CHEMISTRY 161 Chapter 6 www.chem.hawaii.edu/Bil301/welcome.html

THERMODYNAMICS study of the energy associated with change HEAT CHANGE Physical Why does water evaporate? 2. Chemical Why do some molecules react and others do not?

study of the energy associated with chemical reactions THERMOCHEMISTRY study of the energy associated with chemical reactions 2 H2(g) + O2(g) → 2 H2O(l) + energy Hindenburg 1937 Challenger 1986

ENERGY 1. Kinetic energy 2. Radiant energy 3. Potential energy energy of a moving microscopic or macroscopic object E = ½ m u2 2. Radiant energy energy in form of photons (‘light’) (solar energy) E = h  (h = Planck’s constant) 3. Potential energy energy by changing object’s position in height E = m g h (h = height)

LAW OF CONSERVATION OF ENERGY 4. Thermal Energy energy associated with random motion of atoms and molecules Ekin = ½ M u2 = 3/2 R T 5. Chemical Energy energy stored in chemical bonds of substances LAW OF CONSERVATION OF ENERGY THE TOTAL ENERGY OF THE UNIVERSE IS CONSTANT

ENERGY vs. HEAT THERMOCHEMISTRY T2 T1 heat is spontaneous transfer of thermal energy two bodies at different temperatures T1 > T2 spontaneous T2 T1 THERMOCHEMISTRY study of the energy associated with chemical reactions study of the heat change in a chemical system

REFERENCE SYSTEM CLASSIFICATION

THE TOTAL ENERGY OF THE UNIVERSE IS CONSTANT SURROUNDINGS SYSTEM THE TOTAL ENERGY OF THE UNIVERSE IS CONSTANT

ENDOTHERMIC EXOTHERMIC 2 HgO(s) → O2(g) + 2 Hg(l) heat Surrounding System Surrounding heat System ENDOTHERMIC EXOTHERMIC 2 HgO(s) → O2(g) + 2 Hg(l) 2 H2(g) + O2(g) → 2 H2O(l)

O2(g) + 2 Hg(l) 2 H2(g) + O2(g) 2 HgO(s) 2 H2O(l) Energy Exothermic (heat given off by system) Endothermic (heat absorbed by system) 2 HgO(s) 2 H2O(l)

DH = Hproducts - Hreactants QUANTIFICATION Enthalpy of Reaction Enthalpy is the heat release at a constant pressure (mostly atmospheric pressure) DH = Hfinal - Hinitial DH = Hproducts - Hreactants Hfinal > Hinitial : DH > 0 ENDOTHERMIC Hfinal < Hinitial : DH < 0 EXOTHERMIC

Hfinal > Hinitial Hfinal < Hinitial O2(g) + 2 Hg(l) Energy O2(g) + 2 Hg(l) 2 H2(g) + O2(g) Hfinal > Hinitial Hfinal < Hinitial Exothermic Endothermic 2 HgO(s) 2 H2O(l)

DH = Hfinal – Hinitial DH = + 6.01 kJ Energy H2O(l) Hfinal > Hinitial DH = Hfinal – Hinitial DH = + 6.01 kJ Endothermic H2O(s) H2O(s) → H2O(l) ΔH = + 6.01 kJ

DH = Hfinal – Hinitial DH = - 6.01 kJ Energy H2O(l) Hfinal < Hinitial DH = Hfinal – Hinitial DH = - 6.01 kJ Exothermic H2O(s) H2O(l) → H2O(s) ΔH = - 6.01 kJ

THERMOCHEMICAL EQUATIONS H2O(l) → H2O(s) ΔH = - 6.01 kJ Mol relationships 2. Enthalpy change

CH4(g) + 2 O2(g) → 2 H2O(l) + CO2(g) ΔH=-890.4 kJ Calculate the heat evolved when combusting 24.0 g of methane gas.

DH = Hproducts - Hreactants SUMMARY Thermochemistry Enthalpy of Reaction heat released or absorbed by the system at a constant pressure DH = Hproducts - Hreactants Hfinal > Hinitial : DH > 0 ENDOTHERMIC Hfinal < Hinitial : DH < 0 EXOTHERMIC

Homework Chapter 6, p. 205-211 problems