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Chemistry 100

Chemistry 100. Enthalpy and Hess’s Law. Energy Changes in Chemical Reactions. Let’s take a typical reaction CH 4 (g) + O 2 (g)  CO 2 (g) + 2 H 2 O (l) This is an example of ‘burning a fossil fuel.’ This reaction releases energy! What about this reaction? KCl (s)  KCl (aq)

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Chemistry 100

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  1. Chemistry 100 Enthalpy and Hess’s Law

  2. Energy Changes in Chemical Reactions • Let’s take a typical reaction CH4 (g) + O2 (g)  CO2 (g) + 2 H2O (l) • This is an example of ‘burning a fossil fuel.’ This reaction releases energy! • What about this reaction? KCl (s)  KCl (aq) • This reaction requires energy!

  3. Energy Changes System energy surroundings Almost all chemical and physical processes are accompanied by the loss or gain of energy from the system.

  4. Enthalpy Enthalpy H º the heat content of a substance. Enthalpy change - DH thermal energy (heat) evolved or absorbed under constant pressure conditions.

  5. Exothermic vs. Endothermic Processes • Exothermic process ® heat content of system decreases, the system enthalpy decreases (i.e., DH < 0) • e.g., the combustion of methane CH4 (g) + O2 (g)  CO2 (g) + 2 H2O (l) • Endothermic process ® energy of the system is increased (i.e. DH > 0). • e.g., dissolving potassium chloride in water. KCl (s)  KCl (aq)

  6. The Enthalpy Change in a Chemical Reaction • We want to know how much energy is released when we burn ethane! C2H6 (g) + 7/2 O2 (g)  2 CO2 (g) + 3 H2O (l) • We wish to know the difference in the heat content of the reactants vs. the products. • H   npH (products) -  nr H (reactants) • np and nr represent the number of moles of products and reactants, respectively.

  7. The Enthalpy Change (cont’d) • For the ethane combustion reaction 1C2H6 (g) + 7/2 O2 (g)  2 CO2 (g) + 3 H2O (l) H   npH (products) -  nr H (reactants) = 3 H[H2O (l)] + 2 H[CO2 (g)] - (7/2 H[O2(g)] + 1 H[C2H6 (g)] )

  8. Conventions for Writing Thermochemical Equations • For exothermic reactions, H <0; for endothermic reactions, H > 0. • H values are given for processes occurring at 25C and 1 atm pressure • STTP - standard thermodynamic temperature and pressure.

  9. Conventions (cont’d) • The physical state of each substance participating in the process must be stated • s  solid • l  liquid • g  gas • aq  aqueous solutions

  10. Conventions (cont’d) Fractional stoichiometric coefficients are permitted in thermochemical equations. The stoichiometric coefficients = the number of moles of each substance involved in the transformation

  11. Conventions (cont’d) • Multiply or divide a chemical equation by a factor, the H value must also be multiplied or divided by that factor • Enthalpy is an extensive property. • When an equation is reversed, the sign of the H value is changed but its magnitude stays the same.

  12. DH Values and Hess’s Law • Calculating enthalpy changes for physical and chemical transformations. • tabulated values of reaction enthalpies (largely measured with a calorimeter). • indirect Method – Hess’s Law.

  13. Hess’s Law • Hess’s Law - enthalpy changes for sequences of reactions. • Enthalpy change accompanying a reaction is the same whether the reaction occurs in a single step or in many steps.

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