Catalyst Given the following equations and H o values, determine the heat of reaction (kJ) at 298 K for the reaction: B 2 H 6 (g) + 6 Cl 2 (g)  2 BCl 3 (g) + 6 HCl(g) BCl 3 (g) + 3 H 2 O(l)  H 3 BO 3 (g) + 3 HCl(g) Δ H = kJ B 2 H 6 (g) + 6 H 2 O(l)  2 H 3 BO 3 (s) + 6 H 2 (g Δ H = kJ 1/2 H 2 (g) + 1/2 Cl 2 (g)  HCl(g) Δ H = kJ

Lecture 5.3 – Enthalpies of Formation and Bond Enthalpies

Today’s Learning Targets LT 5.6 – For a given chemical bond, I can characterize and calculate its bond enthalpy. Furthermore, I can apply this value to the type and length of bond that the molecule contains. LT 5.8 – For a phase change, I can characterize the enthalpy of formation and what this value indicates about a given phase change. LT 5.9 – I can apply Hess’s Law to a chemical reaction and calculate the enthalpy for a combination of chemical reactions.

Enthalpies of Formation ( Δ H f ) The enthalpy change for creating a compound from its constituent elements is known Δ H of formation ( Δ H f ) 2 C (s)+ H 2 (g)  C 2 H 2 Δ H f =226.7 kJ/mol The value of Δ H f depends on temperature, pressure, and state of products/reactants

Standard Enthalpies In order to compare Δ H f of different reactions, we define standard state of 1 atm and 298 K Don’t confuse with STP! (1 atm and 273 K) Standard Enthalpies ( Δ H o ) are Δ H values calculated under these standard conditions Δ H f o is the change in enthalpy to produce one mole of the compound from the constituent elements All elements that are stable at room temperature have a Δ H f o of 0 kJ/mol

Class Example Write the equation corresponding to the standard enthalpy of formation of liquid carbon tetrachloride (CCl 4 ).

Table Talk Write the equation corresponding to the standard enthalpy of formation of gaseous propane (C 3 H 8 ).

Using Δ H f to calculate Δ H rxn We can use known Δ H f o and Hess’s Law to calculate the Δ H rxn o for a given reaction of interest. For the combustion of propane gas, C 3 H 8 (g), to CO 2 (g) and H 2 O (l) under standard conditions: C 3 H 8 (g) + 5 O 2 (g)  3 CO 2 (g) + 4H 2 O (l) Δ H rxn o = ? You know that: 3 C (s) + 4H 2 (g)  C 3 H 8 (g) Δ H f o = kJ/mol C (s) + O 2 (g)  CO 2 (g) Δ H f o = kJ/mol H 2 (g) + ½ O 2 (g)  H 2 O (l) Δ H f o = kJ/mol

Shortcut for Using Δ H f to calculate Δ H rxn We can quickly calculate the Δ H rxn by using the equation: Note that n and m represent the coefficient from the balanced chemical reaction.

Class Example Calculate the standard enthalpy change for the combustion of 1 mole of benzene (C 6 H 6 ) to CO 2 (g) and H 2 O (l). Note: Δ H f o (CO 2 ) = kJ/mol Δ H f o (C 6 H 6 ) = 49.0 kJ/mol Δ H f o (H 2 O) = kJ/mol

Table Talk Calculate the standard enthalpy change for the combustion of 1 mole of ethanol (C 2 H 5 OH) to CO 2 (g) and H 2 O (l). Note: Δ H f o (CO 2 ) = kJ/mol Δ H f o (C 2 H 5 OH) = kJ/mol Δ H f o (H 2 O) = kJ/mol

Bond Enthalpies and Δ H rxn We can use bond enthalpies to estimate the Δ H rxn where certain bonds are broken and new bonds are formed. Allows for a quick determination of exo- or endothermic NOTE – This is an estimate This is an application of Hess’s Law Assume all bonds are broken and then all bonds reform:

Class Example You run the reaction: 2 C 2 H 6 (g) + 7 O 2 (g)  4 CO 2 (g) + 6 H 2 O Using the following Δ H values, calculate Δ H rxn for the reaction and determine if it is endothermic or exothermic: Δ H C – H = 413 kJ/mol Δ H C – C = 348 kJ/mol Δ H O=O = 495 kJ/mol Δ H C =O = 799 kJ/mol Δ H H– O = 463 kJ/mol

Table Talk You run the reaction: 2 N 2 H 4 (g)  N 2 (g) + 2 H 2 Using the following Δ H values, calculate Δ H rxn for the reaction and determine if it is endothermic or exothermic: Δ H N – H = 391 kJ/mol Δ H N – N = 163 kJ/mol Δ H N2 = 941 kJ/mol Δ H H-H = 436 kJ/mol

Problem Set 5.3 This is a sampling of free response type questions that have been observed on past AP exams This will be due next Monday/Tuesday!

Rate Yourself We have finished almost all of chapter 5! Rate yourself 1 – 4 on LTs 5.1 to 5.9. It is your responsibility to study any areas that are a 2 or lower over the long weekend

Happy Thanksgiving!

Closing Time Read 5.7, 8.8 Do book problems: