Jenalyn D. Osuna

The synthesis of Copper(I) tetraiodomercurate(II) and the Study of its Solid State Properties I. Introduction Metals are opaque and shiny materials that are usually solid at room temperature. At this physical state, metallic compounds possess ordered arrays of atoms which form a crystal lattice structure. A crystal lattice is defined as a three-dimensional, infinite array of points. The lattice points are the basic repeating structure of a crystal. This means that each of the lattice points is surrounded by another neighboring lattice points that are arranged in an identical manner. (Shriver & Atkins, 2010) The smallest repeating unit of the structure which carries all the information necessary for the unambiguous construction of an infinite crystal lattice is referred to as a unit cell. These can either be close-packed or non-closed-packed. The former contains interstitial sites which can either be an octahedral or a tetrahedral hole. On the other hand, the latter involves unit cells that are simple cubic and body-centered cubic lattice. The most common crystalline structures are cubic-closed packed, hexagonal closed-packed and body-centered cubic. (House, J. E., 2008) Metals are also malleable, ductile and good electrical and thermal conductors. These properties are the consequence of their crystal structure, which are affected by several forces of interaction as well as its surrounding conditions. The bonding in metals is essentially covalent where the bonding electrons are delocalized over the whole crystal. This accounts for the high thermal and electrical conductivity of metals. (House, J. E., 2008) Aside from that, metals are also capable of existing in various forms under specified conditions of pressure and temperature. That is, an alteration on these parameters may cause changes in the lattice structure of a given metal complex. This phenomenon is referred to as polymorphism wherein the metal ion assumes a formation that leads to its stability under a given condition. Its ability to do so can be attributed to the low directionality of the bonds that metal atoms may form. (Heslop R. B. and Robinson, P. L., 1967) Depending on the temperature, polymorphs of metals may be labeled as alpha, beta or gamma. Different polymorphs have different arrangements of atoms within the unit cell, and this can have a great effect on the properties of the final crystallized compound. The change that takes place between crystal structures of the same chemical compound is called polymorphic transformation. (Heslop R. B. and Robinson, P. L., 1967) This exercise was designed to study the polymorphic transformation of a metal complex. The specific objectives of this exercise were to synthesize and prepare Cu 2HgI4, determine its the transition temperature and study its the electrical conductivity. One of the interesting characteristics of copper(I) tetraiodomecurate(II) is that it has two distinct forms under different conditions. Each form is identified using several parameters such as color, crystalline structure, density, solubility and electrical conductivity. Copper (I) tetraiodomercurate is considered as a thermochromic compound which means that it reversibly changes color with accordance to changes in temperature. The change in color happens at a determined temperature which can be varied by doping the material. The temperature at which the compound changes color is called the transition temperature. Two forms of this complex differ in electrical conductivity. Experimentally, electrical conductivity is determined by placing a potential difference between two electrodes. (Shriver & Atkins, 2010) The response will be the current, I. This parameter can be related to resistance by using Ohm’s law: Where is the current measured in amperes; voltage measured in volts. is the resistance measured in ohms and is the The resistance can then be related to resistivity using according to the following equation: Where is the resistivity; is the length of wire and is the cross-sectional area. Given the resistivity, the conductivity, σ, can be calculated using the following equation (Shriver & Atkins, 2010): II. Methodology The experiment was divided into three parts which involves the preparation of Cu2HgI4, determination of its transition temperature, and the study of its electrical conductivity. The first part involves the preparation of copper(I) iodide and mercury(II)iodide and the synthesis proper of Cu2HgI4. First, copper(I) iodide was prepared by mixing together 1.25 millimoles of copper(II) sulfate using a 0.5M solution, 20% excess potassium iodide using a 1M solution, several drops of 6M acetic acid and 25mL distilled water in a beaker. Second, about 0.1g sodium sulfite in 5mL distilled water was added to the mixture while stirring. Third, the supernatant was decanted off the mixture taking into account that not more than 1-2% of the copper(I) iodide precipitate was lost. Next, the stoichiometric amount of mercury(II)iodide required for the synthesis of Cu2HgI4 was prepared by allowing the appropriate amount of 0.05M mercuric nitrate to react with 20% excess of 1M potassium iodide in 50mL distilled water. Then, the copper(I) iodide was then added quantitatively to mercury(II) iodide and the resulting suspension was heated to almost boiling for about 20 minutes while vigorously stirring the mixture. Following that, the mixture was then filtered through coarse filter paper while it was still hot. Then, the precipitate was washed several times with water and was allowed to drain thoroughly. Finally, the moist solids were transferred onto an evaporating dish, and settled on a steam bath for about 10 minutes or until the solids were dried. The resulting product was pressed into powder. Meanwhile, for the determination of the transition temperature, a small amount of the synthesized Cu2HgI4 was put into a test tube where it is directly touching a thermometer. The tube was then partially submerged into a water bath and the transition temperature was determined by subjecting and removing the water bath into a hot plate. The set up resembles heating and cooling mode where a change of color from bright red to dark brown and dark brown to bright red was carefully observed, respectively. The temperature was also carefully monitored during the process. The apparent transition temperature was recorded and it was compared to the theoretical transition temperature. Lastly, an electrical conductivity apparatus was used to study the electrical conductivity of Cu2HgI4. The apparatus consisted of a 4-cm length 4-mm-OD glass tube and two brass electrodes of about 2.5mm diameter connected in series with a high resistance volt-ohmmeter. A small portion of the dry sample was packed tightly between the two electrodes inside the glass tube by pushing the two electrodes against each other. In order to hold the two electrodes together, a rubber band was used. The sample was heated inside the glass tube using a lighter flame while taking care not to blacken the glass tube. The resistance reading was observed and recorded. The sample was allowed to cool without disassembling the set up. The resistance reading as well as the color change exhibited by the sample was again recorded. III. Results and Discussion The Crystalline Solid State Solids can be classified as amorphous and crystalline. Amorphous solids are those having no particular arrangement of molecules while crystalline solids are those that are arranged in an orderly manner. Crystalline solids have atoms, ions or molecules packed in regular geometric arrays, with the structural unit called unit cell that are arranged in lattice structures. These are patterns that are formed by the points which represent the location of the units within the crystal. (Heslop R. B. and Robinson, P. L., 1967) Unit cell can be classified as primitive, body centered, face centered and side centered while the crystal formed can be classified as cubic, tetragonal, hexagonal, trigonal, orthorhombic, monoclinic and triclinic. Figure 5.1. The 14 Bravais lattices grouped according to the 7 crystal systems. Some of the properties of crystalline solids that are directly related to their crystal structures are polymorphism, thermochromism and ionic conductivity. These properties are affected by several factors including forces of attraction and the surrounding conditions. Polymorphism refers to the ability of compounds to adopt different crystal structures at a specified condition of pressure and temperature. (Heslop R. B. and Robinson, P. L., 1967) The natural occurrence of allotropes in nature proves this phenomenon. Allotropes are compounds of the same composition but of different crystal structure. Generally, metals tend to assume closed packed phases at low temperatures. However, at higher temperatures, they tend to assume loosely packed phases. Moreover, the molar volumes of metals decrease at high pressure conditions. (Nelson, P. G., 2011) Copper(I) tetraiodomercurate(II) is one of the several substances which occur in two distinct forms under different conditions. Each allotropic modification is distinguished from the other by color, density, crystalline structure, solubility and other physical properties. Cu2HgI4 became unique because it belongs to the group of solid state conductors which shows different thermal conductivity at its two forms. Synthesis of Cu2HgI4 In this exercise, Cu2HgI4 was produced from the reaction mixture of pre-synthesized copper(I) iodide and mercury(II) iodide. copper(I) iodide was prepared by mixing together 1.25 mmol of copper(II) sulfate using a 0.5M solution, 20% excess potassium iodide using a 1M solution, several drops of 6M acetic acid and 25mL distilled water in a beaker. Second, about 0.1g sodium sulfite in 5mL distilled water was added to the mixture while stirring. Third, the supernatant was decanted off the mixture taking into account that not more than 1-2% of the copper(I) iodide precipitate was lost. For the synthesis of copper(I) iodide, 1.25 mmol of 0.5 M CuSO 4 was allowed to react with 20% excess of 1 M KI solution according to the following reaction: In the reaction, formation of CuI is more favored than the formation of CuI2 and it involves the reduction of Cu2+ to Cu+. This reaction follows the Hard/Soft Acid/Base (HSAB) rule which states that the most stable oxidation state of the Cun+ ions in water is manipulated to be the softer Cu+ by the addition of the soft base I -. The existence of an element in two or more oxidation states in an aqueous solution is a matter of their relative thermodynamic stability, which is most conveniently expressed in terms of the standard reduction potential of the reaction: The reduction potential for Cu (II) and Cu (I) solution is governed by the Nernst equation: However, in the presence of excess iodide ion, the iodine undergoes further reaction, forming triiodide ions as described in the following reaction: Hence, the true net ionic equation involved is as follows: Presence or addition of any species to the solution causes a decrease in the concentrations of either Cu2+ or Cu+ which in effect, alters their stoichiometric ratio. Hence, an observable change in the potential and relative stability of the ions will be manifested. A decrease in the concentration of Cu2+ will result to a less positive potential and the higher oxidation state will become more stable. On the other hand, a decrease in the concentration of Cu + will result to a more positive potential and the lower oxidation state will become more stable. Considering the solubility of the formed salts, copper (I) iodide has a very low solubility product constant (K sp = 5.1x10-12). Hence, further addition of KI decreases the concentration of Cu + so that the reduction of Cu2+ to Cu+ will become more favored. CuSO4 served as an oxidizing agent as well as the source of Cu2+. On the other hand, KI served as the source of I-. KI was added in excess in order to ensure that all CuSO4 reacted. The addition of acetic acid to the mixture prevented the formation of hydroxide salts of Cu(OH)2. It also acted as a catalyst in the formation of copper (I) iodide. The net reaction involved for the synthesis of copper(I) iodide is as follows: Since the reaction is in equilibrium, the triiodide formed can act as an oxidizing agent taking back Cu+ to Cu2+. The addition of sodium sulfite, which also served as the reducing agent, removed the formed iodide ion. According to the following reaction: Under aqueous medium, reduction is less favored since it causes a decrease in the concentration of Cu2+ that will result to its preferred stability than Cu+. On the other hand, Cu+ can undergo disproportionation wherein the oxidation state is continuously lowered and raised. Disproportionation reaction of an element allows that element to serve both as a reducing and oxidizing agent. Hence, further addition of KI to the solution will cause a decrease in the concentraion of Cu+, making it more stable in the solution compared to Cu 2+. Due to this, reduction is favored. Anion metathesis, which is an exchange reaction, was utilized in the synthesis of HgI2. An exchange between the nitrate ion and iodide ion happened during the reaction of Hg(NO3)2 with KI in aqueous environment. For this reaction, Hg(NO 3)2 served as the source of Hg 2+ while KI served as the source of I-. KI was added in excess in order to make sure that all Hg(NO3)2 have reacted. The reaction involved is described by the following equation: The synthesis proper of Cu2HgI4 happened during the mixture of a stoichiometric amount of HgI2 with the CuI solution. Stoichiometric addition was done in order to prevent the formation of K2HgI4, which is a possible side product. This could happen if excess KI and Hg(NO 3)2 were added according to the following reaction: Aside from that, excess amounts of the reagents may cause the incorporation of soluble but unreacted reagents to the solid which are considered impurities. This in effect may alter some of the physical properties of the product which includes darkening of its color and lowering the transition temperature range. After mixing the CuI and HgI2 solution, the mixture turned dark orange and eventually turned colloidal white. It was heated to almost boiling for about 20 minutes with vigorous stirring. This was done in order to remove the I 2 in the solution and force the reaction towards the product formation while preventing the formation of impurities. The mixture eventually turned brick red then turned dark brown. The mixture was then filtered while hot using coarse filter paper. Filtering the mixture while hot prevents the incorporation of soluble impurities that may precipitate out at lower temperature. The filtrate appeared colorless solution while the precipitate, which is the crude product, appeared dark-brown but eventually turned brick red. The product was then washed with water to remove the adhered impurities and was allowed to drain continuously. The moist solid was dried by transferring it to an evaporating dish and placing in a steam bath. The product was pressed dried, and weighed. The general reaction involved is for the synthesis of Cu2HgI4 is as follows: Table 5.1 summarizes the observations during the synthesis of Cu 2HgI4. On the other hand, Table 5.2 summarizes the data on the yield of the synthesized product. Table 5.1. Step by step observation on the preparation of Cu2HgI4. STEP/ACTION TAKEN OBSERVATIONS 1.25 mmol CuSO 4 n/a CuI was provided + 20% excess KI n/a CuI was provided + 6M acetic acid n/a CuI was provided + 25mL water n/a CuI was provided + 0.1g sodium sulfite in 5mL water n/a CuI was provided Supernatant liquid n/a CuI was provided Decant Residue n/a CuI was provided + Hg(NO3)2, KI and water Turbid light yellow mixture Mixing of CuI and HgI2 Opaque orange mixture Upon heating with stirring Red solution Filtration Filtrate Residue Washing with water Residue after steam bath Product Faint pink solution Red solids Red solids Red solids Red Solids Table 5.2. Data on the theoretical, actual and percent yield. Parameter Actual Yield, g Theoretical Yield, g Percent Yield, % Polymorphism and Thermochromism Value- One of the characteristics of Cu2HgI4 is that it exhibits polymorphism and thermochromism. Its allotropic forms are the α and β-form. Thermochromic compounds are substances that have the ability to reversibly change color depending on the changes in temperature. (House, J. E., 2008) The observable rapid change in its color from dark-brown to brick red upon filtration and standing is a manifestation of this property. The β-form is the more dominant form at low temperature, which is brick red in color. The arrangement of its unit cells is tetragonal where the iodide ion occupies the face centered cubic unit cells. On the other hand, Cu + and Hg 2+ occupy tetrahedral holes formed by iodide ions and they are arranged in separate alternating layers packed between the close-packed layers of iodide ions. Figure 5.2. The β form of Cu2HgI4. However, the more dominant form at high temperature is the α-form, which is dark-brown in color. The arrangement of its unit cells is cubic where the iodide ions occupy the face centered cubic unit cells. On the other hand, Cu + and Hg 2+ randomly occupy all the tetrahedral holes and certain vacant positions in the iodide array. On the average, each tetrahedral site contains ¼ Cu+ and ⅛ Hg2+. Figure 5.3. The α form of Cu2HgI4. The transition between one crystal form to another takes place at a sharply defined temperature which is referred to as the transition temperature. (House, J. E., 2008) The transition temperature of Cu2HgI4 was determined through two modes which are the increasing and decreasing modes, respectively. This was done by subjecting the setup, consisting of a test tube containing Cu2HgI4 with a thermometer directly touching the substance, on a water bath. The temperature was monitored from 40-90ºC and vice versa for the increasing and decreasing modes, respectively. Within this temperature, the transition of Cu2HgI4 from brick-red to darkbrown was observed. Table 5.3 summarizes the data for the determination of the transition temperature. It was determined that the transition temperatures of Cu2HgI4 for the increasing and decreasing modes are 62°C and 56°C, respectively. Table 5.3. Determination of the transition temperature. Literature Value Upon heating Upon cooling 67 55 Transition Temperature, °C Increasing Temp. Mode Temperature Observation Brick red to 62 brown Decreasing Temp Mode Temperature Observation Brown to 56 brick-red Theoretically, at increasing temperature, the cations can move freely but when the transition temperature was hit, the cations tend to randomly distribute itself about all of the tetrahedral holes in the structure. This is accompanied by a color change fro m brick-red to dark-brown as well as an increase in its electrical conductivity. (Heslop R. B. and Robinson, P. L., 1967) The color change is attributed to a small decrease in the band gap (2.1 to 1.9 eV) with the change in structure. Upon cooling, the cations begin to occupy lattice positions permanently and the color changes from dark-brown to brick-red. The transition at increasing temperature mode is more spontaneous than at decreasing mode. This is dictated by the 2nd law of thermodynamics, which deals with entropy. (House, J. E., 2008) It is easier for the molecules to move excitedly than arrange themselves in their permanent position. This is the possible explanation for the observed temperature lag during the decreasing temperature mode. This temperature lag is referred to as hysteresis. It occurs when the forces acting upon a system are changed or when the product is impure. (House, J. E., 2008) Theoretically, the lag in the cooling mode should be around 5˚C. There are two kinds of thermochromism which are the continuous and discontinuous in nature. Continuous thermochromism occurs when there is a gradual change in color observed as the temperature rises over time. On the other hand, discontinuous thermochromism occurs when there is a dramatic change in color observed at a specific temperature or over a very small temperature range. (House, J. E., 2008) At increasing temperature mode, the observed type of thermochromism is continuous since abrupt change in color was observed. On the other hand, discontinuous thermochromism is observed at decreasing temperature mode since the change in color is evident for a wider range. Ionic Conductivity Another property of Cu2HgI4 that was studied on this exercise is the electrical conductivity. Electrical conductivity refers to the ability of a material to conduct electric current upon application of voltage. The materials that transport current due to the ability of their ions to move in response to an applied voltage are called ionic conductors. They are the solid analog of electrolytes. The parameter that was measured is the resistance. Resistance refers to the opposition of the flow of the charge. It is the reciprocal of conductance which refers to the permission on the flow of the electric charge. (Heslop R. B. and Robinson, P. L., 1967) Resistance and conductance are related to each other according to the following equation: The set-up involved for the measurement of the resistance was described on the methodology part. Table 5.4 summarizes the data on the determination of the electrical conductivity of the sample. From the data, it was observed that the resistance at stable condition (unheated) is off scale. It is too high to be read by the apparatus. When it is heated, the resistance reading was 57.17 MΩ which corresponds to a conductance of 0.01749 MΩ-1. Table 5.4. Study of electrical conductivity of Cu2HgI4 upon heating. Resistance, MΩ Conductivity, MΩ-1 57.17 0.01749 Observations Color change from brick red to brown Theoretically, a decrease in conductivity of a material can be observed as the temperature applied to it increases. (Heslop R. B. and Robinson, P. L., 1967) However, the data analysis suggests that as the temperature of Cu2HgI4 increases, its conductance also increases and the opposite is true when the temperature decreases. This observation can be attributed to the dominance of the α-form of Cu2HgI4 at higher temperatures. This form exhibits ionic conductivity due to its more disordered structure which gives the ions space for movement causing them to easily conduct electricity. Moreover, the number of available lattice points is greater than the number of cations and this enhances conductivity. (Heslop R. B. and Robinson, P. L., 1967) On the other hand, upon cooling, there is a gradual transformation of the crystal structure from the disordered α-phase to the ordered β-phase. Mobile cations begin to occupy lattice positions permanently. Cations eventually possess a smaller amount of kinetic energy. Since the ions are less mobile, which has low kinetic energy and no more vacant positions are anymore available, they are hindered from moving and conducting charge. (Heslop R. B. and Robinson, P. L., 1967) Hence, the α-form of Cu2HgI4 is a better conductor as compared to the β-form. Ionic conductivity of Cu2HgI2 has a lot of applications including its use as a semi-conductor due to its thermal conductivity. Because it exhibits thermochromism, it is also used as an indication of temperature change in other materials. Aside from Cu 2HgI2, other examples of thermochromic compounds are the vanadium oxide, zinc oxide and lead oxides. The former is used as a window coating to block infrared transmission and reduce the loss of building interior heat. On the other hand, the latter two are used as semiconductors. IV. Summary, conclusion and recommendation In this exercise, Cu2HgI4 was synthesized and its solid state properties such as polychromism, thermochromism and thermal conductivity were studied. The copper(I) iodide) was prepared by mixing 0.5M CuSO 4 and 1M KI solution in the presence of acetic acid and sodium sulfite. Also, the mercury(II) iodide was prepared by mixing 12.5mL of 0.05M mercuric nitrate and 1M KI solution in water. Both the synthesis reaction uses an excess of 20% KI solution. The prepared CuI and HgI 2 were then mixed to yield Cu2HgI4. The synthesized compound exhibits the property of thermocromism, thus there is a color transition from brick red to dark-brown when the sample was heated and reached its transition temperature. The color transition was reversible as observed when it was allowed to cool back to room temperature. The percent yield obtained was 67.66%. As for the transition temperature, in the exercise, color change occurred when the sample reached the temperature of 62°C upon heating and at 56°C upon cooling. The phenomenon of hysteresis is a temperature lag that happens when the compound cools down. It was experienced when the sample was allowed to cool back to room temperature. Also, the thermal conductivity of the α and β form of the product was studied by measuring their resistance. The α-form has higher conductivity compared to the β-form because at a highly disordered structure, there are many spaces to which the cations can freely move and conduct electricity. The synthesis involves stoichiometric amounts of reagents, thus, washing should be highly monitored so that maximum amount of product is recovered. Difference in the data of the two groups depended on the manner of recovering the product from the washings and the amount of reagents used. V. References 1. Atkins, P. et al., (2010). Shriver and Atkins Inorganic Chemistry. 5th ed. New York, USA: W. H. Freeman and Company. pp. 66-73 2. Heslop R. B. and Robinson, P. L. (1967). Inorganic Chemistry: A Guide to Advanced Study. 3rd ed. Amsterdam, Netherlands: Elsevier Publishing Company. pp-. House, J. E. (2008). Inorganic Chemistry. Canada: Elsevier Publishing Company. pp-. Nelson, P. G. (2011). Introduction to Inorganic Chemistry: Key Ideas and their Experimental Basis. London, UK: Peter G. Nelson and Ventus Publishing Aps. pp. 40-42