Science league topic 7 bond energy and resonance structure
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Science league topic 7: bond energy and resonance structure. Bond Energies. chemical reactions involve breaking bonds in reactant molecules and making new bond to create the products the amount of energy it takes to break one mole of a bond in a compound is called the bond energy (D)

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Science league topic 7: bond energy and resonance structure

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Science league topic 7 bond energy and resonance structure

Science league topic 7: bond energy and resonance structure


Bond energies

Bond Energies

  • chemical reactions involve breaking bonds in reactant molecules and making new bond to create the products

  • the amount of energy it takes to break one mole of a bond in a compound is called the bond energy (D)

    • Bond energy is measured in the gas state

    • homolytically – each atom gets ½ bonding electrons


Bond lengths

Bond Lengths

  • the distance between the nuclei of bonded atoms is called the bond length

  • because the actual bond length depends on the other atoms around the bond we often use the average bond length

    • averaged for similar bonds from many compounds


Trends in bond energies and bond length

Trends in Bond Energies and bond length

  • the more electrons two atoms share, the stronger the covalent bond

  • Energy: Triple bonds > double bonds > single bond

    • C≡C (837 kJ) > C=C (611 kJ) > C−C (347 kJ)

  • the shorter the covalent bond, the stronger the bond

    • F−F (237 kJ) > Cl−Cl (218 kJ) > Br−Br (193 kJ)

    • bonds get weaker down the column

  • Bond length: Triple bonds > double bonds > single bond


Using bond energies to estimate d h rxn

Using Bond Energies to Estimate DH°rxn

  • the actual bond energy depends on the surrounding atoms and other factors

  • we often use average bond energies to estimate the DHrxn

    • works best when all reactants and products in gas state

  • bond breaking is endothermic, DH = +

  • bond making is exothermic, DH = −

    DHrxn= ∑nr(D(reactants)) + ∑np (D(products))


Estimate the enthalpy of the following reaction

Estimate the Enthalpy of the Following Reaction


Estimate the enthalpy of the following reaction1

Estimate the Enthalpy of the Following Reaction

H2(g) + O2(g) ® H2O2(g)

reaction involves breaking 1mol H-H and 1 mol O=O and making 2 mol H-O and 1 mol O-O

bonds broken (energy cost)

(+436 kJ) + (+498 kJ) = +934 kJ

bonds made (energy release)

2(464 kJ) + (142 kJ) = -1070

DHrxn = (+934 kJ) + (-1070. kJ) = -136 kJ

(Appendix DH°f = -136.3 kJ/mol)


Resonance structures

Resonance structures


Resonance

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O S O

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O S O

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••

Resonance

  • when there is more than one Lewis structure for a molecule that differ only in the position of the electrons, they are called resonance structures

  • the actual molecule is a combination of the resonance forms – a resonance hybrid

    • it does not resonate between the two forms, though we often draw it that way

  • look for multiple bonds. Most oxyanions and oxyacidshave resonance structures.


Resonance1

Resonance

  • The bonds between Os have exactly the same length!

    • The resonance bond length is < single bond, but > double bond.


Rules of resonance structures

Rules of Resonance Structures

  • Resonance structures must have at least one double bond and at least two same surrounding atoms.

    • One of the same surrounding atoms should be connected to the double bond.

    • only electron positions can change

  • C, N, O, and F elements have a maximum of 8 electrons

    • bonding and nonbonding

    • Others don’t follow octet rule

  • Resonance structures of a molecule are the same. All the bonds which could form a double bond are the same with same bond energy.


Drawing resonance structures

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+1

Drawing Resonance Structures

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  • draw first Lewis structure that maximizes octets

  • assign formal charges

  • move electron pairs from atoms with (-) formal charge toward atoms with (+) formal charge

  • Make the resonance structure: move in electrons to form the double bond from a different surrounding atom after you can move out electron pairs from one double bond.

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+1


Exceptions to the octet rule

Exceptions to the Octet Rule

  • There are three types of ions or molecules that do not follow the octet rule:

    • Atoms in compounds with less than an octet. Boron and Hydrogen

    • Center atom in compounds with more than eight valence electrons (an expanded octet). All the nonmetals from period 3 and below.

      • The empty d orbitals in the outermost shell can be used to accommodate the e-


Fewer than eight electrons

Fewer Than Eight Electrons

  • Consider BF3:

    • Giving boron a filled octet places a negative charge on the boron and a positive charge on fluorine.

    • Fluorine only forms single bond in the compounds.

  • Therefore, structures that put a double bond between boron and fluorine are much less important than the single-bond structure.


More than eight electrons

More Than Eight Electrons

  • The only way PCl5 can exist is if P has 10 electrons around it.

    • If Cl is negative charged in the compounds, it forms single bond like F.

  • It is allowed to expand the octet of atoms on the 3rd row or below.

    • Presumably dorbitals in these atoms participate in bonding.


More than eight electrons1

More Than Eight Electrons

Even though we can draw a Lewis structure for the phosphate ion that has only 8 electrons around the central phosphorus, the better structure puts a double bond between the phosphorus and one of the oxygens.


More than eight electrons2

More Than Eight Electrons

  • This eliminates the charge on the phosphorus and the charge on one of the oxygens.

  • The lesson is: When the central atom is on the 3rd row or below and expanding its octet eliminates some formal charges, do so.


Something to remember

Something to remember

  • H and F can only form single bond.

  • When negatively charged, Cl, Br and I can only form single bond.


Type of covalent bonds

Type of covalent bonds


Types of bonds

Types of Bonds

  • sigma (s) bond: the bonding points along the axis connecting the two bonding nuclei

    • s-to-sor p-to-p

  • pi (p) bond: the bonding is parallel to each other and perpendicular to the axis connecting the two bonding nuclei

    • between parallel porbitals

  • the interaction between parallel orbitals is not as strong as between orbitals that point at each other; therefore s bonds are stronger than p bonds


Science league topic 7 bond energy and resonance structure

Tro, Chemistry: A Molecular Approach


How many s and p bonds in formaldehyde

How many s and p bonds in Formaldehyde?


Bond rotation

Bond Rotation

  • the sigma (s)bond can rotate along the internuclear axis without breaking the bond.

  • the pi (p)bond can NOT rotate around the axis without the breaking of the pi bond.


Homework

Homework

  • Page 400: 61 b, c and d, 62 c and d, 71 a, 73, 77

  • How many sigma and pi bonds in the CO2?


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