IB TOPIC 14: Bonding 14. 1 Shapes of molecules. 14.1.1 Predict the shape and bond angles for species with five and six negative charge centres using the VSEPR theory. You should be able to predict the electron distribution, molecular shape and bond angles of molecules or molecular ions with 5 or 6 negative centres such as PCl5, SF6, XeF4, and PF6. 14. 2 Hybridisation 14.2.1 Describe σ and π bonds. Covalent bonds The sharing of electrons occurs as a result of the interaction or overlapping of a half-filled atomic orbital (= an orbital containing an unpaired electron) of one atom with a half-filled atomic orbital of another atom. Both single/unpaired electrons now become paired as they share each others overlapping atomic orbitals which now form a new single molecular orbital. A molecular orbital is a region of space around both nuclei with a great probability of finding the shared electron pair; each molecular orbital can hold two electrons. Most of the time, the shared electron pair is found in the region between both nuclei as this is where the atomic orbitals have overlapped; this is also the region where the attraction between the pair of electrons and the nucleus is the greatest. So in a molecular orbital the electron density is the highest in the internuclear region, which is the area, surrounding the axis between the nuclei of both atoms. Covalent bonds and the energy state of the particles involved Electrons in bond orbitals are attracted by both nuclei. As a result, they experience a larger attraction than when they were unpaired. Therefore, after the overlapping, these bonding electrons are at a lower energy (more negative) level then non-bonding valence electrons in separate atoms as the non-bonding valence electrons are only attracted by one nucleus (see figure below). After the bond formation, a more stable substance is formed as the molecule is at a lower energy level than what the separate atoms were at before the bonding. This is because its shared electrons are now attracted more strongly. This also explains why bond making is an exothermic process and bond breaking is endothermic because when we want to break a bond, we have to supply the energy the electrons lost in forming the bond. The greater the difference in energy, the stronger the bond. The diagram below shows the energy changes that occur in the course of bond formation between 2 hydrogen atoms. The hydrogen molecule is at a lower energy state, and therefore more stable, than the original atoms. energy H(g) + H(g) electron in H atomic orbital 436 kJ/mol electron in H atomic orbital H2(g) Molecular orbital 1 | Page Topic 14 (In a way, in ionic bonding the bond orbital has strongly moved towards one atom) Multiple bonds Normally, the number of covalent bonds an atom makes = number of its unpaired electrons (provided all covalent bonds are normal covalent and not dative bonds!). For instance hydrogen has one unpaired electron so it makes one covalent bond; oxygen has two so…. Sometimes the number of other atoms to bond with are limited and more than one bond (a multiple bond) is formed with the same atom. The formation of a multiple bond involves a different type of covalent bond. The first type of covalent bond formed between two atoms is always a sigma, , bond. Any subsequent bonds formed are always pi bonds, . This is the case because, as the area along the axis between both nuclei is already occupied by the bond/electrons, any further overlap of atomic orbitals has to take place above and below this axis (=sideways overlap). So bonds only form when there is already a bond. They are only formed by p orbitals!!! What is the difference between sigma, , and pi, , bonds? In sigma, , bonds the electron cloud (space of high probability of finding bonding electrons) or molecular orbital, formed as a result of the end-on overlap or ‘head-on overlap’ of two atomic orbitals, lies along the axis between the two nuclei. The electron cloud or molecular orbital encompasses both nuclei and both electrons can move around both nuclei although they are most likely to be found on the axis between both nucelii. A bond can be formed between two s orbitals, 1 s and 1 p orbital, 2 p orbitals and so on. The diagram on the left shows parallel p orbitals of two separate carbon atoms overlapping sideways in a ethene molecule. Notice that there is already a sigma bond between both carbon atoms. The diagram on the right shows the outcome which is a pi, , bond. (from http://www.chemguide.co.uk/analysis/uvvisible/bonding.html on 15/10/09) bonds are symmetrical ‘sausage’ shaped electron clouds lying above and below the axis between the two nuclei and are formed by the sideways overlap of 2 parallel p orbitals. As p orbitals have two lobes, the bond has two regions of overlap, one above and below the axis between both nuclei. Summary: a single covalent bond double bond triple bond The diagram below shows the triple bond in ethyne. 2 | Page Topic 14 Type of bond 1 bond 1 bond + 1 bond 1 bond + 2 bonds (from http://www.science.uwaterloo.ca/~cchieh/cact/c120/hybridcarbon.html on 15/10/09) Comparison of bond strength of and pi bonds. As the electron density of the bond is above and below the axis between the two nuclei of the bonding atoms, the electrons are attracted less strongly than the two electrons in a bond; ‘on average’ they are always further away than the ‘ electrons’ from both nuclei. In addition. the area of overlap (sideways overlapping only) is also less although this in not what the diagram of a pi bond shows; the greater the overlap the stronger the bond. Both these factors explain why a bond is weaker than a bond i.e. less energy is needed to break it and less energy is released when it is made. Because they are attracted less strongly, the electrons in a bond also have more freedom and kinetic energy. In a bond the centre of charge density is above and below the internuclear axis. For any bond to form and remain both atoms must lie in the same plane!!! And will need to stay in the same plane after it has been formed. If these atoms are moved and there is no overlap anymore, the bond is broken. A triple bond is made up of a bond and two bonds, which are at right angles to each other; as there are 3 areas of overlap this triple bond is the stronger compared to a double or single covalent bond (but remember that the strongest component is the bond). Hybridisation 14.2.2 Explain hybridization in terms of the mixing of atomic orbitals to form new orbitals for bonding. 14.2.3 Identify and explain the relationships between Lewis structures, molecular shapes and types of hybridization (sp, sp2 and sp3). Use: http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/hybrv18.swf The theory of hybridisation is a theory which was developed to explain some of the shortcomings of the valence bond theory. The valence bond theory states that the chemical behaviour of an element is determined by its valence electrons, e.g. covalent bonds (sigma or pi) are formed when half-filled atomic orbitals (containing unpaired electrons) from 2 different atoms overlap to form electron pairs so that both atoms achieve the noble gas electronic configuration. Therefore the number of unpaired electrons in an atom determines the number of covalent bonds that the atom makes in a molecule or molecular ion. But the valence bond theory has not always been able to explain all of the experimental data (on bonding and shapes of molecules) obtained from the study of the physical properties of molecules or molecular ions. 3 | Page Topic 14 Some examples of some of these shortcomings of the valence bond theory: Why does boron make 3 covalent bonds with 3 different atoms although it has only 1 unpaired valence electron? Why does beryllium make two covalent bonds although it does not have any unpaired electrons? Beryllium although being a metal forms a covalent bond with halogens. This is because of its small size its 2s electrons are attracted too strongly by the nucleus so that a complete transfer can not occur. Why does carbon make 4 bonds although it has only 2 unpaired electrons? Why do carbon compounds like methane have a perfectly tetrahedral arrangement around each carbon atom i.e. all angles are 109? all 4 bonds have the same length and all electron pairs are distributed evenly? This can only be as a result of all bonds having equal repulsion which can only occur if they are all on the same energy level. When applying the VSEPR theory to determine the shape of molecules, we give the same amount of repulsion to all bonding pairs although they cannot all be on the same energy level, equally we give the same amount of repulsion to all non-bonding pairs although again they cannot all be on the same energy levels to justify the same force of repulsion. In water one of the lone pair is on the 2s orbital which is at a lower energy level than the 2p orbitals but when explaining the shape using the VSEPR we have considered both lone pairs (one on 2s and one on 2p) to be on a par in terms of energy (=equivalent) i.e. they cause the same repulsion which should not be the case if one is on a lower energy level. New ideas (by Linus Pauling) were added and these include the idea of hybridisation. Hybridisation is the mixing of a number of non-equivalent (=not the same) but similar atomic orbitals (e.g. 2s and 2p orbitals) to form the same number of new equivalent atomic orbitals (hybrid orbtials) for bonding. These new orbitals are hybrids (= a mixture); they are degenerate/equivalent (= of the same energy level) and have some s and p orbital character i.e. their energy level is in between the two. Each hybridized orbital has a different orientation i.e. points in a different direction. Hybridised orbitals have their own shapes (different from the normal s and p orbitals) i.e. they have 1 large lobe which extends between the two interacting atoms; these large lobes allow them to overlap better - area of overlap is larger - and form stronger bonds than unhybridised orbitals. So more energy is released when they are formed as opposed to unhybridised orbitals; that energy is used to initially promote the electrons during the process. Hybridisation involves 2 stages: the unpairing of an electron and the promotion of an electron e.g. an s electron to a p orbital; the actual hybridisation (or mixing) in which all orbitals become degenerate/the same. Example: hybridisation in boron promotion hybridisation non- 2s 4 | Page 2p 2s Topic 14 2p sp2 hybridised Example: hybridisation in carbon promotion 2s 2p hybridisation 2s sp3 2p Example : hybridisation in beryllium promotion 2s 2p hybridisation 2s 2p sp nonhybridised The energy needed for the unpairing of some electrons to a higher energy level is provided when a bond is made (bond making releases energy) as a result of the hybridisation. The energy released when making a new bond makes up for the energy needed to promote and hybridise electrons. Multiple bonds All sigma covalent bonds in polyatomic molecules are formed as a result of the overlap of two hybridised orbitals or one hybridised and one unhybridised (e.g. hydrogen). Pi, , bonds are always formed from unhybridised p orbitals, usually the pz, so they have no impact on the shape. Determining the type of hybridization that has occurred in a molecule or molecular ion Types of hybridization: sp, sp2 and sp3 How can we now identify the type of hybridisation (or which orbitals have hybridised) in a molecule or molecular ion? 1. draw the Lewis structure 2. determine electron pair geometry or valence electron pair distribution around central atom by counting the number of electron pairs (lone and bonding pairs) but consider a multiple bond as one; 3. the number you obtained in step 2 = number of hybrid orbitals 4. if 2 hybrid orbitals = sp hybridisation (180) 5. if 3 hybrid orbitals = sp2 hybridisation(120 bond angles between hybrid orbitals) 6. if 4 hybrid orbitals = sp3 hybridisation(109/ bond angles between hybrid orbitals) The type of hybridization that occurs depends on the number of sigma bonds and lone pairs that have to be accommodated around the atom. Hybrid orbitals are responsible for all the σ bonding overlaps and non-bonding pairs in a molecule or molecular ion. Unhybridised orbitals are responsible for all the bonding overlaps in a molecule. 5 | Page Topic 14 We can also work out the type of hybridization from the shapes of the molecules or the angles between bonding pairs. Shapes and orientation of hybridised orbitals type of hybridisation sp sp2 sp3 how many orbitals have been hybridized one s and one p orbital orientation of hybridized orbitals 1 s and 2 p orbitals 120 bond angles 1 s and 3 p orbital shape of molecule 180 bond angles 109 bond angles linear trigonal planar all 4 orbitals carry bonding pair: tetrahedral 1 orbital carries a lone pair: pyramidal 2 lone pairs: bent Good website: http://www.chemguide.co.uk/atoms/bonding/covalent.html#top Link the hybridisation and formation of sigma and pi bonds in this tutorial: http://www.mhhe.com/physsci/chemistry/animations/chang_7e_esp/bom5s2_6.swf 14. 3 Delocalisation of electrons 14.3.1 Describe the delocalization of π electrons and explain how this can account for the structures of some species. See: http://www.mikeblaber.org/oldwine/chm1045/notes/Bonding/Resonan/Bond07.htm http://www.mpcfaculty.net/mark_bishop/resonance.htm The valence bond theory could also not explain why the bonds in an ozone molecule were of the same length and strength. If a Lewis structure is drawn of an ozone molecule there should be 1 double bond and one single bond which means in an ozone molecule there should be two bonds of different lengths which is not backed up by experimental evidence. In fact, experimental evidence shows both bonds between the 3 oxygen atoms to be the same in length and strength. Resonance or delocalization of electrons occurs when 2 or more equivalent Lewis structures can be drawn with different electron positions for the same positions of atoms. It can only happen in molecules or molecular ions in which there are unhybridised p orbitals. Resonance structures are Lewis structures which do not exist and which differ only in the location of electron pairs but not in the number of valence electrons or the arrangement of the atoms within a molecule. Each resonance structure shows a different way in which lone and bonding pairs are arranged; bonding pairs and lone pairs are interchanged between the 2 or more resonance structures that can be drawn. Resonance then is a concept in which the actual structure of a molecule or molecular ion is taken to be the average of all possible Lewis structures. This average structure is called the resonance hybrid. When drawing resonance structures it is important that each one is separated by the symbol The reason why different arrangements of electrons can be drawn is because sometimes bonds extend over more than 2 atoms and this allows some of the electrons to move between more than two atoms, 6 | Page Topic 14 they are not localised or fixed between two atoms but they are delocalised over the entire ion or molecule (or parts of it). The advantage of this arrangement is that it lowers the energy of the ion/molecule making it more stable as the negative charge carried by the electrons is more spread out over the molecule or ion; the delocalised bonding pairs are shared equally between more than two atoms. The delocalisation or sharing of bonding electrons between three or more atoms occurs as a result of the overlapping of more than two unhybridised p orbitals, forming the bond; each orbital overlaps with the orbitals of the atoms on each side creating a larger continuous molecular orbital.; this only occurs if the p orbitals are in the same plane(eg trigonal planar) As the electron in the delocalised bond move around more - have more freedom - their energy of attraction is less than the attraction experienced by electrons in a localised sigma and pi bond which is why the delocalised bond is the weakest bond of all 3 types. As both a pi bond or a delocalised bond can only occur between two atoms that already have a sigma bond, the order of bond strength/length is the following: single bond (weakest ; longest bond length); single bond and delocalised bond (although a delocalised bond on its own – but this does never occur – would theoretically be the weakest); a single and delocalised bond together are sometimes referred to as 1 1/3 bond if the delocalisation is over 3 atoms as e.g. in the ethanoate ion double bond; triple bond; Experimental evidence for resonance: The theory of resonance has been developed as a result of X-ray measurements indicating equal bond length and bond strength within molecules which were thought to contain a mixture of double and single bonds e.g. in sulphur dioxide, ozone, and benzene. When measured the bond length is found to be intermediate between the single and the double bond. (see data booklet) The actual bond consists of sigma and part of a pi bond which explains its intermediate strength and length. So resonance occurs whenever there is a question as to which atom(s) has/have the double bond(s) between them. A Lewis structure can then be drawn for each possible arrangement changing the location of the double bonds; each such a Lewis structure in which the number of electron pairs between two atoms must differs is referred to as a resonance structure. It must be realised that any of the possible resonance structures does not actually exist and that the molecule does not continuously interchanges between the different resonance structures; a more accurate representation is the resonance hybrid diagram which represents an average of all Lewis structures. The resonance structures are merely structures showing the delocalisation. The resonance hybrid which is the most accurate presentation refers to the diagram which summarises in one structure the various arrangements of valence electrons and which shows the equal bond length; the resonance hybrid` also shows the only form in which the molecule exists. The more resonance structures that can be drawn, the greater the delocalisation – and as delocalisation gives stability - more stable the molecule. So delocalised bonding gives stability to a molecule Resonance is also used to explain properties such as reactivity (e.g. in the case of benzene and phenol), stability (benzene) and acidity (ethanoate ion). 7 | Page Topic 14 IB past paper questions PAPER 1 1. (M08) What is the shape of PF5? A. tetrahedral 2. B. trigonal pyramidal C. square planar D. trigonal bipyramidal (M08) The formula of tetrachloromethane is CCl4. Which combination correctly shows the number of lone pairs of electrons in the Lewis structure, the shape of the molecule and the type of hybridization of the carbon atom? 3. (N07) What is the shape of the CO32− ion and the approximate O–C–O bond angle? A. linear, 180 pyramidal, 109 B.trigonal planar, 90 C. trigonal planar, 120 D. 4. (N07) In the molecules N2H4 , N2H2 , and N2, the nitrogen atoms are linked by single, double and triple bonds, respectively. When these molecules are arranged in increasing order of the lengths of their nitrogen to nitrogen bonds (shortest bond first) which order is correct? 5. (N07) What is the molecular geometry and the Cl–I–Cl bond angle in the ICl4− ion? 8 | Page Topic 14 6. (N07) 7. (M07) Which molecule is square planar in shape? A. XeO4 B. XeF4 C. SF4 D. SiF4 8. (M07) 9. (M06) Which is the most volatile substance? A. Chlorine B. Fluorine C. Sodium chloride D. Sodium fluoride 10. (M06) Which is the smallest bond angle in the PF5 molecule? A. 90° B. 109.5° C. 120° D. 180° 11. (M06) Which types of hybridization are shown by the carbon atoms in the compound CH2= CH-CH3? II.sp2 I. sp A. I and II only 9 | Page B. I and III only Topic 14 III. sp3 C. II and III only D. I, II and III 12. (M01) Which species is/are sp2 hybridised? I. C2H4 A. I only II. C2H6 B. I and II only III. C3H6 C. I and III only D. II and III only 13. (M01) Which species contains no delocalised electrons? B. NO3- A. O3 C. CO32- D. H2SO4 14. (M00) In which of the following is there at least one double bond? I. O2 II. CO2 A. I only 15. B. III only III. C2H4 C. II and III only D. I, II and III (M01) Which is the best description of metallic bonding? A. B. C. D. The attraction between oppositely charged ions The attraction between protons and electrons The attraction between positive ions and delocalised electrons The attraction between nuclei and electron pairs 16. (N01) The length of bond between carbon and oxygen is shortest in A. CO B. CO2 C. CH3CH2OH D. CH3CHO 17. (N01) In which species can the bonding NOT be described in terms of delocalisation of ρi electrons A. CH3CH2O- B. CH3CO2- D. NO3- C. O3 18. (M00) According to the VSEPR theory, which molecule would be expected to have the smallest bond angle? A. H2O B. H2CO C. CH4 D. NH3 19. (M00) In which of the following would hydrogen bonding be expected to occur? I. CH4 II. CH3COOH III. CH3OCH3 A. II only B. I and III only C. II and III only D. I, II and III 20. (M01) In which of the following are the compounds BF3, SiH4, CO2 and SF6 arranged in decreasing order of bond angle? A. BF3, SiH4, CO2, SF6 B. BF3, SF6, CO2, SiH4 C. CO2, BF3, SiH4, SF6 D. SF6, CO2, SiH4, BF3 10 | P a g e Topic 14 21. (N00) Which molecule has the largest bond angle? A. BF3 B. CF4 C. NF3 D. OF2 22. (M02) The shape of the triiodide ion, I3-, is best described as A. bent B. linear C. T-shaped D. triangular 23. (M02) The delocalisation of electrons is most likely to be significant in A. CO2 B. SO2 C. HCOOH D. TiO2 24. (M02) The carbon-carbon bond angle in CH3CHCH2 is closest to A. 180 C. 120 C. 109 D. 90 25. (M02) When a Lewis structure for HCOOCH3 is drawn, how many bond pairs and how many lone pairs of electrons are present Bond pairs A. B. C. D. Lone pairs 8 7 7 5 4 5 4 5 26. (N02) Which intermolecular forces exist in dry ice, CO2 (s)? A. Covalent bonds B. Dipole-dipole attractions C. Van der Waals forces D. Hydrogen bonding 27. (N02) The elements X and Y have the following electronic configurations: X 1s2 2s2 2p6 3s2 3p6 4s2 Y 1s2 2s2 2p6 3s2 3p5 What is the formula of the compound formed between X and Y? A. XY2 B. X5Y2 C. X2Y5 D. XY5 28. (N02) Which statements about the following molecule are correct? (CH3)2CHCH=CHC≡CCH=CH2 I. Three carbon atoms are sp3 hybridized. II. Three carbon atoms are sp2 hybridized. III. Two carbon atoms are sp hybridized. A. I and II only B. I, II and III C. II and III only D. I and III only 29. (M01) Which molecule has the longest nitrogen-nitrogen bond length? 11 | P a g e Topic 14 A. N2 PAPER 2 B. N2F2 1. (M08/TZ2) 2. (M07/TZ1) 3. (M07/TZ1) 12 | P a g e Topic 14 C. N2H4 D. N2H2 4. (M06) 5. (N05) This question is about Period 3 elements and their compounds. (a) Explain, in terms of their structure and bonding, why the element sulfur is a non-conductor of electricity and aluminium is a good conductor of electricity. [4] (b) Explain, in terms of its structure and bonding, why silicon dioxide, SiO2, has a high melting point. [2] (c) Silicon tetrachloride, SiCl4, reacts with water to form an acidic solution. (i) Explain why silicon tetrachloride has a low melting point. [2] (ii) Write an equation for the reaction of silicon tetrachloride with water. [1] 13 | P a g e Topic 14 6. (N06) 7. (a) Explain the meaning of the term hybridization. [1] (b) State the type of hybridization shown by the carbon atom in the H–C≡N molecule, and the number of σ and π bonds present in the C≡N bond. [2] (c) Describe how σ and π bonds form. [4] (N03) (a) The boiling points of the hydrides of group 6 elements increase in the order H2S H2Se H2Te H2O Explain the trend in the boiling points in terms of bonding. [3] (b) (i) Draw the Lewis structures for carbon monoxide, carbon dioxide and the carbonate ion, CO 32-. [3] (ii) Identify the species with the longest carbon-oxygen bond and explain your answer. [3] (iii) Draw the Lewis structure of ClF3 and predict its shape. [2] (c) Hydrazoic acid, N3H, can be represented by two possible Lewis structures in which the atoms can be arranged as NNNH. (i) Draw the two possible Lewis structures of N3H. [2] (ii) Predict the N-N-N and H-N-N bond angles in each case and give your reasoning. [6] (iii) Predict the hybridization of the N atom bonded to the hydrogen atom in each case. [2] 8. (N01) (a) (i) Draw Lewis structures to represent BF3, NF3 and BF4- . [3] (ii) Use the principles of the Valence Shell Repulsion Electron Pair (VSEPR) theory to predict the shapes of the above three species. Compare and account for the bond angles in NF3 and BF4- in terms of VSEPR theory. [5] (iii) Explain the meaning of the term hybridization. State the type of hybridization shown by the central atoms in BF3 and NF3. [3] (iv) Explain the term polar bond. Predict and explain the polarity of the bonds within BF3 and NF3. State whether BF3 and NF3 are polar molecules. Explain your answer. [5] (b) (i) Explain what is meant by a sigma () and a pi ( ) bond. Describe a double bond and a triple bond in terms of and bonds. [4] (ii) Define the term delocalisation. 14 | P a g e Topic 14 [1] 9. (M01) (a) For each of the molecules C2H2, C2Cl4 and SF4, draw their Lewis structure, and use the VSEPR theory to predict their shape and bond angles. [10] (b) State the type of hybridization in C2H2 and C2Cl4. [2] (c) Draw two resonance structures for each of the ethanoate ion (CH3CO2-). [2] (d) Comment on the carbon to oxygen bond length in the ethanoic acid molecule and the ethanoate ion. [4] 10. (N02) (a) Draw Lewis structures of the following species. BF4- H2CO NO2+ NO2- [4] (b) Predict the shape and bond angle of each of the species in (a), explaining your choice using VSEPR. [8] (c) Explain the term hybridization and state the type of hybridization in each of the species in (a). [3] (d) Ethanoic acid contains two different types of carbon to oxygen bond. Explain how the carbon and oxygen atoms combine to form each of those bonds and compare their bond lengths and strengths. [7] (e) The structure of the ethanoate ion can be written as shown below The stability of the ethanoate ion suggests a different type of carbon to oxygen bond. (i) Describe the actual carbon to oxygen bond in the ethanoate ion. [2] (ii) Predict a value for the bond length of the carbon to oxygen bon din the ethanoate ion. [1] 15 | P a g e Topic 14 Mark scheme TOPIC 14 Paper 1 1 2 3 4 5 6 D C C D A B 7 8 9 10 11 12 Paper 2 1. (M06) 16 | P a g e Topic 14 B B B A C C 13 14 15 16 17 18 D D C A A A 19 20 21 22 23 24 A C A B B C 25 26 27 28 29 30 A C A D C 17 | P a g e Topic 14 2. (N05) 3. (N06) 18 | P a g e Topic 14 4. 19 | P a g e Topic 14 20 | P a g e Topic 14 5. 21 | P a g e Topic 14 6. (M01) 22 | P a g e Topic 14 23 | P a g e Topic 14 7. 24 | P a g e Topic 14 25 | P a g e Topic 14 26 | P a g e Topic 14