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Chapter 11

Chapter 11 . Thermochemistry. Energy. Thermochemistry - concerned with heat changes that occur during chemical reactions Energy - capacity for doing work or supplying heat weightless, odorless, tasteless if within the chemical substances- called chemical potential energy.

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Chapter 11

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  1. Chapter 11 Thermochemistry

  2. Energy • Thermochemistry - concerned with heat changes that occur during chemical reactions • Energy - capacity for doing work or supplying heat • weightless, odorless, tasteless • if within the chemical substances- called chemical potential energy

  3. Gasoline contains a significant amount of chemical potential energy • Heat - represented by “q”, is energy that transfers from one object to another, because of a temperature difference between them. • only changescan be detected! • flows from warmer  cooler object

  4. Exothermic and Endothermic Processes • Essentially all chemical reactions, and changes in physical state, involve either: • release of heat, or • absorption of heat

  5. Studying heat changes…

  6. The Law of Conservation of Energy states that in any chemical or physical process, energy is neither created nor destroyed. • All the energy is accounted for as work, stored energy, or heat.

  7. A calorie is defined as the quantity of heat needed to raise the temperature of 1 g of pure water 1 oC. • Used except when referring to food • a Calorie, written with a capital C, always refers to the energy in food • 1 Calorie = 1 kilocalorie = 1000 cal.

  8. Imagine two college freshman who find themselves in that most difficult (yet prevalent) of all college situations: trying to pull an all-nighter with no food anywhere around.  Being well versed in the college rituals, they do what all such unfortunate souls do: order pizza at 3:00 am!   Now, our two coeds are both the same size and weight, but there is a significant difference: Student1 eats two slices, and starts to feel full.  Student2 eats two slices and feels like she is just getting started.  After 3 slices, student1 is stuffed, while it takes almost 6 slices to fill student2.   It is obvious to the observer that these two students do not have the same capacity for pizza.  We might even say that student2 has a larger pizza capacity!  

  9. The calorie is also related to the joule, the SI unit of heat and energy • named after James Prescott Joule • 4.18 J = 1 cal • Heat Capacity - the amount of heat needed to increase the temperature of an object exactly 1 oC

  10. Specific Heat Capacity - the amount of heat it takes to raise the temperature of 1 gram of the substance by 1 oC (abbreviated “C”) • often called simply “Specific Heat” • Water has a HUGE value, compared to other chemicals

  11. Specific Heat • For water, • C = 4.18 J/(g oC) • C = 1.00 cal/(g oC) • Thus, for water: • it takes a long time to heat up, and • it takes a long time to cool off!

  12. To calculate, use the formula: • q = mCT • m is the mass in grams • heat is abbreviated as “q” • T = change in temperature • C = Specific Heat - units are either J/(g oC) or cal/(g oC) • Problem: The temperature of a piece of copper with a mass of 95.4g increases from 25.0oC to 48.0oC when the metal absorbs 849 J of heat. What is the specific heat of copper?

  13. Calorimetry • Calorimetry - the accurate and precise measurement of heat change for chemical and physical processes. • The device used to measure the absorption or release of heat in chemical or physical processes is called a Calorimeter

  14. Calorimetry • Foam cups are excellent heat insulators, and are commonly used as simple calorimeters • For systems at constant pressure, the heat content is the same as a property called Enthalpy (H) of the system • Thus, q = H = m x C x T • H is negative for an exothermic reaction • H is positive for an endothermic reaction

  15. C + O2 Energy C O2 Reactants ® Products + 395 kJ C + O2CO2 395kJ

  16. Heat content of the system decreases… Energy DH is <0 Reactants ® Products

  17. O C O O C O O C O In terms of bonds O C O Breaking this bond will require energy. Making these bonds will give off energy. In this case making the bonds gives off more energy than it takes to break them.

  18. CaO + CO2 Energy CaCO3 Reactants ® Products CaCO3® CaO + CO2 CaCO3 + 176 kJ ® CaO + CO2 176 kJ

  19. Energy Heat content of the system increases… DH is > 0 Reactants ® Products

  20. Chemistry happens in MOLES • An equation that includes energy is called a thermochemical equation • CH4 + 2O2 CO2 + 2H2O + 890 kJ • 1 mole of CH4 releases 890 kJ of energy. • When you make 890 kJ you also make 2 moles of water

  21. CH4 + 2 O2 CO2 + 2 H2O + 890 kJ • If 10. 3 grams of CH4 are burned completely, how much heat will be produced? 1 mol CH4 890 kJ 10. 3 g CH4 16.05 g CH4 1 mol CH4 = 571 kJ

  22. CH4 + 2 O2 CO2 + 2 H2O + 890 kJ • How much energy will be produced from the burning of methane with 55 L of O2? • How many grams of water would be produced with 506 kJ of heat?

  23. Heats of Fusion and Solidification • Molar Heat of Fusion (Hfus) - the heat absorbed by one mole of a substance in melting from a solid to a liquid • Molar Heat of Solidification(Hsolid) - heat lost when one mole of liquid solidifies

  24. Heat absorbed by a melting solid is equal to heat lost when a liquid solidifies. • Thus, Hfus = -Hsolid • Problem: How many grams of ice at OoC and 101.3kPa could be melted by the addition of 2.25 kJ of heat?

  25. Heats of Vaporization and Condensation • When liquids absorb heat at their boiling points, they become vapors. • Molar Heat of Vaporization(Hvap) - the amount of heat necessary to vaporize one mole of a given liquid.

  26. Heats of Vaporization and Condensation • Condensation is the opposite of vaporization. • Molar Heat of Condensation(Hcond) - amount of heat released when one mole of vapor condenses • Hvap = - Hcond

  27. Heats of Condensation • The large values for Hvapand Hcondare the reason hot vapors such as steam is very dangerous • You can receive a scalding burn from steam when the heat of condensation is released! "ARRRRGGHHH!!!Steam burn!! Steam burn!!!!!"

  28. Problem: How much heat (in kJ) is absorbed when 24.8 g H2O(l) at 100oC is converted to steam at 100oC? Try it!

  29. Heat of Solution • Heat changes can also occur when a solute dissolves in a solvent. • Molar Heat of Solution (Hsoln) - heat change caused by dissolution of one mole of substance

  30. Heat of Solution H2O(l) NaOH(s) Na1+(aq) + OH1-(aq) Hsoln = - 445.1 kJ/mol • The heat is released as the ions separate and interact with water, releasing 445.1 kJ of heat as Hsoln thus becoming so hot it steams! • How much heat is released when 2.5 moles of NaOH(s) is dissolved in water?

  31. Hess’s Law • If you add two or more thermochemical equations to give a final equation, then you can also add the heats of reaction to give the final heat of reaction. Called Hess’s law of heat summation. 2NH3(g)  N2(g) + 3H2(g) H= 138KJN2(g) + 2O2(g)  2NO2(g) H= -99KJH2(g) + 1/2O2(g)  H2O(l) H= 52.5KJ Calculate the enthalpy change for the following reaction: 2NH3(g) + 4H2O(l)  2NO2(g) + 7H2(g) ΔH = ??

  32. One for you to TRY!! Use the following thermochemical equations: C2H5OH (l) + 3O2(g) 2CO2 (g) + 3H2O (l) H° = –1367 kJ H2(g) + 1/2 O2(g) H2O(l) H° = –286 kJ C (s) + O2(g) CO2(g) H° = –394kJ to calculate the enthalpy change for the following reaction: 2C (s) + 3H2 (g) + 1/2 O2 (g) C2H5OH (l) H° = ? Answer: -279 kJ

  33. Standard Heats of Formation • - the DH for a reaction that produces 1 mol of a compound from its elements at standard conditions • Standard conditions: 25°C and 1 atm. • Symbol is • The standard heat of formation of an element = 0 • This includes the diatomics

  34. What good are they? • The heat of a reaction can be calculated by: • subtracting the heats of formation of the reactants from the products according to the following equation. DHo = ( Products) - ( Reactants)

  35. CH4 (g) = - 74.86 kJ/mol O2(g) = 0 kJ/mol CO2(g) = - 393.5 kJ/mol H2O(l) = - 285.8 kJ/mol Example • CH4(g) +2O2(g)CO2(g) + 2 H2O(l) • DH= [-393.5 + 2(-285.8)] - [-74.68 +2 (0)] DH= ? kJ -890.42 kJ

  36. Can you handle this one? • Problem: What is the standard heat of reaction (DHo) for the reaction of gaseous carbon monoxide with oxygen to form gaseous carbon dioxide? 2CO(g) + O2(g) 2CO2(g)

  37. Thanks for watching!

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