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Chemistry 17.4

Chemistry 17.4. Section 17.4. Calculating Heats of Reaction. Calculating Heats of Reaction. 17.4.

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Chemistry 17.4

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  1. Chemistry 17.4 Section 17.4 Calculating Heats of Reaction

  2. Calculating Heats of Reaction 17.4 • Emeralds are composed of the elements chromium, aluminum, silicon, oxygen, and beryllium. What if you wanted to determine the heat of reaction without actually breaking the gems down to their component elements? You will see how you can calculate heats of reaction from known thermochemical equations and enthalpy data.

  3. 17.4 Hess’s Law • Hess’s Law • What are two ways that you can determine the heat of reaction when it cannot be directly measured?

  4. 17.4 Hess’s Law • Hess’s law allows you to determine the heat of reaction indirectly. • Hess’s law of heat summation states that 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.

  5. 17.4 Hess’s Law

  6. 17.4 Hess’s Law

  7. 17.4 Standard Heats of Formation • Standard Heats of Formation • For a reaction that occurs at standard conditions, you can calculate the heat of reaction by using standard heats of formation.

  8. 17.4 Standard Heats of Formation • The standard heat of formation (∆Hf0) of a compound is the change in enthalpy that accompanies the formation of one mole of a compound from its elements with all substances in their standard states at 25°C.

  9. 17.4 Standard Heats of Formation

  10. 17.4 Standard Heats of Formation • The Standard Heat of Formation of Water

  11. 17.7

  12. 17.7

  13. 17.7

  14. 17.7

  15. 17.4 Standard Heats of Formation

  16. 17.4 Section Quiz. • 1. According to Hess’s law, it is possible to calculate an unknown heat of reaction by using • heats of fusion for each of the compounds in the reaction. • two other reactions with known heats of reaction. • specific heat capacities for each compound in the reaction. • density for each compound in the reaction.

  17. 17.4 Section Quiz. • 2. The heat of formation of Cl2(g) at 25°C is • the same as that of H2O at 25°C. • larger than that of Fe(s) at 25°C. • undefined. • zero.

  18. 17.4 Section Quiz. • 3. Calculate  H0 for NH3(g) + HCl(g)  NH4Cl(s). Standard heats of formation:NH3(g) = 45.9 kJ/mol, HCl(g) = 92.3 kJ/mol, NH4Cl(s) = 314.4 kJ/mol • 176.2 kJ • 360.8 kJ • 176.2 kJ • 268 kJ .

  19. The End

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