CHM012 Chemistry for Engineers PDF

CHM012 Chemistry for Topic 3: Engineers Molecular Structure and Orbitals Department of Chemistry College of Science and Mathematics MSU-Iligan Institute of Technology | Page...

CHM012 Chemistry for Topic 3: Engineers Molecular Structure and Orbitals Department of Chemistry College of Science and Mathematics MSU-Iligan Institute of Technology | Page  Molecular Structure Topic 3: Molecular Structure and Orbital Montalban, B. | Page 2  Valence Shell Electron-Pair Repulsion (VSEPR) ⮚ By Gillespie and Nyholm, 1957 “…that the electrostatic repulsion is reduced to a minimum when various regions of electron density position are as far as possible.” Regions of electron density: a. Covalent bonds (bonding pairs) b. Unshared pair of electrons (lone pairs) ⮚ The structure around a given atom is determined principally by minimizing electron pair repulsions Topic 3: Molecular Structure and Orbital Montalban, B. | Page 3  Bonding Pairs and Lone Pairs ✔ Bonding pairs are shared between two nuclei Electrons can be close to either nucleus They are relatively confined between the two nuclei ✔ Lone pairs center around just one nucleus, and both electrons choose that nucleus ✔ Lone pairs need more space than bonding pairs They compress the angles between bonding pairs Topic 3: Molecular Structure and Orbital Montalban, B. | Page 4  Valence Shell Electron-Pair Repulsion (VSEPR) 1. Rules on repulsion bond pair – bond pair < Lone pair – bond pair < Lone pair – Lone pair ❖ The bond angle between bonding pairs decreases as the number of lone pairs increases on the central atom CH4 NH3 H2O Note: decreasing electron pair repulsion Number of Lone Pairs 0 1 2 increases bond angle Bond Angle 109.5° 107° 104.5° Topic 3: Molecular Structure and Orbital Montalban, B. | Page 5  Valence Shell Electron-Pair Repulsion (VSEPR) 2. Effect of electronegativity - Increasing size and lower electronegativity of the central atom permit the lone pairs to be drawn out further, thus decreasing the repulsion between bonding pairs. H2O H2S H2Se 104.5o 92.1o 90.6o strongest weakest bp-bp repulsion bp-bp repulsion Topic 3: Molecular Structure and Orbital Montalban, B. | Page 6  Valence Shell Electron-Pair Repulsion (VSEPR) 3. Repulsion exerted by bonding pairs decreases as the electronegativity of the bonded atoms increases. H2O F2 O 104.5o 103.2o decreasing bp-bp repulsion Topic 3: Molecular Structure and Orbital Montalban, B. | Page 7  Example Arrange the following molecules in the increasing order of bond angle: H2O, CH4, SF6, NH3 Topic 3: Molecular Structure and Orbital Montalban, B. | Page 8  Valence Shell Electron-Pair Repulsion (VSEPR) 4. Repulsion exerted by triple bonds are greater than those in single bonds. ❖ While using the VSEPR model, a double bond must be considered as one effective pair ✔ The two pairs involved in the double bond are not independent pairs ✔ The double bond acts as one center of electron density that repels other electron pairs ❖ With molecules that exhibit resonance, any one of the resonance structures can be used to predict its molecular structure using the VSEPR model 180o 120o 109o decreasing bond angle due to decreasing repulsion Topic 3: Molecular Structure and Orbital Montalban, B. | Page 9 | Page  Valence Shell Electron-Pair Repulsion (VSEPR) 5. In trigonal bipyramidal ❖ When there are five electron pairs, the structure that produces minimal repulsion is a trigonal bipyramid. It consists of two trigonal-based pyramids that share a common base a. Lone pair occupy the equatorial position (120o away) b. Double bonds occupy equatorial positions c. Less electronegative atoms occupy equatorial positions axial F F Cl S F P Cl F Cl F F equitorial SF4 PF2Cl3 seesaw Trigonal bypyramid Topic 3: Molecular Structure and Orbital Montalban, B. | Page 11  Valence Shell Electron-Pair Repulsion (VSEPR) 6. In octahedral ❖ The best arrangement for six pairs of electrons around a given atom is the octahedral structure. This structure has 90-degree bond angles a. A lone pair occupy in any position b. 2 lone pairs, the 2nd lone pair must be in opposite position of the 1st lone pair (180o) F F F F F Br X F e F F F BrF5 XeF4 Topic 3: Molecular Structure and Orbital Montalban, B. | Page 12  Valence Shell Electron-Pair Repulsion (VSEPR) 7. Multiple bonds do not affect the gross stereochemistry. The geometry is primarily determined by: a. Lone pairs b. Bonding pairs The presence of multiple bond will only affect, bond angle and bond distance. Topic 3: Molecular Structure and Orbital Montalban, B. | Page 13  Molecules Containing No Single Central Atom The VSEPR model can accurately determine the structure of complicated molecules such as methanol. The Lewis structure is: ✔ There are four pairs of electrons around the C and O atoms, which give rise to a tetrahedral arrangement ✔ Space requirements of the lone pairs distort the arrangement (a) The arrangement of electron pairs and atoms around the carbon (b) The arrangement of bonding and lone pairs around oxygen (c) The molecular structure Topic 3: Molecular Structure and Orbital Montalban, B. | Page 14  Accuracy of the VSEPR Model ✔ It aptly predicts the molecular structures of most molecules formed from non-metallic elements ✔ It can be used to predict the structures of molecules with hundreds of atoms ✔ It fails to determine the molecular structure in certain instances ⮚ Phosphine (PH3) and ammonia (NH3) have similar Lewis structures but different bond angles—94 degrees and 107 degrees, respectively Topic 3: Molecular Structure and Orbital Montalban, B. | Page 15  Example ⮚ Determine the shape and bond angles for each of the following molecules: a) HCN b) PH3 c) SF4 d) O3 e) KrF4 Topic 3: Molecular Structure and Orbital Montalban, B. | Page 16  Example When phosphorus reacts with excess chlorine gas, the compound phosphorus pentachloride (PCl5) is formed. In the gaseous and liquid states, this substance consists of PCl5 molecules, but in the solid state it consists of a 1:1 mixture of PCl4+ and PCl6 − ions. Predict the geometric structures of PCl5, PCl4+ , and PCl6−. Topic 3: Molecular Structure and Orbital Montalban, B. | Page 17  Dipole Moment ✔ A molecule that has a center of positive charge and a center of negative charge is said to be dipolar or to possess a dipole moment. ✔ It is represented by an arrow pointing to the negative charge center. The tail indicates the positive charge center. ✔ Electrostatic potential diagrams can also be used to represent dipole moment. ✔ The colors of visible light are used to show variation in distribution of charge. Red - Most electron-rich region Blue - Most electron-poor region Topic 3: Molecular Structure and Orbital Montalban, B. | Page 18  Bond Polarity ✔ Any diatomic molecule with polar bonds will exhibit dipole moments ✔ This behaviour can also be exhibited by polyatomic molecules ✔ Few molecules possess polar bonds but lack dipole moment Occurs when the individual bond polarities are arranged in a manner that they cancel each other out Topic 3: Molecular Structure and Orbital Montalban, B. | Page 19  Comparison of the Polarity of Two Molecules ⮚ A POLAR molecule ⮚ A NONPOLAR molecule Topic 3: Molecular Structure and Orbital Montalban, B. | Page 20  Example Predict the molecular structure of the sulfur dioxide molecule. Is this molecule expected to have a dipole moment? Topic 3: Molecular Structure and Orbital Montalban, B. | Page 21  Example For each of the following molecules, show the direction of the bond polarities and indicate which ones have a dipole moment: a) HCl b) Cl2 c) SO3 (planar molecule with the oxygen atoms spaced evenly around the central sulfur atom) d) CH4 (tetrahedral with the carbon atom at the center) e) H2S (V-shaped with the sulphur atom at the point) Topic 3: Molecular Structure and Orbital Montalban, B. | Page 22  Valence Bond Theory ✔ In Valence-Bond Theory, electrons of two atoms begin to occupy the same space. ✔ This is called “overlap” of orbitals. ✔ The sharing of space between two electrons of opposite spin results in a covalent bond. ✔ Increased overlap brings the electrons and nuclei closer together until a balance is reached between the like charge repulsions and the electron- nucleus attraction. ✔ Atoms can’t get too close because the internuclear repulsions get too great. Topic 3: Molecular Structure and Orbital Montalban, B. | Page 23  Hybrid Orbitals ✔ Hybrid orbitals form by “mixing” of atomic orbitals to create new orbitals of equal energy, called degenerate orbitals. ✔ When two orbitals “mix” they create two orbitals; when three orbitals mix, they create three orbitals; etc. Topic 3: Molecular Structure and Orbital Montalban, B. | Page 24  sp hybrid orbital ✔ When we look at the orbital diagram for beryllium (Be), we see that there are only paired electrons in full sublevels. ✔ Beryllium makes electron deficient compounds with two bonds for Be. Why? sp hybridization (mixing of one s orbital and one p orbital) ✔ Mixing the s and p orbitals yields two degenerate orbitals that are hybrids of the two orbitals. – These sp hybrid orbitals have two lobes like a p orbital. – One of the lobes is larger and more rounded, as is the s orbital. Topic 3: Molecular Structure and Orbital Montalban, B. | Page 25  sp hybrid orbital ✔ These two degenerate orbitals would align themselves 180° from each other. ✔ This is consistent with the observed geometry of Be compounds (like BeF2) and VSEPR: linear. Topic 3: Molecular Structure and Orbital Montalban, B. | Page 26  sp2 hybrid orbital ✔ Using a similar model for boron leads to three degenerate sp2 orbitals. ✔ Gives a trigonal planar arrangement of atomic orbitals with bond angles of 120 degrees ✔ It occurs on the combination of one 2s and two 2p orbitals ✔ One p orbital is not used it is oriented perpendicular to the plane of the sp2 orbitals 1s sp2 2p Topic 3: Molecular Structure and Orbital Montalban, B. | Page 27  sp3 hybrid orbital ✔ With carbon, we get four degenerate sp3 orbitals. ✔ It can be observed upon combination of one 2s and three 2p orbitals ✔ Whenever an atom requires a set of equivalent tetrahedral atomic orbitals, this model assumes that the atom adopts a set of sp3 orbitals. The atom becomes sp3 hybridized 1s sp3 Topic 3: Molecular Structure and Orbital Montalban, B. | Page 28  What Happens with Water? ❖ In H2O molecule, the angle question: Why is it 104.5° instead of 90°? ❖ Oxygen has two bonds and two lone pairs—four electron domains. ❖ The result is sp3 hybridization! Topic 3: Molecular Structure and Orbital Montalban, B. | Page 29  Types of Bond - How does a double or triple bond form? Two types of bonds: ⮚ Sigma (σ) bonds – are characterized by: – head-to-head overlap. – cylindrical symmetry of electron density about the internuclear axis. ⮚ Pi (π) bond – are characterized by: – side-to-side overlap. – electron density above and below the internuclear axis. Topic 3: Molecular Structure and Orbital Montalban, B. | Page 30  Double and Triple Bonds ✔ Single bonds are always σ- bonds. ✔ Multiple bonds have one σ- bond, all other bonds are π- bonds. ⮚ Double Bond ⮚ Triple Bond Topic 3: Molecular Structure and Orbital Montalban, B. | Page 31  Bonding orbitals in CO2 Topic 3: Molecular Structure and Orbital Montalban, B. | Page 32  Example a. Draw the Lewis structure for HCN b. Which of the hybrid orbitals are used? c. Draw HCN and: Show all the bonds between the atoms Label each σ or π bond Topic 3: Molecular Structure and Orbital Montalban, B. | Page 33  Hybrid Orbitals – Hypervalent Molecules ⮚ The elements which have more than an octet ⮚ Valence-Bond model would use d orbitals to make more than four bonds. ⮚ This view works for period 3 and below. ⮚ Theoretical studies suggest that the energy needed would be too great for this. Topic 3: Molecular Structure and Orbital Montalban, B. | Page 34  dsp3 Hybridization ⮚It is a combination of one d, one s, and three p orbitals ⮚It results in a trigonal bipyramidal arrangement of five equivalent hybrid orbitals ⮚The image illustrates hybrid orbitals in a phosphorus atom Topic 3: Molecular Structure and Orbital Montalban, B. | Page 35  d2sp3 Hybridization ⮚An atom is d2sp3 hybridized when there is a combination of two d, one s, and three p orbitals ⮚It results in an octahedral arrangement of six equivalent hybrid orbitals ⮚The image illustrates the orbitals in a sulfur atom Topic 3: Molecular Structure and Orbital Montalban, B. | Page 36  Example How is the xenon atom in XeFe4 hybridized? Solution XeFe4 has six pairs of electrons around xenon that are arranged octahedrally to minimize repulsions. An octahedral set of six atomic orbitals is required to hold these electrons, and the xenon atom is d2sp3 hybridized Topic 3: Molecular Structure and Orbital Montalban, B. | Page 37  Hybrid Orbitals - Summary Topic 3: Molecular Structure and Orbital Montalban, B. | Page 38  Example For each of the following molecules or ions, predict the hybridization of each atom, and describe the molecular structure. a. CO b. BF4− c. XeF2 Topic 3: Molecular Structure and Orbital Montalban, B. | Page 39  Molecular Orbital Theory ⮚ Wave properties are used to describe the energy of the electrons in a molecule. ⮚ Molecular orbitals have many characteristics like atomic orbitals: – maximum of two electrons per orbital – Electrons in the same orbital have opposite spin. – Definite energy of orbital – Can visualize electron density by a contour diagram ⮚ They differ from atomic orbitals because they represent the entire molecule, not a single atom. Whenever two atomic orbitals overlap, two molecular orbitals are formed: one bonding, one antibonding. ✔ Bonding orbitals are constructive combinations of atomic orbitals. ✔ Antibonding orbitals are destructive combinations of atomic orbitals. They have a new feature unseen before: A nodal plane occurs where electron density equals zero. Topic 3: Molecular Structure and Orbital Montalban, B. | Page 40  Molecular Orbital Theory ⮚ Whenever there is direct overlap of orbitals, forming a bonding and an antibonding orbital, they are called sigma (σ) molecular orbitals. The antibonding orbital is distinguished with an asterisk as σ*. Here is an example for the formation of a hydrogen molecule from two atoms. Note: The number of orbitals are conserved. The number of MOs will always be equal to the number of atomic orbitals used to construct them. Topic 3: Molecular Structure and Orbital Montalban, B. | Page 41  Molecular Orbital Diagram An energy-level diagram, or MO diagram shows how orbitals from atoms combine to give the molecule. In H2 the two electrons go into the bonding molecular orbital (lower in energy). Bond order - It refers to the difference between the number of bonding electrons and the number of antibonding electrons divided by 2. ✔ Larger bond order is generally related to greater bond strength Topic 3: Molecular Structure and Orbital Montalban, B. | Page 42  Can H2- and He2 form? Hydrogen ion, H2- Helium molecule, He2 Topic 3: Molecular Structure and Orbital Montalban, B. | Page 43  Homonuclear Diatomic Molecules ⮚ These molecules are composed of two identical atoms ⮚ The valence orbitals significantly contribute to the MO of a particular molecule. ⮚ Electron probability in such molecules is high above and below the line between the nuclei. ❖ Both orbitals are pi (π) molecular orbitals ✔ Bonding MO - π2p ✔ Antibonding MO - π2p* ⮚ For atoms with both s and p orbitals, there are two types of interactions: ✔ The s and the p orbitals that face each other overlap in σ fashion. ✔ The other two sets of p orbitals overlap in π fashion – These are, again, direct and “side-ways” overlap of orbitals. Topic 3: Molecular Structure and Orbital Montalban, B. | Page 44  Homonuclear Diatomic Molecules The resulting MO diagram: – There are σ and σ* orbitals from s and p atomic orbitals. – There are π and π* orbitals from p atomic orbitals. – Since direct overlap is stronger, the effect of raising and lowering energy is greater for σ and σ*. Topic 3: Molecular Structure and Orbital Montalban, B. | Page 45  s and p Orbital Interactions ⮚ In some cases, s orbitals can interact wit the pz orbitals more than the px and py orbitals. ⮚ It raises the energy of the pz orbital and lowers the energy of the s orbital. ⮚ The px and py orbitals are degenerate orbitals. Topic 3: Molecular Structure and Orbital Montalban, B. | Page 46  MO Diagram and Magnetism Diamagnetism is the result of all electrons in every orbital being spin paired. These substances are weakly repelled by a magnetic field. Causes a substance to be repelled from the inducing magnetic field Paramagnetism is the result of the presence of one or more unpaired electrons in an orbital. Causes a substance to be attracted into the inducing magnetic field Example: Is oxygen (O2) paramagnetic or diamagnetic? Look back at the MO diagram! It is paramagnetic. Topic 3: Molecular Structure and Orbital Montalban, B. | Page 47  MO Diagram of Homonuclear Diatomic Molecules Topic 3: Molecular Structure and Orbital Montalban, B. | Page 48  Example Use the molecular orbital model to predict the bond order and magnetism of each of the following molecules: a) Ne2 b) P2 Topic 3: Molecular Structure and Orbital Montalban, B. | Page 49  Heteronuclear Diatomic Molecules ⮚ Diatomic molecules can consist of atoms from different elements. ⮚ The atomic orbitals have different energy, so the interactions change slightly. ⮚ The more electronegative atom has orbitals lower in energy, so the bonding orbitals will more resemble them in energy. Example: HF molecule ✔ Consider the HF molecule and assume that fluorine uses only its 2p orbitals to bond to hydrogen ✔ The MOs for HF are composed of fluorine 2p and hydrogen 1s orbitals ✔ The fluorine 2p orbital is lower in energy than the hydrogen 1s orbital ✔ The σ MO holding the binding electron pairs will show higher electron probability closer to fluorine Partial MO energy-level Electron probability ✔ Electron pairs are not equally shared diagram distribution Topic 3: Molecular Structure and Orbital Montalban, B. | Page 50  Example Use the molecular orbital model to predict the magnetism and bond order of the NO, NO+ and CN− ions Topic 3: Molecular Structure and Orbital Montalban, B. | Page 51