Solution Experiment Collin College

Transcription

1 Solution Experiment Collin College Christian E. Madu, PhD and Michael Jones, PhD Objectives Predict the polarity of a molecule using the Lewis Dot Formula and molecular shape. Determine the polarity of solvents using the Like Dissolves Like rule. Determine the type of Mixtures Create a solubility curve for KNO3 Safety: SAFETY GOGGGLES MUST BE WORN WHILE WORKING IN THE LABORATORY Introduction A mixture is composed of two or more pure substances that are physically mixed, but not chemically combined. For example, a sugar solution is a mixture of sugar and water. Also, the air we breathe is a mixture of oxygen, nitrogen, and small amounts of carbon dioxide, argon, water vapor and other gases. Although the proportions by mass of the components may vary from sample to sample, the mixture retains most of the properties of its components. There are basically two classes of mixtures: - Heterogeneous mixture (has no uniform composition) - Homogeneous mixture (has uniform composition) A solution is a homogeneous mixture in which a varying proportion of particles of a pure substance (solute) are uniformly dispersed in another substance (solvent). The particles of the solvent are usually present in a larger quantity while the particles of the solute are present in a smaller quantity. An aqueous solution is a solution with water as the solvent and the solute could be a soluble solid, liquid or gas. In making a solution, the polarity of the solute and solvent must be the same. Usually, polar solutes are soluble in polar solvents while non-polar solutes are soluble in nonpolar solvents. This is called the like dissolves like rule.

2 We can use the knowledge of the electron dot formula to predict the 3D-shape of the molecule. Then the bond polarity and the shape can in turn be used to predict the polarity of the molecule. In the valence shell electron-pair repulsion (VSEPR) theory, the electron groups (lone pairs, single, or multiple bonds) around the central atom are arranged as far apart as possible to minimize the repulsion between their negative charges. The total number of electron groups around the central atom is determined by drawing the electron-dot formula of the molecule, while the 3D shape of the molecule is determined by the number of atoms and lone-pair of electrons within the electron group. The polarity of a bond is determined by the electronegativity difference between the two bonding atoms. Electronegativity is the ability of the bonding atom to attract the bonding electrons to itself. A bond between two atoms with similar or identical electronegativities is said to be a nonpolar bond. A bond between two atoms with different electronegativities and therefore an unequal sharing of the bonding electrons is said to be a polar covalent bond. The polar bond shows a charge separation called a dipole. A molecule with two or more covalent bonds could be a nonpolar molecule if: There are no polar bonds in the molecule. There are polar bonds with symmetrical arrangement in the molecule (a symmetrical arrangement cancels the dipoles of the bonds, making the molecule as a whole nonpolar). A molecule with two or more polar covalent bonds will be a polar molecule if there are polar bonds with unsymmetrical arrangements within the molecule. A. Predicting the Polarity of Molecule We are going to use the electron-dot formula, VSEPR theory, 3D shape, and bond polarity to determine the molecular polarity of water (H2O). Answer the following questions in the table provided below. 1. Draw the electron-dot formula of H2O. 2. How many electron groups are around the central atom? 3. What is the molecular shape of H2O? 4. What types of bonds are in a water molecule and what is the polarity of the bonds (polar or nonpolar bonds)? 5. Is H2O a polar or a nonpolar molecule?

3 Table 1 Molecule Electrondot formula 3D shape Indicate the bonds present Polarity of bonds Molecular polarity H2O B. Determining the polarity of solvents using the Like Dissolves Like Rule We will use the result obtained from part A above to determine the polarity of other solvents. When a liquid dissolve in another liquid they are said to be miscible liquids. If they do not dissolve in each other and instead form different layers when mixed they are said to be immiscible liquids. Materials: Test tubes (7), test tube rack, stirring rods, H2O, hexane, methanol, pentane, cyclohexane. B.1 Label the test tubes 1-7 and set them up in a test tube rack. B.2 To each test tube 1-4 add 3 ml of water and to each test tube 5-7 add 3 ml of hexane. B.3 Add the following materials to the specified test tubes: Table 2 Test tube # Add 3 ml Then add 2 ml Miscibility 1 H2O Hexane 2 H2O Methanol 3 H2O Pentane 4 H2O Cyclohexane 5 Hexane Methanol 6 Hexane Pentane 7 Hexane Cyclohexane

4 B.4 Use the result in the table above to determine the polarity of the solvents in the table below Table 3 Solvent Hexane Methanol Pentane Cyclohexane Molecular Polarity ** Please note that all waste should be discarded into the appropriate waste containers provided in the lab, not in the sink. C. Polarity of solvents and solutes using the Like Dissolves Like Rule Here we are going to determine the solubility of some solids in polar and nonpolar solvents and then use the like dissolves like rule to determine the polarity of the solids and the type of mixture formed (homogeneous or heterogeneous mixture). Materials: Test tubes (6), test tube rack, spatulas, stirring rods, H2O, hexane, NaCl, sucrose, biphenyl. C.1 Label the test tubes 1-6 and set them up in a test tube rack. C.2 To each test tube 1-3 add 2 ml of water and to each test tube 4-6 add 2 ml of hexane. C.3 Add a few crystals of the following materials to the specified test tubes: Table 4 Test tube # Add 2 ml Then add few crystals of 1 H2O NaCl 2 H2O Sucrose 3 H2O Biphenyl 4 Hexane NaCl 5 Hexane Sucrose 6 Hexane Biphenyl Solubilty Type of mixture

5 C.4 Use the result in the table above to determine the polarity of the solutes in the table below Table 5 Solvent NaCl Sucrose Biphenyl Molecular Polarity ** Please also note that all waste should be discarded into the appropriate waste containers provided in the lab, not in the sink. D. Solubility curve of KNO3 Solubility of a compound is defined as the maximum amount of solute that can be dissolved in 100 g of water at a given temperature. Solubility is usually stated in g/100 g H2O. The solubility of solids generally increases as the temperature increases. When the solution that holds a maximum amount of solute at a particular temperature (saturated solution) is cooled to a lower temperature, the excess solute will precipitate out at the lower temperature. A solubility curve is the plot of solubility (g/100 g H2O) versus temperature (degree Celsius). Materials: Large test tube, weighing bowl, 400 ml beaker, buret clamp, 10 ml graduated cylinder, KNO3(s), and hot plate. To reduce the amount of KNO3 used, each group of students will be assigned a given amount of KNO3 to use, and the results obtained by each table will be shared with the entire class to be used for plotting the solubility curve. D.1 Each table will be assigned to carefully weigh out one of the following amounts of KNO3 (4 g, 5 g, 6 g, 7 g, 8 g, or 9 g KNO3). Place the weighed out amount of KNO3 in the large test tube. D.2 Add 10 ml of H2O to the test tube at room temperature and clamp the test tube to a ring stand and place the test tube in a beaker of water.

6 D.3 Use the hot plate to heat the water while stirring the mixture in the test tube until all the KNO3 solid dissolves completely. Continue heating to 60 C. D.4 Remove the test tube from the hot water and allow the test tube and content to cool while stirring gently with the thermometer. Look closely for the first appearance of crystals and record the temperature of the solution at the appearance of the crystals. D.5 Repeat the heating and cooling steps until you get two or three temperature readings that agree. D.6 Record your result and those of the other lab groups in the table below and use the table to plot a solubility curve of solubility (g solute/100 g H2O) versus temperature ( C). Table 6 Mass KNO3 (g) Temperature ( C) Crystals Appear Solubility (g KNO3/100 ml H2O)

7 References 1. Karen C. Timberlake; Essential Laboratory Manual for general, organic, and biological chemistry. Second Edition, Pearson Prentice Hall, Molecular structure