Thermodynamics Think Energy Chemical Reactions Energy & Temperature Measuring Energy Kinetic Energy and Heat ©2011 University of Illinois Board of Trustees ght

Basic Thermodynamics Gain Energy to Change Phase The study of energy and its transformations Think Energy Lose Energy to Change Phase Energy Holds the Atom Together Energy Holds Atoms Together ©2011 University of Illinois Board of Trustees ght

Review Phase Change Click the picture to observe the motion of the atoms. Atoms and Molecules MOVE because they have … ENERGY They even break away from each other when they absorb enough energy. ©2011 University of Illinois Board of Trustees ght

Gain ENERGY to Change Phase solid to liquid (s → l)liquid to gas (l → g) ©2011 University of Illinois Board of Trustees ght

Lose ENERGY to Change Phase liquid to solid (l → s)gas to liquid (g → l) ©2011 University of Illinois Board of Trustees ght

How do we know? Melting Curve (s → l) Vaporization Curve (l → g) Remember the demonstration with the temperature probes. The temperature of the water did not increase during phase change. You observed ENERGY being used to do work! ©2011 University of Illinois Board of Trustees ght

Elements can exist as a s, l or g. An Interesting FACT… It all depends on the amount of ENERGY! Click the periodic table to play ©2011 University of Illinois Board of Trustees ght

ENERGY holds the atom together… Since protons carry a positive charge, they repel each other. For a nucleus to remain stable, energy is needed to overcome the forces of repulsion. This energy is known as the Nuclear Binding Energy ©2011 University of Illinois Board of Trustees ght

How do we know? ENERGY is released when the atom splits…because more stable nuclei are formed ©2011 University of Illinois Board of Trustees ght

Energy is released during bond formation This energy is know as a CHEMICAL BOND! Log into WebMO and determine the bond energy ©2011 University of Illinois Board of Trustees ght

How do we know? Click on the skull to play At End of Movie - Click to Return HomeReturn Fe 2 O 3 + 2Al -> 2Fe + Al 2 O 3 + ENERGY click ©2011 University of Illinois Board of Trustees ght

 Thermochemistry – the study of the transfer of energy that accompanies chemical reactions and physical changes. ©2011 University of Illinois Board of Trustees ght

Heat and Temperature  Calorimeter – Instrument used to measure heat in a chemical or physical change.  Temperature – the measure of the average kinetic energy of the particles in a sample of matter. (The greater the kinetic energy the higher the temperature.) ©2011 University of Illinois Board of Trustees ght

 Joule (J) – the SI unit of heat and energy. J = N x m = kg x m 2 s 2  Heat – the energy transferred between samples of matter because of temperature differences. ©2011 University of Illinois Board of Trustees ght

 Energy transferred as heat always moves spontaneously from matter at a higher temperature to matter at a lower temperature. ©2011 University of Illinois Board of Trustees ght

Heat Capacity and Specific Heat  Specific heat – is the amount of energy required to raise the temperature of one gram of substance by one degree Celsius or one Kelvin. The most common unit is J/(g x °C). (C p is used when the specific heat is at a given pressure. “q” is heat, while “m” is mass.) C p = q___ m x ΔT ©2011 University of Illinois Board of Trustees ght

Heat of Reaction  Heat of reaction – the amount of energy released or absorbed as heat during a chemical reaction.  Thermochemical equation – an equation which includes energy released or absorbed. 2H 2 + 2O 2 → 2H 2 O kJ ©2011 University of Illinois Board of Trustees ght

Thermochemistry and Heat Capacity Coffee Lab – Virtual Lab  /vlab.php /vlab.php Select File Load Homework Thermochemistry Coffee ©2011 University of Illinois Board of Trustees ght

 Enthalpy change (ΔH) – the amount of energy absorbed or lost by a system as heat during a process at constant pressure. Only changes in enthalpy can be measured. ΔH = H products – H reactants ©2011 University of Illinois Board of Trustees ght

+ ΔH = endothermic Heat enters the system, the initial energy of the reactants is lower than the final energy. Course of Reaction → ©2011 University of Illinois Board of Trustees ght

- ΔH = exothermic Heat leaves the system, the initial energy of the reactants is greater than the final energy of the products. ©2011 University of Illinois Board of Trustees ght

Concepts using thermochemical equations. 1. The coefficients in a balanced thermochemical equation represent the numbers of moles of reactants and products. 2. The physical state of the product or reactant involved is an important factor. 3. The change in energy represented by a thermochemical equation is directly proportional to the number of moles. 4. The value of the energy change, ΔH, is usually not significantly influenced by changing temperature. ©2011 University of Illinois Board of Trustees ght

Heat of Formation  The molar heat of formation is the energy released or absorbed as heat when one mole of a compound is formed by combination of its elements in their standard states. Heats of formation are given using their standard states, and atmospheric pressure. (Δ f H°) ©2011 University of Illinois Board of Trustees ght

Stability and Heat of Formation  If a large amount of energy is released when a compound is formed, the compound has a high negative heat of formation. These are very stable.  Compounds with relatively positive values of heats of formation, or only slightly negative values, are relatively unstable and will spontaneously decompose. ©2011 University of Illinois Board of Trustees ght

Heat of Combustion  The energy released as heat by the complete combustion of one mole of a substance. ©2011 University of Illinois Board of Trustees ght

Heat of combustion vs. Heat of formation  Heat of combustion is in terms of one mole of REACTANT.  Heat of formation is in terms of one mole of PRODUCT. ©2011 University of Illinois Board of Trustees ght

Calculating Heats of Reaction  Hess’s Law: The overall enthalpy change in a reaction is equal to the sum of enthalpy changes for the individual steps in the process. ©2011 University of Illinois Board of Trustees ght

General Principles for thermochemical equations. 1. If a reaction is reversed, the sign of ΔH is also reversed. H 2 + ½O 2  H 2 O ΔH c ° = kJ/mol H 2 O  H 2 + ½O 2 ΔH c ° = kJ/mol 2. You can multiply the coefficients, but they must remain proportional throughout the equation. H 2 + ½O 2  H 2 O ΔH c ° = kJ/mol 2H 2 + O 2  2H 2 O ΔH c ° = 2(-285.8) kJ/mol ©2011 University of Illinois Board of Trustees ght

Hess’s Law  Practice problems: _help/Hesslaw.html _help/Hesslaw.html ©2011 University of Illinois Board of Trustees ght

Determining Heat of Formation Given: C (s) + O 2(g)  CO 2(g) Δ f H ° = kJ/mol H 2(g) + ½O 2(g)  H 2 O (l) Δ f H ° = kJ/mol C 5 H 12(g) + 8O 2(g)  5CO 2(g) + 6H 2 O (l) Δ f H ° = kJ/mol Unknown: 5C (s) + 6H 2(g)  C 5 H 12(g) Δ f H ° = ? ©2011 University of Illinois Board of Trustees ght

Resources 1. Modern Chemistry (Holt, Rinehart, and Winston) 2. b/vlab.php b/vlab.php 3. ee/ExptG1.gif ee/ExptG1.gif 4. e_help/Hesslaw.html The End Click to Return HomeReturn ©2011 University of Illinois Board of Trustees ght