Thermochemistry

1. Thermochemistry Study of energy changes that occur during chemical reactions and changes in state.

2. First Law of Thermodynamics • Role of energy in a chemical reaction: rE system + rE surroundings =rE universe =0 • Another way of expressing the Law of Conservation of Energy!

3. Heat of Reaction (rHrxn) • Quantity of heat released or absorbed during a chemical reaction: 2CO (g) +O2(g) 2CO2(g)+566.8 kJ • Where rH = H products – H reactants Here H represents enthalpy, which is the heatcontent of a system at constant pressure. Energy stored in several forms: PE of chemical bonds PE related to volume and pressure KE of random motions

4. Table I: Heats of Reaction • Rxns 1-7: Combustion rxns or rxn with O2 Hc = Heat of Combustion • Rxns 8-18: Formation Reactions (Synthesis) 1 mole of substance is formed from its elements. Hf = Heat of Formation • Rxns 19-24: Dissolving Equations Hsol = Heat of Solution

5. CH4(g) + O2(g)  CO2(g) + 2H2O (l)H = -890.4 kJ 1 mole of methane + 1 mole of oxygen yields 1 mole of carbon dioxide gas + 2 moles of liquid water • The reaction is exothermic. (negative sign) • 890.4 kJ of energy is releasedper mole of CH4(g) burned.

6. Thermochemical Equations Coefficients represent the number of MOLES of reactants and products and never the number of molecules. The physical state must be included. The change in enthalpy is directly proportional to the number of moles of a substance. The value of H is independent of temperature.

7. Enthalpy Diagrams

8. Standard Conditions: rHo • Standard Thermodynamic Conditions: • 100.00 kPa • Generally at 298.15 K (25oC) • All reactants and products are present in their pure states. • Standard Heat of Reaction: rHorxn • Measured in kilojoules (kJ)

9. Heat of Combustion, rHc • The amount of heat released in a combustion reaction per one mole of a reactant or product. • Always exothermic, so rHcis negative. • Carbon burns according to the equation, C(s) + O2(g) CO2(g). rHc= -393.5kJ/mol CO2. If 2.00 mol of CO2 forms, how much heat is released?

10. Standard Heat of Formation • Special type of heat of reaction • Symbol is rHof • Defined as the heat released or absorbed when 1 mole of a compound is formed from its elements under standard conditions. • rHofof an element is assigned a value of 0 kJ/mole. • These values are then used to calculate the rHo of a reaction involving these substances.

11. Heat of Formation,rHf • The enthalpy change when one mole of a compound is formed from its elements in their standard state. Note: Standard state is NOT the same as STP (Standard Temperature and Pressure). • STABILITY and rHf • Compounds with a large, negative rHfare very stable, whereas positive values tend to be unstable.

12. Calculating rHorxn • For any chemical reaction, the enthalpy change can be calculated by this equation: rHorxn= (sum of rHof of products) – (sum of rHof of reactants) • When using this equation, follow these rules: • rHof= 0 for an element in its standard state. • If a reactant or product has a coefficient in the balanced equation, multiply its rHof by its coefficient. Then enter this amount in the rHorxn equation.

13. Enthalpy Diagram

14. Enthalpy Diagram

15. Enthalpy Diagram

16. Hess’s Law • Basis for this type of calculation is known as Hess’s Law: • The overall enthalpy change in a reaction is equal to the sumof enthalpy changes for the individual steps in the process. • Calculating rHrxnusing Hess’s Law: • Thermochemical equations are rearranged • Terms cancelled • Equations added

17. Bond Dissociation Energy • Can be used to estimate rHrxn • Chemical reactions involve the making and breaking of bonds • This is verified by the close agreement between estimated and measured values of rHrxn

18. Heat in Changes of State • Molar Heat of Fusion & Solidification • The amount of heat absorbed by a melting solid is exactly the same as the quantity of heat released when the liquid solidifies. rHfusion = -rHsolidification • Molar Heat of Vaporization & Condensation rHvaporization = -rHcondensation • Molar Heat of Solution rHsolution = -/+

19. Energy Changes on Dissolving • Dissolving may be either endothermic or exothermic. Cannot predict ahead of time. • Two things going on – it’s a balancing act! • Pulling solute molecules apart from each other and pulling solvent molecules apart from each other. This costs you energy – breaking bonds or interactions is endothermic. • Forming interactions between solute & solvent molecules. This gives you energy – making bonds or interactions is exothermic.

20. Solute-solvent attraction: releases energy. Pulling solute particles apart: absorbs energy. Also have to pull solvent particles apart.

21. Second Law of Thermodynamics • Role of energy in a spontaneous process: rS system + rS surroundings =rS universe >0 • Entropy of the universe must always increase! • Unlike energy, entropy is NOT conserved in a spontaneous process.

22. Gibbs Free-Energy Change • Denoted by the symbol, rG, enables us to predict whether a reaction will be spontaneous under a given set of conditions. rG = rH – TrS • Note that temperature plays an important part and is associated with the disorder factor.

23. Sign of rG • Negative rG  rxn is spontaneous • Positive rG  rxn is non-spontaneous Note: Temperature is measure in Kelvin and is therefore always positive. Rxn is an abbreviation for the word reaction.

24. Free Energies of Formation • Negative rGof • indicates that a substance forms spontaneously. • Positive rGof • indicates that a substance cannot form spontaneously from its elements under the conditions specified.