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Energetics

Energetics. 6.1 What is Energetics? 6.2 Ideal Enthalpy Changes Related to Breaking and Formation of Bonds 6.3 Standard Enthalpy Changes. 6.4 Experimental Determination of Enthalpy Changes by Calorimetry 6.5 Hess’s Law 6.6 Calculations involving Enthalpy Changes of Reactions. 6.1.

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Energetics

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  1. Energetics 6.1 What is Energetics? 6.2 Ideal Enthalpy Changes Related to Breaking and Formation of Bonds 6.3 Standard Enthalpy Changes

  2. 6.4 Experimental Determination of Enthalpy Changes by Calorimetry 6.5 Hess’s Law 6.6 Calculations involving Enthalpy Changes of Reactions

  3. 6.1 What is Energetics? Energetics is the study of energy changes associated with chemical reactions. Thermochemistry is the study of heat changes associated with chemical reactions. Some terms Enthalpy(H) = heat content in a substance Enthalpy change(H)= heat content of products - heat content of reactants= Hp - Hr

  4. 6.1 Change in internal energy Heat change atconstant pressure Work done on the surroundings = + (Heat change at constant volume) Internal Energy and Enthalpy e.g. Zn(s) + 2HCl(aq)  ZnCl2(aq) + H2(g) Enthalpy change

  5. 6.1 Exothermic and Endothermic Reactions An exothermic reaction is a reaction that releases heat energy to the surroundings. (H = -ve)

  6. 6.1 Exothermic and Endothermic Reactions An endothermic reaction is a reaction that absorbs heat energy from the surroundings. (H = +ve)

  7. 6.2 Enthalpy Changes Related to Breaking and Forming of Bonds e.g. CH4 + 2O2 CO2 + 2H2O In an exothermic reaction, the energy required in breaking the bonds in the reactants is less than the energy released in forming the bonds in the products (products contain stronger bonds).

  8. 6.2 Enthalpy Changes Related to Breaking and Forming of Bonds In an endothermic reaction, the energy required in breaking the bonds in the reactants is more than the energy released in forming the bonds in the products (reactants contain stronger bonds).

  9. 6.2 Bond Enthalpies To be discussed in later chapters.

  10. ø Enthalpy change under standard conditions denoted by symbol: H Standard Enthalpy Changes CH4(g) + 2O2(g)  CO2(g) + 2H2O(g) H = -802 kJ mol-1 CH4(g) + 2O2(g)  CO2(g) + 2H2O(l) H = -890 kJ mol-1 As enthalpy changes depend on temperature and pressure, chemists find it convenient to report enthalpy changes based on an internationally agreed set standard conditions: 1. elements or compounds in their normal physical states;2. a pressure of 1 atm (101325 Nm-2); and3. a temperature of 250C (298 K)

  11. ø The standard enthalpy change of neutralization (Hneut) is the enthalpy change when one mole of water is formed from the neutralization of an acid by an alkali under standard conditions. e.g. H+(aq) + OH-(aq)  H2O(l) Hneut = -57.3 kJ mol-1 ø Standard Enthalpy Changes of Reactions Standard Enthalpy Change of Neutralization e.g. The standard enthalpy change of neutralization between HNO3 and NaOH is -57.3 kJ mol-1 e.g. The standard enthalpy change of neutralization between HCl and NaOH is -57.1 kJ mol-1

  12. Acid Alkali Hneu ø HCl HCl HCl HF NaOH KOH NH3 NaOH -57.1 -57.2 -52.2 -68.6 Standard Enthalpy Changes of neutralization H+(aq) + OH-(aq)  H2O(l)

  13. 6.3 The standard enthalpy change of solution (Hsoln) is the enthalpy change when one mole of a solute is dissolved to form an infinitely dilute solution under standard conditions. ø e.g. NaCl(s) + water  Na+(aq)+Cl-(aq) Hsoln=+3.9 kJ mol-1 ø e.g. LiCl(s) + water  Li+(aq) + Cl-(aq) Hsoln=-37.2 kJ mol-1 ø Standard Enthalpy Changes of Reactions Standard Enthalpy Change of Solution Note that enthalpy changes of solution associate with physical changes.

  14. 6.3 Salt Hsoln(kJ mol-1) ø NaOH NaCl KOH KBr -44.7 +3.9 -57.8 +20.0 Standard Enthalpy Changes of solution

  15. 6.3 The standard enthalpy change of formation (Hf) is the enthalpy change of the reaction when one mole of the compound in its standard state is formed from its constituentelements under standard conditions. ø ø e.g. 2Na(s) + Cl2(g)  2NaCl(s)H = -822 kJ mol-1 ø Na(s) + ½Cl2(g)  NaCl(s)Hf = -411 kJ mol-11 mole Standard enthalpy change of formation of NaCl is -411 kJ mol-1. Standard Enthalpy Changes of Reactions Standard Enthalpy Change of Formation

  16. 6.3 What is Hf [N2(g)] ? ø Hf [N2(g)] = 0 ø Standard Enthalpy Changes of Reactions N2(g)  N2(g) The enthalpy change of formation of an element is always zero.

  17. 6.3 Standard Enthalpy Changes of Reactions Standard Enthalpy Change of Combustion e.g. C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(l) H1 = -2220 kJ 2C3H8(g) + 10O2(g) 6CO2(g) + 8H2O(l) H2 = ? H2 = -4440 kJ  It is more convenient to report enthalpy changes per mole of the main reactant reacted/product formed.

  18. 6.3 The standard enthalpy change of combustion (Hc) of a substance is the enthalpy change when one mole of the substance burns completely under standard conditions. ø e.g. C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(l) Hc = -2220 kJ mol-1 1 mole ø The standard enthalpy change of combustion of propane is -2220 kJ mol-1 Standard Enthalpy Changes of Reactions Standard Enthalpy Change of Combustion

  19. 6.3 ø Substance Hc(kJ mol-1) H2(g) C (diamond) C (graphite) CO(g) CH4(g) -285.8 -395.4 -393.5 -283.0 -890.4 Standard Enthalpy Changes of Reactions

  20. 6.4 Experimental Determination of Enthalpy Changes by Calorimetry (p. 151) Experimental Determination of Enthalpy Changes by Calorimetry Calorimeter = a container used for measuring the temperature change of solution Determination of Enthalpy Change of Neutralization

  21. 6.4 Experimental Determination of Enthalpy Changes by Calorimetry (p. 151) Heat evolved = (m1c1 + m2c2) ΔT Where m1 is the mass of the solution, m2 is the mass of calorimeter, c1 is the specific heat capacity of the solution, c2 is the specific heat capacity of calorimeter, And Δ T is the temperature change of the reaction.

  22. 6.4 Experimental Determination of Enthalpy Changes by Calorimetry (p. 153) Determination of Enthalpy Change of Combustion

  23. 6.4 Experimental Determination of Enthalpy Changes by Calorimetry (p. 153) Heat evolved = (m1c1 + m2c2) ΔT Where m1 is the mass of water in the calorimeter, m2 is the mass of the calorimeter, c1 is the specific heat capacity of the water, c2 is the specific heat capacity of calorimeter, And Δ T is the temperature change of the reaction.

  24. 6.5 Hess’s Law (p. 157) Route 1 H1 H2 H3 E Route 2 Hess’s Law A + B C + D H1 = H2+H3 Hess’s Law states that the total enthalpy change accompanying a chemical reaction is independent ofthe route by which the chemical reaction takes place. Why?

  25. 6.5 Hess’s Law (p. 158) Importance of Hess’s Law The enthalpy change of some chemical reactions cannot be determined directly because: • the reactions cannot be performed in the laboratory • the reaction rates are too slow • the reactions may involve the formation of side products But the enthalpy change of such reactions can be determined indirectly by applying Hess’s Law.

  26. 6.5 Hess’s Law (p. 158) Given: Hf [CO2(g)] = -393.5 kJ mol-1; Hc [CO(g)] = -283.0 kJ mol-1 ø ø Hf [CO(g)] C(graphite) + ½O2(g) CO(g) ø + ½O2(g) + ½O2(g) H1 H2 CO2(g) Hf [CO(g)] +H2=H1 ø Hf [CO(g)] =H1 -H2 ø Enthalpy Change of Formation of CO(g) = -393.5 -(-283.0 ) = -110.5 kJ mol-1

  27. 6.5 Hf CO(g)] +H2=H1 ø Enthalpy Change of Formation of CO(g)

  28. 6.5 ΔH ø MgSO4(s) + 7H2O(l) MgSO4·7H2O(s) aq ΔH1 ΔH2 Mg2+(aq) + SO42-(aq) + 7H2O(l) ø ΔH = enthalpy of hydration of MgSO4(s) ΔH1 = molar enthalpy change of solution of anhydrous magnesium sulphate(VI) ΔH2 = molar enthalpy change of solution of magnesium sulphate(VI)-7-water ΔH = ΔH1 - ΔH2 ø Enthalpy Change of Hydration of MgSO4(s) aq

  29. 6.6 Calculations involving Enthalphy Changes of Reactions (p. 161) Hreaction products reactants ø ø  Hf [products] -  Hf [reactants] elements Hreaction =  Hf [products] -  Hf [reactants] ø ø Calculations involving standard enthalpy changes of reactions

  30. The END

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