Thermochemistry

1. Thermochemistry Changes in Matter and Energy

2. Learning Goals • use proper scientific terminology to describe changes in energy and matter • explain types of changes that occur and the energy release in those changes • list and describe types of energy • list and describe types of changes of state • use proper scientific terminology to describe energy transfer • calculate the heat released or absorbed by chemical reactions

3. Success Criteria • define the terms: thermochemistry, physical change, chemical change, nuclear change, thermal energy, chemical energy, mechanical energy, sound energy, radiant energy, kinetic energy, potential energy, heat, temperature, exothermic, endothermic, open systems, closed systems, isolated systems, changes of state • compare physical, chemical and nuclear changes • list and describe types of energy • list and describe types of changes of state • define the terms: calorimetry, specific heat capacity, heat transfer, enthalpy/enthalpy changes • calculate the heat released or absorbed by chemical reactions using proper symbols and SI units

4. Thermochemistry • the study of the energy changes that accompany physical and chemical changes in matter Figure 1: Physical: Hydrogen boils at -252°C Chemical: Burning hydrogen gas Nuclear: Hydrogen undergoes nuclear fusion in the Sun to produce helium

5. Heat and Energy Changes • Thermal Energy – a form of kinetic energy that results from the motion of molecules (e.g. melting of metal – when the increased vibration of metal particles causes them to break out of their ordered solid pattern) Figure 2: Molten steel

6. Chemical system – a set of reactants and products under study, usually represented by a chemical equation • Surroundings – all matter around the system that is capable of absorbing or releasing thermal energy Surroundings Chemical System Figure 3: Chemical system and its surroundings

7. When a reaction occurs, heat is transferred between substances. • Heat (q) is the amount of energy transferred between substances • Exothermic change involves releasing thermal energy as heat flows out of the system • Endothermic change involves absorbing thermal energy as heat flows into the system Figure 4: In exothermic changes, energy is released from the system, usually causing an increase in the temperature of the surroundings. In endothermic changes, energy is absorbed by the system, usually causing a decrease in the temperature of the surroundings.

8. Open System • A system in which both matter and energy can move in or out Figure 5: A campfire represents an open system

9. Isolated System • an ideal system in which neither matter nor energy can move in or out Closed System • a system in which energy can move in or out, but not matter Figure 6: A bomb calorimeter is a device in which a fuel is burned inside an insulated container to obtain accurate measurements of heat transfer during chemical reactions. Because neither mass nor energy can escape, the chemical system is described as isolated.

10. Measuring Energy Changes - Calorimetry • Calorimetry is the technological process of measuring energy changes in a chemical system • During a combustion reaction, heat is transferred from the chemical system into the surroundings (exothermic) • The “surroundings” refers to the system’s environment

11. Three factors are combined to represent the quantity of heat (q) transferred: q = mc∆T q = quantity of heat transferred (J) m = mass (g) c = specific heat capacity (J/g●°C) ΔT = temperature change (°C)

12. Specific heat capacity is the quantity of heat required to raise the temperature of a unit mass of a substance by one degree Celsius

13. Learning Checkpoint Practice p. 300 UC # 1, 2, 3, 4, 5

14. Heat Transfer and Enthalpy Change • What is energy? • Kinetic energy (KE) is the energy of motion. • Potential energy (PE) is stored energy of position possessed by an object • The term chemical energy is the special name often given to PE that arises from the forces of attraction that bind atoms together in compounds • These forces of attractions are called chemical bonds

15. It is important to note that when chemicals react to form new substances, atoms exchange as old bonds break and new bonds form (this process actually changes the PE of the atoms) • Sometimes the reaction’s products have more potential energy than it’s reactants; in other reactions the products have less potential energy than the reactants • In general all chemical reactions either liberate (exothermic reactions) or absorb (endothermic reactions) energy

16. Chemists study the enthalpy change, or the energy absorbed from or released to the surroundings when a system changes from reactants to products • Enthalpy change is given the symbol ∆H • The enthalpy change of the system equals the quantity of heat that flows from the system to its surroundings, or from the surroundings to the system ∆H system = ± ׀q surroundings׀

17. Law of Conservation of Energy states that energy may be converted from one form to another, or transferred from one set of molecules to another, but the total energy of the system and its surroundings remains the same Figure 8: In this example of an exothermic change, the change in potential energy of the system (H ) equals the change in kinetic energy of the surroundings (q). This is consistent with the law of conservation of energy.

18. We show exothermic reactions by q = -vemeaning that energy has been lost from the system • Endothermic reactions are documented by q = +vemeaning that energy was absorbed by the system Figure 7: In exothermic changes, energy is released from the system, usually causing an increase in the temperature of the surroundings. In endothermic changes, energy is absorbed by the system, usually causing a decrease in the temperature of the surroundings.

19. Learning Checkpoint Practice p. 302 UC # 8, 9, 10, 11, 12, 13 p. 304 UC # 14, 15

20. Medical Cold Packs LABpp. 347 - 348

22. Figure 1: Reactions that involve a change in energy

23. Learning Checkpoint Section 5.1 Questions p. 305 # 1, 2, 3, 4, 5, 6