Chemical Reactions: Pigment Synthesis PDF

Chemical Reactions: Pigment Synthesis Learning Objectives − Observe chemical reactions and identify evidence of a chemical change. − Gain some familiarity with chemistry of transition metals....

Chemical Reactions: Pigment Synthesis Learning Objectives − Observe chemical reactions and identify evidence of a chemical change. − Gain some familiarity with chemistry of transition metals. − Practice filtration as one of the separation techniques − Solve stoichiometry problems, including the determination of limiting reagents and the calculation of theoretical yield and percent yield. Textbook Chemistry 2nd , Chapter 4 Stoichiometry of Chemical Reactions Introduction Chemical and Physical Changes In this experiment, yo will observe chemical reactions. Chemistry deals with the study of matter and its properties, its changes, and the energy associated with those changes. Properties are identifying characteristics of matter. Chemical properties are determined from the chemical reactions a substance undergoes. Chemical reac- tions are written as chemical equations and utilize symbols to indicate conditions in the reactions. The common symbols used in chemical equations are explained in Table LF.1. In a chemical reaction, changes occur in the properties of the reactants as they become new substances. The formation of new substances is called a chemical change. Certain changes in properties can be used as evidence Table LF.1: Symbols Used in Chemical Equations Symbol Meaning + Separates substances on the same side of the chemical equation −−−→ Separates reactants and products, indicating a reaction −−−→ Δ Indicates reactants are heated to cause reaction (s) Indicates solid state of substance (l) Indicates liquid state of substance (g) Indicates gaseous state of substance (aq) Indicates aqueous state of substance Laboratory Manual Prepared by Catalyst Education, LLC for the University of Manitoba LF.1 Department of Chemistry.  Pigment Synthesis that a chemical change has taken place. On the other hand, if new chemicals are not formed, the change is only a physical change. Water boiling into steam is a physical change because steam is the gaseous form of water (H2 O). However, if electricity is passed through water, it splits into its component elements, H2 and O2. While these are also gases, they are not the same chemical substance as steam. Thus, the electrolysis of water is a chemical change. A chemical reaction (chemicals change) when: a. The reaction mixture’s colour changes. b. The heat is evolved (the reaction mixture warms up), or consumed from the environment. c. Light is emitted (luminescence). d. The precipitate is formed. e. The gas formation is observed (without external heating). It is important to record proper descriptions when observing chemical changes. Recording Observations The basis of the scientific method of investigation is experiment and observation. Observations provide you with clues to what has happened in an experiment. Careful interpretation is important: these clues can provide both definite and ambiguous answers. Observations may not answer all queries and can lead you to ask new questions. Colours are very important and should be recorded accurately. For example, a silver-grey strip of vanadium metal is heated in the presence of pale yellow-green chlorine gas to give a granular purple solid. The changes in colour and in crystal structure indicate that a reaction has occurred. Three chlorides of vanadium are known: at room temperature, VCl2 and VCl3 are granular purple solids, while VCl4 is a bright red liquid. Thus, while it can be concluded that VCl4 is not formed, further investigation is required to determine whether VCl2 and VCl3 (or a combination of both) was formed in this experiment. You could also ask why VCl2 and/or VCl3 are or ed in tead o l hat reaction condition o ld lead to the e cl i e or ation o l or l or l and hy l and l ha e the a e phy ical appearance. inally yo ho ld al o con ider the pro a ility that there a another co po nd pre ent in a hi h eno h concentration that co ld alter thi concl ion. o e a le to pport concl ion yo r o er ation ha e to e thoro h and na i o. or e a ple an a eo ol tion o ion i l e- reen the addition o a onia t rn the ol tion a deep l e colo r indicatin that a co ple ation ha occ rred. ecordin oth ol tion a l e ill not di tin i h et een the and th there i no pporta le e idence o a reaction. or ettin to record the colo r chan e al o lea e yo itho t the e idence re ired to pport the correct concl ion. a tance ha no colo r and i tran parent li e ater the proper ter i clear colourless. ll o er ation t e recorded a they are ade yo r e ory i not o ecti e and other people o er ation do not apply to yo r e peri ent. hat yo o er ed i hat happened re ardle o hat yo e pected to happen. there a an error in proced re that yield an ne pected re lt the only accepta le ol tion i to redo the e peri ent. Laboratory Manual Prepared by Catalyst Education, LLC for the University of Manitoba LF.2 Department of Chemistry.  Pigment Synthesis Types of Chemical Reactions When a chemical reaction occurs, the atoms in the reactants rearrange to produce new combinations of atoms that are referred to as products. However, because the atoms are only rearranging, there is no change in the number of atoms between the reactants and the products. The total number of atoms going into the reaction must be present in the product(s). This is known as the Law of Conservation of Matter, which states that atoms of matter are not created or destroyed during a chemical reaction. This same law is used to balance chemical equations that use formulas to represent the chemical reactions. Only balanced equations can be used in chemical calculations. In equations, the reactants are shown on the left side and products are shown on the right. An arrow between sides indicates that a chemical reaction takes place from left to right. Symbols written in parentheses are used to indicate the physical states of substances in the equation. These symbols include (s) for a solid, (l) for liquid, (g) for gas, and (aq) to indicate a substance dissolved in water. There are multiple types of chemical reactions classified by the kinds of reactants and products present. The most common chemical reaction types are: 1. Synthesis (combination) when two or more elements and/or compounds react to form a more complex C(s) + O2 (g) −−−→ CO2 (g) single product. (Reaction LF.1) 2. Decomposition when a single complex reactant breaks apart into multiple simple products CaCO3 (s) −−−→ CaO(s) + CO2 (g) (Reaction LF.2) 3. Single replacement when one element reactant replaces an element in a compound reactant. This type always has two reactants, an element and a compound. There are always two products, a different element and a different compound. Mg(s) + 2 HCl (aq) −−−→ MgCl2 (aq) + H2 (g) (Reaction LF.3) 4. Double replacement (metathesis) when elements in two reactant compounds switch places. These re- actions usually occur in aqueous solution and result in the formation of a precipitate or a molecular compound such as a gas or water. AgNO3 (aq) + NaCl(aq) −−−→ AgCl(s) + NaNO3 (aq) (Reaction LF.4) CH4 (g) + 2 O2 (g) −−−→ CO2 (g) + 2 H2 O(g) 5. Combustion (Reaction LF.5) Laboratory Manual Prepared by Catalyst Education, LLC for the University of Manitoba LF.3 Department of Chemistry.  Pigment Synthesis Chemical reactions that are carried out in water, are among the most common and important since water is the main constituent of Earth’s surface and the fluids of all living organisms. An aqueous solution is one in which water acts as a solvent. In this experiment we will explore two different classes of reactions that occur in aqueous solutions – precipitation and acid-base reactions. Precipitation reaction is the formation of an insoluble compound when we mix two solutions containing soluble compounds. For more information read:. Equations for Ionic Reactions -. riting and alancing hemical quations. Precipitation Reactions and Solubility Rules - 4.2 lassifying hemical eactions. Reaction Yields - 4.4 eaction ields History of Pigments A pigment is a coloured substance that is completely or nearly insoluble in water. Pigment’s main applications are in paints (particularly artist paints), printing inks, and plastics. They are also used in cosmetics, rubber, concrete, glass, and ceram- ics. Almost all pigments are inorganic compounds. Natural inorganic pigments have been used as paints since prehistoric times. The most important pigments are titanium(IV) oxide (TiO2, white), carbon black (C, black), different iron and chromium oxides, complex salts of cobalt, copper, lead and cadmium. Figure LF.1: Pigments. 1 By Dan Brady - Brady, CC BY 2.0, pigments Early pigments were simply as ground earth or clay, and they were mixed with saliva or animal fat to make a paint. Egyptian Blue (calcium copper tetrasilicate, CaCuSi4 O10 ) and vermilion (mercury(II) sulfide, HgS) were among the earliest of the artificially produced pigments (Figure LF.2). By 3000 B.C.E., the Egyptians had used in their arts malachite (basic copper(II) carbonate, CuCO3 ⋅ Cu(OH)2 ), orpiment (arsenic trisulfide, As2 S3 , yellow pigment), red ochre (iron(III) oxide, Fe2 O3 ), and charcoal (carbon). Chemistry and Colours. Structural features of coloured compounds Many different materials, natural and synthetic, can be coloured. Chlorophyll from green plants and hemoglobin from blood are the coloured materials from living organisms. The world of minerals and gems is full of wide variety of colours (Figure LF.3). Laboratory Manual Prepared by Catalyst Education, LLC for the University of Manitoba LF.4 Department of Chemistry.  Pigment Synthesis Figure LF.2: The earliest known and most widely used pigments: Top, from left to right: Egyptian bluea , vermilionb , malachitec , and red ochred. Bottom, from left to right: Egyptian blue in Egyptian faience (produced 750-700 BC)e , vermilion in a Chinese carved laquer box from Qing dynastyf , and an ancient Egyptian painting using both malachite and red ocherg. a ByFK1954 - Own work, Public Domain, Egyptian blue b ByKardinal9 - Own work, CC BY-SA 3.0, vermilion c By Metabisulfite - Own work, CC BY-SA 4.0, malachite d By Wotlarx - Own work, CC BY-SA 4.0, red ochre e By Bairuilong - Own work, CC BY-SA 4.0, Egyptian faience f CC BY-SA 2.0, Chinese laquer box g By Norman de Garis Davies, Nina Davies (Copy of an 15th century BC Picture) - Matthias Seidel, Abdel Ghaffar Shedid: Das Grab des Nacht. Kunst und Geschichte eines Beamtengrabes der 18. Dynastie in Theben-West, von Zabern, Mainz 1991 ISBN 3805313322, Public Domain, Egyptian painting Figure LF.3: The minerals malachitea (green), azuriteb (blue), and proustitec (red). a By AlixSaz - Own work, CC BY-SA 4.0, malachite b By Géry PARENT - Own work, Public Domain, azurite c By Rob Lavinsky, iRocks.com – CC-BY-SA-3.0, CC BY-SA 3.0, prousite Laboratory Manual Prepared by Catalyst Education, LLC for the University of Manitoba LF.5 Department of Chemistry.  Pigment Synthesis As mentioned previously (Experiment 2), colour is produced when an object absorbs certain wavelengths of a visible light. The reflected light defines the colour of the object. Once atoms or molecules absorb light, their electrons are excited to the higher-energy levels (orbitals), but not all electronic transitions fall into the visible region. For many main group atoms, the absorbed photons are in the ultraviolet range of the electromagnetic spectrum, which human eye cannot detect. We say that they are colourless or white. However, most of the compounds formed by transition metals (d-block) can absorb photons in the visible range. Nickel salts are often green, copper salts are blue to blue-green, and chromium compounds may be red, green, yellow, or blue. Transition metals are characterized by having unfilled 𝑑 (or 𝑓) orbitals, and they typically form coordination compounds, which are usually coloured. Colour emerges in these compounds because of a phenomenon called 𝑑- orbital splitting – the five 𝑑 orbitals in a transition metal atom or ion do not all have the same energy when the atom is a part of the compound. The existence of 𝑑- orbitals with several different energies allows for excitation of electrons from one level to another. The energies required for these transitions, often correspond to the wave-lengths of visible light. The colour of many minerals and gems such as emerald, ruby, turquoise, azurite, and malachite are the result of these electron transitions. In this experiment, you will synthesize two chemical pigments: Malachite and Verdigris. Numerous historic pigments are copper-based, and different recipes can be found in artists’ works. Blue azurite, Egyptian Blue, Malachite, Verdigris were widely used until 17th century. A new generation of pigments was introduced in the late 18th century with the development of synthetic chemistry. Malachite is the oldest known green pigment. It is a basic copper(II) carbonate mineral of green colour with the formula CuCO3 ⋅ Cu(OH)2. In ancient Egypt malachite was used in tomb paintings since the Fourth dynasty. The green colour of malachite represented protection. It was produced by grinding a natural mineral to a powder and washed. In European paintings, malachite has been of importance in the 15th and 16th centuries. In the laboratory, malachite can be made by reacting hydrated copper(II) sulfate with sodium carbonate (Reac- tion LF.6): 2 CuSO4 ⋅ 5 H2 O(aq) + 2 Na2 CO3 (aq) −−−→ CuCO3 ⋅ Cu(OH)2 (s) + 2 Na2 SO4 (aq) + CO2 (g) + 9 H2 O(l) (Reaction LF.6) The green-blue basic copper(II) carbonate is isolated, dried, and then used to make paint. For more information about malachite, see malachite. Verdigris (copper(II) acetate monohydrate, Cu(CH3 COO)2 ⋅ H2 O) is one of the first artificial pigments used pri- marily by Greeks. Verdigris was the most vibrant green pigment widely used from antiquity through the Middle Ages, Renaissance, and Baroque until 19th century. The name “Verdigris” comes from Old French meaning green of Greece. Historic recipes for verdigris are very simple: copper plates were exposed to the vapours of fermenting grapes. Acetic acid produced as a by-product of wine-making reacts with metallic copper to form the oxidized Laboratory Manual Prepared by Catalyst Education, LLC for the University of Manitoba LF.6 Department of Chemistry.  Pigment Synthesis blue-green corrosion crust which is scraped off and ground. The synthesis of verdigris can be performed in one of two ways: the simple or the multistep synthesis. For the simple synthesis, pure copper objects are suspended over acetic acid inside sealed container. The synthesis can take between 2 days and 6 months depending on the acetic acid concentration, as shown in this video. You will use the multistep synthesis to make this pigment starting with copper(II) sulfate: with ammonia: a) First, convert the copper(II) sulfate into basic copper(II) sulfate. 4 CuSO4 ⋅ 5 H2 O(aq) + 6 NH3 (aq) −−−→ CuSO4 ⋅ 3 Cu(OH)2 (s) + 3 (NH4 )2 SO4 (aq) + 14 H2 O(l) (Reaction LF.7) b) Then, sodium hydroxide is used to form copper(II) hydroxide. CuSO4 ⋅ 3 Cu(OH)2 (s) + 2 NaOH(aq) −−−→ 4 Cu(OH)2 (s) + Na2 SO4 (aq) (Reaction LF.8) c) Acetic acid is added to convert copper(II) hydroxide to copper(II) acetate. Cu(OH)2 (s) + 2 CH3 COOH(aq) −−−→ Cu(CH3 COO)2 (aq) + 2 H2 O(l) (Reaction LF.9) Because copper(II) acetate is somewhat water soluble (7.2 g/100 mL cold water), we will use a glacial acetic acid which is water-free form of the acetic acid. Making Paint For use as a paint or artists’ colour, a pigment must be mixed with a substance to make a liquid or paste like phase which allows it to adhere to a surface. This “substance” is called a binder. Throughout the ages, artists have used different binders for their paints. In ancient Egypt paintings, beeswax was used as a binder. Until the Middle Ages, the artists used tempera paint where pigments were mixed with an aqueous solution of egg yolk. Tempera paint was slowly replaced by oil-based paints with linseed, walnut, poppy seed, hemp, and safflower oils used as binders. To make watercolours, natural gums were used, such as Arabic gum. Nowadays, acrylic-based paints are commonly used. Although any pigment can be used with any binder, in some cases a superior paint can be made with a particular binder. For example, malachite gives brighter colours when mixed with egg tempera than with oil. Hydrates In this experiment you will work with hydrates which are ionic compounds that contain water molecules as integral components of their crystal structure. To name the hydrate, you use the name of the salt and add the word which begins with a Greek prefix denoting the number of water molecules and ends with“hydrate”. For example, CuSO4 ⋅ 5 H2O is named copper(II) sulfate pentahydrate. When you measure the mass of a hydrate, mass of the water molecules must be taken into account! Laboratory Manual Prepared by Catalyst Education, LLC for the University of Manitoba LF.7 Department of Chemistry.  Pigment Synthesis Sometimes the water can be removed with heat and/or vacuum, however unless the water is purposefully removed, it is present. Thus, when measuring the mass of a sample that is hydrated, we must consider the water molecules. For example, if a reaction requires cobalt(II) chloride to be dissolved in water and then reacted, but only solid cobalt(II) chloride hexahydrate (CoCl2·6H2O) is available, we must consider that even though we’re only interested in the cobalt(II) chloride, we cannot measure just the mass of the cobalt(II) chloride and so the water must be taken into account for the molar mass. To calculate the molar mass of CoCl2·6H2O, I will calculate the molar mass of CoCl2 and add the molar mass of water multiplied by 6. Please pay attention: the formula contains 6 molecules of water and I multiply molar mass of water by 6. Thus, M (CoCl2·6H2O) = M(CoCl2) + 6*M(H2O) = 129.839 g/mol + 6*18.015 g/mol = 237.929 g/mol Coordination Compounds When dissolved in water, ionic compounds dissociate into their constituent ions. For example, solid CuSO4 dis- solves in water to give Cu2+ and SO4 2 – : CuSO4 (s) −−−→ Cu2+ (aq) + SO4 2− (aq) (Reaction LF.10) Once dissolved, these ions are no longer associated with each other. In aqueous solution, both cations and anions are surrounded by water molecules, which interact with the ions via strong intermolecular interactions. For transition metal cations such as Cu2+ , these interactions are more covalent in character: a water molecule can share a lone pair of electrons with the electron-deficient cation. In aqueous solution, Cu2+ bonds to six water molecules (called ligands) to form a hexaaquo complex: Cu2+ (aq) + 6 H2 O (l) −−−→ [Cu(H2 O)6 ]2+ (aq) [Cu(H2 O)6 ]2+ is called a complex ion. The water molecules play the role of a ligand. Ligands are any molecule or ion that binds to the metal. A complex ion is stabilized with ions of opposite charge by formation of a coor- dination compound (e.g., [Cu(H2 O)6 ]SO4 ). We use square brackets to write the formula of a complex ion, while counterions are written outside the brackets. It shows that ligands are bound directly to the metal ion while counterions are not directly bound. The complex cation of copper(II) is shown in Figure LF.5. It has a skewed octahe- dral shape and exchanges water molecules rapidly with the surrounding solution. A lone pair on each O atom donates electron density to the electron-deficient Cu2+ cation, resulting in a bonding interaction. This type of bond is called a coordi- nate covalent bond or a dative bond and is often represented as an arrow point- ing from the lone pair donor to the lone pair acceptor (shown on the picture). The Cu−OH2 bonds are strong enough that they are maintained upon the forma- Figure LF.5: Complex cation of copper(II). tion of ionic salts. For example, the hydrated salt copper (II) sulphate pentahy- drate (CuSO4 ⋅ 5 H2O) maintains four Cu−OH2 bonds. Other species that contain Laboratory Manual Prepared by Catalyst Education, LLC for the University of Manitoba LF.8 Department of Chemistry.  Pigment Synthesis lone pairs, such as ammonia (NH3) and chloride anion can also act as ligands and form coordination complexes with transition metal cations. For example, in this lab you will make the [Cu(NH3)4(H2O)2]2+ ion. Although it is present in the initial solution used in this experiment, SO42 – does not participate in any of the reactions that are conducted in this lab. Such species are called spectator ions, and they are not included in net ionic equations. The colour of transition metal compounds and their solutions depends on the ligands bonded to the transition metal cations. For example, CuSO4 ⋅ 5 H2 O crystals have a deep blue colour. Upon heating, the water ligands are released, leading to anhydrous CuSO4 which is white. In solution, [Cu(H2 O)6 ]2+ has a blue-green colour; [CuCl3 (H2 O)3 ] – has a red colour while [CuCl4 (H2 O)2 ]2 – has a yellow-green colour. Colour can thus be used to determine which coordination complex has been formed in a solution or as a solid. Labware and Laboratory Techniques used in this Experiment Stirring bars are small pieces of iron (or magnetic steel) encased in Teflon. There is a large variety of sizes and shapes. Always make sure that the size of the stir bar reasonably matches the vessel and purpose of its use. Large stir bars develop significantly larger force upon contact with magnetic s tirrers, a nd, t hus, can either break an inappropriate glass container, or jump out of it, causing spills. To stir the solutions and reaction mixtures we will use a magnetic mixer coupled with a heating element (hot plate). There is a constant bar-magnet under the plate attached to the electric motor axis. The speed of rotation is regulated via the knob in the front panel. Watch the video Using a Hot and Stir Plate on Labflow. Chemical Synthesis involves the separation and purification of the desired product from unwanted contaminants. Common methods of separation are filtration, sedimentation, decantation, extraction, and sublimation. A separation of solids from liquids, called filtration, is the most common laboratory procedure. For that purpose, a variety of filtering devices and filters are used. To filter aqueous solutions with pH close to neutral, we use paper filters of a different porosity and diameter. An appropriate-size paper filter is folded and then fitted into the glass funnel for a gravity filtration. Figure LF.6: Filtration by gravity. Image on the left is the gravity filtration setup while the image on the right shows a properly folded or fluted, filter paper. Laboratory Manual Prepared by Catalyst Education, LLC for the University of Manitoba LF.9 Department of Chemistry.  Pigment Synthesis Another widely used method of paper filtration in the chemical lab- oratory is the application of the Büchner funnel and Büchner flask method, shown in Figure LF.7. Procedure Don't print the procedure. You will obtain detailed Figure LF.7: Büchner funnel vac- instructions in the lab. uum filtration setup. References: Safety Precautions 1. Solomon, S.D.; Rutkowsky, S.A.; Mahon, M.L.; Halpern, E.M. J. Chem. Ed. 2011, 88, 1694-1697. Wear safety glasses and gloves throughout this experiment (provided in the lab). 2. Wiggins, M.B.; Heath, E.; Alcantara-Garcia, J. J. Chem. Ed. 2019, 96, 317-322. Copper sulfate (CuSO4) stains the skin. MalachiteA.C.; 3. Gaquere-Parker, is a low toxicity Hill, pigment. P.S.; Haaf, M.P.; Parker, C.D.; Doles, N.A.; Yi, A.K.; Kaminski, T.A. J. Chem. Ed. 2017, 94,is 235-239. Verdigris toxic by ingestion and may cause irritation. NaOH 4. Vyhnal, solutions should C.R.; Mahoney, E.H.R;beLin, handled with care. Y.; Radpour, R.; Wadsworth, H. J. Chem. Ed. 2020, 97, 1272-1282. Glacial acetic acid (100%) is highly corrosive, and it causes severe skin burns and eye damage. You shouldA.; 5. Vyboishchik, always useM. Popov, thisMaterials chemicalToday: under Proceedings a fume hood.2021, 38, 1560-1563. 6. Corbeil, M.C.; Charland, J.P.; Moffatt E.A. Studies in Conservation, 2002, 47, 237-249. Don’t forget to record all your observations in the lab notebook! Take pictures of all your precipitates. Part 1: Synthesis of Malachite Guidelines: In the first p art o f t his l ab, y ou w ill m ake t wo s olutions, o ne o f c opper(II) s ulfate ( CuSO4 ) and one of sodium carbonate (Na2 CO3 ). Pay attention that you will use copper(II) sulfate in its hydrated form, CuSO4 ⋅ 5 H2 O. You will slowly add sodium carbonate solution to the copper(II) sulfate solution, while vigorously stirred on a stir plate. Record your observations! You need to find a way to isolate and weigh out your precipitate. This mass will allow you to calculate the number of moles of the product formed. You can use this information to determine, first, which one of the reagents was limiting, and second, what was a percent yield of your reaction. Propose the procedure for the malachite synthesis using the materials available and the following chemical reac- tion: 2 CuSO4 ⋅ 5 H2 O(aq) + 2 Na2 CO3 (aq) −−−→ CuCO3 ⋅ Cu(OH)2 (s) + 2 Na2 SO4 (aq) + CO2 (g) + 9 H2 O(l) Laboratory Manual Prepared by Catalyst Education, LLC for the University of Manitoba LF.10 Department of Chemistry.