Types of Chemical Reactions and Electrolytes: A Demonstration

Transcription

1 Exercise 4 Page 1 Illinois Central College Chemistry 130 Laboratory Section Name Types of Chemical Reactions and Electrolytes: A Demonstration Objectives To study four classes of chemical reactions and to learn more of how to write correct chemical formulas and balanced equations. Background Many chemical reactions fall into one of the following four categories: A. Combination: A + B AB B. Decomposition: AB A + B C. Displacement: AB + C CB + A D. Double Displacement: AB + CD AD + CB Note: Because you will investigate examples of each reaction type and will write balanced equations representing each reaction you carry out, a Periodic Table of the Elements and your valence sheet will be helpful as you write the required formulas for the compounds in the equations. You are reminded that the free (elemental) forms of hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, and iodine are diatomic; that is, they are symbolized as: H 2, N 2, F 2 O 2, C1 2, Br 2, and I 2 respectively. A thorough knowledge in balancing chemical reactions is required and helpful in predicting the outcome of a chemical reaction. Complete the prelab exercise at the end of this section before you come to your laboratory class. An answer sheet is also provided at the end of this exercise for you to complete during the lab. In order to complete the net ionic equations, you must refer to the solubility table in your text.

2 Exercise 4 Page 2 Procedure A. Combination (or Addition) Reaction: An example: Ca + S CaS In the above example two elements combine to form a compound. The combination of two smaller compounds to form a larger one also meets this criterion. 1. When copper metal reacts with hot sulfur vapors it yields copper(i) sulfide. Write the equation for this simple combination reaction. 2. The products of simple combustion reactions results in the addition of oxygen in the air to form the oxides of each of the elements in the compound, carbon dioxide and water. Write a balanced equation for the burning of propane, C 3 H Using tongs hold a small piece of magnesium metal in the flame. The magnesium reacts with the oxygen in the air to form magnesium oxide. Describe what happens. Write a balanced equation for this reaction. 4. The burning of magnesium occurs at such a high temperature that some nitrogen in the air can react with the magnesium to form some magnesium nitride. Write a balanced equation for this reaction. B. Decomposition Reaction: An example: 2 KClO 3 2 KCl + 3 O 2 The starting material, usually a single compound, is broken into its component elements or simpler compounds. 1. When mercury(ii) oxide is heated, it decomposes to metallic mercury and oxygen gas. Write a balanced equation for this simple decomposition reaction. 2. In the exercise on physical/chemical changes you heated ammonium chloride. Recall or repeat the experiment where approximately one gram of ammonium chloride is gently heated in a large test tube over a flame. Carefully smell the vapor. Ammonia is detected along with another gas. Write the balanced equation for this reaction.

3 Exercise 4 Page 3 C. Displacement Reaction: An example: CuCl 2 + Zn Cu + ZnCl 2 Refering to the solubility rules in you text,you should also be able to write the net ionic equation for such a reaction. For instance: Cu +2 + Zn Cu + Zn +2 An element reacts with a compound in this type of reaction. The free element, which is chemically more active, forces an element to leave and takes its place. 1. Place a few ml of dilute hydrochloric acid in a test tube. Drop a small piece of aluminum metal into the acid. Aluminum is displacing the hydrogen in the hydrogen chloride. The hydrogen bubbles out as a gas as the aluminum dissolves. Write a balanced equation for the reaction and a net ionic equation. 2. Place a few ml of water in a small beaker. Add a small calcium turning. Calcium is very active and even displaces one of the hydrogens in water. Write a balanced equation and the net ionic equation for the reaction. D. Double Displacement (Metathesis) Reaction: An example: NiCI 2 + CaS CaCl 2 + NiS (s) Net Ionic Ni +2 + S -2 NiS (s) This is a sort of "partner exchange" reaction. It frequently occurs when two ionic substances in aqueous solution are mixed together. If there is no interaction among the ions, no reaction occurs. However, if the ions can combine to form an insoluble solid, a gas, or a non-ionized product, such as water, then the double displacement reaction takes place. 1. To 1 ml of potassium chloride solution add 1 ml of silver nitrate solution. An insoluble precipitate of silver chloride appears. Write a balanced equation for this reaction and the net ionic equation. 2. To 1 ml of sodium carbonate add 1 ml of dilute hydrogen chloride. In this case one of the products of the ion partner exchange reaction is unstable and decomposes to form water and carbon dioxide gas. Write a balanced reaction equation and the net ionic equation. 3. To 1 ml of barium hydroxide add 1 ml of dilute hydrogen sulfate (sulfuric acid). Notice both the heat released due to the formation of water and observe the formation of the precipitate. Write a balance reaction equation and the net ionic equation.

4 Exercise 4 Page 4 IONIC AND MOLECULAR SOLUTES An Electrolyte Demonstration Electrodes will be immersed in each of the following substances (solutions, solvents, solids, and molten solids). If ions are present in sufficiently high concentration, the light bulb(s) connected to the electrodes will glow. If the concentration of ions is high, the bulb will glow brightly, and the substance dissolved in water is described as a "strong electrolyte." If the concentration of ions is low, the bulb will glow dimly, and the solute is described as a "weak electrolyte", since it conducts electricity weakly. A weak electrolyte when dissolved in water yields some ions, but is mostly composed of molecules. If the bulb remains unlit, ions are either absent (the substance is composed entirely of molecules), or cannot move toward the electrodes (for example, in a solid in which the ions cannot move), or ions are in too small concentration to allow appreciable current. Each substance listed will be tested by your instructor. Write an "X" in the appropriate column ("strong," "weak," or "non," which describes your observations of the ability of the substance listed to conduct electricity). Next, explain your observations by describing the particles) present which account for the conductivity of each substance. Substance Distilled water Conductivity strong weak non Conductivity due to the presence of the following ions. Tap water C 12 H (solid) C 12 H (aq) NaCl (solid) NaCl (aq) NaCl (molten) NaOH (aq) CH 3 OH CH 3 OH (aq) HC 2 H HC 2 H (aq) Ba(OH) 2 HCl NH 3

5 Exercise 4 Page 5 Illinois Central College Chemistry 130 Laboratory Section Name REPORT SHEET Types of Chemical Reactions and Electrolytes: A Demonstration A. Combination Reactions Result: 4. B. Decomposition Reactions 1. 2.

6 Exercise 4 Page 6 C. Displacement Reactions 1. Result: Net Ionic 2. Result: Net Ionic D. Double Displacement Reactions 1. Result: Net Ionic 2. Result: Net Ionic 3. Result: Net Ionic

7 Exercise 4 Page 7 QUESTIONS: 1. What is the difference between an ordinary chemical equation and the net ionic equation? 2. By what mechanism does current flow from one electrode to another? 3. Briefly explain the difference between a weak electrolyte and a strong electrolyte. 4. Pure liquid hydrogen chloride and pure liquid ammonia are both non-conductors of electricity, yet both of these substances are classified as electrolytes. Explain why this is true. 5. The HCl solution made the light bulb glow very brightly, whereas the NH 3 solution made it glow much more dimly. Both solutions were made up in the same concentration. How do you explain the fact that the light was so much brighter with the HCl solution?

8 Exercise 4 Page 8 6. Pure acetic acid is a non-conductor. Does this mean that acetic acid is a non-electrolyte? Explain briefly. 7. Crystals of NaCl are made up of Na + and Cl - ions. How do you explain the fact that the crystalline NaCl is a non-conductor? 8. Why does molten NaCl conduct an electrical current? 9. Based on your observations in this experiment, do you think crystalline sugar is made up of covalent molecules or ions? Explain.

9 Exercise 4 Page 9 Illinois Central College Chemistry 130 Laboratory Section Name PRELAB: Exp.4 Types of Chemical Reactions Balance the following chemical reactions after determining the correct formulas for each of the elements or compounds. 1. Aluminum + Oxygen Aluminum oxide 2. Hydrogen sulfate + Ammonium hydroxide Water + Ammonium sulfate 3. Butane (C 4 H 10 ) + oxygen Carbon Dioxide + Water 4. Zinc + Hydrogen chloride Zinc chloride + Hydrogen (g) 5. Barium acetate + Sodium phosphate Barium phosphate + Sodium acetate Balance the following net ionic equation: 6. Ba +2 + PO 4-3 Ba 3 (PO 4 ) 2(S) 7. H 2 CO 3 + OH - H 2 O + CO 3-2

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