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Energy and Chemical Reactions Energetics

Energy and Chemical Reactions Energetics. Using Bond Enthalpies to calculate energy changes in a reaction. 1. Energy Changes. Breaking chemical bonds requires energy Forming new chemical bonds releases energy. 2. Exothermic and Endothermic Processes. Exothermic processes release energy

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Energy and Chemical Reactions Energetics

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  1. Energy and Chemical ReactionsEnergetics Using Bond Enthalpies to calculate energy changes in a reaction 1

  2. Energy Changes • Breaking chemical bonds requires energy • Forming new chemical bonds releases energy 2

  3. Exothermic and Endothermic Processes • Exothermic processes release energy C3H8 (g) + 5 O2 (g)  3 CO2 (g) + 4H2O (g) + 2043 kJ • Endothermic processes absorb energy C(s) + H2O (g)+113 kJ CO(g) + H2 (g) 3

  4. Exothermic reactions

  5. Examples of exothermic reactions • Combustion • Neutralization: acid + base  salt + water

  6. Endothermic Reactions

  7. Energy Changes in endothermic and exothermic processes In an endothermic reaction there is more energy required to break bonds than is released when bonds are formed. The opposite is true in an exothermic reaction. 7

  8. Enthalpy • Enthalpy is the heat absorbed or released during a chemical reaction where the only work done is the expansion of a gas at constant pressure • DH 8

  9. Enthalpy Changes • The change in enthalpy is designated by the symbol DH. • If DH < 0 the process is exothermic. • If DH > 0 the process is endothermic. 9

  10. Bond Enthalpies • The energy to required to break a covalent bond in the gaseous phase is called a bond enthalpy. • Bond enthalpy tables give the average energy to break a chemical bond. Actually there are slight variations depending on the environment in which the chemical bond is located 10

  11. Bond Enthalpy Table The average bond enthalpies for several types of chemical bonds are shown in the table below: 11

  12. Bond Enthalpies • By comparing the energies for breaking bonds in the reactants and forming bonds in the products one can determine the enthalpy change for a reaction 12

  13. Bond Enthalpies • Energy is required to break chemical bonds. Therefore when a chemical bond is broken it carries a positive charge • Energy is released when chemical bonds form. When chemical bonds are formed their energy change carries a negative charge 13

  14. Exothermic reaction: more energy is released in forming the bonds than in breaking the bonds.

  15. The endothermic reaction absorbs more energy breaking bonds than it releases in bond formation.

  16. Bond Enthalpy Calculations Example 1: Calculate the enthalpy change for the reaction N2 + 3 H2 2 NH3 • Draw the structural formulas • List the number and types of each kind of bond broken and formed • Bond breaking is endothermic, so add these • Bond formation is exothermic, so subtract these • Add the result to find the enthalpy of the overall reaction. 16

  17. Bonds broken • N=N = 944 • H-H 3(436) = 1308 • Total = 2252 kJ • Bonds formed • 2x3 = 6 N-H 6(388) = - 2336 kJ • Net enthalpy change • = + 2252 - 2336 = - 190 kJ

  18. Another problem • Example 2: Calculate the enthalpy change for the reaction O2 + 2 H2 2H2O

  19. Add bonds broken and subtract bonds formed Bonds broken (add endothermic) • O = O 496 • 2 H-H 2(436) Bonds formed (subtract exothermic) • 4 H-O 4(463) • 496 +2(436) – 4(463) = -484 kJ mol-1

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