Thermochemistry 2 Hesss Law Heat of Formation Heat

Thermochemistry 2 Hess’s Law Heat of Formation Heat of Combustion Bond Enthalpy

Hess’s Law Want: C(graphite) C(diamond) Given: C(graphite) + O 2 CO 2 C (diamond) + O 2 CO 2 H = ? H = -394 KJ H = -396 KJ

Hess’s Law Want: 2 B(s) + 3 H 2 (g) B 2 O 3 (g) Given: H = ? 2 B (s) + 3/2 O 2 (g) B 2 O 3 (s) H = -1273 KJ B 2 H 6 (g) + 3 O 2 (g) B 2 O 3 (s)+ 3 H 2 O (g) H = -2035 KJ H 2 (g) + ½ O 2 (g) H 2 O (l) H = -286 KJ H 2 O(l) H 2 O(g) H = 44 KJ

Standard Enthalpy of Formation ( Hf) • Standard Enthalpy of ______ - enthalpy change when one mole of a substance is formed from its elements, in their standard states, under standard conditions.

Standard Enthalpy of Formation ( Hf) • Write equations to represent the following processes. • The standard enthalpy of formation of CH 3 Br • The standard enthalpy of formation of CH 3 COC 2 H 5 • The standard enthalpy of formation of Ca. CO 3

Standard Enthalpy of Formation ( Hf) • What is the Hf for Cu. S (s)? § Look up in Appendix • What is the Hf for 2 moles of Fe. Cl 3 (s)? • Remember, these are heats of FORMATION which means that the compound is being produced. If it on the reactant side, you must flip the sign!

Standard Enthalpy of Formation ( Hc) • Standard Enthalpy of ______ - enthalpy • change when one mole of a substance is completely burned in oxygen under standard conditions. Energy is usually released in such a reaction Hc, so it will usually be negative.

Standard Enthalpy of Formation ( Hc) • Write the reaction for the following… • Hc [C 2 H 6(g)] = - 1565 k. J/mol • The standard enthalpy of combustion of CH 4 (g) • The standard enthalpy of combustion of Al(s)

Example 1 (do not use Appendix) • Calculate the standard enthalpy of • • • formation of ethane (C 2 H 6), given the following combustion data… C(graphite) = -393 k. J/mol H 2(g) = -286 k. J/mol C 2 H 6(g) = -1560 k. J/mol

Example 2 (do not use appendix) • Calculate the standard enthalpy of • • combustion of propan-2 -ol (CH 3 CH(OH)CH 3), given the following data Enthalpies of combustion for C(graphite) = -393 k. J/mol and H 2(g) = -286 k. J/mol. Enthalpy of formation of propan-2 -ol = -318 k. J/mol

Examples • Calculate the H for the following reaction using the appendix • C 3 H 8 (g) + 5 O 2 (g) 3 CO 2 (g) + 4 H 2 O (l) • Is the reaction endothermic or exothermic? Why?

Examples • How much heat will be released from the combustion of 1. 80 g of C 6 H 6. Use the heat of formation data in the appendix.

Example • Thermite reaction can be used to • • produce molten iron for welding railway tracks together. Fe 2 O 3(s) + 2 Al(s) Al 2 O 3(s) + 2 Fe(s) Calculate the enthalpy change in the Thermite reaction, given the standard enthalpies of formation of iron (III) oxide and aluminum oxide are -823 and -1675 k. Jmol-1, respectively.

Bond Enthalpies • The strength of the bond in a diatomic covalent molecule is given by the bond dissociation energy. • For example hydrogen, H 2 or H-H • H 2(g) 2 H(g) BDE= +436 k. J

Bond Enthalpies • In order to ______ a bond, energy must be put in (______ process) • When ______ a bond, energy is released (______ process).

Bond Enthalpy Example 1 • Calculate the standard enthalpy of the • reaction below. CH 3 CH=CH 2 + H 2 CH 3 CH 2 CH 3

Bond Energy Data

Bond Enthalpy Example 2 • Calculate the enthalpy change for the • reaction below. CH 3 CH=CH 2 + Br 2 CH 2 Br. CH 3

Bond Energy Data

Phase Diagrams • An area on a phase diagram represents one ______ , a line represents the conditions under which two phases can exist in ______. • The ______ point describes the conditions under which all three phases can coexist. • The ______ point describes the maximum temperature that a liquid of the substance can exist. § Above this temperature the difference between the liquid and the gas disappear and the substance is referred to as a ______.

Phase Diagrams

Heating Curve