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Understanding Thermochemistry: Energy Forms, Laws, and Enthalpy

Thermochemistry explores the nature of energy, including kinetic and potential energy, along with how energy is transferred through work and heat. It explains the First Law of Thermodynamics, stating that energy is conserved in processes, and discusses concepts like enthalpy, calorimetry, and the laws governing chemical reactions. The chapter delves into exothermic and endothermic processes, calorimetry methods, Hess’s Law, and enthalpies of formation, aiming to provide a thorough understanding of how energy dynamics play a crucial role in physical and chemical changes.

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Understanding Thermochemistry: Energy Forms, Laws, and Enthalpy

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  1. Chapter 5Thermochemistry

  2. The Nature of Energy • Kinetic Energy and Potential Energy • Kinetic energy is the energy of motion: • Potential energy is the energy an object possesses by virtue of its position. • Potential energy can be converted into kinetic energy. Example: a bicyclist at the top of a hill.

  3. The Nature of Energy • Kinetic Energy and Potential Energy • Electrostatic potential energy, Ed, is the attraction between two oppositely charged particles, Q1 and Q2, a distance d apart: • The constant  = 8.99  109 J-m/C2. • If the two particles are of opposite charge, then Ed is the electrostatic attraction between them.

  4. Units of Energy • SI Unit for energy is the joule, J: • We sometimes use the calorie instead of the joule: • 1 cal = 4.184 J (exactly) • A nutritional Calorie: • 1 Cal = 1000 cal = 1 kcal

  5. Systems and Surroundings • Analyzing Energy Changes • System: part of the universe we are interested in. • Surroundings: the rest of the universe.

  6. Transferring Energy • Work and Heat • Force is a push or pull on an object. • Work is the product of force applied to an object over a distance: • Energy is the work done to move an object against a force. • Heat is the transfer of energy between two objects. • Energy is the capacity to do work or transfer heat.

  7. The First Law of Thermodynamics • Energy is neither created or destroyed • Total energy lost by a system equals the total energy gained by a system. • Internal Energy: total energy of a system (kinetic + potential). • Cannot measure absolute internal energy. • Change in internal energy,

  8. Relating DE to Heat and Work • Energy of (system + surroundings) is constant. • Any energy transferred from a system must be transferred to the surroundings (and vice versa). • From the first law of thermodynamics: • when a system undergoes a physical or chemical change, the change in internal energy is given by the heat added to or absorbed by the system plus the work done on or by the system:

  9. Sign Conventions

  10. Exothermic and Endothermic Processes • Endothermic: absorbs heat from the surroundings. • Exothermic: transfers heat to the surroundings. • An endothermic reaction feels cold. • An exothermic reaction feels hot.

  11. State Functions • State function: depends only on the initial and final states of system, not on how the internal energy is used.

  12. Work • Chemical reactions can absorb or release heat. • However, they also have the ability to do work. • For example, when a gas is produced, then the gas produced can be used to push a piston, thus doing work. • Zn(s) + 2H+(aq)  Zn2+(aq) + H2(g) • The work performed by the above reaction is called pressure-volume work. • When the pressure is constant,

  13. Enthalpy • Enthalpy, H: Heat transferred between the system and surroundings carried out under constant pressure. • Enthalpy is a state function. • If the process occurs at constant pressure,

  14. Enthalpy • Since we know that • We can write • When DH, is positive, the system gains heat from the surroundings. • When DH, is negative, the surroundings gain heat from the system.

  15. Enthalpy

  16. Enthalpies of Reaction • For a reaction: • Enthalpy is an extensive property (magnitude DH is directly proportional to amount): • CH4(g) + 2O2(g)  CO2(g) + 2H2O(g) DH = -802 kJ • 2CH4(g) + 4O2(g)  2CO2(g) + 4H2O(g) DH = -1604 kJ

  17. Enthalpies of Reaction • When we reverse a reaction, we change the sign of DH: • CO2(g) + 2H2O(g)  CH4(g) + 2O2(g) DH = +802 kJ • Change in enthalpy depends on state: • H2O(g)  H2O(l) DH = -88 kJ

  18. Calorimetry • Heat Capacity and Specific Heat • Calorimetry = measurement of heat flow. • Calorimeter = apparatus that measures heat flow. • Heat capacity = the amount of energy required to raise the temperature of an object (by one degree). • Molar heat capacity = heat capacity of 1 mol of a substance. • Specific heat = specific heat capacity = heat capacity of 1 g of a substance.

  19. Constant Pressure Calorimetry • Atmospheric pressure is constant!

  20. Constant Pressure Calorimetry

  21. Bomb Calorimetry (Constant Volume Calorimetry) • Reaction carried out under constant volume. • Use a bomb calorimeter. • Usually study combustion.

  22. Hess’s Law • Hess’s law: if a reaction is carried out in a number of steps, H for the overall reaction is the sum of H for each individual step. • For example: • CH4(g) + 2O2(g)  CO2(g) + 2H2O(g) H = -802 kJ • 2H2O(g)  2H2O(l) H = -88 kJ • CH4(g) + 2O2(g)  CO2(g) + 2H2O(l) H = -890 kJ

  23. Hess’s Law Note that: H1 = H2 + H3

  24. Enthalpies of Formation • If 1 mol of compound is formed from its constituent elements, then the enthalpy change for the reaction is called the enthalpy of formation, Hof . • Standard conditions (standard state): 1 atm and 25 oC (298 K). • Standard enthalpy, Ho, is the enthalpy measured when everything is in its standard state. • Standard enthalpy of formation: 1 mol of compound is formed from substances in their standard states.

  25. Enthalpies of Formation • If there is more than one state for a substance under standard conditions, the more stable one is used. • Standard enthalpy of formation of the most stable form of an element is zero.

  26. Standard Enthalpies of Formation at 298 K

  27. Using Enthalpies of Formation of Calculate Enthalpies of Reaction • We use Hess’ Law to calculate enthalpies of a reaction from enthalpies of formation.

  28. Using Enthalpies of Formation of Calculate Enthalpies of Reaction • For a reaction

  29. Foods and Fuels • Foods • Fuel value = energy released when 1 g of substance is burned. • 1 nutritional Calorie, 1 Cal = 1000 cal = 1 kcal. • Energy in our bodies comes from carbohydrates and fats (mostly). • Intestines: carbohydrates converted into glucose: • C6H12O6 + 6O2 6CO2 + 6H2O, DH = -2816 kJ • Fats break down as follows: • 2C57H110O6 + 163O2 114CO2 + 110H2O, DH = -75,520 kJ

  30. Fuels • In 2000 the United States consumed 1.03  1017 kJ of fuel. • Most from petroleum and natural gas. • Remainder from coal, nuclear, and hydroelectric. • Fossil fuels are not renewable.

  31. Fuels

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