9 10 9 11 Bond Energies Bond Lengths

9. 10 -9. 11 Bond Energies, Bond Lengths and Bonding in Metals

Bond Energy • Bond energy – the energy required to break one mole of the bond in the gas phase. • Bond energies are always positive because it takes energy to break bonds. Higher bond energies means that a stronger bond exists. • Average bond energies can be used to calculate ΔHrxn = ∑(ΔH’s bonds broken) + ∑(ΔH’s bonds formed) • A reaction is exothermic when weak bonds break and strong bonds form. • A reaction is endothermic when strong bonds break and weak bonds form.

Lets Try a Practice Problem! The reaction between hydrogen and oxygen to form water is highly exothermic. Which statement is true of the energies of the bonds that break and form during the reaction? a. ) The energy needed to break the required bonds is greater than the energy released when the new bonds form. b. ) The energy needed to break the required bonds is less than the energy released when the new bonds form. c. ) The energy needed to break the required bonds is about the same as the energy released when new bonds form. b. ) The energy needed to break the required bonds is less than the energy released when the new bonds form.

Let’s Try Another! A potential future fuel is methanol (CH 3 OH). Write a balanced equation for the combustion of gaseous methanol and use bond energies to calculate the enthalpy of combustion of methanol in k. J/mol. 2 CH 3 OH(g) + 3 O 2(g) 2 CO 2(g) + 4 H 2 O(g) Now look at your units (k. J/mol). We have to reduce the equation to show the combustion of one mole of methanol. CH 3 OH(g) + 3/2 O 2(g) CO 2(g) + 2 H 2 O(g) ΔHrxn = ∑(ΔH’s bonds broken) + ∑(ΔH’s bonds formed)

• For question on previous slide: – Draw the Lewis structure for each molecule. – Calculate the bond energies for each bond that is broken or formed. Avg. bond energies are on pg. 410 in your textbook. – Calculate ΔHrxn Bonds Broken: 3(C-H) = 3 (414 k. J/mol) 1(C-O) = 1 (360 k. J/mol) 1(H-O) = 1(464 k. J/mol) 1. 5(O=O) = 1. 5 (498 k. J/mol) ∑(ΔH’s bonds broken) = 2813 k. J/mol Bonds Formed 2 (C=O) = 2 (-799 k. J/mol) 4 (H-O) = 4 (-464 k. J/mol) ∑(ΔH’s bonds formed) = -3454 k. J/mol ΔHrxn = ∑(ΔH’s bonds broken) + ∑(ΔH’s bonds formed) ΔHrxn = ∑(2813 k. J/mol) + ∑(-3454 k. J/mol) = -641 k. J/mol

Bond Length • The average bond length has been calculated as the length of a bond between two atoms in a large number of compounds. The unit is pm = 1. 0 X 10 -12 m. • Size of bonds: Triple bonds are shorter than double bonds, and double bonds are shorter than single bonds. • Strength of bonds: Triple bonds are stronger than double bonds, which are shorter than single bonds. • As bond length decreases, bond strength increases.

Bonding in Metals • Occurs between metals. • Each metal donate its valence electrons to form a sea of electrons. • Once the metal donates electrons, there is an attraction between the metal cations and the sea of electrons. • Since the electrons are mobile, the metal can conduct electricity, and heat. • The sea of electrons also shield the positively charged ions and allow them to slide past each other, making the metal both malleable and ductile.

9. 10 -9. 11 pg. 420 #’s 79 -82 Study for quiz on Chapter 9