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Thermodynamics

Thermodynamics. 1.2.1 Enthalpy. 2.1 Enthalpy. Enthalpy is the heat content of a system, or the amount of energy within a substance , both kinetic and potential.

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Thermodynamics

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  1. Thermodynamics 1.2.1 Enthalpy

  2. 2.1 Enthalpy • Enthalpy is the heat content of a system, or the amount of energy within a substance, both kinetic and potential. • Every substance possesses both potential energy, and kinetic energy from to the constant motion of the particles. This total amount of energy is enthalpy.

  3. Enthalpy • Symbol for enthalpy: H • Unit for enthalpy:the joule, J

  4. Heat of Reaction • It isn’t possible to directly measure the heat content of a substance- chemists can only measure how much enthalpy changes • Therefore, we will generally refer to the change in enthalpy, or ΔH. This is also know as the heat of the reaction.

  5. Exothermic Reactions • During exothermic reactions, more energy is released during bond formation than is required to break bonds. An exothermic reaction is written like this: • Cu(s) + Cl2(g) → CuCl2(g) + 220.1 kJ • There is a net release of 220.1 kJ of energy, this appears on the product side.

  6. Exothermic reactions • Exothermic reactions release energy because the reactants have more potential energy than the products. • The "excess" energy is released to the surroundings.

  7. _ • This can be shown graphically. • The graph illustrates the amount of potential energy of the products is less than that of the reactants; potential energy decreased.

  8. When the amount of entropy in a system decreases a negative sign to indicate the drop. • Thus, we could rewrite the equation shown above as: • Cu(s) + Cl2(g) → CuCl2(g) + 220.1 kJ • Cu(s) + Cl2(g) → CuCl2(g) ΔH° =  -220.1 kJ

  9. Endothermic Reactions • During endothermic reactions, energy is absorbed from the surroundings. An endothermic reaction is written with the energy term on the reactant side: • H2O(g) + C(s) + 132 kJ → CO2(g) + 2H2(g) • H2O(g) + C(s) → CO2(g) + 2H2(g) ΔH° =132 kJ

  10. Endothermic reactions • An input of energy is required • The potential energy of the reactants is lower than the potential energy of the products; energy must be added to the reaction for it to occur.

  11. A potential energy graph ofan endothermic reaction

  12. We can remove the energy term from the equation and write it as a positive value, indicating that enthalpy increased: • H2O(g) + C(s) → CO2(g) + 2H2(g) ΔH° =  +132 kJ

  13. Practice problems 1. Identify each of the following reactions as exothermic or endothermic. • CH4(s) + 2O2(g) → CO2(g) + 2H2O(l) ΔH° = -890 kJ • 2HCl(g) → H2(g) + Cl2(g) ΔH° = 185 kJ • 4NH3(g) + 5O2(g) → 4NO(g) + 6H2O(l) ΔH° = -1169 kJ 2. When potassium nitrate dissolves in water, the beaker containing the solution gets cooler. Is dissolving this salt an exothermic or an endothermic process?

  14. Answers • 1. Identify each of the following reactions as exothermic or endothermic. • CH4(s) + 2O2(g) → CO2(g) + 2H2O(l) ΔH° = -890 kJ Exothermic • 2HCl(g) → H2(g) + Cl2(g) ΔH° = 185 kJ Endothermic • 4NH3(g) + 5O2(g) → 4NO(g) + 6H2O(l) ΔH° = -1169 kJ Exothermic • 2. When potassium nitrate dissolves in water, the beaker containing the solution gets cooler. Is dissolving this salt an exothermic or an endothermic process? • This is an endothermic process. The reaction requires heat, which it absorbs from the surroundings (the water). This causes the water temperature to decrease.

  15. Assignment • Assignment 1.1.1

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