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Enthalpy changes and calorimetry

Enthalpy changes and calorimetry. Enthalpy changes in reactions Calorimetry and heat measurement Hess’s Law Heats of formation. Learning objectives. Describe the standard state for thermodynamic functions Explain sign of enthalpy change for changes of state

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Enthalpy changes and calorimetry

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  1. Enthalpy changes and calorimetry Enthalpy changes in reactions Calorimetry and heat measurement Hess’s Law Heats of formation

  2. Learning objectives • Describe the standard state for thermodynamic functions • Explain sign of enthalpy change for changes of state • Calculate enthalpy changes for reactions • Use specific heat and heat capacity in calorimetric problems • Apply Hess’ law to calculations of enthalpy change • Use standard heat of formation in calculations of enthalpy of reaction • Use bond energies to estimate heats of reaction

  3. The importance of state • The heat change in a reaction will depend upon the states of the reactants and/or products • States must be specified • ΔH(1) = - 2043 kJ • ΔH(2) = - 2219 kJ • Additional heat is released in (2) because condensation of H2O releases 176 kJ

  4. Thermodynamic standard state • Heat changes will also depend upon P and T • The Standard State: Most stable form of a substance at 1 atm pressure and 25ºC(usually); 1 M concentration for solutions • Conventionally, enthalpy change under standard conditions is represented by: ΔHº • Any thermodynamic function referring to standard conditions is identified by º

  5. Calculating ΔU from ΔHº • Reaction between N2 and H2 at P = 40 atm and ΔV – 1.12 L • What is ΔU? ΔU = ΔH – PΔV ΔU > ΔH • Internal energy change is less negative by amount of work done by surroundings on compressing system N2(g) + 3H2(g) = 2NH3(g) ΔHº = -92.2 kJ

  6. Enthalpies of change of state • Conversion of a substance from one state to another involves heat input or output • Heat is absorbed when breaking bonds • Solid → liquid → gas • Heat is released when making bonds • Gas → liquid → solid • Heat of fusion, vaporization etc. (formerly known as “latent” heats – heat consumed with no change in T)

  7. State functions ignore path • Transition from solid to gas may lie through liquid phase... • Or transition may be direct – sublimation • Enthalpy change is the same by either route ΔHsubl = ΔHfusion + ΔHvap

  8. Heat of reaction • Enthalpy change for a reaction is commonly known as heat of reaction • Exothermic refers to reaction where heat is given out by system (we warm our hands): ΔH < 0 • Endothermic refers to reaction where heat is absorbed by system and surroundings get cool (hands get cold): ΔH > 0 • NOTE SIGNS OF ΔH!

  9. Calculations with ΔHº • ΔHº may be quoted for a given equation or for one mole of a reactant or product • Either way, it can be used to calculate the heat change for a given amount of substance

  10. Example • What is the heat evolved from the combustion of 15.5 g of C3H8? • Combustion of one mole of C3H8 yields -2219 kJ • Molar mass of C3H8 = 44.0 g/mol • No moles in 15.5 g = 15.5/44.0 = 0.352 mol • Heat produced = - 2219 x 0.352 = - 782 kJ

  11. Calorimetry: the universe in the palm of your hand • Heat given out or taken in by process is measured by temperature change of water in calorimeter (surroundings) • Insulation confines heat exchange to process and calorimeter

  12. Heat capacity and specific heat • Heat capacity, C: Heat required to raise temperature of body by 1ºC (extensive) • Specific heat (capacity), c (σ): Heat required to raise 1 gram of the substance by 1ºC (intensive)

  13. Constant pressure and constant volume • Heat capacity can be defined at constant pressure and constant volume • For solids and liquids they are similar • For gases the values differ by R (the ideal gas constant): CP = CV + R

  14. Molar heat capacity • Related to specific heat (sometimes known as specific heat capacity) is the molar heat capacity, Cm: the heat required to raise the temperature of 1 mole of a substance by 1ºC

  15. Some examples • The heat capacity of water is much larger than many other substances • Consequence of strong H-bonding • Important consequences for climate, the quality of life, and life itself

  16. Hess’s law and the many pathways • Enthalpy being a state function means that enthalpy change for process is independent of pathway • Hess’s law states that: Overall enthalpy change for reaction equals sum of enthalpy changes for individual steps

  17. Application: determining ΔH for unknown process in terms of ΔH for known process • Consider the reaction Proceeds in two steps ΔH1 = ΔH2 + ΔH3 • Reaction 1 is sum of reactions 2 + 3

  18. Obtain ΔH2 from ΔH1 and ΔH3 • ΔH2, which is hard to determine experimentally, is obtained using ΔH1 and ΔH3 • ΔH2 is endothermic, the others are exothermic

  19. Combustion of CH4 by alternate routes • Route 1: complete combustion to CO2 • Route 2: intermediate product is CO • Different pathways, same heat

  20. Standard heats of formation • Standard heat of formation, ΔHfº: The enthalpy change for the formation of 1 mole of a substance in its standard state from it constituent elements in their standard states • ΔHºf for elements in most stable state is by definition 0

  21. Reactions are often hypothetical • ΔHºf (CH4) is -74.8 kJ/mol although direct reaction between C and H2 does not occur • Important to know which is the standard state for any substance • ΔHºf values are tabulated for many substances

  22. Heats of reaction, ΔHfº, and Hess’s law • Application of ΔHºf to calculation of heats of reaction (ΔHºRXN) • For reaction: aA + bB = cC + dD • Products – reactants

  23. Estimating ΔHºf using bond energies • What if ΔHºf value(s) for reaction is (are) unknown? • Estimate enthalpy change as difference between bonds broken (energy cost) and bonds made (energy gain) • ΔHºRXN = D(reactant bonds) – D(product bonds) • (Bond energies are positive values) GAIN COST

  24. Consider reaction: • On reactant side: • 3 C-H bonds and 3 Cl-Cl bonds are broken • On product side: • 3 C-Cl bonds and 3 H-Cl bonds are made • Reaction is exothermic • H-Cl bond is much stronger than Cl-Cl

  25. More than enthalpy • Some reactions are exothermic • Some reactions are endothermic • Both kinds of reaction can occur spontaneously. Or not. Why? • The enthalpy is not the arbiter of spontaneity • There is more to energy than heat • We must also consider entropy • Stay tuned for the Second Law…

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