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Enthalpy

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Enthalpy

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  1. Enthalpy Most reactions we investigate occur under conditions of constant PRESSURE (not Volume) ENTHALPY: Heat of reaction at constant pressure! Use a “coffee-cup” calorimeter to measure it Heat capacity Exercise: When 50mL of 1M HCl is mixed with 50mL of 1M NaOH in a coffee-cup calorimeter, the temperature increases from 21°C to 27.5°C. What is the enthalpy change, if the density is 1g/mL and specific heat 4.18 J/g.K?

  2. Perfect gas enthalpy • Use intensive property of molar enthalpy and internal energy • At 25°C, RT = 2.5 kJ/mol Thermicity-Revision Endothermic reaction (q>0) results in an increase in enthalpy (DH>0) Exothermic reaction (q<0) results in an increase in enthalpy (DH<0) NB: Internal energy and Enthalpy are STATE FUNCTIONS

  3. Temperature variation of enthalpy Convenient empirical expression to use for heat capacity is: Exercise: What is the change in molar enthalpy of N2 when it is heated from 25°C to 100 °C, given that:

  4. Relation between heat capacities

  5. Thermochemistry Chemists report data for a set of standard conditions: The standard state of a substance (°) is the pure substance at exactly 1 bar It is conventional (though not obligatory) to report data for a T of 298.15K Standard enthalpies of phase transition Energy that must be supplied (or is evolved) as heat, at constant pressure, per mole of molecules that undergo the phase transition under standard conditions (pure phases), denoted DH° Note: the enthalpy change of a reverse transition is the negative of the enthalpy change of the forward transition

  6. DH°

  7. Sublimation • Direct conversion of a solid to a vapour The enthalpy change of an overall process is the sum of the enthalpy changes for the steps into which it may be divided

  8. Enthalpies of ionisation (kJ/mol) DionH°(T)= Ionisation energy(0) + (5/2)RT (see Atkins & de Paula, Table 3.2)

  9. Problems • Ethanol is brought to the boil at 1 atm. When the electric current of 0.682 A from a 12.0 V supply is passed for 500 s through a heating coil immersed in the boiling liquid, it is found that the temperature remains constant but 4.33 g of ethanol is vapourised. What is the enthalpy of vapourisation of ethanol at its boiling point at 1 atm? • Calculate the standard enthalpy of sublimation of ice at 0°C given that DfusH° is 6.01 kJ/mol and DvapH°is 45.07 kJ/mol, both at 0°C. • DsubH° for Mg at 25°C is 148 kJ/mol. How much energy as heat must be supplied to 1.00 g of solid magnesium metal to produce a gas composed of Mg2+ ions and electrons?

  10. Bond enthalpies (kJ/mol)

  11. Problem • Estimate the standard reaction enthalpy for the formation of liquid methanol from its elements as 25°C

  12. Enthalpies of combustion Enthalpies (heats) of combustion: complete reaction of compounds with oxygen. Measure using a bomb calorimeter. Most chemical reactions used for the production of heat are combustion reactions. The energy released when 1g of material is combusted is its Fuel Value. Since all heats of combustion are exothermic, fuel values are reported as positive. • Most of the energy our body needs comes from fats and carbohydrates. • Carbohydrates are broken down in the intestines to glucose. Glucose is transported in the blood to cells where it is oxidized to produce CO2, H2O and energy: • C6H12O6(s) + 6O2(g)  6CO2(g) + 6H2O(l) DcH°=-2816 kJ • The breakdown of fats also produces CO2 and H2O • Any excess energy in the body is stored as fats

  13. Heats of formation If one mole of the compound is formed under standard conditions from its elements in their reference state then the resulting enthalpy change is said to be the standard molar enthalpy (Heat) of formation, fH° where the subscript indicates this. The reference state is the most stable form under the prevailing conditions.

  14. Hess’s Law To evaluate unknown heats of reaction The standard enthalpy of a reaction is the sum of the standard enthalpies for the reactions into which the overall reaction may be divided rxnHo = nDfHom(products) - nDfHom(reactants)

  15. Variation of DrH° with T DrH°(T2) = DrH°(T1) + DrCp°(T2-T1) Kirchoff’s Law DrCp° = S nCp,m°(products) - S nCp,m°(reactants) If heat capacity is temperature dependent, we need to integrate over the temperature range

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