UW Department of Chemistry Lab Lectures Online Chem 162/164 1 of 14 Lab 1: Chemical Models & Magnetism Part I. Chemical Models Part II. Magnetic Susceptibility Part III. Ferromagnetism and Ferrofluids Part I. Chemical Models Introduction Molecules are three-dimensional objects. Paper and projection screens are two-dimensional. These simple facts-of-life mean that students of chemistry seldom get to ?see? molecules as they actually are and, consequently, often have difficulty understanding why they behave as they do. In the first part of this lab you will construct three-dimensional models of a number of simple molecules using plastic balls and sticks. You will then answer questions about the electron-pair geometry, molecular structure, and orbital hybridization, and examine the models to see what molecular properties are consequences of shape. First, you will draw the Lewis dot structure for each molecule. If you do this in preparation for the lab, then when you are in lab, you will be able to quickly assemble the model and draw the 3-D Lewis structure. Recall the general steps for drawing Lewis structures: 1) Sum the valence electrons from all the atoms 2) Add or subtract electrons depending upon the overall charge on the molecule 3) Use a pair of electrons to form a bond between each pair of bound atoms 4) Arrange the remaining electrons to satisfy the duet rule for hydrogen and the octet rule for the second-row elements, keeping in mind that there are some elements that may exceed the octet rule. Once you have the model assembled, you can work through the list of tasks and questions associated with each molecule. You will use the valence shell electron-pair repulsion (VSEPR) model, which is based on the idea that minimizing electron-pair repulsion within the molecule dictates the structure of the molecule, to determine the position of the atoms around the central atom(s). Recall from the Zumdahl textbook, the steps for using the VSEPR model: 1) Draw the Lewis dot structure of the molecule 2) Count the electron pairs (not the atoms) around the central atom(s) and arrange them so that they are as far apart as possible ? this is the electron-pair geometry 3) Based on how the electron pairs are shared in bonds, determine the positions of the atoms around the central atom(s) 4) Determine the molecular structure/shape based on the position of the atoms UW Department of Chemistry Lab Lectures Online Chem 162/164 2 of 14 Actual molecular shapes can deviate from ideal electron-pair geometries because lone electron-pairs take up more space than atoms. Therefore, the electron-pair geometry, which accounts for all electron pairs around an atom, whether lone pairs or in bonds with other atoms, determines the structure of the molecule. The table below summarizes the possible electron-pair geometries: Number of Electron Pairs Arrangement of Electron Pairs 2 Linear 3 Trigonal planar 4 Tetrahedral 5 Trigonal bipyramidal 6 Octahedral A given electron-pair arrangement can have different molecular shapes, some with just bonding groups and others with both bonding and non-bonding groups. Methane (CH4) and ammonia (NH3) each have four electron groups, but the C in CH4 is bound to four hydrogens while the N in NH3 is bound to three hydrogens and has one non-bonding pair. To help classify the various shapes, a specific AXmEn designation (shape classification) is assigned to the central atom of the molecule, or, in the case of molecules with more than one central atom, to each central atom. AXmEn designation is based on the Lewis structure of molecules, not the molecular formula, although sometimes, by coincidence, the designation may appear to be related to the formula. A is the central atom X is a surrounding atom E is a nonbonding valence-electron group (typically a lone pair) m and n are integers Examples: CH4 ? AX4 NH3 ? AX3E H2O ? AX2E2 C2H4 ? AX3, AX3 Draw the Lewis structures of these molecules and prove the AXmEn designation to yourself. As stated above, the name of the molecular structure or shape depends on the arrangement of the atoms around a central atom, taking into account the effect of lone pair(s) on the position of the atoms. Electron-Pair Geometry Possible Molecular Structures (with AXmEn classification) Linear Linear (AX2) Trigonal planar Trigonal planar (AX3), Bent/V-shaped (AX2E) Tetrahedral Tetrahedral (AX4), Trigonal pyramidal (AX3E), Bent/V-shaped (AX2E2) Trigonal bipyramidal Trigonal bipyramidal (AX5), See-saw (AX4E), T-shaped (AX3E2), Linear (AX2E3) Octahedral Octahedral (AX6), Square pyramidal (AX5E), Square planar (AX4E2) UW Department of Chemistry Lab Lectures Online Chem 162/164 3 of 14 Hybridization is the mixing of atomic orbitals to form special orbitals for bonding. The s and p valence orbitals of a central atom are combined to form ?hybrid? orbitals that are then used to form bonds to other atoms. Considering methane as an example, the 2s and 2p valence orbitals combine to form four sp3 hybrid orbitals that allow carbon to make 4 bonds and the molecule to take on a stable tetrahedral structure. When there is a double bond present, as between the two carbon atoms in ethene (C2H4), the 2s and 2p orbitals combine to form three sp2 hybrid orbitals that can each bond to an atom, with a 2p orbital left over to form a second bond with a p orbital from another central atom. If a triple bond is present, as between the two carbons in ethyne (C2H2), the 2s and 2p orbitals combine to form two sp hybrid orbitals that can each bond to an atom, with two 2p orbitals left over to form the second and third bonds with p orbitals from the other central atom. Electronegativity describes the relative affinities of atoms for the electrons in a bond. When electrons are unequally shared between two atoms in a bond, the result is a polar bond. A molecule with a charge distribution resulting in a center of positive charge at one end and a center of negative charge at the other has what is called a dipole moment. Used to form a second bond with a p orbital from a second central atom. Used to form bonds to three atoms. Used to form second and third bonds with available p orbitals from a second central atom. UW Department of Chemistry Lab Lectures Online Chem 162/164 4 of 14 Non-polar bond: C C 2.2 2.2 Electronegativities are equal. Polar bond: Cl C 3.2 2.2 Electronegativities are unequal; Cl is the negative end of ?- ?+ the bond. ? represents a partial positive or negative charge. An arrow with a + sign at the tail end is used to indicate the direction of the dipole moment of the bond. A polar molecule exists when the combined effect of individual polar bonds results in an overall dipole moment for the molecule. The dipole moment provides important information about the bonding and electron distribution within a molecule. For the examples below, consider the net effect of the individual polar bonds in each molecule. Molecules whose atoms are connected by single bonds tend to be rather 'floppy.' This is because single bonds, unlike double bonds, allow the atoms to twist, or rotate, along the single bond. The rate at which rotation occurs about a single bond is almost unbelievable: the two CH3 groups in the molecule CH3-CH3 spin at a rate of approximately 107 times a second! This sort of 'free rotation' is not possible in large molecules because the various parts bump into each other. As you build molecular models in this lab, you will find that atoms can freely rotate The symmetry of CCl4 cancels out all of the individual dipole moments, resulting in a non-polar molecule. The dipole moment between C and H is much smaller than between C and Cl. Thus, CHCl3 is polar and has an overall dipole moment. UW Department of Chemistry Lab Lectures Online Chem 162/164 5 of 14 around single bonds but cannot rotate around double or triple bonds. Whether a molecule is 'rigid' or not has a big effect on its properties! Free rotation around the C-C bond below causes individual dipole moments to cancel themselves out and results in a non-polar molecule. The inability of a molecule to rotate around a double bond makes the molecules below different from one another. The molecule on the left is a non-polar due to symmetry. The molecule on the right is polar because the bromine atoms are on the same side of the molecule. When you are building a model, you will want to evaluate whether or not you get a different molecule when you interchange some of the atoms. If you do, then you have a set of isomers, which are different molecules constructed from the same set of atoms, or, put another way, different molecules with the same chemical formula. As an example, consider molecules with the formula C2H2Cl2. C C H Cl H Cl C C H Cl Cl H C C H H Cl Cl t r a n s? 1 , 2 d i c h l o r o e t h e n e 1 ,1 d i c h l o r o e t h e n ec i s ? 1 , 2 d i c h l o r o e t h e n e The first two configurations are different because in one case the H atoms and Cl atoms are on the same side (i.e. 'cis'), while in the other case they are diagonal from each other (i.e. 'trans'). This difference is only meaningful if the 'ends' of the molecule (i.e. the CHCl parts) are connected in such a way that they cannot rotate; if the ends can rotate 180?, then the two molecules are really identical. Double bonds (as you will see when you build the model) always keep the atoms 'locked in place.' Isomers that result from double bonds are called geometric isomers. On the other hand, the third isomer shown above, is truly different from the other two because the H and Cl atoms are attached to different C atoms. The official names for these molecules, as you will learn later in the Zumdahl text, are: cis-1,2-dichloroethene, trans-1,2-dichloroethene and 1,1-dichloroethene, respectively. The numbers indicate which UW Department of Chemistry Lab Lectures Online Chem 162/164 6 of 14 carbon atoms the two chlorines ('dichloro') are attached to, counting not from left-to-right, but in whatever way minimizes the sum of the numbers! Thus the third example is '1,1', not '2,2.' You should learn the names of all the molecules you build for this experiment. Formal charge is defined as the hypothetical charge on an atom in a molecule or an ion. It is equal to the number of valence electrons minus the sum of all the unshared electrons and half the shared valence electrons. The easiest way to calculate this number is to: 1) Circle the atom in question, cutting all bonds to that atom in half. 2) Count the number of electrons within the circle (bonds that are cut in half count as 1 electron). 3) Subtract this number from the number of valence electrons of the atom. 4) Make sure the total of all formal charges matches the overall charge on the molecule or ion. For example: NH4+ Part II. Magnetic Susceptibility Introduction There are many pieces of evidence that point to the validity of electronic configurations. One is successive ionization energies. These show large jumps whenever an electron from a different orbital is removed and even larger jumps when an electron from a lower energy level is removed. For example, consider Mn with electronic configuration [Ne]3s23p64s23d5. The successive ionization energies in electron-volts (1eV = 1.60 x 10-19J) are: IE1 IE2 IE3 IE4 IE5 IE6 IE7 IE8 7.4 15.6 33.7 51.2 72.4 95.5 119 194 Removal of the 4s2 electrons Removal of the 3d5 electrons Removal of the 3p6 electron Big jump Big jump UW Department of Chemistry Lab Lectures Online Chem 162/164 7 of 14 The measurement of ionization energies is not easy. One difficulty is that the species must be atomic and in the gas phase. There is, however, an aspect of electronic configuration that we can measure in a general chemistry laboratory: the number of unpaired electrons as predicted by the aufbau principle and Hund's rule. According to Hund's rule, the five 3d5 electrons in the Mn atom should each be in a different 3d5 orbital. When these atoms bond (as in metallic bonding), these electrons have a tendency to pair with electrons from the other atoms. Many transition metal compounds have one or more unpaired electrons and are, therefore, termed paramagnetic. Diamagnetic compounds have only paired electrons and are, therefore, unaffected by a magnetic field. Consider an unpaired electron orbiting a nucleus (e.g. unpaired 3d1 electron in Ti3+) Because the electron is spinning while traveling around the nucleus, it behaves like a tiny magnet. Paired electrons cancel out the magnetic fields of each other, however, when the atom is placed in an external magnetic field, any unpaired electrons will align, creating an induced magnetic field. The magnitude of this induced magnetic field is essentially the magnetic susceptibility of the atom. The electron spin magnetic moment is only observed when you have unpaired electrons. How do you find out if something is magnetic? The answer is to bring together a known magnet and the test substance and look for interaction between the two. The figure below contrasts the behavior of paramagnetic and diamagnetic substances in a magnetic field. Note that a paramagnetic substance has the effect of increasing the magnetic field. E l e c t r o n h a s o r b i t a l m o t i o n a n d s p i n m o t i o n T h e s p i n n i g a c t i o n m a k e s t h e e l e c t r o n t o b e h a v e a s t i n y m a g n e t . G o i n g a r o u n d t h e n u c l e u s S p i n n i n g o n i t s a x i s F i g u r e 1 : T h e o r i g i n o f t h e s p i n m a g n e t i c m o m e n t The spinning action makes the electron behave like a tiny magnet. UW Department of Chemistry Lab Lectures Online Chem 162/164 8 of 14 Figure 2. A: Magnetic lines of force showing the effect of diamagnetic and paramagnetic materials on the magnetic field. B: Microscopic explanation of paramagnetism. The magnetic moment of the sample tends to draw the magnetic field through the sample. The force of attraction between the small magnet (single paramagnetic atom, molecule, or ion) and the large magnetic field can be measured. It is however better to talk about interaction energy. An example is the attraction between ions being discussed in terms of energy (lattice energy, not joules but actually joules/mole). We shall describe the interaction between a paramagnetic ions (or atoms or molecules) in terms of energy. However, energy by itself is not enough of a description since a larger magnetic field will induce a greater interaction energy than a weaker magnetic field. Therefore, we will describe the interaction between a paramagnetic ion (or atom or molecule) in Joules/Tesla, J/T. (Magnetic fields are measured in units of Tesla, T.) The spin magnetic interaction energy (usually called spin magnetic moment) due to a single unpaired electron (as in Ti3+) is 1.73 uB, where uB is the Bohr Magneton, which is equal to 9.273 x 10-24 J/T. The use of uB units allows us to avoid small numbers in very much the same way a dipole moment is discussed in Debye units (1D = 3.338 x 10-30 Cm). For reasons beyond the scope of this course, when a species (atom, molecule, or ion) has two unpaired electrons, the effective magnetic interaction energy is not 2 x 1.73 uB but rather 2.83 uB. The table below relates the number of unpaired electrons (n) to various uB values. n ? eff (uB units) 1 1.73 2 2.83 3 3.87 4 4.90 5 5.92 6 6.93 If we can measure the Bohr magnetons (uB), we can assign the number of unpaired electrons. Because there is no direct way to measure uB, we will make a number of independent UW Department of Chemistry Lab Lectures Online Chem 162/164 9 of 14 measurements from which we can calculate uB. This approach is similar to determination of the 'measurement' of density -- you measure the mass and the volume and calculate the density! Let us begin by making a pictorial view of the interaction between the outside field and the spin magnetic moment. In the figure below (Figure 3), we have unpaired electrons from TiF3 units and each of the unpaired electrons is represented by an arrow. Without an external field, the spin direction is random and the net magnetization is zero. When an external filed is applied, most electron magnetic moments align and the substance is magnetized. For substances like iron, the magnetization remains even after the external field is turned off resulting in a permanent magnet (See Part III). Consider the factors that affect the magnitude of I. 1) The number of unpaired electrons (or small magnets) per unit volume. The density and molar mass of the substance determine this value. 2) The temperature has the effect of randomizing the spins against the alignment imposed by the external field. Thus the higher the temperature, the lower the net magnetization (I). If the applied magnetic field has magnitude H, we make the following definitions: Volume Susceptibility (XV) = I/H, where I is the intensity of magnetism induced in the substance and H is the intensity of the applied magnetic field. It is a good idea to define a ratio of this kind because while I will vary with the strength of the magnetic field (H), the ratio I/H will remain constant. A p p l y e x t e r n a l m a g n e t i c f i e l d D i r e c t i o n o f e x t e r n a l f i l e d o f s t r e n g t h H I n d u c e d M a g n e t i c f i e l d d u e t o a l l i g n m e n t o f s p i n s ( = I ) R a n d o m s p i n o r i e n t a t i o n F i g u r e 3 . M a g n e t i z a t i o n o f a p a r a m a g n e t i c s u b s t a n c e UW Department of Chemistry Lab Lectures Online Chem 162/164 10 of 14 Mass Susceptibility (XG) = Xv/d, where d is the density of the substance, and Molar Susceptibility (XM) = XGMM, where MM is the molar mass. Using the Johnson-Matthey susceptibility balance, the molar susceptibility (XM ) is determined according to the formula: oM 9M .C .L ( R R )X 1 0 m ?? Where: M = molar mass (g/mole) L is the sample length (cm) m is the sample mass (grams) R is the magnetic balance reading for the tube plus sample Ro is the magnetic balance empty tube reading C is the calibration constant provided next to the balance. It is convenient to ignore the units at this stage. Suffice it to say that the units do work out thanks to the 109 divisor. If molar susceptibility (XM) can be measured using the Johnson-Matthey balance, all that remains are the equations that will take us from XM to uB. The product of XM and the temperature T (in K) is a constant. It is this product that is related to the effective magnetic moment (? eff) and to the number of unpaired electrons (n): Meff? = 2.83 X T = n(n+ 2) The table that allows easy transformation from ? eff values to the number of unpaired electrons has already been provided. Part III. Ferromagnetism and Ferrofluids Introduction You have considered the concepts of paramagnetism and diamagnetism. There is another form of magnetism, ferromagnetism. Ferromagnetism is technologically important and extensively exploited in the use of permanent magnets and magnetic recording media. The difference between simple paramagnetism and ferromagnetism is shown in Figure 4 where unpaired electrons are depicted as arrows. In a paramagnet, and in the absence of a magnetic field, the spins are randomly oriented because of thermal motion (Figure 4A) (i.e. thermal motion is sufficient to prevent any form of interaction between the spins.) The UW Department of Chemistry Lab Lectures Online Chem 162/164 11 of 14 application of a magnetic field aligns the spins along or in opposition to the magnetic field, and the overall magnetism is small (Figure 4B). In both cases, the spins disorder upon the removal of an applied magnetic field. Figure 4. A diagram showing the orientation of electrons in: (A) a paramagnetic solid in the absence of a magnetic field, (B) a paramagnetic solid in the presence of a magnetic field, (C) a ferromagnet in the absence of a magnetic field, and (D) a ferromagnet solid in the presence of a magnetic field. In a ferromagnet, the unpaired electrons strongly interact with one another (even in the absence of an external magnetic field) in large regions known as magnetic domains (Figure 4C). Magnetic domains involve a large number of atoms or ions. Because these domains are randomly oriented, the net magnetism is small or zero. Application of a magnetic field aligns all the domains in the direction of the magnetic field (Figure 4D), leading to a large, overall magnetism. If the orientation of spins is retained after the applied field is removed, a permanent magnet is produced. Ferromagnetic materials form the basis for audio and VCR tape technology. The tape itself consists of a polymer that is impregnated with crystals of ?-Fe2O3, CrO2, or a similar compound. The recording device consists of an electromagnetic head that creates a varying magnetic field as it receives signal from the microphone. The tape is magnetized as it passes through the magnetic field of the recording head. The strength and magnetization varies with the frequency of the sound being recorded. When the tape is played back, the magnetization of the moving tape induces a varying current whose signal is amplified and sent to the speakers. UW Department of Chemistry Lab Lectures Online Chem 162/164 12 of 14 The magnetic behavior of a ferromagnet is temperature-dependent because of the interplay of thermal energy and the stability gained by aligning the electrons in the ferromagnetic state. Above a critical temperature, known as the Curie temperature (Tcurie), thermal energy is sufficient to break the alignment of the spins and the material exhibits simple paramagnetism. Below Tcurie, the aligning forces overcome thermal effects to maintain ferromagnetic behavior. In other words, above Tcurie, the magnetism will vary with T and below Tcurie, the magnetism will be fixed and independent of T. The table below lists some common ferromagnets and their Tcurie values. Material Tcurie (K) Fe 1043 Co 1388 Ni 627 Gd 293 CrBr3 37 EuO 77 Gadolinium has Tcurie at about room temperature (20 oC). The electronic configuration of gadolinium is [Xe]6s24f75d1. With such a large number of unpaired electrons, it should not come as a surprise that when a small amount of gadolinium is placed in our magnetic susceptibility balances, the reading is off the scale. When a sample of gadolinium in a small tube is placed between poles of a magnet, it will stick to one side. The interaction is strong enough for the sample to stick to the magnet. When heated above the Curie temperature of 20oC, the sample changes from a ferromagnet to a paramagnet. The decline in the net magnetic moment (compare Figure 4B and 4D) is no longer strong enough to support the weight of the sample and it drops off. Ferrofluids have the fluid properties of a liquid and the magnetic properties of a solid. Ferrofluids actually contain small particles (~100 ?A) of a magnetic material suspended in a liquid medium (solvent). Ferrofluids were first discovered in the 1960?s at the NASA Research Center, where scientists were investigating possible methods of controlling liquids in space. The benefits of a magnetic fluid were immediately obvious: the location of the fluid could be precisely controlled by the simple application of a magnetic field, and by varying the strength of the magnetic field, the fluids could be forced to flow. The most important ferrofluids are based on magnetite, Fe3O4. (Magnetite is actually ferromagnetic. There are two spin sets, Fe2+ and Fe3+, in opposing directions that do not cancel and, therefore, leave a net spin.) To ensure that particles do not agglomerate, a surfactant (dual nature molecule: -COOH polar head and long, non-polar hydrophobic chain), such as oleic acid, is added. The polar head (-COOH) is attracted, or binds, to the magnetite particle. The hydrophobic chain prevents the particle from agglomerating (Figure 5). UW Department of Chemistry Lab Lectures Online Chem 162/164 13 of 14 Technically, a ferrofluid is a soft magnetic material as opposed to a hard magnetic material. Hard materials are permanent magnets and they either attract or repel (N-N repel, S-N attract). A soft magnetic material is always attracted to a permanent magnet independent of its polarity. A ferrofluid moves towards the source of a magnetic field created by a permanent magnet. It collects at the location where the magnetic field has the highest value and also changes rapidly in direction. Ferrofluids follow the lines of force of a magnet, and in doing so break up into ?spikes? because of the effect of surface tension. The spikes typically are hexagonal in shape and are equidistant from each other. The prelab assignment on WebAssign addresses the following: ? Assigning AXmEn notation to classify the molecular shape around the central atom(s) in various molecules. ? Determining the shape of various molecules, based on the arrangement of atoms around a central atom, and taking into account the effect of non-bonding electron pairs. Helpful information ? The chapters in Zumdahl covering bonding (general and covalent), molecular structure, and electron configurations (aufbau principle, Hund?s rule) will be extremely helpful in understanding much of this lab (see chapters 12, 13, 14, and 19). Pay special attention to the parts of Ch 19 that discuss the electron configurations of transition metals. ? Be sure to prepare for Part I by reading the prelab information in this document as well as the relevant sections of the Zumdahl text so that you can work through the in-lab report worksheet efficiently. ? The bonding chapter of Zumdahl has several tables containing drawings of the electron- pair geometries and some of the molecular structures for the VSEPR model approach to determining molecular structures. H O O C H O O C H O O C C O O H l o n g c h a i n s i n t e r a c t i n g w i t h t h e o i l a n d w i t h e a c h o t h e r p r e v e n t a g g l o m e r a t i o n . F i g u r e 5 . S t a b i l i z a t i o n o f m a g n e t i t e p a r t i c l e s b y o l e i c a c i d . UW Department of Chemistry Lab Lectures Online Chem 162/164 14 of 14 ? Here is an example of a complete molecular description of NH3: 3-D Lewis Dot Structure Electron-pair geometry: tetrahedral Molecular shape: trigonal pyramidal AXmEn classification: AX3E Hybridization on N: sp3 4 hybrid orbitals for the 4 electron pairs Polarity: polar Permanent dipole in the direction of the lone electron pair (shown in red) ? The experimental procedures for Parts II and III are quite simple, so make sure that you take the time to understand the terminology, equations, calculations, etc. that are necessary to complete the post-lab report for these sections. Also make sure that you record ALL of the data necessary to complete the table in the post-lab report. ? For a little more information about ferrofluids, take a look at the following websites (links valid as of the revision date of this document): o http://www.ferrotec.com/technology/ferrofluid/ (more background information) o http://www.youtube.com/watch?v=9SKVHv63SlM (video of a ferrofluid in a closed tube being manipulated by a magnet brought into close proximity to the sample) chemistry Percent Copper and Formula Weight of a Copper Compound
Want to see the other 14 page(s) in intro lab 1.pdf?JOIN TODAY FOR FREE!