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Thermochemical equations

Thermochemical equations. 16.3. 16.3 Thermochemical equations. Thermochemical equation = a balanced chemical equation that includes the physical states of all reactants and products and the enthalpy change (∆H). Remember that enthalpy is the heat content of a system at constant pressure.

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Thermochemical equations

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  1. Thermochemical equations 16.3

  2. 16.3 Thermochemical equations • Thermochemical equation = a balanced chemical equation that includes the physical states of all reactants and products and the enthalpy change (∆H). • Remember that enthalpy is the heat content of a system at constant pressure.

  3. EXOTHERMIC 4 Fe (s) + 3O2(g) 2Fe2O3(s)+ 1625 kJ 4 Fe (s) + 3O2(g) 2Fe2O3(s)∆H= -1625 kJ The negative value for ∆H means that the system (reaction) is losing energy, giving off heat, is exothermic.

  4. ENDOTHERMIC 27 kJ + NH4NO3(s) NH4+(aq)+ NO3-(aq) NH4NO3(s) NH4+(aq)+ NO3-(aq) ∆H = 27 kJ The positive value for ∆H means that the system (reaction) is gaining energy, taking in heat, is endothermic.

  5. Enthalpy of Combusion • ∆Hcomb is the enthalpy change for the complete burning of one mole of substance. • Combustion of glucose: C6H12O6(s) + 6O2 6CO2 + 6H2O ∆Hcomb = - 2808 kJ The negative value means heat is given off (exothermic.) For 1 mole of glucose, 2808 kJ is given off.

  6. Table 16-5 on page 501 • ∆Hcomb for octane = -5471 kJ/mol • Translation: 5471 kJ is given off from the combustion of one mole of octane. • Which gives off more energy: combustion of propane or methane? • Propane (-2219 kJ/mol) • (Methane is only -891 kJ/mol)

  7. CHANGES OF STATE • Solid to liquid; liquid to gas; solid to gas • All take in energy (endothermic) • ∆H is + • Gas to liquid; liquid to solid; gas to solid • All lose energy (exothermic) • ∆H is -

  8. Changes of State • ∆Hvap (enthalpy of vaporization) = heat required to vaporize one mole of a liquid. (liquid to gas) • ∆Hfus (enthalpy of fusion) = heat required to melt one mole of a solid substance (solid to liquid)

  9. ∆Hfus and ∆Hvap Both are endothermic, so their values are positive. Table 16-6 on page 502 Does it take more heat to vaporize one mole of water or one mole of ammonia? Water (Hvap = 40.7 kJ/mol). (Ammonia is only 23.3 kJ/mol)

  10. STOICHIOMETRY • It’s back!! • Remember 1 mol = 6.02 x 1023 r.p. 1 mol = 22.4 L (gas at STP) 1 mol = molar mass (periodic table) NOW ADD: 1 mol = __kJ (from ∆H)

  11. How do you use it? • Write the given with UNIT. • Use unit multipliers to cancel units until you get your answer. • Look up correct ∆H on table 16-5 or 16-6, that number of kJ = 1 mol. • Round to the correct significant digits.

  12. EXAMPLE • Calculate the heat required to melt 25.7 g of solid methanol at its melting point. (∆H= 3.22kJ/mol) 25.7 g CH3OH x 1 mol x 3.22 kJ = 32.05 g 1 mol 2.58 kJ

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