Experiment Eight Acids and Bases

Transcription

1 Name: Lab Section: Experiment Eight Acids and Bases Objective Identifying and understanding the nature of acids and bases is an important part of the laboratory toolbox, the purpose of this lab is to help understand the different properties of acids and bases. Classifying substances as acids or bases based on their characteristic properties, determining ph values, and examining the reactivity are the goals set forth in this experiment. Introduction There are many theories and definitions that attempt to explain what constitutes an acid or a base. An early and still useful theory, developed by Arrhenius in the late 1800s, defined an acid as a substance that produces hydrogen ions, H+, when dissolved in water. A base, according to Arrhenius, is a species that produces hydroxide ions, OH-, when dissolved in water. For example, hydrogen chloride is an acid in the Arrhenius theory because hydrogen chloride dissociates in water, producing hydrogen ions. HCl H + + Cl Hydrogen chloride is called a strong acid because virtually every HCl molecule ionizes when dissolved. Other substances, although they do in fact produce hydrogen ions when dissolved, do not completely dissociate when dissolved, these are known as weak acids. For example, the acidic component of vinegar (acetic acid) is a weak acid: HC 2 H 3 O 2 C 2 H 3 O 2 + H + The double-ended arrow in the equation for the ionization of acetic acid indicates that this substance reaches equilibrium when dissolved in water, at which point a certain fixed concentration of hydrogen ions is present. The concentration of hydrogen ion produced by dissolving a given amount of weak acid is typically several orders of magnitude less than if the same amount of strong acid is dissolved. In addition to different types of acids: strong acid dissociates completely into its ions, weak acid partially dissociates leaving some of the original acid still intact; the same occurs for bases. A strong base will completely dissociate into its respective ions. For example, NaOH is a strong base: NaOH(s) Na + (aq) + OH (aq) For every molecule of NaOH that is dissolved in water, an equivalent number of OH- is produced. In contrast, ammonium hydroxide, NH4OH, is a weak base meaning it only partially breaks into its components when dissolved in water: NH 4 OH NH OH Once again notice the double sided arrow which means that both the forward and reverse reactions are happening, NH4OH is not completely gone, it s still present in solution. 79 P a g e

2 Name: Lab Section: Reactions of acids The most important type of reaction that occurs with an acid and/or base and another substance is a neutralization reaction. The hydrogen ion from an aqueous acid will combine with the hydroxide ion from an aqueous base, producing a salt and water. For example: HCl + NaOH NaCl + H 2 O Or HNO 3 + NH 4 OH NH 4 OH + H 2 O Regardless of what the acid and base are for a neutralization reaction the net ionic reaction will always be the same: Procedure H + + OH _ H 2 O 1. ph of Acid-Base solutions. Place four small pieces of ph paper on a watch glass. Using the four solutions present at the station, place one drop of a solution on one piece of the ph paper. If the solution is in a bottle and doesn t have a dropper, use a glass stirring rod to apply the acid/base solution to the ph paper. Using the color coded insert on the ph paper determine the ph of each solution, and record it in the data sheet. If you think the color is between two different ph values, then split the difference. (For example, if the solution looks to be in between 4 and 5, then you should record 4.5 in your data sheet.) Each solution, should be placed on one (and only one) ph strip. Each piece of ph paper should be no greater than the representative strip like this 2. Production of Salts from Acid-Base Solutions. Combine five drops of 6M HCl and five drops of 6M NaOH on a clean, dry watch glass. Stir the mixture with a glass stirring rod. Place the watch glass in the oven and allow it to dry, continue the rest of the lab and come back for this before you finish the lab. Record your observations in the data sheet, be sure to use a magnifying glass to observe the shape/color of the resulting solid. (Saying white solid is not good enough, provide some indication of the shape of the solid. colorless small plate like crystals would be a better observation than white solid ) 3. Neutralization. Add 2.0 ml of 0.1 HCl in a clean test tube, add one drop of universal indicator. Record the color of the indicator and the ph of the solution in the data sheet. This indicator changes color gradually with ph and can be used to monitor the ph of a solution during a neutralization reaction. Keep handy the color chart provided with the indicator. It shows the color of the indicator under different ph conditions. Add the 0.1M NaOH solution drop wise with the disposable plastic pipette, keeping track of the number of drops used. Record the color after three drops have been added. Continue adding 0.1M NaOH dropwise, record the color and ph at three-drop incruments until the ph has risen to ph greater than P a g e

3 Name: Lab Section: Answer the questions on the data sheet using the following information. 20 drops = 1mL Repeat the process using 0.1M acetic acid and adding 0.1M NaOH dropwise. 4. Heat of Neutralization. Obtain approximately 2mL (the plastic pipettes have ml graduation marks on them) of each 6M HCl and 6M NaOH in separate clean test tubes. Using a thermometer, determine the temperature of the acid, record the result in your data sheet. Quickly pour the base solution, 6M NaOH, directly into the acid solution. Stir the solution with the thermometer and monitor the temperature change. Record the highest temperature achieved by the mixture in your data sheet. Calculate the change in temperature on your data sheet. Repeat the process with 2mL of 6M Acetic Acid and 2ml of 6M NaOH. 5. Reactions of Acids and Metals and Calcium Carbonate. Using the porcelain spot plate with 12 divots, place a small quantity of zinc, magnesium, copper, and calcium carbonate in two rows of the plate. To the first row add enough 6M HCl to cover each of the metals. Record your observations in the data section. Repeat the process with 6M acetic acid, and record the results in your data section. WASTE: All of the waste from part 5 should be placed in the designated area of lab. Be sure to rinse the spot plate before putting it back to dry. All other acid/base reaction materials can go down the sink with running water. 81 P a g e

4 Name: Lab Section: Data/Results 1. ph of Acid-Base solutions. Solution ph measured Solution ph measured 0.1M HCl 0.1M HC2H3O2 0.1M NaOH 0.1M NH 4 OH 2. Production of Salts from Acid-Base Solutions. Observation of HCl/NaOH product (Hint: this is a Neutralization Rxn) 82 P a g e

5 Name: Lab Section: 3. Neutralization. (Rxn #1) 0.1M HCl (Hydrochloric Acid) color observed ph observed Initial solution after indicator added After three drops 0.1M NaOH Drops of 0.1M NaOH required to reach ph 7 Drops of 0.1M NaOH required to reach ph 10 How many ml of NaOH were required to reach ph 7 if 1ml = 20 drops? (Show your work for this calculation below to receive full credit) (Rxn #2) 0.1M HC2H3O2 (Acetic Acid) color observed ph observed Initial solution after indicator added After three drops 0.1M NaOH Drops of 0.1M NaOH required to reach ph 7 Drops of 0.1M NaOH required to reach ph 10 How many ml of NaOH were required to reach ph 7 if 1ml = 20 drops? (Show your work for this calculation below to receive full credit) 83 P a g e

6 Name: Lab Section: 4. Heat of Neutralization. (Rxn #1) 6 M HCl (Hydrochloric Acid) Initial temperature of 6M HCl Highest temperature reached on adding 2ml 6M NaOH Change in temperature? (Rxn #2) 6M HC2H3O2 (Acetic Acid) Initial temperature of 6M Acetic Acid Highest temperature reached on adding 2ml 6M NaOH Change in temperature? Temperature ( C) Temperature ( C) 5. Acids and Metals and Calcium Carbonate. (Write observations) 6M Hydrochloric acid (HCl) 6M Acetic Acid (HC2H3O2) Zinc (Zn) Magnesium (Mg) Copper (Cu) Calcium Carbonate (CaCO3) 84 P a g e

7 Name: Lab Section: Post-Lab Questions (Each question number corresponds with the section from data/results) 1. Explain the results of the ph paper your knowledge of ph and the ideas of strong/weak acids/bases and dissociation of ions. 2. Based on the results from part 2 of the lab: a. What salt was obtained from the reaction you did in lab? b. Write the balanced chemical equation for the reaction of H2SO4 and NaOH. c. What is the salt that will result from this reaction? 3. In your own words, what does reaching a ph of 7 mean? 85 P a g e

8 Name: Lab Section: 4. For the neutralization for the strong and weak acid: a. Did either reaction show a higher temperature? Rxn #1 Rxn #2 Neither b. Was Rxn #1 endothermic or exothermic? Endothermic Exothermic 5. Based on the reactions with metals, answer the following: a. Summarize the reactivity of the metals ( Zn, Mg, Cu) from most to least reactive: Most Reactive > > > > > > Least Reactive b. Write the balanced chemical equation for the reaction of HCl with sodium carbonate. 86 P a g e