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Enthalpy (H)

Enthalpy (H). H - amount of heat change that occurs during a chemical reaction Change in enthalpy ΔH = H products – H reactants Negative -ΔH = exothermic Positive ΔH = endothermic. Practice Problem.

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Enthalpy (H)

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  1. Enthalpy (H) H - amount of heat change that occurs during a chemical reaction Change in enthalpy ΔH = H products – H reactants • Negative -ΔH = exothermic • Positive ΔH = endothermic

  2. Practice Problem • Calculate the amount of heat generated when 5.2 grams of C2H6O is burned according to the following equation. C2H6O + O2 CO2 + H2O ΔH = -276 KJ

  3. Practice Problem • How many grams of sugar should a person eat to obtain their needed 2000 calories of intake for a day? C6H12O6 + 6 O2  6 CO2 + H2O ΔH = -2801 KJ

  4. 1 food Calorie C = 1000 calories = 4184 J

  5. Calorimetry – the measurement of heat change • Specific heat (s) – the amount of heat needed to raise the temperature of one gram of the substance by 1 ºC. • Heat capacity (C) - the amount of heat needed to raise the temperature of a given quantity of a substance by 1 ºC.

  6. Calorimetry heat = (mass) (specific heat) (ΔT) q = m s Δt

  7. Standard enthalpy of formation ΔHfº =the heat change that results when one mole of a compound is formed from its elements at 1 atm.

  8. Standard enthalpy of formation • Heat of formation is calculated based on an arbitrary reference point like that of sea level • With the most stable form of the element is set at 0

  9. Direct method of determination Cgraphite + O2(g) CO2(g) ΔHfº = -393.5

  10. Indirect - by calculation • Hess’s Law – when reactants are converted to products, the change in enthalpy is the same whether the reaction takes place in one step or in a series of steps.

  11. Practice exercise page 221 • Calculate the standard enthalpy of formation of cabon disulfide (CS2) from its elements, given that Cgraphite + O2(g) CO2(g) ΔHfº = -393.5 Srhombic + O2 (g) SO2(g) ΔHfº = -296.1 CS2 (l) + 3O2(g) CO2(g) + 2SO2 ΔHfº = -1072

  12. Calculate the ΔHrxnº ΔH º rxn = ΔHfº products - ΔHfº reactants

  13. Practice problem page 222 • Benzene (c6H6) burns in air to produce carbon dioxide and liquid water. Calculate the heat released in KJ/g of the compound reacted with oxygen. The standard enthalpy of formation of benzene is 49.04 KJ/mol

  14. Homework Chapter 6 12, 13, 18, 20, 30, 36, 38, 42, 44, 46, 48, 56, 58, 60, 66, 72, 78, 80

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