Chemical Bonding Chapter 8

Transcription

1 Chemical Bonding Chapter 8 Get your Clicker, 2 magnets, goggles and your handouts Nov 15 6:15 PM Recall that: Ionic-Involves the transfer of electrons - forms between a metal and a nonmetal Covalent-Involves the sharing of electrons - forms between 2 nonmetals Nov 15 6:18 PM 1

2 What type of bond is this? What type is this? Covalent bond Nov 15 6:24 PM Which of the following is an ionic compound? a) CO 2 b) MgCl 2 c) SO 2 d) PCl 3 Nov 15 8:16 PM 2

3 Chemical Bonds Forces that hold groups of atoms together and make them function as a unit. A bond will form if the energy of the aggregate is lower than that of the separated atoms. Nov 5 12:56 PM The interaction of 2 H atoms Nov 5 12:57 PM 3

4 As the atoms approach each other, the energy decreases until the distance reaches nm and then begins to increase again due to repulsions. Nov 5 12:58 PM Coulomb's Law The energy of interaction for an ion pair can be calculated using Coulomb's Law. E= 2.31 x J nm ( Q 1 Q 2) r E= energy in joules r = the distance between the ion centers in nm Q 1 and Q 2 = numerical ion charges Nov 9 10:29 AM 4

5 Example: Calculate the energy of interaction in kj/mol between Mg 2+ and O 2 if the distance between the centers of them is nm (2.05 Angstroms). Nov 9 10:38 AM Compare the energy of the MgO bond to the energy of the NaCl bond: NaCl Why is MgO so much higher? MgO has a higher charge leads to more electrostatic attraction MgO bond = Nov 18 2:17 PM 5

6 Ionic Bonding electrons are transferred Covalent Bonding electrons are shared equally by nuclei What about intermediate cases? Polar Covalent Bond Unequal sharing of electrons between atoms in a molecule. Results in a charge separation in the bond (partial positive and partial negative charge). Nov 5 1:01 PM Polar molecules align in an electric field.mp4 Nov 9 9:29 AM 6

7 Nov 9 9:35 AM equal sharing unequal sharing of electrons Jan 23 2:50 PM 7

8 Electronegativity The ability of an atom in a molecule to attract shared electrons to itself. For a molecule HX, the relative electronegativities of the H and X atoms are determined by comparing the measured H X bond energy with the expected H X bond energy. On the periodic table, electronegativity generally increases across a period and decreases down a group. The range of electronegativity values is from 4.0 for fluorine (the most electronegative) to 0.7 for cesium (the least electronegative). Jan 23 3:14 PM Nov 9 9:51 AM 8

9 If lithium and fluorine react, which has more attraction for an electron? Why? In a bond between fluorine and iodine, which has more attraction for an electron? Why? The fluorine has more attraction for an electron than does lithium. Both have valence electrons in the same principal energy level (the 2nd), but fluorine has a greater number of protons in the nucleus. Electrons are more attracted to a larger nucleus (if the principal energy level is the same). The fluorine also has more attraction for an electron than does iodine. In this case the nuclear charge of iodine is greater, but the valence electrons are at a much higher principal energy level (and the inner electrons shield the outer electrons). Nov 9 9:51 AM What is the general trend for electronegativity across rows and down columns on the periodic table? Explain the trend. Generally speaking, the electronegativity increases in going from left to right across a period because the number of protons increases which increases the effective nuclear charge. The electronegativity decreases going down a group because the size of the atoms increases as you go down so when other electrons approach the larger atoms, the effective nuclear charge is not as great due to shielding from the current electrons present. Nov 9 9:53 AM 9

10 Nov 9 9:55 AM If ENeg Diff <0.4 it's a non polar covalent bond If ENeg Diff > 0.5 and <1.8 it's a polar covalent bond If ENeg Diff > 1.8 it's an ionic bond Jan 27 1:58 PM 10

11 AP Chapter 8 Bonding.notebook Arrange the following bonds from most to least polar: a) N F b) C F c) Cl Cl O F N O B Cl C F Si F S Cl a) C F, b) Si F, c) B Cl, N F, C F, S Cl, O F N O Cl Cl Nov 9 9:56 AM Which of the following bonds would be the least polar yet still be considered polar covalent? Mg O C O O O Si O N O The correct answer is N O. To be considered polar covalent, unequal sharing of electrons must still occur. Choose the bond with the least difference in electronegativity yet there is still some unequal sharing of electrons. Nov 9 9:56 AM 11

12 Which of the following bonds would be the most polar without being considered ionic? Mg O C O O O Si O N O The correct answer is Si O. To not be considered ionic, generally the bond needs to be between two nonmetals. The most polar bond between the nonmetals occurs with the bond that has the greatest difference in electronegativity. Nov 9 9:59 AM Bond Polarity and Dipole moment Dipole Moment Property of a molecule whose charge distribution can be represented by a center of positive charge and a center of negative charge. Use an arrow to represent a dipole moment. Point to the negative charge center with the tail of the arrow indicating the positive center of charge Nov 9 10:00 AM 12

13 Nov 9 10:06 AM No net dipole (poles cancel out) Nov 9 10:07 AM 13

14 Lewis Dot Diagrams What are Lewis dot diagrams? ~element symbol with dots around it to represent the outer shell electrons Jan 27 1:40 PM Dot diagrams should be drawn with maximum number of unpaired dots. Ex Draw the dot structure for the following elements: B Si N S F Jan 27 5:16 PM 14

15 Lewis diagrams for the representative elements Jan 27 5:21 PM Dot diagrams for ions Positive ions ( metals ) lose all their outer electrons So we place a bracket around the symbol and give it a charge Ex Al loses 3 e to become [Al] 3+ atom ion Jan 27 1:46 PM 15

16 Negative ions ( nonmetals) Since negative ions gain electrons, they will gain enough to fill up their outer shell (the 8) Ex O has 6 dots We place a bracket and the charge [ O ] 2 Jan 27 5:37 PM Steps for drawing Lewis Structures for molecules 1) Determine the # and types of atoms and total number of dots Ex H 2 O 2) Draw dot structures to determine center atom the one that can form the most bonds 3) Draw a skeleton and connect all atoms to center with a pair of dots, then surround all atoms (except H) with 8 dots 4) Count the dots and compare to step 1 If equal then stop Jan 27 7:27 PM 16

17 Multiple Bonds Sometimes you need more dots than are supplied by the atoms to satisfy all atoms with 8 dots. When you count the total number of dots, there will be too many. When this happens, it means you have multiple bonds. Only C,O,N,S, and P commonly have double bonds. 1 extra pair means a double bond X X 2 extra pairs means either 2 double bonds or X X X 1 triple bond X X Only C and N commonly have stable triple bonds Jan 27 7:22 PM When this happens, what to do? 1. Decide where your double or triple bond is located. 2. For each extra pair, take one unshared pair from each atom involved in the multiple bond. 3. Replace the two pairs you took away with one pair between them. Ex CO 2 Follow the steps in red above for each extra pair. Now that you have followed the steps and satisfied all atoms with 8 dots, you have 20 dots. That's more than you calculated in step one. This means you have multiple bonds. Two extra pairs means either you have two doubles or one triple bond. Since carbon is bonded to two identical atoms, it makes more sense that you have two doubles that one triple. Remove these two unshared pairs and put one pair in the middle for them to share. Do the same thing for the second extra pair. so the final structure looks like this: O C O O C O Each atom has access to 8 dots Jan 27 7:42 PM 17

18 Draw Dot diagrams for the following: a. O 2 b. N 2 Jan 27 6:35 PM Jan 29 8:01 AM 18

19 Jan 29 9:06 AM Lewis Structures for polyatomic ions. Jan 27 6:40 PM 19

20 What does it mean if you have an ion with a -2 charge? with a +1 charge? In step one, add for negative charge, subtract for positive. Ex. Draw Lewis Structures for each of the following: Ammonium ion Polyatomic ions Sulfate ion Jan 27 6:39 PM Resonance structures: When there is only one double bond with identical atoms attached, the double bond resonates back and forth between the atoms NO 3 1 Jan 27 6:50 PM 20

21 Octet exceptions and paramagnetism. Jan 27 8:14 PM Exceptions to Octet Rule: Boron and Beryllium are metals but sometimes form molecular compounds only forms 3 bonds B When boron forms a molecular compound, it only contributes 3 electrons so it only gets a total of 6 only forms 2 bonds Be Be only has 2 electrons so when it forms a molecular compound, it only gets a total of 4 electrons Some elements can have more than 8 electrons: other exceptions: can form 5 bonds P can form 6 bonds S hydrogen can only form one bond!!1 H Jan 27 7:50 PM 21

22 More exceptions to Octet Rule NO 2 molecule has an odd number of valence electrons (17) it has an unpaired electron substances that contain one or more unpaired electrons are called Paramagnetic they show a strong attraction to an external magnetic field If a substance is paramagnetic, its mass will appear to be greated in a magnetic field than in the absence of one In diamagnetic substances, all electrons are paired. These substances are weakly repelled by a magnetic field Jan 27 7:58 PM Even though O 2 can be drawn with paired electrons, experiments have shown it is paramagnetic, so it is believed that O 2 is actually a hybrid of the following 2 structures O O O O When liquid oxygen is poured between the poles of a magnet, it sticks until it boils away. Jan 27 7:58 PM 22

23 Coordinate covalent bonds one atom contributes both bonding electrons Ex. CO C O C O can be shown this way in structural formulas C O Bond dissociation energies the total energy required to break the bond between two covalently bonded atoms there a table in your text that lists bond dissociation energies for several covalent bonds Ex H H bond dissociation energy is 435 kj/mol H H kj H + H Jan 27 7:56 PM Bond Energy Can be used as another way to calculate approximate energies for reactions. What other ways can we do this? Breaking bonds requires energy endothermic Forming bonds releases energy exothermic H = sum of the energies requried to break old bonds (positive sign) plus the sum of the energies released in the formation of new bonds (negative sign) Nov 13 2:47 PM 23

24 Nov 16 8:13 AM Example 1 Using data from the table, calculate H for the following reaction. Compare this to the H f calculated for the thermodynamic values in the Appendix. CH 4(g) + 4 F 2(g) > CF 4(g) + 4 HF Bonds Broken Energy per bond (kj) Total Energy Bonds Formed Energy per bond (kj) Total Energy H = (Energy to break bonds) ( Energy released from forming bonds) Nov 16 8:01 AM 24

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26 Molecular geometry VSEPR Theory ( Valence Shell Electron Pair Repulsion ) states that electrons in a molecule repel each other and will C try and get as far away from each other as possible. is based on the number electron domains around a central atom. can be used to predict structures of molecules or ions that contain only non metals does fail in some cases. Each area where electrons exist is called an "electron domain" or simply "domain." It does not matter how many electrons are present, from one to six, it is still just one domain. Now a domain with six electrons in it (a triple bond) is bigger (and more repulsive) than a lone electron domain. However, it is still just one domain. Keep in mind that the domains are all attached to the central atom and will pivot so as to maximize the distance between domains. A this is the central atom of the molecule (or portion of a large molecule being focused on). B this letter represents the ligands or atoms attached to the central atom. No distinction is made between atoms of different elements. For example, AB 4 can refer to CH 4 or to CCl 4. E this stands for nonbonding electron pairs. Jan 27 7:55 PM Nov 16 8:29 AM 26

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29 Two domains 2 AB 2 Linear O 180 C O sp hybrids = Three domains AB 3 trigonal planar sp 2 hybrids AB 2 E Bent Four domains AB 4 tetrahedral sp 3 orbitals AB 3 E pyramidal 107 AB 2 E 2 bent Jan 27 7:55 PM 5 Domains AB 5 Trigonal bipyramidal sp 3 d hybrid orbitals 6 Domains AB 6 Octahedral 6 sp 3 d 2 hybrid orbitals Jan 29 5:41 PM 29

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31 Feb 19 11:38 AM Shape quiz for clicker Get out your periodic table. Put everything else away. Feb 2 11:28 PM 31

32 a) AB 2, linear b) AB 2 E, bent c) AB 2 E 2, bent d) AB 3, trigonal planar Feb 19 10:43 AM a) linear b) bent c) tetrahedral d) trigonal pyramidal e) octahedral f) trigonal planar g) trigonal bipyramidal Feb 19 10:57 AM 32

33 a) linear b) bent c) tetrahedral d) trigonal pyramidal e) octahedral f) trigonal planar g) trigonal bipyramidal Feb 19 10:47 AM a) linear b) bent c) tetrahedral d) trigonal pyramidal e) octahedral f) trigonal planar g) trigonal bipyramidal Feb 19 10:47 AM 33

34 a) linear b) bent c) tetrahedral d) trigonal pyramidal e) octahedral f) trigonal planar g) trigonal bipyramidal Feb 19 10:47 AM a) linear b) bent c) tetrahedral d) trigonal pyramidal e) octahedral f) trigonal planar g) trigonal bipyramidal Feb 19 10:48 AM 34

35 a) linear b) bent c) tetrahedral d) trigonal pyramidal e) octahedral f) trigonal planar g) trigonal bipyramidal Feb 19 10:49 AM a) linear b) bent c) tetrahedral d) trigonal pyramidal e) octahedral f) trigonal planar g) trigonal bipyramidal Feb 19 10:50 AM 35

36 a) linear b) bent c) tetrahedral d) trigonal pyramidal e) octahedral f) trigonal planar g) trigonal bipyramidal Feb 19 10:50 AM Feb 19 11:08 AM 36

37 a) linear b) bent c) tetrahedral d) trigonal pyramidal e) octahedral f) trigonal planar g) trigonal bipyramidal Feb 19 10:51 AM a) linear b) bent c) tetrahedral d) trigonal pyramidal e) octahedral f) trigonal planar g) trigonal bipyramidal Feb 19 10:51 AM 37

38 a) linear b) bent c) tetrahedral d) trigonal pyramidal e) octahedral f) trigonal planar g) trigonal bipyramidal Feb 19 10:52 AM a) linear b) bent c) tetrahedral d) trigonal pyramidal e) octahedral f) trigonal planar g) trigonal bipyramidal Feb 19 10:54 AM 38

39 What is the geometry of the SO2 molecule? A) tetrahedral b) bent c) trigonal pyramidal d) T shaped e) linear f) trigonal planar g) square pyramidal h) octahedral Feb 19 8:57 AM What is the shape? A) tetrahedral b) bent c) trigonal pyramidal d) T shaped e) linear f) trigonal planar g) square pyramidal h) octahedral Feb 19 8:59 AM 39

40 What is the shape of the molecule below? A) tetrahedral b) bent c) trigonal pyramidal d) T shaped e) linear f) trigonal planar g) square pyramidal h) octahedral Feb 19 9:10 AM What is the shape of the PF 5 molecule? A) tetrahedral b) bent c) trigonal pyramidal d) T shaped e) linear f) trigonal planar g) square pyramidal h) square planar Feb 19 9:00 AM 40

41 What is the shape of the NH 3 molecule? A) tetrahedral b) bent c) trigonal pyramidal d) T shaped e) linear f) trigonal planar g) trigonal bipyramidal h) square planar Feb 19 8:59 AM What is the shape of the ICl 3 molecule? A) tetrahedral b) bent c) trigonal pyramidal d) T shaped e) linear f) trigonal planar g) trigonal bipyramidal h) square planar Feb 19 8:57 AM 41

42 What is the shape of the BCl 3 molecule? A) tetrahedral b) bent c) trigonal pyramidal d) T shaped e) linear f) trigonal planar g) trigonal bipyramidal h) square planar Feb 19 9:08 AM Polar Bonds and Molecules Recall that : In a nonpolar covalent bond, the bonding electrons are shared equally between the two atoms nucleus + shared electron pair + nucleus In a polar covalent bond, the bonding electrons are shared unequally between the two atoms + + electronegativity the ability of an atom in a bond to attract electrons The more electronegative atom will have the stronger electron attraction and will acquire a slightly negative charge, and the less electronegative atom will acquire a slightly positive charge. To determine whether a bond is polar, look up the electronegativity of both atoms in the bond and subtract. If the difference is , the bond is polar. Jan 27 7:58 PM 42

43 Just because a molecule has polar bonds does not necessarily make the molecule polar. The effect of polar bonds on the polarity of an entire molecule depends on the shape of the molecule and the orientation of the polar bonds. A molecule can possess polar bonds and still be nonpolar. If the polar bonds are evenly (or symmetrically) distributed, the bond dipoles cancel and do not create a molecular dipole. For example, the three bonds in a molecule of BF 3 are significantly polar, but they are symmetrically arranged around the central fluorine atom. No side of the molecule has more negative or positive charge than another side, and so the molecule is nonpolar: Jan 27 7:59 PM In a polar molecule, one end of the molecule is slightly negative and the other end is slightly positive.. A molecule that has two poles is called a dipolar molecule or dipole. Jan 27 8:00 PM 43

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45 Determining Molecular Polarity Get a valid VSEPR structure A molecule can be nonpolar in either of two ways: A) All the bonds are nonpolar B) Some bonds are polar and the bond vectors cancel out (A bond is polar if the difference in electronegativity between atoms is greater than about 0.4 ) A molecule can be polar only if it meets both of the following conditions: A) It must have polar bonds AND B) The bond vectors do not cancel out To determine if a molecule that has polar bonds is polar overall, add the bond vectors according to the rules of vector addition. This means you must consider both the length and the direction in space. Vectors are added head to tail (i.e. move the tail of the second vector to the head of the first vector without changing the directions of either vector. The line from the tail of the first vector to the head of the second vector is the vector sum. Feb 21 12:07 PM Feb 21 12:12 PM 45

46 Feb 10 10:21 AM Decide whether the molecules represented by the following formulas are polar or nonpolar. (You may need to draw Lewis structures and geometric sketches to do so.) a. CO 2 b. OF 2 c. CCl 4 d. CH 2 Cl 2 e. HCN Solution: a. The Lewis structure for CO 2 is The electronegativities of carbon and oxygen are 2.55 and The 0.99 difference in electronegativity indicates that the C O bonds are polar, but the symmetrical arrangement of these bonds makes the molecule nonpolar. If we put arrows into the geometric sketch for CO 2, we see that they exactly balance each other, in both direction and magnitude. This shows the symmetry of the bonds. Jan 27 8:01 PM 46

47 b. The Lewis structure for OF 2 is difference of 0.54 If a bent molecule has even one polar bond it will be a polar molecule. The electronegativities of oxygen and fluorine, 3.44 and 3.98, respectively, produce a 0.54 difference that leads us to predict that the O F bonds are polar. The molecular geometry of OF 2 is bent. Such an asymmetrical distribution of polar bonds would produce a polar molecule. Jan 27 8:01 PM c. The molecular geometry of CCl 4 is tetrahedral. Even though the C Cl bonds are polar, their symmetrical arrangement makes the molecule nonpolar. All tetrahedrals are non polar if the surrounding atoms are the same Jan 27 8:02 PM 47

48 d. The Lewis structure for CH 2 Cl 2 is The following geometric sketches show that the polar bonds are asymmetrically arranged, so the molecule is polar. (Notice that the Lewis structure above incorrectly suggests that the bonds are symmetrically arranged. Keep in mind that Lewis structures often give a false impression of the geometry of the molecules they represent.) Jan 27 8:02 PM e. The Lewis structure and geometric sketch for HCN are the same: differences The electronegativities of hydrogen, carbon, and nitrogen are 2.20, 2.55, and The 0.35 difference in electronegativity for the H C bond shows that it is essentially nonpolar. The 0.49 difference in electronegativity for the C N bond tells us that it is polar. Molecules with one polar bond are always polar. Jan 27 8:03 PM 48

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51 Feb 3 8:54 PM Isoelectronic series A series of ions/atoms containing the same number of electrons. O 2, F, Ne, Na +, Mg 2+, and Al 3+ Nov 21 11:21 AM 51

52 Formal Charge Sometimes several Lewis structures can be drawn when following the rules. How do we select the most appropriate structure? By using the formal charge method The formal charge of an atom in a molecule is the difference between the # of valence electrons on the free atom and the number of valence e assigned the to the atom in the molecule. Formal charge= (# of valence e on free atom) ( number of valence e assigned to the atom in the molecule) Lone e + 1/2 shared e The formal charge of a molecule or polyatomic ion must add up to the charge of the ion of molecule. Feb 6 1:52 PM Formal charge= (# of valence e on free atom) ( number of valence e assigned to the atom in the molecule) Lone e + 1/2 shared e Consider the Lewis structure for POCl 3. Assign the formal charge for each atom in the molecule. P: 5 4 = +1 O: 6 7 = 1 Cl: 7 7 = 0 Nov 17 11:24 AM 52

53 GK3s Nov 18 2:41 PM Two possible Lewis structures for the sulfate ion are shown below. Use formal charges to determine which one is preferred. Formal charge= (# of valence e on free atom) ( number of valence e assigned to the atom in the molecule) Lone e + 1/2 shared e Nov 17 11:23 AM 53

54 Formal charge= (# of valence e on free atom) ( number of valence e assigned to the atom in the molecule) Lone pairs + 1/2 shared e For single bonded oxygens: For double bonded oxygens For sulfur: Which is preferred? Nov 17 10:46 AM Nov 18 2:48 PM 54

55 Attachments imgres.webloc search.webloc hydrogen bonding electronegativity and polarity clip Shape quiz for clicker Polar molecules align in an electric field.mp4