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Thermochemistry

Thermochemistry. Part II : Enthalpy. Review: Temperature. A measure of the average kinetic energy of the particles that make up a substance or system Change in temp represents a change in kinetic energy. Systems and surroundings:.

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Thermochemistry

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  1. Thermochemistry Part II : Enthalpy

  2. Review: Temperature • A measure of the average kinetic energy of the particles that make up a substance or system • Change in temp represents a change in kinetic energy

  3. Systems and surroundings: • System: area where reaction takes place (generally is the reaction) • Surroundings: outside of the system • Closed verses open systems

  4. Open-system • both matter and energy can freely cross from the system to the surroundings and back. • Ex: an open test tube

  5. Closed-System • energy can cross the boundary, but matter cannot. • Ex: a sealed test tube

  6. Isolated-System • neither matter nor energy can cross between the system and the surroundings. • Ex: The universe • there are no surroundings to exchange matter or energy with (as far as we know!)

  7. Enthalpy (∆H) • a.k.a heat of reaction • Chemical rxn (and phase changes) involve changes in potential energy not kinetic • Enthalpy is the potential energy change of a system during process like chem rxn (or phase change) • Units: kJ/mol

  8. Chemical Rxns: • The internal energy of a reactant or product cannot be measured, but their change in enthalpy can • Enthalpy changes represent the diff between PE of products and PE of reactants • PE changes result from bonds being broken and/or formed

  9. Breaking a bond requires energy!

  10. Creating a bond releases energy ! (Strength of bond determines the energy needed to break or create bond)

  11. Exothermic and Endothermic Rxns:

  12. Exo verses Endo: • If energy is released out of the system then it is considered to be an exothermic enthalpy change (exit). • If energy is absorbed into the system then it is considered to be an endothermic enthalpy change (enter).

  13. There are several different enthalpies that we will be looking at : • Enthalpy of Rxn (∆Hrxn) • Standard enthalpy of Rxn (∆H0rxn) -- at standard temp (25°C) and pressure (100 kPa) • Standard molar enthalpy of formation (∆H0f) • Standard molar enthalpy of combustion (∆H0comb) • We use tables to find these VALUES!

  14. Standard enthalpy of Rxn (∆H0rxn) • Exothermic and endothermic • 3 ways to represent: • Thermochemical equation (includes energy in eq) • Chem eq + ∆H0rxn beside • Enthalpy diagrams

  15. Exothermic: • ∆H0is always negative!!!!! • Energy written on product side • H2 + ½ O2 H2O + 285.8 kJ • H2 + ½ O2 H2O ∆H0rxn= -285.8 kJ/mol

  16. Exothermic cont’d: Hint: • Arrow down for neg and up for pos • arrow goes toward the products

  17. Endothermic: • ∆H0is always positive!!!!! • Energy written on reactant side • 117.3 kJ + MgCO3 MgO + CO2 • MgCO3 MgO + CO2∆H0rxn= 117.3 kJ/mol

  18. Endothermic cont’d: Hint: • Arrow down for neg and up for pos • arrow goes toward the products

  19. Compare:

  20. Enthalpy dependent on molar amounts: • Linearly dependent (multiply by the stoich coefficients) • Ex:

  21. Standard molar enthalpy of formation (∆H0f) • Is the quantity of energy that is absorbed (+) or released (-) when one mole of a compound is formed directly from its elements (so this is why you see ½ as coefficient) • Ex:

  22. Standard molar enthalpy of combustion (∆H0comb) • Enthalpy associated with combustion of 1 mol of substance • Ex:

  23. HW  • Pg 643 # 15, 16, 17, 18

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