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
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_(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.
