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Energy, Kinetics and Equilibrium in Chemical Reactions

Energy, Kinetics and Equilibrium in Chemical Reactions. ENERGY. ENTHALPY. e nergy absorbed as heat during a chemical reaction at constant pressure cannot be measured directly, it is not the same as temperature Unit joules (J) Symbol H. ENTHALPY.

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Energy, Kinetics and Equilibrium in Chemical Reactions

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  1. Energy, Kinetics and Equilibrium in Chemical Reactions

  2. ENERGY

  3. ENTHALPY • energy absorbed as heat during a chemical reaction at constant pressure • cannot be measured directly, it is not the same as temperature • Unit • joules (J) • Symbol • H

  4. ENTHALPY • theamount of heat absorbed or lost by a system during a process • Enthalpy of reaction is the quantity of energy transferred as heat during a chemical reaction • the difference between the stored energy of the reactants and the products

  5. SYSTEM VS. SURROUNDINGS

  6. ENTHALPY • DH = H products – H reactants

  7. ENTHALPY EXAMPLE Ex: Given the equation: C2H4(g) + H2(g) C2H6(g) The product of the reaction has an enthalpy value of -285.8 kJ/mol and the reactants enthalpy values are -1410.9 kJ/mol and +1559.7 kJ/mol, respectively. What is the enthalpy of this reaction?

  8. ENTHAPY PROBLEM • H products= -285.8 kJ/mol • H reactants = (-1410.9kJ/mol + 1559.7 kJ/mol) = 148.8 kJ/mol DH = H products – H reactants DH = -285.8 kJ/mol - 148.8 kJ/mol = -434.6kJ/mol

  9. SIGNIFICANTS OF SIGN • DH = + • Endothermic reaction • heat is absorbed by the system • the energy of the products is greater than the energy of the reactants

  10. ENDOTHERMIC RXN DIAGRAM

  11. SIGNIFICANTS OF SIGN • DH = - • Exothermic reaction • heat is released to the surroundings • the energy of the reactants is greater than the energy of the products

  12. EXOTHERMIC RXN DIAGRAM

  13. THERMOCHEMICAL EQUATIONS • Shows the energy involved in the reaction • If the reaction is exothermic the DH value is on the product side • If the reaction is endothermic the DH value is on the reactant side

  14. EXAMPLES • 2 H2(g) + O2(g)  2 H2O + 483.6 kJ • Is this reaction exothermic or endothermic? • 2C(s) + 2H2(g)+ 52.3 kJ  C2H4(g) • Is this reaction exothermic or endothermic?

  15. Energy per mole • Because the enthalpy values represent kJ/mol we can calculate the energy released or absorbed in a chemical reaction. • Ex: Given the following equation find the energy released when 0.25 moles of C2H2 reacts in a complete combustion reaction. The enthalpy of combustion of C2H2 is -1301.1 kJ/mol.

  16. EXAMPLE • Step 1: write the balanced equation. (remember the DH value is per mol: this equation has 2 mols of C2H2(g) so we have to double the DH value in the equation) • 2 C2H2(g) + 5 O2(g)  2 CO2(g) + 2 H2O(l) + 2602.2 kJ • Step 2: solve the problem: in the reaction • 2 molsC2H2(g) have a DH = -2602.2 kJ (exothermic) • I want to know what is the DH value for 0.25 mol C2H2(g)

  17. EXAMPLE • 2 C2H2(g) + 5 O2(g)  2 CO2(g) + 2 H2O(l) + 2602.2 kJ • 0.25 mol X 2602.2 kJ/2 mol = -325.2 kJ • Remember in the balanced equation the energy value is for 2 moles of C2H2(g)

  18. ENTROPY • A measure of the degree of randomness of the particles, such as molecules in a system. • Reactions favor progress towards greater randomness

  19. ENTROPY • The entropy of a pure crystalline solid is zero at absolute zero • As energy is added molecular motion increases • The change is enthalpy can be calculated

  20. ENTROPY • Change in entropy is the difference between entropy of the products and the reactants • Increase in entropy is a positive value • Decrease in entropy is a negative value

  21. ENTROPY • Symbol for entropy S • Symbol for change in entropy DS • Unit kJ/mol * K • Entropy is temperature dependent

  22. ENTROPY • The process of forming a solution involves an increase in entropy • Solids are more ordered, they have low entropy • Liquids are less ordered than solids, the molecules have more movement, they have more entropy than solids

  23. ENTROPY • Gases have very little order, they move very fast and take the shape of their container. They have the most entropy. • (s) < (l) < (g)

  24. ENTROPY • CH4(g) + H2O(g)  CO(g) + 3H2(g) • Does this reaction become more random or less random? • Will the reaction have a positive DS or a negative DS?

  25. EXAMPLE • Determine if the entropy change will be positive or negative for the following reactions:(NH4)2Cr2O7(s) → Cr2O3(s) + 4 H2O(l) + CO2(g)2 H2(g) + O2(g) → 2 H2O(g)PCl5(g)→ PCl3(g)+ Cl2(g)

  26. FREE ENERGY (DG) • difference between the change in enthalpy and the product of the kelvin temperature and the entropy change • DG = DH – TDS • This can be used to make predictions as to the reactions ability to progress spontaneously.

  27. SPONTANEOUS REACTION PREDICTION

  28. SPONTANEOUS REACTION PREDICTION

  29. EXAMPLE • The gas-phase reaction of H2 and CO2 to produce H2O and CO2 has DH = +11 kJ and DS = +41 J/mol*K. Is the reaction spontaneous at 298.15K? What is the DG?

  30. ANSWER • DG = DH – TDS • DG = 11kJ – (298.15K * 0.041 kJ/mol *K) • DG = -1.2 kJ • The reaction will be spontaneous

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