Slide 1 / 36 New Jersey Center for Teaching and Learning Progressive Science Initiative This material is made freely available at www.njctl.org and is intended for the non-commercial use of students and teachers. These materials may not be used for any commercial purpose without the written permission of the owners. NJCTL maintains its website for the convenience of teachers who wish to make their work available to other teachers, participate in a virtual professional learning community, and/or provide access to course materials to parents, students and others. Click to go to website: www.njctl.org Slide 2 / 36 AP Chemistry Unit 4: Presentation A Covalent Bonding www.njctl.org Slide 3 / 36 Covalent Bonding When the N 2 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. Covalent Bond(F 2 ) Ionic Bond (NaCl) e- e- 9 p+ 9 p+ Cl- Na + Nuclei are attracted to Charged ions are attracted to shared pairs of electrons each other between nuclei. 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. Repulsions predominate Nuclei are too far apart to be attracted to each Distance between nuclei others electrons Potential Energy (kJ/mol) Attractions developing Coulombic attractions maximized, repulsions minimized Slide 6 / 36 Where do we find chemical bonds? Coulombic attractions develop both between atoms within molecules but also between neighboring molecules. Intramolecular Attractions Intermolecular Attractions (within molecules) (between molecules) Hydrogen Ionic Bonds bonding between neighboring O water molecules Covalent Bonds H H
Slide 7 / 36 Intramolecular Attractions How ionic or covalent a bond is depends on the electronegativity difference between the atoms involved. Electronegativity Difference 0 0.4 1.7 3 non-polar ionic polar covalent covalent 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 on a more ionic character. Slide 8 / 36 1 When atoms form bonds, energy is released. True False Slide 8 (Answer) / 36 1 When atoms form bonds, energy is released. True False Answer True [This object is a pull tab]
Slide 9 / 36 2 Which of the following processes would NOT be exothermic? A H + H --> H 2 B Na + (g) + Cl - (g) --> NaCl(s) C MgF 2 (s) --> Mg 2+ (g) + 2F - (g) D A and B E A and C Slide 9 (Answer) / 36 2 Which of the following processes would NOT be exothermic? A H + H --> H 2 B Na + (g) + Cl - (g) --> NaCl(s) C MgF 2 (s) --> Mg 2+ (g) + 2F - (g) Answer C D A and B E A and C [This object is a pull tab] Slide 10 / 36 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
Slide 10 (Answer) / 36 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 Answer E C The potential energy increases as repulsions become significant D A and B [This object is a pull tab] E B and C Slide 11 / 36 4 Which of the following involves the breaking of intermolecular forces? A 2H 2 O(l) --> 2H 2 (g) + O 2 (g) B C(s) + 2H 2 (g) --> CH 4 (g) C I 2 (s) --> I 2 (g) D CO 2 (g) --> C(s) + O 2 (g) E None of these Slide 11 (Answer) / 36 4 Which of the following involves the breaking of intermolecular forces? A 2H 2 O(l) --> 2H 2 (g) + O 2 (g) B C(s) + 2H 2 (g) --> CH 4 (g) Answer C I 2 (s) --> I 2 (g) C D CO 2 (g) --> C(s) + O 2 (g) E None of these [This object is a pull tab]
Slide 12 / 36 5 As the electronegativity difference between atoms becomes greater, the bond takes on a more covalent character. True False Slide 12 (Answer) / 36 5 As the electronegativity difference between atoms becomes greater, the bond takes on a more covalent character. True False Answer False [This object is a pull tab] Slide 13 / 36 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
Slide 13 (Answer) / 36 6 Which of the following bonds is most ionic in character? A Al-O B Si-O Answer C Li-O E D H-O E Rb-O [This object is a pull tab] Slide 14 / 36 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 Slide 14 (Answer) / 36 7 Which of the following bonds would be most covalent in character? A C-H B C-S Answer C C-O B D C-F E C-N [This object is a pull tab]
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) + 1/2F 2 (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 1/2F 2 (g) --> F(g) +157 kJ/mol Ionization of F(g) F(g) + e- --> F-(g) -328 kJ/mol -1036 kJ/mol Bond formation Li+(g) + F-(g) --> LiF(s) 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, Substance Charges Lattice Energy the greater the coulombic NaF(s) +1 and -1 -923 kJ/mol attraction and the higher MgO(s) +2 and -2 -3791 kJ/mol the lattice energy. The smaller the ionic Substance Ionic radii Lattice Energy radii, the greater the NaF(s) F- = 117 pm -923 kJ/mol coulombic attraction and NaCl(s) Cl- = 167 pm -786 kJ/mol the higher the lattice energy
Slide 18 / 36 8 Which of the following would have the highest lattice energy? A BeO B MgS C MgCl 2 D MgI 2 E NaF Slide 18 (Answer) / 36 8 Which of the following would have the highest lattice energy? A BeO B MgS C MgCl 2 Answer A D MgI 2 E NaF [This object is a pull tab] Slide 19 / 36 9 Rank the following from lowest to highest lattice energy. A I < II < III B I < III < II I. NaBr II. LiBr C II < III < I III. LiF D II < I < III E III < II < I
Slide 19 (Answer) / 36 9 Rank the following from lowest to highest lattice energy. A I < II < III B I < III < II I. NaBr II. LiBr C II < III < I Answer E III. LiF D II < I < III E III < II < I [This object is a pull tab] Slide 20 / 36 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 Slide 20 (Answer) / 36 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 Answer B C The calcium ion has a smaller nuclear charge than magnesium ion D CaO has higher ionic charges than MgO [This object is a pull tab] E None of these
Slide 21 / 36 11 Which of the following BEST explains why the lattice energy of MgF 2 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 MgF 2 D The charge density of the anion is less in MgO E None of these Slide 21 (Answer) / 36 11 Which of the following BEST explains why the lattice energy of MgF 2 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 Answer C The charge density of the anion is less in MgF 2 C D The charge density of the anion is less in MgO E None of these [This object is a pull tab] Slide 22 / 36 Covalent Bond Formation The sharing of electrons allows atoms to lower their potential energy by achieving a complete valence shell. Consider F 2 e- + + e- [Ne] __ __ __ __ __ __ __ __ [Ne] shared pair of electrons provides both flourine atoms with a full valence shell (electrons are of opposite spin to minimize repulsions)
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