4.4 Covalent Bonding: Electronegativity and Bond Strength
In a polar covalent bond, the atom that attracts the electrons less strongly has:
the partial positive charge In a polar covalent bond, the atom that attracts the electrons less strongly has the partial positive charge.
The change in enthalpy for a reaction can be given by calculating:
the sum of the bond dissociation energies of the reactants minus the sum of the bond dissociation energies of the products It will be bonds broken minus bonds formed, so the bond dissociation energies of the reactants minus the bond dissociation energies of the products.
Non polar covalent bonds
with equal sharing of the bond electrons, arise when the electronegativities of the two atoms are equal.
2CO(g)+O2(g)⟶2CO2(g) Bond energy of (C≡O)=1074 kJ Bond energy of (O=O)=499 kJ Bond energy of (C=O)=802 kJ What is the enthalpy of the reaction using bond energies?
ΔH=∑Dbonds broken−∑Dbonds formed ΔH=[2(C≡O)+(O=O)]−4(C=O) ΔH =2(1074 kJ)+(499 kJ)−4(802 kJ) ΔH=−561 kJ
Covalent Compounds
compounds that contain covalent bonds exhibit different physical properties than ionic compounds. Because the attraction between molecules, electrically neutral, is weaker than that between electrically charged ions. Covalent compounds generally have much lower melting and boiling point than ionic compounds. In fact, many covalent compounds are liquids or gases at room temperature, and, in their solid states.
Bond Length
defined as the distance between the centers of two covalently bonded atoms Determined by the number of bonded electrons . The higher the bond order, the stronger the pull between the two atoms and the shorter the bond length.
Lewis Symbols
describe valence electron configurations of atoms and monatomic ions. Consisting of an element symbol surrounded by one dot for each of its valence electrons.
Electronegativity is measured in:
electronegativity is dimensionless There are no units on electronegativity, it is an arbitrary relative scale designed by Linus Pauling.
Pure Covalent Bond
exists when there is no difference between the two atoms sharing the electrons. The electronegativity of the two atoms is identical.
If the bonds in the products are stronger than the bonds in the reactants, the reaction must be:
exothermic If the bonds are stronger in the products, they must sit at a lower potential energy and thus energy must have been released in forming them.
triple bond
forms when three electron pairs are shared by a pair of atoms
What is the bond type in the molecule MgO?
ionic Locate the elements on the periodic table and record their electronegativities. Mg=1.2 O=3.5 Calculate the difference between the electronegativities by subtracting. 3.5−1.2=2.3 Use this calculated value to figure out the type of bond between the atoms. 2.3>1.7 which means there is an ionic bond between Mg and O
The greater the electronegativity difference, the more the bond will exhibit:
ionic character As the difference in electronegativity increases, the ionic character of the bond increases.
Bond Energy
measure of bond strength in a chemical bond. as the average value of the gas-phase bond dissociation energies for all bonds of the same type within the same chemical space
Electronegativity
measure of the tendency of an atom to attract in a bond. The more strongly an atom attracts the electron in its bonds, the larger its electronegativity. The more electronegative atom is the one with the partial negative charge. The greater the difference in electronegativity, the more polarized the electron distribution and the larger the partial charges of the atoms.
The ________________ an atom attracts the electrons in its bonds, the _______________ its electronegativity.
more strongly, larger Electronegativity is a measure of the tendency of an atom to attract electrons (or electron density) towards itself, and it determines how the shared electrons are distributed between the two atoms in a bond.
Use the figure provided to determine which of the following describes Cl2
nonpolar covalent Using the figure, we can determine that Cl2 is nonpolar and covalent (0 electronegative difference)
In a polar covalent bond, the more electronegative atom will bear a:
partial negative charge The more electronegative atom will be partially negative.
Which type of covalent bond is the longest?
single Single bonds are the longest, as they contain the fewest electron pairs.
single bond
single shared pair of electrons
octet rule
tendency of main group atoms to form enough bonds to obtain eight valence electrons is known as the octet rule.
Enthalpies of Formation
(Product) - (reactants) Step 1: Write a balanced chemical equation C6H12O6(s) + O2(g) = 6CO2(g) + 6H2O(I)
Polar Covalent Bond
Bonds that are partly ionic. Greater the electronegativity difference, the more ionic the bond is
Which of the following does NOT contain a polar covalent bond?
Br2 The bond between bromine atoms Br2 is not a polar covalent bond, but rather a pure covalent bond, since each bromine atom is identical and the electrons that form the bond will not be preferentially closer to one bromine atom more than the other.
Use the figure provided to determine which of the following is the least polar.
CO
Which of the following are true about electronegativity? (select all that apply)
Electronegativity generally increases as you move across the periods (from left to right) on the periodic table. Electronegativity is a measure of the tendency of a bonded atom to attract electrons towards itself. Atoms with higher electronegativity have a stronger attraction to the electrons in their bonds.
When the bonds in the products are weaker than those in the reactacts, the reaction is:
Endothermic Bond dissociation energy is the energy required to separate a pair of bonded atoms. If the bonds of the products are weaker than the bonds in the reactacts, then heat is absorbed meaning H is positive and the reaction is endothermic
When the bonds in the products are stronger than the bonds in the reactants, the reaction is:
Exothermic Bond dissociation energy is the energy required to separate a pair of bonded atoms. If the bonds of the products are stronger than the bonds in the reactacts, then heat is released meaning H is negative and the reaction is exothermic.
Covalent Bonding
Results from mutual attraction of atoms for a "shared" pair of electrons. Formed between two atoms when both have a similar tendencies to attract electrons to themselves Ex: two hydrogen atoms bond covalently to form an H2 molecule
electronegativity and bond type
When the difference is very small or zero, the bond is covalent and non polar. When it is large, the bond is polar covalent or ionic.
Bond Enthalpies
also known as bond energy is defined as the amount of energy required to break one mole of the stated bond Ex: the bond energy of a O-H single bond is 463 kJ/mol, meaning it requires 463 kJ of energy to break one mole of O-H bonds.
When comparing H2 and HCl, the hydrogen atom in the HCl molecule will
be surrounded by less electron density A chlorine atom will draw electron density away from the hydrogen atom, so hydrogen will have lost some electron density in the HCl molecule, whereas in a hydrogen molecule, each hydrogen has the same amount of electron density
The energy needed to break a covalent bond is known as:
bond dissociation energy In order to separate bonded atoms, a certain amount of energy is required. The more energy required, the stronger the bond. We can measure the strength of a covalent bond by determining how much energy would be needed in order to break it. The necessary amount of energy to break this bond is known as the bond dissociation energy.
Which of the following nitrogen-nitrogen bonds is the strongest?
N−N =160 N=N =418 N≡N =946
What type of bond will form between two atoms that have a difference in electronegativity equal to 0.2?
A pure covalent bond A difference in electronegativity of 0.2 is not a large enough difference to classify the bond as a polar covalent bond; therefore, it will be a pure or nonpolar covalent bond.
Use the figure provided to determine which of the following has bonds with the greatest bond-polarity?
H2O Calculating the difference between the electronegativities for each element of each molecules, we find that H and O have the greatest difference, so H2O has bonds with the greatest polarity.
H2(g) + C2H4(g) -> C2H6(g)
H= Sum of bonds broken− Sum of bonds formed =[(H-H)+4(C-H)+(C=C)]−[6(C-H)+ (C-C)] =[436+4(413)+614] − [6(413)+348] H =−124 kJ
What is the enthalpy of the reaction using bond energies?
H= Sum of bonds broken− Sum of bonds formed H=[(H-H)+ (F-F)]−2(H-F) H=(432+154)−2(565) H= −544 kJ
Use the figure provided to determine which of the following is the least polar.
ICl The least polar bonds are formed between elements with the smallest difference in their electonegativities. In this case, ICl, has the smallest difference of 0.5.
Who is credited with deriving the electronegativity values of the periodic table?
Linus Pauling
Determining Lewis Structures
Molecules, we add the number of valence electrons on each atom in the molecule. - SiH4 Si: 4 Valence electrons per atom x 1 atom = 4 + H: 1 Valence electron per atom x 4 atoms = 4 = 8 Valence electrons Negative Ions, add number of valence electrons on the atoms to the number of negative charges on the ion (one electron is gained for each single negative charge) -CHO2 C: 4 VE/atom x 1 atom = 4 H: 1 VE/atom x 1 atom = 1 atom O: 6 VE/atom x 2 atoms = 12 + 1 additional electron = 1 = 18 Valence Electrons Positive ions, we add the number if valence electrons on the atoms in the ions and then subtract the number of positive charges on the ion 9one electron is lost for each single positive charge) from the total number of valence electrons. -NO+ N: 5 VE/atom x 1 atom = 5 O: 6 VE/atom x 1 atom = 6 + -1 electron (positive charge) = 10 VE Since OF2 is a neutral molecule we simply add the number of valence electrons O: 6 VE/atom x 1 atom = 6 F: 7 VE/atom x 2 atom = 14 = 20 VE
Which molecule will have the strongest bond?
N2 A nitrogen molecule has a triple bond, while the rest have double or single bonds.
