Answer
Truong-Son N.
May 1, 2018

#DeltaH_(rxn) = -"8.75 kJ"#, exothermic reaction

Heat flow at constant pressure is given by:

#q = mc_sDeltaT#,

where:

  • #m# is the mass of the solution in #"g"#.
  • #c_s# is the specific heat capacity of the solution in #"J/g"^@ "C"#.
  • #DeltaT = T_f - T_i# is the change in temperature in #""^@ "C"#. (In #"K"# the intervals are the same.)

The mass involved is:

#240.0 cancel"mL" xx "1.25 g"/cancel"mL" = "300.0 g soln"#

Therefore, the heat that is absorbed is:

#q_(sol n) = "300.0 g soln" cdot "3.74 J/g"^@ "C" cdot (7.80^@ "C")#

#=# #"8751.6 J"#

At constant pressure, #q = DeltaH#, so the enthalpy change of the surroundings (solution) is simply

#q_(sol n) = DeltaH_(sol n) = +"8.75 kJ"#

since heat was absorbed by the surroundings, hence increasing the thermal energy the surroundings (solution) have.

Therefore, by conservation of energy,

#color(blue)(DeltaH_(rxn) = -"8.75 kJ")#,

because the reaction releases heat from the reaction out towards the solution.

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