Energy: Heat, Nutrients, and Enthalpy Page 13

Transcription

1 V. Thermochemistry A. Thermochemistry is concerned with the B. Such reactions and changes are. VI. Chemical Reactions A. Chemical compounds contain B. Chemical potential energy. C. In chemical reactions, D. Exothermic reactions. Example: E. Endothermic reactions. Example: VII. Enthalpy A. Enthalpy ( ) is defined as the. B. The in enthalpy (not the actual enthalpy) is measured and represents The ( ) is the change in enthalpy for a reaction. C. Hrxn = C. THE SIGN OF Hrxn 1. Endothermic: Reactants heat; therefore, Hproducts Hreactants, and Hrxn is. 2. Exothermic: Reactants heat; therefore, Hproducts Hreactants, and Hrxn is. E. Hrxn = (at constant pressure) VIII. Thermochemical Equations A. Express the amount of during reactions. B. Balanced thermochemical equations include and, expressed as the. Energy: Heat, Nutrients, and Enthalpy Page 13

2 IX. Endothermic Processes 1. Energy is. Certain chemical reactions and phase changes require energy. 2. Activation energy is the. Energy must be. 3. Reactants have energy than products. 4. Example: A cold pack can be used to ice down a knee injury. When the pack is opened and squeezed, an inner membrane releases ammonium nitrate into water. Heat is the knee and is by the cold pack. X. Exothermic Processes A. Energy is. According to the Law of Conservation of Energy, B. Once sufficient activation energy is added, the reaction. C. Reactants have energy than products. D. Example: A heat pack can warm your hands on a cold day. When the pack is opened, oxygen from the air reacts with iron in the pack. Energy is. Energy: Heat, Nutrients, and Enthalpy Page 14

3 XI. Changes of Phase A. Phase changes are. B. Phase changes always occur with and can be. XII. Molar Enthalphies Phase Changes for Water H2O(g) Hvap = kj Enthalpy Hcond = 40.7 kj H2O(l) Hfusion = kj Hsolid = 6.01 kj H2O(s) The energy of the system increases as ice melts and then vaporizes, as illustrated by the upward arrows. The downward arrows indicate that the energy of the system decreases as water vapor condenses and then solidifies. A. Molar enthalpy (heat) of vaporization: = 1. Vaporization or ( ): H 2 O ( ) H 2 O ( ) 2. process 3. Hvap is. Energy: Heat, Nutrients, and Enthalpy Page 15

4 B. Molar enthalpy (heat) of fusion: = 1. Fusion or ( ): H 2 O ( ) H 2 O ( ) 2. process 3. Hfus is. C. Molar enthalpy (heat) of condensation: = 1. Condensation ( ): H 2 O ( ) H 2 O ( ) 2. process 3. Same numerical value but opposite sign of Hvap. Hcond is. D. Molar enthalpy (heat) of solidification: = 1. Solidification or ( ): H 2 O ( ) H 2 O ( ) 2. process 3. Same numerical value but opposite sign of Hfus. Hsolid is. E. Sublimation ( ) and deposition ( ) encompass two phase changes. XIII. Heating Curve for Water 140 C 120 C T E M P E R A T U R E ( C) 100 C 80 C 60 C 40 C 20 C 0 C -20 C -40 C ENERGY Kinetic Energy: Potential Energy: Energy: Heat, Nutrients, and Enthalpy Page 16

5 Phase Change State Change IMF Energy Endo/Exo Sign on q or ΔH To calculate heat released or absorbed during phase changes, use the equation. Example: How much energy is required to melt 36.0 grams of water? The heat capacity for water varies for different phases. Specific heat (c) for liquid water = Specific heat (c) for solid water (ice) = Specific heat (c) for gaseous water (steam) = Example: How much heat is required to heat 5 grams of ice from 10 C to 70 C? Water is going through three processes: Heat (q) must be calculated for each process, and the three values must be added together to determine the total amount of heat required. XIV. Phase Diagram A. Defined: Energy: Heat, Nutrients, and Enthalpy Page 17

6 B. Diagram differs for different substances because of different and points. 1. Freezing point: 2. Boiling point: C. Triple point: D. Critical point: XV. Combustion A. Combustion is the. 1. For biological systems, are used as fuel. 2. also undergo combustion reactions and provide warmth and fuel for transportation. B. Combustion is. D. STANDARD ENTHALPIES OF COMBUSTION:. When symbolized by, the changes were determined with all reactants and products at standard conditions ( ). Substance Formula H 0 comb Sucrose C 12 H 22 O 12 (s) 5644 kj/mol Octane C 8 H 18 (l) 5471 kj/mol Glucose C 6 H 12 O 6 (s) 2808 kj/mol Propane C 3 H 8 (g) 2219 kj/mol Methane CH 4 (g) 891 kj/mol Hydrogen H 2 (g) 286 kj/mol Combustion Calculation Practice 1. How much heat is evolved when 54.0 grams of glucose (C6H12O6) is burned? C6H12O6 (s) + 6O2(g) 6CO2 (g) + 6H2O (l) Hcomb = 2808 kj/mol 2. How much thermal energy is produced by burning 12 g of octane? Energy: Heat, Nutrients, and Enthalpy Page 18

7 3. The heat of combustion of carbon to carbon dioxide is kj/mol. Calculate the heat released upon formation of 35.2 g CO2 from carbon and oxygen gas. C (s) + O2 (g) CO2 (g) Hcomb = kj/mol 4. How much heat is released by the combustion of 206 g of hydrogen gas? [ Hcomb = 286 kj/mol]. 5. How many kilojoules of heat would be provided by the combustion of 10.0 g of butane? [ Hcomb = 2878 kj/mol]. 6. What mass of propane (C3H8) must be burned in a barbecue grill to release 4560 kj of heat? The Hcomb of propane is 2219 kj/mol. X. Entropy and Spontaneous Processes A. Entropy ( ) is a measure of. It is also referred to as a measure of the or of a system. Entropy as the number of possible arrangements of particles. Entropy of Solid Entropy of Liquid Entropy of Gas B. Processes that result in greater entropy are, like ice melting. For example, messing up your bedroom is not hard to do! C. Processes that result in less entropy are ; energy must be supplied. For example, cleaning up your messy bedroom takes energy; it doesn t happen automatically! D. The states that processes always proceed in such a way that the entropy of the universe. E. ( ) is similar to change in enthalpy. S system = If entropy increases, S products S reactants and S system is. If entropy decreases, S products S reactants and S system is. Energy: Heat, Nutrients, and Enthalpy Page 19

8 F. Spontaneity can sometimes be. Increases entropy ( Ssystem > 0) Decreases entropy ( Ssystem < 0) Prediction Practice Indicate whether the Ssystem is greater than (>) or less than (<) zero, and predict the sign of Ssystem (+ or ) for each of the following. 1. ClF (g) + F2 (g) ClF3 (g) 2. NH3 (g) NH3 (aq) 3. CH3OH (l) CH3OH (aq) 4. C10H8 (l) C10H8 (s) 5. Fe (s) + Zn 2 + (aq) Fe 2 + (aq) + Zn (s) 6. CaCl 2 (s) Ca+ (aq) + 2Cl (aq) 7. H 2 O (l) H 2 O (g) 8. 2H2 (g) + O2 (g) 2H2O (l) 9. N2H4 (l) + 2H2O2 (l) N2 (g) + 4H2O (g) 10. HgS (s) + O2 (g) Hg (l) + SO2 (g) Energy: Heat, Nutrients, and Enthalpy Page 20