To Do 5th HW assignment due Friday by 10 pm. Begin reading Chapter 7 and doing the homework: we will start Chapter 7 on Tuesday.

Determining Hrxn Since H = qp, determine the heat transferred at constant pressure. This is termed calorimetry. Since energy is conserved, we can often determine Hrxn for one reaction given values of Hrxn for others. Uses the principle in Hess’s Law. Use standard enthalpies of formations for reactants and products (we have tables of these).

A Coffee-Cup Calorimeter Made of Two Styrofoam Cups

Clicker Question You drop 50.0 g of 100.0C metal into 100.0 g of 22.0C water. The final temperature of the water is 29.5C. Identify the metal. c(water) = 4.18 J/g C a) aluminum (c=0.89 J/gC) b) iron (c=0.45 J/gC) c) copper (c=0.39 J/gC) d) lead (c=0.13 J/gC) e) I do not know how to do this.

Constant-Pressure Calorimetry Can also determine enthalpy change for chemical reactions that occur in solution. See Example 6.5 on page 256 of your text. In Chemistry 103 you will also determine enthalpy change of a salt dissolving (Lab 7), enthalpy changes for phase changes (Lab 8), and explore Hess’s law (Lab 9).

Bomb Calorimeter: Constant V

Hess’s Law Due to first law and H being a state function. If a reaction is reversed, the sign of H is also reversed The magnitude of ΔH is directly proportional to the quantities of reactants and products in a reaction. That is, if the coefficients in a balanced equation are multiplied by an integer, the value of H is multiplied by the same integer.

Hess’s Law

Clicker Question Given the following data 2NH3(g)  N2(g) + 3H2(g) H = 92kJ 2H2(g) + O2(g)  2H2O(g) H = -484kJ Determine ΔH for 2N2(g) + 6H2O(g)  3O2(g) + 4NH3(g) a) -1268 kJ b) 1268 kJ c) 1636 kJ d) -1636 kJ e) I did not/cannot get any of these answers.

Determining Hrxn Since H = qp, determine the heat transferred at constant pressure. This is termed calorimetry. Since energy is conserved, we can often determine Hrxn for one reaction given values of Hrxn for others. Uses the principle in Hess’s Law. Use standard enthalpies of formations for reactants and products (we have tables of these).

Standard Enthalpy of Formation: Hf° Change in enthalpy that accompanies the formation of 1 mole of a compound from its elements with all substances in their standard states. Example: H2(g) + (1/2)O2(g)  H2O(g) Hf° = -242kJ

Pathway for the Combustion of Methane

Schematic Diagram of Energy Changes