Covalent Bonding A chemical bond results from the coulombic - - PDF document

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Unit 4: Presentation A Covalent Bonding

AP Chemistry Slide 3 / 36

Covalent Bonding

When the N2 bond forms, enormous amounts of energy are released, making Nitrogen widely used in explosives.

Slide 4 / 36 What is a chemical bond?

A chemical bond results from the coulombic attraction of one atom or ion for another.

Cl- Na+ 9 p+ 9 p+

e- e- Covalent Bond(F2) Ionic Bond (NaCl) Nuclei are attracted to shared pairs of electrons between nuclei. Charged ions are attracted to each other

Slide 5 / 36 Why do atoms form bonds?

Atoms form bonds because it lowers their potential energy. Bond formation is exothermic as energy will be released.

Distance between nuclei

Nuclei are too far apart to be attracted to each

• thers electrons

Attractions developing Coulombic attractions maximized, repulsions minimized Repulsions predominate

Potential Energy (kJ/mol)

Slide 6 / 36 Where do we find chemical bonds?

Coulombic attractions develop both between atoms within molecules but also between neighboring molecules. Intramolecular Attractions (within molecules)

Ionic Bonds

H O H

Covalent Bonds

Intermolecular Attractions (between molecules)

Hydrogen bonding between neighboring water molecules

Slide 7 / 36 Intramolecular Attractions

How ionic or covalent a bond is depends on the electronegativity difference between the atoms involved. Electronegativity Difference 0.4 1.7 3 The more similar the electronegativities, the more likely the electrons will be shared. This becomes less likely as the electronegativity difference becomes great and the bond takes

• n a more ionic character.

non-polar covalent polar covalent ionic

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1 When atoms form bonds, energy is released. True False

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1 When atoms form bonds, energy is released. True False

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Answer True

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2 Which of the following processes would NOT be exothermic? A H + H --> H2 B Na+(g) + Cl-(g) --> NaCl(s) C MgF2(s) --> Mg2+(g) + 2F-(g) D A and B E A and C

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2 Which of the following processes would NOT be exothermic? A H + H --> H2 B Na+(g) + Cl-(g) --> NaCl(s) C MgF2(s) --> Mg2+(g) + 2F-(g) D A and B E A and C

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Answer C

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3 Which of the following is TRUE regarding bond formation? A Bonding results from nuclei - nuclei coulombic attractions B Bonding results from nuclei - electron coulombic attractions C The potential energy increases as repulsions become significant D A and B E B and C

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3 Which of the following is TRUE regarding bond formation? A Bonding results from nuclei - nuclei coulombic attractions B Bonding results from nuclei - electron coulombic attractions C The potential energy increases as repulsions become significant D A and B E B and C

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Answer E

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4 Which of the following involves the breaking of intermolecular forces? A 2H2O(l) --> 2H2(g) + O2(g) B C(s) + 2H2(g) --> CH4(g) C I2(s) --> I2(g) D CO2(g) --> C(s) + O2(g) E None of these

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4 Which of the following involves the breaking of intermolecular forces? A 2H2O(l) --> 2H2(g) + O2(g) B C(s) + 2H2(g) --> CH4(g) C I2(s) --> I2(g) D CO2(g) --> C(s) + O2(g) E None of these

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Answer C

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5 As the electronegativity difference between atoms becomes greater, the bond takes on a more covalent character. True False

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5 As the electronegativity difference between atoms becomes greater, the bond takes on a more covalent character. True False

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Answer False

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6 Which of the following bonds is most ionic in character? A Al-O B Si-O C Li-O D H-O E Rb-O

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6 Which of the following bonds is most ionic in character? A Al-O B Si-O C Li-O D H-O E Rb-O

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Answer E

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7 Which of the following bonds would be most covalent in character? A C-H B C-S C C-O D C-F E C-N

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7 Which of the following bonds would be most covalent in character? A C-H B C-S C C-O D C-F E C-N

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Answer B

Slide 15 / 36 Ionic Bond Formation

Ionic bonding occurs in a series of steps, most of which require energy but occur because they are coupled to the highly exothermic formation of the bond. Formation of LiF(s) from it's elements in their standard state. Li(s) + 1/2F2(g) --> LiF(s)

Event Reaction Energy Change Sublimation of Li(s) Li(s) --> Li(g) +180 kJ/mol Ionization of Li(g) Li(g) --> Li+(g) + e- +520 kJ/mol Breaking of existing F-F bond 1/2F2(g) --> F(g) +157 kJ/mol Ionization of F(g) F(g) + e- --> F-(g)

• 328 kJ/mol

Bond formation Li+(g) + F-(g) --> LiF(s)

• 1036 kJ/mol

Overall energy change = -505 kJ/mol Highly exothermic

Slide 16 / 36 Ionic Bond Formation

The thermochemical steps for ionic bond formation are often summarized in a Born-Haber Cycle Diagram The energy released when the gaseous ions combine is known as the Lattice Energy Hf represents the overall energy change of the process.

Slide 17 / 36 Lattice Energy

The magnitude of the lattice energy is influenced by the charge and size of the ions involved. The higher the charges, the greater the coulombic attraction and the higher the lattice energy.

Substance Charges Lattice Energy NaF(s) +1 and -1

• 923 kJ/mol

MgO(s) +2 and -2

• 3791 kJ/mol

The smaller the ionic radii, the greater the coulombic attraction and the higher the lattice energy

Substance Ionic radii Lattice Energy NaF(s) F- = 117 pm

• 923 kJ/mol

NaCl(s) Cl- = 167 pm

• 786 kJ/mol

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8 Which of the following would have the highest lattice energy? A BeO B MgS C MgCl2 D MgI2 E NaF

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8 Which of the following would have the highest lattice energy? A BeO B MgS C MgCl2 D MgI2 E NaF

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Answer A

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9 Rank the following from lowest to highest lattice energy. A I < II < III B I < III < II C II < III < I D II < I < III E III < II < I

• I. NaBr
• II. LiBr
• III. LiF

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9 Rank the following from lowest to highest lattice energy. A I < II < III B I < III < II C II < III < I D II < I < III E III < II < I

• I. NaBr
• II. LiBr
• III. LiF

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Answer E

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10 Which of the following BEST explains why the lattice energy of CaS is lower than that of MgO? A CaO has lower ionic charges than MgO B The calcium ion has more shielding than the magnesium ion C The calcium ion has a smaller nuclear charge than magnesium ion D CaO has higher ionic charges than MgO E None of these

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10 Which of the following BEST explains why the lattice energy of CaS is lower than that of MgO? A CaO has lower ionic charges than MgO B The calcium ion has more shielding than the magnesium ion C The calcium ion has a smaller nuclear charge than magnesium ion D CaO has higher ionic charges than MgO E None of these

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Answer B

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11 Which of the following BEST explains why the lattice energy of MgF2 is lower than that of MgO? A The oxide ion is smaller than the flouride ion B The charge of the cation is higher in MgO C The charge density of the anion is less in MgF2 D The charge density of the anion is less in MgO E None of these

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11 Which of the following BEST explains why the lattice energy of MgF2 is lower than that of MgO? A The oxide ion is smaller than the flouride ion B The charge of the cation is higher in MgO C The charge density of the anion is less in MgF2 D The charge density of the anion is less in MgO E None of these

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Answer C

Slide 22 / 36 Covalent Bond Formation

The sharing of electrons allows atoms to lower their potential energy by achieving a complete valence shell. Consider F2 [Ne] __ __ __ __ __ __ __ __ [Ne] + + e- e- shared pair of electrons provides both flourine atoms with a full valence shell (electrons are of opposite spin to minimize repulsions)

Slide 23 / 36 Covalent Bond Formation

The electronegativity difference between the atoms involved determines how equally the electrons are shared. Non-polar covalent bond

H-H

There is no difference in electronegativity so the electrons are shared equally. Polar covalent bond

H-Cl

There is a significant difference in electronegativity between the atoms (3.2 - 2.1) so the bond is polar, meaning opposite charges develop across the bond

partially - partially +

Slide 24 / 36 Covalent Bond Strength

The strength of a covalent bond is influenced by the radii of the atoms, the polarity of the bond, and by the # of electron pairs being shared. Effect of Atomic Radii Smaller atoms result in smaller distances between charges thereby increasing the coulombic attractions Bond Atomic Radii Bond Enthalpy H-H H = 53 pm 436 kJ/mol Cl-Cl Cl = 79 pm 243 kJ/mol

Slide 25 / 36 Covalent Bond Strength

The strength of a covalent bond is influenced by the radii of the atoms, the polarity of the bond, and by the # of electron pairs being shared. Effect of polarity The more polar the bond, the stronger the coulombic attractions Bond EN difference Bond Enthalpy H-Cl 0.9 431 kJ/mol H-S 0.5 344 kJ/mol

Note: The sizes are not a constant in this comparison, however, the projected enthalpy of an H-Cl bond would be the average of an H-H and a Cl-Cl bond or 340 kJ/mol. The observed enthalpy is much higher and explained by the polarity of the bond.

Slide 26 / 36 Covalent Bond Strength

The strength of a covalent bond is influenced by the radii of the atoms, the polarity of the bond, and by the # of electron pairs being shared. Effect of multiple shared pairs Atoms often share more than one pair of electrons to realize a full valence shell. As we will learn later, these pairs do not all form the same kind of bonds but the net effect is to increase the coulombic attractions between the nuclei. Bond # of shared pairs Bond Enthalpy O-O 1 142 kJ/mol O=O 2 498 kJ/mol

Slide 27 / 36 Covalent Bond Length

The length of a covalent bond is influenced by enthalpy of the bond. A high bond enthalpy is an indication of a strong coulombic attraction between nuclei, thereby indicating a small bond length between nuclei. Bond Enthalpy Length H-H 436 kJ/mol 74 pm H-Cl 431 kJ/mol 127 pm C-C 347 kJ/mol 154 pm C=C 611 kJ/mol 121 pm

Note: The C=C is longer than the H-H bond despite having the higher bond enthalpy due to the increased radii of the C atoms over the H atoms in the H-H bond.

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12 Which of the following would be considered a polar covalent bond? A H-H B H-S C Cl-Cl D C-S E C-O

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12 Which of the following would be considered a polar covalent bond? A H-H B H-S C Cl-Cl D C-S E C-O

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Answer E

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13 Which of the following elements, if bonded to S would produce the most polar covalent bond? A H B P C Cl D F E C

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13 Which of the following elements, if bonded to S would produce the most polar covalent bond? A H B P C Cl D F E C

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Answer D

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14 Which of the following bonds would be expected to have the smallest bond enthalpy? A F-F B Cl-Cl C C-C D C-O E C-H

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14 Which of the following bonds would be expected to have the smallest bond enthalpy? A F-F B Cl-Cl C C-C D C-O E C-H

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Answer B

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15 Which of the following would be expected to have the longest bond length? A H-O B H-S C H-Cl D H-C E H-F

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15 Which of the following would be expected to have the longest bond length? A H-O B H-S C H-Cl D H-C E H-F

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Answer B

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16 All else being equal, the more polar the bond, the shorter the bond length. True False

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16 All else being equal, the more polar the bond, the shorter the bond length. True False

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Answer True

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17 What frequency of light would be required to break an H-H bond with an enthalpy of 436 kJ/mol? A 6.6 x1032 s-1 B 6.6 x 1029 s-1 C 1.1 x 1015 s-1 D 1.1 x 1012 s-1 E 1.1 x103 s-1

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17 What frequency of light would be required to break an H-H bond with an enthalpy of 436 kJ/mol? A 6.6 x1032 s-1 B 6.6 x 1029 s-1 C 1.1 x 1015 s-1 D 1.1 x 1012 s-1 E 1.1 x103 s-1

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Answer C

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18 A higher frequency of light would be required to break an H-Cl bond compared to an H-Br bond. True False

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18 A higher frequency of light would be required to break an H-Cl bond compared to an H-Br bond. True False

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Answer True

Slide 35 / 36 In the next notebook, we will examine covalent bonding in molecules and see how it affects the properties of those molecules. Slide 36 / 36