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Thermochemistry

Thermochemistry is the study of heat flow in chemical systems, focusing on energy changes during reactions. This field distinguishes between endothermic processes (heat absorbed by the system) and exothermic processes (heat released). Key concepts include the role of state properties, calorimetry for measuring heat flow, and changes in enthalpy (H). Through examples involving calorimetry and thermochemical equations, we explore how to calculate heat flow and understand its significance in various chemical reactions.

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Thermochemistry

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  1. Thermochemistry Some say the world will end in fire, Some say in ice, From what I’ve tasted of desire I hold with those who favor fire. Robert Frost Fire and Ice

  2. Principles of Heat Flow • Thermochemistry is the study of heat flow • System: the part of the universe on which we are focused • Surroundings: exchange energy with system (in close contact with system)

  3. Composition Temperature Pressure (remind you of stp?) Heat flow (q) q is + when heat flows into the system from the surroundings (heating of a cup of coffee) q is - when heat flows out of the system into the surrounds (cooling of a cup of coffee) State Properties if X is a state property, then ∆X = Xfinal-Xinitial Heat Flow is NOT a state property

  4. Endo- and Exothermic Processes • An endothermic process (q>0) in which heat flows form the surroundings into the reaction system. • And exothermic process(q<0) in which heat flows from the reaction system into the surroundings surroundings system OUT exothermic IN endothermic

  5. q = C x ∆t ∆t = tfinal - tinitial For a pure substance q = m x c x ∆t m is mass c is specific heat Specific heat is the amount of energy necessary to raise the temperature of 1 gram of a substance 1 degree C. Magnitude of Heat Flow t is temperature q is heat flow C is heat capacity q is measured in joules

  6. Example • How much heat is needed to raise the temperature of a 3000 gallon pool from 10 C to 20 C? The specific heat of water is 4.18 J/goC 1. Q = m x c x ∆t 2. Plug and chug

  7. Calorimetry • Calorimeter: device that measures the heat flow in a reaction • qreaction = -q calorimeter • qreaction = -Ccal x ∆t • “old” heat unit is the calorie • 1 cal = 4.184 J (c) • 1 kcal = 4.184 kJ (C)

  8. Coffee Cup Calorimeter • Heat from the reaction inside the innermost cup flows out to the water in the outer cup. • Ccal = mwater x cwater • qreaction =-mwater x cwater x ∆t

  9. Bomb Calorimetry

  10. The reaction between H and Cl : H2(g) + Cl2(g) --> 2HCl(g) can be studied in a bomb calorimeter. It is found that when a 1.00 g sample of H2 reacts completely the temperature rises from 20.00 C to 29.82 C. Taking the heat capacity of the calorimeter to be 9.33 kj?oC, calculate the amount of heat evolved in the reaction. Which equation to use? Example qreaction = -Ccal x ∆t Plug and chug Answer: -91.6 kJ exothermic

  11. When 1.00 g of CaCl2 is added to 50.0 g of water in a coffee-cup calorimeter, it dissolves CaCl2(s) --> Ca2+(aq) + 2Cl-(aq) And the temperature rises from 25.00 C to 28.51 C. Assuming that all the heat given off by the reaction is transferred to the water, calculate q for the reaction system. Which equation? Example qreaction = -mwater x cwater Plug and chug Answer: -734 J • exothermic

  12. Enthalpy (H) • Enthalpy is a type of chemical energy (heat content) • Under constant P, heat flow for a reaction system is equal to the difference in enthalpy between products and reactants qreaction= ∆H= Hproducts- Hreactants

  13. Enthalpy continued • For an exothermic reaction q = ∆H< 0 • For an endothermic reaction q = ∆H> 0 The enthalpy of a substance, like its volume, is a state property

  14. A chemical equation which shows the enthalpy relation between products and reactants NH4NO3(s) --> NH4+(aq)+ NO3-(aq) ∆H = +28.1 kJ The sign of ∆H indicates endothermic (+) or exothermic (-) Thermochemical Equations The coefficients represent numbers of moles The phases must be specified--significant differences at different phases The value quoted for H applies when products and reactants are at the same temp (usually 25C) H2(g)+ Cl2(g) --> 2HCl ∆H = -185kJ

  15. The magnitude of ∆H is directly proportional to the amount of reactant or product The ∆H for a reaction is equal in magnitude but opposite in sign to ∆H for the reverse reaction. Rules of Thermochemistry H2(g)+ Cl2(g) --> 2HCl ∆H = -185kJ H2(g)+ Cl2(g) --> 2HCl(g) ∆H = -185 kJ -185 kJ1 mol H2 1 mol Cl2 -185 kJ -185 kJ 2 mol HCl 2HCl (g)--> H2 (g) + Cl2 (g) ∆H = 185 kJ Heat of fusion : heat absorbed when a solid melts Heat of vaporization: heat absorbed when a liquid vaporizes

  16. Example

  17. Enthalpy of Formation

  18. Example

  19. Example

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