Unit 6: Thermochemistry

1. Unit 6: Thermochemistry • Introduction • Heat and Work • Specific Heat • Enthalpy (DH) • Enthalpy of Reaction • Phase Diagram

2. Introduction • Most daily activities involve processes that either use or produce energy: • Activities that produce energy • Metabolism of food • Burning fossil fuels • Activities that use energy: • Photosynthesis • Pushing a bike up a hill • Baking bread

3. Introduction • Thermodynamics • The study of energy and its transformations • Thermochemistry: • A branch of thermodynamics • The study of the energy (heat) absorbed or released during chemical reactions

4. Introduction • Objects can have two types of energy: • Kinetic energy • Energy of motion • Thermal energy • The type of kinetic energy a substance possesses because of its temperature • Potential energy • Energy of position • “stored” energy resulting from the attractions and repulsions an object experiences relative to other objects

5. Introduction • Units of Energy • SI unit = joule (J) • 1 J = the kinetic energy of a 2 kg mass moving at a speed of 1 m/s • A very small quantity • Kilojoule (kJ) • 1 kJ = 1000 J

6. Introduction • Units of Energy (cont) • Calorie (cal) • Originally defined as the amount of energy needed to raise the temperature of 1g of water from 14.5oCto 15.5oC. • 1 cal = 4.184 J (exactly) • Kilocalorie (kcal) • 1 kcal = 1000 cal

7. 1 kJ = 1000 J J = 3.02 kJ x 1000 J = 3020 J 1 kJ Introduction Example: Convert 3.02 kJ to J. Given: 3.02 kJ Find: J

8. 1 cal = 4.184 J 1 kJ = 1000 J J = 725 cal x 4.184 J x 1 k J = 3.03 kJ 1 cal 1000 J Introduction Example: Convert 725 cal to kJ. Given: 725 cal Find: kJ

9. Introduction • When using thermodynamics to study energy changes, we generally focus on a limited, well-defined part of the universe. • System: • The portion of the universe singled out for study • Surroundings: • Everything else

10. Introduction The system is usually the chemicals in the flask/reactor. The system The flask and everything else belong to the surroundings.

11. Introduction • Open system: • A system that can exchange both matter and energy with the surroundings • Closed system: • A system that can exchange energy with the surroundings but not matter A cylinder with a piston is one example of a closed system.

12. Introduction • In a closed system energy can be gained from or lost to the surroundings as: • Work • Heat • Work: • Energy used to cause an object to move against a force • Lifting an object • Hitting a baseball

13. Introduction • Heat: • The energy used to cause the temperature of an object to increase • The energy transferred from a hotter object to a cooler one • Energy: • The capacity to do work or to transfer heat

14. work Introduction • The potential energy of a system can be converted into kinetic energy and vice versa. • Energy can be transferred back and forth between the system and the surroundings as work and/or heat. Potential energy Kinetic energy

15. The First Law of Thermodynamics • Although energy can be converted from one form to another and can be transferred between the system and the surroundings: Energy cannot be created or destroyed. (First Law of Thermodynamics) • Any energy lost by the system must be gained by the surroundings and vice versa.

16. The First Law of Thermodynamics • The First Law of Thermodynamics can be used to analyze changes in the Internal Energy (E) of a system. • The sum of all kinetic and potential energy of all components of a system • For molecules in a chemical system, the internal energy would include: • the motion and interactions of the molecules • the motion and interactions of the nuclei and electrons found in the molecules

17. The First Law of Thermodynamics • Internal Energy: • Extensive property • depends on mass of system • Influenced by temperature and pressure • Has a fixed value for a given set of conditions • State function

18. The First Law of Thermodynamics • The internal energy of a system is a state function. • A property of the system that is determined by specifying its condition or its state in terms of T, P, location, etc • Depends only on its present condition • Does not depend on how the system got to that state/condition

19. The First Law of Thermodynamics • The internal energy of a system can change when: • heat is gained from or lost to the surroundings • work is done on or by the system. • The change in the internal energy D E = Efinal - Einitial DE = change in internal energy Efinal = final energy of system Einitial = initial energy of system

20. The First Law of Thermodynamics • If Efinal > Einitial, • DE >0 (positive) • the system has gainedenergy from the surroundings. • endergonic

21. The First Law of Thermodynamics • The decomposition of water is endergonic (DE > 0): 2 H2O (l) 2 H2 (g) + O2 (g) H2 (g), O2(g) Energy must be gained from the surroundings. final E H2O (l) initial

22. The First Law of Thermodynamics • If Efinal < Einitial, • DE < 0 (negative) • the system has lostenergy to the surroundings. • exergonic

23. H2 (g), O2(g) H2O (l) The First Law of Thermodynamics • The synthesis of water is exergonic (DE < 0) 2 H2 (g) + O2 (g) 2 H2O (l) Energy is lost to the surroundings in this reaction. initial E final

24. The First Law of Thermodynamics • The internal energy of a system can change when energy is exchanged between the system and the surroundings • Heat • Work • The change in internal energy that occurs can be found: D E = q + w Where q = heat w = work

25. The First Law of Thermodynamics • By convention: • q = positive • Heat added to the system • w = positive • Work done on the system by the surroundings • q = negative • Heat lost by the system • w = negative • Work done by the system on the surroundings

26. The First Law of Thermodynamics Example: Calculate the change in internal energy of the system for a process in which the system absorbs 140. J of heat from the surroundings and does 85 J of work on the surroundings. Given:system absorbs 140. J heat = system does 85 J work = Find:D E + 140.J - 85 J

27. The First Law of Thermodynamics D E = q + w D E = +140 J + (-85 J) D E = +55 J