Ap-chemistry lab, indentify substance
Sodium chloride white solid NaCl ionic noyes natural no reaction 801 Stearic acid white solid C18H3602 Slightly polar covlaent no bad x yes nox 69 Copper sulfate pentahydrate blue solid CuS04 ionic yes yes natural nox x 650 Material: Conductor HCI Distilled water Test tubes Melting point tester Ph paper Glass stick Procedure Test 1 :Tested the conductivity when all the materials are in solid state Put the conductor’s conductive end in to the material #3 Recorded the results. Repeat 1-2 for all the material Test 2:Test the solubility in water Put a little material #3in a test tube.
Put 2-3 ml of distilled water. Stirred with the glass stick. Observed if the material was dissolving. Use a PH paper to test the PH of the solution. Repeat 1-6 for all the materials. Test 3: Test the conductivity in solution Used the conductor to touch the solution with the material#3. Record the result. Repeat for #7 and #16.
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Test 4: Test the solubility in hexane Put a little material#3 in a test tube. Added 2 ml of hexane. Stirred with a glass stick Observed if the material was dissolving, recorded the results.
Repeat 1-4 for #7 and #16s. Test 5:Reaction with HCI Put a little material #3 in a test tube Added 2ml of HCI. Observed if there was reaction(which will show in bubbles). Recorded the result. Repeat 1-4 for all the materials Test 6: Melting points Used the cellular tube to poke the material# the tube. 3 until there was some substance inside Put the cellular tube in to the melting point tester, set the temperature Waited until the substance started to melt. Record the melting point. Repeat 1-4 for #7 and #16.
Result experimental to 200 conductivity when solid solubility in water ph reaction in hexane melting points(below 200t) hcl conductivity when solution name of the substance #7 noyes 3, acidic no 165 noyes salicylic acid 3 no no not able to test yes 87 no no naphthalene #16 noyes acidic no not melting noyes sodium chloride #6 yes no no no not melting lot of bubble no magnesium Theoretical name of the substance type of compound solubility in water ph reaction in hexane melting pointsCC) hcl conductivity when solution #7 salicylic acid polar covalent yes acidic no 165 no yes #3 naphthalene non-polar covalent no neutral yes 85 no no #16 sodium chloride ionic yes neutral n0801 noyes #6 magnesium metal no neutral n0650 lot of bubble no Discussion For #7 could not conduct electricity when it was solid, but conduct when it was in olution so it was not metal. It was soluble in water so it could not be non-polar compound, because non-polar compound would not dissolve in polar solvent such as water. #7 substance did not dissolve in hexane which proved that it could only be polar covalent or ionic compound, because only non- polar compound could dissolve in hexane which was a non- polar solvent. The PH of the water solution of #7 was acidic, which meant it could only be polar covalent, because all the ionic compound in the list would only have a neutral or base solution. And it could only be either benzoic acid or salicylic acid.
Benzoic acid has a melting point of 122’C, and salicylic acid has a melting point of 159’C. #7 substance melted at 165t which was closer to the salicylic acid. According to all the test results, #7 was proved to be salicylic acid. #3 could not conduct electricity when it was solid, so it was not metal. It was insoluble in water but soluble in hexane, so it could only be a non-polar covalent compound, since polar solvent only dissolve polar solute, non polar solvent only dissolve non-polar solute. And it couldn’t conduct electricity when it was in solution state, which proved that it was a non polar compound. The only non polar compound had all these properties in the list are naphthalene and stearic acid.
The melting point for naphthalene is 85t , and the melting point for stearic acid is 69’C. The #3 substance melted at 8TC, which was closer to naphthalene’s melting point, so # 3 snou d be naphthalene. Fo #16 could not conduct electricity when it was solid, but it could conduct electricity when it was solution, so it should be polar covalent compound or ionic compound. However, there was a experiment error in the PH test. In the experiment, the PH of the solution was acidic, which meant that it was a polar covalent compound, but the elting point of the substance was much higher than all the polar covalent compounds that had a acidic solution in the list.
Ionic compound usually have a high melting point because it requires a lot of energy to break the ionic bonds. so I suspected it to be ionic compound. The ionic compounds in the list that didn’t have a base solution are CaC03 , NaCl and Copper(ll) Sulphate pentahydrates. However, Copper(ll) Sulphate pentahydrates will have a blue solution, but the substance had a colorless solution, so the substance could not be Copper(ll) Sulphate pentahydrates. To test if the substance was CaC03 , I used HCI to test. Because CaC03(s) + 2 HCl(aq) –> CaC12(aq) + C02(g) + H20(l), so if the substance was CaC03 it would react with HCI and produce C02 gas which would show in bubbles.
The substance did not reaction with HCI, and there was no bubble appeared, so the substance #16 was NaCl. For #6 #6 had a silver color and it conducted electricity while it was in solid state, so it was metal. The only metal with a silver color in the list were silver, zinc and magnesium. The magnesium is the most reactive metal in these three metals, because it had less valence and total electrons, so it had lower ionization energy. Sort these three metals ccording to their ionization, silver>zinc>magnesium. Ionization energy is the energy required to remove one valence electron. When metal react with HCI, a ionic compound will be formed and hydrogen gas will be released.
So the metal that has a lower ionization energy will react quicker with HCI and more bubbles will appear. After 1. 0M HCI was added to #6, lots of bubbles appeared immediately. Through the reaction speed, we can know that #6 should be magnesium. Discussion questions 1. How do the melting points of ionic compounds compare to covalent compounds? What evidence from the investigation supports your conclusion? The melting points of ionic compounds are higher than covalent compounds. In the experiment, #16 NaCl was an ionic compound and it had a higher melting point compared to the other two covalent compound #3 and #7, Because the ionic bond is stronger than the covalent bond.
In an ionic compound like salt NaCl, the sodium gives up one electron which goes to Join with the chlorine atom, it will form a stable crystal structure, all the cation and anion will attract each other , it takes more energy (heat) to make the ions come apart. When they do come apart and melt, one atom will still be more positive and the other more negative. In a covalent bond with the atoms share the electrons and neither atom has to give up its electron completely. Covalent compound will have weak intermolecular attractions but they won’t be as strong as the ionic compound has. It requires less energy to break the bonds in covalent compound, so its melting point is lower than ionic compound. 2. When the solids were placed in water were all the results the same? What types of solids conduct electricity in water? Use your investigations to explain what happened.
The partially positive and negative charges developed on the water molecule attract espectively the negative and the positive ions of the substance to be dissolved thereby breaking its chemical bond. Hence the substance dissolves. Ionic compound would dissolve in water, because water is a polar solution that can attract the cation and anion in the ionic compound. For an example, when #16 NaCl dissolves in water, it looks like the diagram below. Because there are freely moving ions in the water solution, so the solution can conduct electricity. In a polar covalent compound, there are no ions available for the water molecule to attract, because water and polar covalent compound posses the same dipole owers.
Because of this, the water molecule uses Dipole-Dipole Attraction to dissolve the compound. For example, the salicylic acid will use their dipole powers to attract their negatively charges dipoles with the water molecules positively charged dipoles, and hydrogen will do the same. The intermolecular forces in salicylic acid are weaker than the dipoles attraction, especially the extra-strong hydrogen bonds that are formed between the hydrogen atoms. As a result, the salicylic molecule is separated by the dipole attraction and form ions surrounded by water molecules. Because here are freely moving ions in the solution, the solution is able to conduct electricity.
Metal and non-polar covalent compound such as #6 and #7 wouldn’t dissolve in water. Metal didn’t dissolve in water because the metallic bonds are so strong and the metal had a same electronegativity so there wouldn’t be a more negative or positive point in metal, so that the attraction of water could not dissociate the molecule. The metal itself can conduct electricity because it is freely moving electrons, but it wont dissolve in water so there is no freely moving ions in water, so the water won’t conduct electricity. A non- polar covalent compound had a balanced charge, so its atoms were not attracted to water molecules, so that dipole-dipole attraction won’t be about to dissociate the molecule.
Because the non-polar covalent compound did not dissolve in water, so there was no freely moving ion in water to conduct electricity. 3. If the solid is ionic, explain why you cannot make the general statement that “all ionic compounds are soluble in water. ” What evidence from the investigation supports your conclusions? ionic compounds containing highly polar ions (ones that are small and have a high charge) will usually not dissolve in water, but rather react ith it, or Just not dissolve at all. For example, CaC03 has a Ca2+ion and a C032-ion, the ionic bond between these two ions are so strong that the intermolecular attraction from hydrogen bond is not able to separate them. 4. Why was it necessary to use distilled water and not tap water?
It was necessary to use distilled water because tap water might have dissolved ions that can conduct electricity or even react with the substance, and it will effect the experiment result. Naphthalene is a hydrocarbon, a covalent compound. Naphthalene is not soluble in water yet sugar is also a covalent compound and is soluble in water. Look at the structure of both compounds and explain what could Justify these results. Naphthalene is a non-polar covalent compound, because its molecular structure is symmetrical and balanced (fgure 1). Although the electronegativity difference between H and C are 0. 4, but the ring structure of naphthalene makes all the attractive force cancel each other in the opposite direction.
All the atoms are evenly charged, and all the electrons are bonded, so the molecule is not polar and it is very stable. And the dipole- dipole force from water are not able to separate the molecule because the molecule is not polar. The electronegativity for C is 2. 5, O is 3. 5 and H is 2. 1, so between O and C there is 1. 0 difference, and between O and H there is 1. 4 difference. The stronger the electronegativity the atoms has, the more ability for the atom to attract electron towards itself, so the atom will become slightly negative, so the molecule will be polar. In the sucrose molecule, there are lot of OH groups in the molecule, which has a huge electronegativity difference of 1. , so the sharing electron between O and H would be pulled towards O, so O will be slightly negative, and H will be slightly positive, so the molecule is polar.