CHEMISTRY 161 Chapter 6

Thermochemistry Enthalpy of Reaction heat released or absorbed by the system at a constant pressure  H = H products - H reactants H products > H reactants :  H > 0 ENDOTHERMIC H products < H reactants :  H < 0 EXOTHERMIC

Measurement of Heat Changes  H = ΔQ ∞ ΔT heat Surrounding System temperature increase (pressure is constant)

 H = ΔQ ∞ ΔT  H = ΔQ = const × ΔT  H m =ΔQ m = c mp ΔT molar enthalpy change temperature change molar specific heat capacity  H m = ΔQ m = c mp ΔT

molar specific heat capacity capability of substances to store heat and energy  c mp ] = J mol -1 K -1 the J necessary to increase the temperature of 1 mol of a compound by 1 K c mp (H 2 O) = 75.3 J mol -1 K -1

 H m = ΔQ m = c mp ΔT 1.prepare two styrofoam cups 2. carry out chemical reaction in a compound with known c mp c mp (H 2 O) = 75.3 J mol -1 K measure temperature change 4. determine ΔH m calorimeter

 H m = ΔQ m = c mp ΔT The Real Experiment  H = ΔQ = (c mp × n) ΔT / × n  H = ΔQ = (c mp × n + c cup ) ΔT

100 ml of 0.5 M HCl is mixed with 100 ml 0.5 M NaOH in a constant pressure calorimeter (c cup = 335 J K -1 ). The initial temperature of the HCl and NaOH solutions are 22.5C, and the final temperature of the solution is 24.9C. Calculate the molar heat of neutralization assuming the specific heat of the solution is the same as for water. c mp (H 2 O) = 75.3 J mol -1 K -1 1.Neutralization reactions 2.Redox reactions 3. Precipitation reactions

Where does the ‘heat’ go?

Constant Volume Calorimeter ΔQ = (c mv × n + c bomb ) ΔT c mv (H 2 O) ≈ c mp (H 2 O)

Constant Volume Calorimeter  H = ΔQ = c p ΔT ΔE = ΔQ + ΔW ΔQ = c v ΔT ΔW = - p ΔV

 E = ΔQ + Δw CHANGING THE INTERNAL ENERGY C 8 H 18 (l) + (25/2) O 2 (g) 8 CO 2 (g) + 9H 2 O(g) E lost as HEAT and WORK E initial E final E lost as HEAT ENERGY

CHANGING THE INTERNAL ENERGY different combinations of q and w INTERNAL ENERGY, E INITIAL STATE FINAL STATE Q  0 w = 0 Q  0 w  0  E = ΔQ + Δw

SUMMARY measurement of heat changes  H m = ΔQ m = c mp ΔT ΔE = ΔQ + ΔW c mp (H 2 O) = 75.3 J mol -1 K -1

Homework Chapter 6, p problems