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. Slide 2 / 36 AP Chemistry Unit 4: Presentation A Covalent Bonding Click to go to website: 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. www.njctl.org 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) 9 p+ e- e- 9 p+ Ionic Bond (NaCl) Na + Cl- 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. Slide 6 / 36 Where do we find chemical bonds? Coulombic develop both between atoms within molecules but also between neighboring molecules. Potential Energy (kj/mol) Repulsions predominate Distance between nuclei Attractions developing Nuclei are too far apart to be attracted to each others electrons Intramolecular Attractions (within molecules) H Ionic Bonds O H Covalent Bonds Intermolecular Attractions (between molecules) Hydrogen bonding between neighboring water molecules Coulombic maximized, repulsions minimized
Slide 7 / 36 Intramolecular Attractions How ionic or covalent a bond is depends on the electronegativity difference between the atoms involved. Electronegativity Difference Slide 8 / 36 1 When atoms form bonds, energy is released. 0 0.4 1.7 3 non-polar covalent polar covalent ionic 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. 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 () / 36 2 Which of the following processes would NOT be exothermic? Slide 10 / 36 3 Which of the following is TRUE regarding bond formation? 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 C A Bonding results from nuclei - nuclei coulombic B Bonding results from nuclei - electron coulombic C The potential energy increases as repulsions become significant D A and B E B and C
Slide 10 () / 36 3 Which of the following is TRUE regarding bond formation? A Bonding results from nuclei - nuclei coulombic B Bonding results from nuclei - electron coulombic E C The potential energy increases as repulsions become significant Slide 11 / 36 4 Which of the following involves the breaking of intermolecular forces? A 2H 2O(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) D A and B E B and C Slide 11 () / 36 4 Which of the following involves the breaking of intermolecular forces? A 2H 2O(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) C Slide 12 / 36 5 As the electronegativity difference between atoms becomes greater, the bond takes on a more covalent character. Slide 12 () / 36 5 As the electronegativity difference between atoms becomes greater, the bond takes on a more covalent character. 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 () / 36 6 Which of the following bonds is most ionic in character? Slide 14 / 36 7 Which of the following bonds would be most covalent in character? A Al-O B Si-O C Li-O D H-O E Rb-O E A C-H B C-S C C-O D C-F E C-N Slide 14 () / 36 7 Which of the following bonds would be most covalent in character? A C-H 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. B C-S Formation of LiF(s) from it's elements in their standard state. C C-O D C-F B 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 E C-N 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 Slide 17 / 36 Lattice Energy The magnitude of the lattice energy is influenced by the charge and size of the ions involved. The energy released when the gaseous ions combine is known as the Lattice Energy Hf represents the overall energy change of the process. The higher the charges, the greater the coulombic attraction and the higher the lattice energy. The smaller the ionic radii, 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 Substance Ionic radii Lattice Energy NaF(s) F- = 117 pm -923 kj/mol NaCl(s) Cl- = 167 pm -786 kj/mol
Slide 18 / 36 8 Which of the following would have the highest lattice energy? A BeO B MgS Slide 18 () / 36 8 Which of the following would have the highest lattice energy? A BeO B MgS C MgCl 2 D MgI 2 C MgCl 2 D MgI 2 A E NaF E NaF Slide 19 / 36 9 Rank the following from lowest to highest lattice energy. Slide 19 () / 36 9 Rank the following from lowest to highest lattice energy. A I < II < III A I < II < III B I < III < II I. NaBr B I < III < II I. NaBr C II < III < I D II < I < III II. LiBr III. LiF C II < III < I D II < I < III II. LiBr III. LiFE E III < II < I E III < II < I Slide 20 / 36 10 Which of the following BEST explains why the lattice energy of CaS is lower than that of MgO? 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 A CaO has lower ionic charges than MgO B The calcium ion has more shielding than the magnesium ion B C The calcium ion has a smaller nuclear charge than magnesium ion D CaO has higher ionic charges than MgO
Slide 21 / 36 11 Which of the following BEST explains why the lattice energy of MgF 2 is lower than that of MgO? 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 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 Cless in MgF 2 D The charge density of the anion is less in MgO Slide 22 / 36 Covalent Bond Formation The sharing of electrons allows atoms to lower their potential energy by achieving a complete valence shell. [Ne] Consider F 2 e- + e- + [Ne] 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 partially + H-Cl partially - 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 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 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 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 between the nuclei. Bond # of shared pairs Bond Enthalpy O-O 1 142 kj/mol O=O 2 498 kj/mol Slide 28 / 36 12 Which of the following would be considered a polar covalent bond? 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. Slide 28 () / 36 12 Which of the following would be considered a polar covalent bond? A H-H A H-H B H-S B H-S C Cl-Cl C Cl-Cl E D C-S D C-S E C-O E C-O Slide 29 / 36 13 Which of the following elements, if bonded to S would produce the most polar covalent bond? Slide 29 () / 36 13 Which of the following elements, if bonded to S would produce the most polar covalent bond? A H B P A H B P C Cl D F C Cl D F D E C E C
Slide 30 / 36 14 Which of the following bonds would be expected to have the smallest bond enthalpy? Slide 30 () / 36 14 Which of the following bonds would be expected to have the smallest bond enthalpy? A F-F B Cl-Cl A F-F B Cl-Cl C C-C D C-O C C-C D C-O B E C-H E C-H Slide 31 / 36 15 Which of the following would be expected to have the longest bond length? Slide 31 () / 36 15 Which of the following would be expected to have the longest bond length? A H-O B H-S A H-O B H-S C H-Cl D H-C C H-Cl D H-C B E H-F E H-F Slide 32 / 36 16 All else being equal, the more polar the bond, the shorter the bond length. Slide 32 () / 36 16 All else being equal, the more polar the bond, the shorter the bond length.
Slide 33 / 36 17 What frequency of light would be required to break an H-H bond with an enthalpy of 436 kj/mol? A 6.6 x10 32 s -1 B 6.6 x 10 29 s -1 Slide 33 () / 36 17 What frequency of light would be required to break an H-H bond with an enthalpy of 436 kj/mol? A 6.6 x10 32 s -1 B 6.6 x 10 29 s -1 C 1.1 x 10 15 s -1 D 1.1 x 10 12 s -1 C 1.1 x 10 15 s -1 D 1.1 x 10 12 s -1 C E 1.1 x10 3 s -1 E 1.1 x10 3 s -1 Slide 34 / 36 18 A higher frequency of light would be required to break an H-Cl bond compared to an H-Br bond. Slide 34 () / 36 18 A higher frequency of light would be required to break an H-Cl bond compared to an H-Br bond. Slide 35 / 36 Slide 36 / 36 In the next notebook, we will examine covalent bonding in molecules and see how it affects the properties of those molecules.