Representing Enthalpy Changes There are a number of different ways to express the change in the quantity of energy stored in a chemical system. Enthalpy changes, DH, can be expressed using thermochemical equations. For instance the complete combustion of 1.0 mol of methanol releases 726 kJ of heat energy. The balanced thermochemical equation is: CH3OH + 3/2 O2 ---> CO2 + 2H2O + 726 kJ

CH3OH + 3/2 O2 ---> CO2 + 2H2O + 726 kJ DHcombustion = -726 kJ/mol of CH3OH Notice the enthalpy change is negative since the quantity of energy stored in CO2 and H2O is less than the quantity of energy stored in CH3OH and O2.

For the thermochemical equation 2A + B + 120 kJ ---> 3C + 4D What is the DH / mol of A? 120 kJ / 2 mol A = 60 kJ / mol A What is the DH / mol of B? 120 kJ / 1 mol B = 120 kJ / mol B What is the DH / mol of C? 120 kJ / 3 mol C = 40 kJ / mol C What is the DH / mol of D? 120 kJ / 4 mol D = 30 kJ / mol D

For the thermochemical equation A + 3B ---> 4C + 2D + 240 kJ What is the DH / mol of A? -240 kJ / 1 mol A = -240 kJ / mol A What is the DH / mol of B? -240 kJ / 3 mol B = - 80 kJ / mol B What is the DH / mol of C? -240 kJ / 4 mol C = -60 kJ / mol C What is the DH / mol of D? -240 kJ / 2 mol D = -120 kJ / mol D

For the thermochemical equation HCl(aq) + NaOH(aq) ----> H2O(l) + NaCl(aq) + 45 kJ enthalpy change can also be expressed as the molar enthalpy outside the equation DHneutralization = - 45 kJ/mol of HCl or NaOH or H2O or NaCl enthalpy change can also be shown by a graph

For the thermochemical equation HCl(aq) + NaOH(aq) ----> H2O(l) + NaCl(aq) + 45 kJ Potential Energy Diagram for the Neutralization of HCl by NaOH HCl(aq) + NaOH(aq DH Ep Reaction Progress H2O(l) + NaCl(aq)

If the dissolving of 8. 0 g of NH4NO3(s) requires 4 If the dissolving of 8.0 g of NH4NO3(s) requires 4.0 kJ of heat write the thermochemical equation and draw a Potential energy curve. The molar mass of ammonium nitrate is 80 g. the n NH4NO3(s) is 0.10 mol molar enthalpy change is 4.0 kJ / 0.10 mol = 40 kJ / mol of NH4NO3(s) thermochemical equation is NH4NO3(s) + 40 kJ ---> NH4NO3(aq)

NH4NO3(s) + 40 kJ ---> NH4NO3(aq) Potential Energy Diagram for the dissolution of Ammonium Nitrate NH4NO3 (aq) DH Ep Reaction Progress NH4NO3 (s)

When sulfur dioxide reacts with oxygen sulfur trioxide forms When sulfur dioxide reacts with oxygen sulfur trioxide forms. If DHreaction is -198 kJ/mol of oxygen what quantity of heat is released when 256 g of SO2 is burned? Write the balanced equation. SO2(g) + O2 (g) ----> SO3 (g) 2 + 198 kJ Calculate the heat released per mole of SO2. 198 kJ / 2 mol SO2 = - 99 kJ / mol SO2 Multiply the kJ /mol SO2 by nSO2 n SO2 = 256 g / 64 g/mol = 4.0 mol (- 99 kJ / mol ) x (4.0 mol) = - 396 kJ

C4H9OH(l) + O2(g) ----> CO2(g) + H2O(l) 6 4 5 If the molar enthalpy of combustion of butanol is - 639.5 kJ, what quantity of heat energy is released when 7.4 g of butanol is burned? First write the balanced thermochemical equation for the combustion of butanol. C4H9OH(l) + O2(g) ----> CO2(g) + H2O(l) 6 4 5 + 639.5 kJ n C4H9OH(l) is 7.4 g / 74 g/mol = 0.10 mol - 639.5 kJ / mol x 0.10 mol = - 64 kJ

Potential Energy Diagram for the combustion of Glucose Draw a Potential energy Diagram for the combustion of glucose if the molar enthalpy is -2.8 MJ. Potential Energy Diagram for the combustion of Glucose C6H12O6(s) + 6O2(g) DH = -2.8 MJ Ep Reaction Progress 6CO2(g) + 6H2O(l)

Try questions pg. 319 # 1-5, pg. 320 # 1-4