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Chapter 11 Notes, part IV

Chapter 11 Notes, part IV. Heat and Reactions. Why are some reactions endothermic and others exothermic?. All compounds have some potential energy stored in their bonds. What would happen if there was more potential energy in the reactants than in the products? Where would that energy go?.

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Chapter 11 Notes, part IV

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  1. Chapter 11 Notes, part IV Heat and Reactions

  2. Why are some reactions endothermic and others exothermic? • All compounds have some potential energy stored in their bonds. • What would happen if there was more potential energy in the reactants than in the products? Where would that energy go?

  3. Into the surroundings!!!

  4. What about… • If there was more potential energy in the products than in the reactants? Where would that energy come from?

  5. From the surroundings!!!

  6. Heat in Reactions • Enthalpy (H)—the measure of internal heat content of a substance (stored energy). • The change in enthalpy (DH) is the same as heat lost/gained.

  7. Heat of Reaction • In a reaction, if DH is negative the reaction is exothermic. • If DH is positive, the reaction is endothermic. • Why?

  8. Heat of Reaction • A thermochemical equation is an equation that includes the heat change (DH) that occurs during that reaction. • The heat of reaction is the heat change for the equation as written. NOTE: There is absolutely no reason for this animation.

  9. Using Heat of Reaction • Heat of reaction gets used the same way as calculating stoichiometry.

  10. 2NaOH + H2SO4 Na2SO4 + 2H2O DH = -145kJ • How much heat would be produced by the reaction of 12.2g H2SO4 with excess NaOH?

  11. CaCO3 CaO + CO2DH = +110kJ • How much heat is required to decompose 12.5g of CaCO3? • If the temperature is 200oC, what volume of gas would be generated at 1.00atm with 1200kJ heat applied?

  12. Heat of Solution • Heat that is produced or absorbed by the dissolving of a substance into water. • Essentially, it is the same as the heat of reaction, but since it is a physical process, it has a different name.

  13. Practice Problem 3 • How many kilojoules of heat is released when 2500 mol NaOH(s) is dissolved in water? NaOH(s) + H2O(l)  Na+(aq) + OH(aq) ΔHsoln = –445.1 kJ/mole

  14. Finding DH • DH can be found for reactions experimentally, but some reactions are too slow or too difficult to measure. Rusting is exothermic, but takes too long to easily measure!

  15. Hess’s Law • Hess’s Law says that heat of reaction is a state function. • In a nutshell, this means that if you add 2 or more thermochemical equations, you can determine the heat of reaction indirectly!

  16. Hess’s Law • Find the heat of reaction when: C(s, diamond)  C(s, graphite) Given that: C(s, diamond) + O2(g)  CO2(g) DH = -395.4kJ C(s, graphite) + O2(g) CO2(g) DH = -393.5kJ

  17. Practice Problem • Find the heat of reaction for: PbCl2(s) + Cl2(g)  PbCl4(l) • Given that: Pb(s) + 2Cl2(g)  PbCl4(l) DH = -329.2kJ Pb(s) + Cl2(g)  PbCl2(s) DH = -359.4kJ

  18. Finding DHo with DHfo • We can find the heat of reaction by using heat of formation! • What do you think heat of formation is?

  19. DHo = DHfo(products) – DHfo(reactants) • The heat of reaction is equal to the heat of formation of products minus reactants. • Remember, heat of formation for elements is usually zero.

  20. Practice Problem • What is the heat of reaction for sulfur dioxide gas and oxygen gas producing sulfur trioxide gas at 25oC and 101.3kPa?

  21. Practice Problem • What is the heat of reaction for the combustion of methane (a gas) with oxygen gas to form carbon dioxide gas and water vapor?

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