HCl + NaOH NaCl + H 2 O acid base salt water

Transcription

1 1 Comparison of Antacids Introduction Antacids and Neutralization Reactions An antacid is a substance that acts to neutralize excess stomach acid in order to relieve acid indigestion and heartburn. Stomach acid is a component of gastric juice, a digestive fluid secreted by glands located in the mucous membrane lining the stomach. This acid functions to activate certain enzymes necessary for digestion and regulates other aspects of the digestive process. In the case of acid indigestion and heartburn, excess gastric juices are produced, causing burning and discomfort due to the increased acid concentration. Acids (such as the acid in gastric juice) are a source of the H + ion. The higher the concentration of an acid, the more H + present. Acids have a ph below 7. The higher the concentration of H + from the acid, the lower will be the ph. Antacids react with H + and effectively reduce its concentration. Antacids are bases, which have a ph above 7. This reaction between equal amounts (in moles) of a strong acid and an antacid (a strong base) is called a neutralization reaction because the acid and antacid cancel or balance each other, resulting in a neutral solution with a ph of 7. Here is an example of a neutralization reaction: HCl + NaOH NaCl + H 2 O acid base salt water Many bases can be recognized by the presence of OH - (the hydroxide ion) in their formula. For example, Mg(OH) 2 is the base present in many commercial antacids. As in the reaction above, the products of any neutralization reaction are some type of salt and water. When you consume an antacid, such as Tums or Rolaids, the base in the antacid tablet reacts with stomach acid in a neutralization reaction. One way to compare one antacid with another is by cost effectiveness, that is, which product neutralizes more acid per dollar spent. In this experiment you will determine the capacity of two different antacids to neutralize stomach acid per unit cost, in order to determine which is the better buy. Titration To determine the capacity of the antacids to neutralize stomach acid, you will use a method called a titration. A titration is an experimental procedure which allows you to determine the exact amount of one substance that will react with a specific amount of another substance. In this experiment, you will determine the exact amount of acid that will react with 0.5 grams of an antacid. Although the antacid sample has a mass of 0.5 grams, the sample contains fillers (such as flavorings) in addition to the base which acts to neutralize stomach acid. Therefore, you won t know how much of the 0.5 gram sample is the useful base, and how much is just filler. The more filler present, the less effective will be a given amount of the antacid in combating heartburn. In this experiment the titration will consist simply of adding an acid solution to the 0.5 gram antacid sample as acid is added to the sample, a neutralization reaction will occur between the acid being added and the base in the antacid sample. The acid will be added to the antacid slowly by delivering it from an instrument called a buret. The buret has volume markings so that

2 2 it will be easy to determine the exact amount of acid needed to react with the antacid sample in the neutralization reaction. As explained above, neutralization occurs when equal amounts (in moles) of an acid and base react to balance or cancel each other. Therefore, you will add acid to the antacid sample just until the amount of acid added (in moles) is exactly equal to the amount of base (in moles) in the 0.5 gram antacid sample at this point neutralization occurs. The volume markings on the buret will enable you to easily determine the total amount of acid that was needed for neutralization to occur. Since moles of acid are equal to moles of base at the point of neutralization, this amount of acid added from the buret will be an indication of the amount of base (in moles) that must be present in the 0.5 gram antacid sample. The point at which neutralization occurs, when moles of acid added are equal to moles of base, is called the endpoint of the titration. When the endpoint is reached, you will stop adding acid since neutralization has occurred. If you add extra acid past the endpoint, there will be no base for it to react with (since all the base has already reacted with acid and has been used up ). The problem with adding extra acid is that the volume reading on the buret will reflect the total amount of acid added you won t know how much of this total volume was needed to reach neutralization, and how much was extra. Since the amount of acid added is meant to be used as an indication of the amount of base in the antacid sample, an inaccurate total acid volume will give an incorrect indication of the amount of base. One more detail of the titration procedure needs to be explained: How will you know when you have added an amount of acid that is exactly equal to the amount of base (in moles)? In other words, how will you know when the endpoint of the titration has been reached and you should stop adding acid? You will add a chemical called an indicator to the solution, and the indicator will change color when the endpoint has been reached. The solution containing the antacid sample will be green initially, due to the presence of the indicator. As you add acid from the buret, the green color will begin to change to yellow and eventually to peach/orange. When the solution becomes bright Buret peach/orange and remains that color for about 10 seconds, the endpoint has been reached and you should stop adding acid. The titration procedure is then complete, and the amount of base in the antacid sample can be determined. Buret The acid used during the titration will be contained in a buret (pictured at the right). The acid will be delivered out of the bottom of the buret by opening and closing the stopcock near the bottom. Volume markings along the entire length of the buret will enable you to determine how much acid was delivered. stopcock Acid will be delivered by draining it from bottom of buret

3 3 Reading a Buret A buret is an instrument designed to deliver very precise volumes of a liquid reagent. Reading a buret is very much like reading a graduated cylinder in that you should read from the bottom of the meniscus and you should always estimate ONE more digit (between the lines) than you can read from the scale. The difference between a buret and a graduated cylinder is that 0.00 ml is at the TOP of the buret just the opposite of how a graduated cylinder is marked. The purpose of this is so that you can fill the buret completely (to the 0.00 ml mark), deliver a certain volume of liquid out of the buret by opening the stopcock at the bottom, and then read the scale to determine what volume was delivered. In other words, a buret reading shows the volume that was delivered (drained out), rather than the volume that is contained in the buret (a graduated cylinder shows the volume contained in the cylinder). Therefore, you should read the scale on a buret in a downward direction. This is exactly the opposite of reading a graduated cylinder where you read the scale in an upward direction. Take a look at the example below before attempting to do the 3 buret questions on the prelab.

4 4 Comparison of Antacids Prelab Name 1. How does an antacid, such as Rolaids or Tums, work to relieve acid indigestion and heartburn? 2. What are the products of the neutralization reaction below between an acid and a base found in an antacid? Be sure your reaction is balanced. 2HCl + Mg(OH) 2 acid base (from antacid) Which of the compounds is the salt? Explain why this type of reaction is called a neutralization reaction. 3. What does cost effectiveness of an antacid refer to? 4. What is the one-word term for the experimental method you will use to determine how much acid can be neutralized by a small sample of an antacid? What is the name of the piece of laboratory glassware that you will use to measure the volume of acid that is neutralized by the antacid sample? 5. What two quantities will be equal at the endpoint of the titration carried out in this experiment? 6. What type of chemical will be used to signal the endpoint of the titration? Prelab continued on next page

5 5 Comparison of Antacids Prelab Name 7. Assume that the buret at the right was previously filled to the 0.00 ml mark. What volume of reagent has been delivered? (Remember to estimate between the lines to get one more digit than can be read from the scale.) 8. Write down a reading for each buret pictured below. 9. Assume that the buret below was previously filled to the 5.00 ml mark. What volume of reagent has been delivered?

6 6 Comparison of Antacids Procedure Preparation of the Buret. 1. To ensure that your buret is clean and does not contain any chemical residue that will interfere with the titration, rinse it thoroughly with tap water. It can be awkward to try to rinse a buret at the sink, so you may want to ask the instructor for suggestions. Be sure to open the stopcock at some point to allow the rinse water to clean the tip of the buret as well. After thorough rinsing with tap water, rinse once with distilled water and allow all liquid to drain out of the buret. 2. Pour approximately 5 ml of 0.3 M HCl (the acid that will be used in the titration) into the buret. With the stopcock closed, hold the buret in a nearly horizonatal position and roll the solution around to wet the entire inner surface of the buret. Then drain the HCl out of the buret by opening the stopcock. The purpose of this step was to use a little HCl to rinse out any remaining water which would otherwise dilute the HCl that will next be added to the buret. 3. Clamp the buret to a ring stand on your lab bench using a special buret clamp. Be sure the stopcock is closed! Then use a funnel to fill the buret with 0.3 M HCl so that the liquid level is just a little above the 0.00 ml mark. 4. It is necessary to fill the lower tip of the buret with HCl in order to end up with accurate volume readings. To fill the tip with acid, place a clean beaker under the tip of the buret to catch any liquid, and then open the stopcock all the way for just a second or two. After closing the stopcock again, carefully examine the tip of the buret to see if it is entirely filled with liquid or if there are some air bubbles trapped inside. If there are air bubbles present, open the stopcock full blast for a few seconds until the rapid flow of liquid causes the air bubbles to be pushed out. If you are not able to eliminate a stubborn air bubble, ask the instructor for help. Air bubbles in the tip will lead to inaccurate volume readings. If you did not drain much HCl out of the buret in the process of filling the tip, the liquid level will probably be slightly below the 0.00 ml mark. You should begin the titration with a nearly full buret since you don t know how much HCl will be needed. Therefore, if the HCl level has dropped by 5 ml or more, add more HCl to the buret so that the liquid level is just a little below the 0.00 ml mark. You are now ready to take a volume reading from the buret. Be sure that the liquid level in the buret is slightly below the 0.00 ml mark (remember to read from the lowest part of the meniscus). Take an initial volume reading and record it on the report sheet. Remember to read and record the volume to two digits after the decimal. (For example: 0.16 ml) When finished, ask the instructor to check your volume reading to be sure you read the buret correctly.

7 7 Titration of the Base in an Antacid Sample. 1. You will titrate two different antacids in order to determine which is the better buy. Choose the antacid you want to begin with and record its name on the report sheet. Weigh out approximately 0.5 grams of this antacid and record the exact mass (all digits) on the report sheet. Place the powder into a 250 ml flask and add 50 ml of distilled water. Then add 8 drops of the indicator to this solution. Swirl the flask well in order to dissolve the antacid in the water. The solution should be green due to the presence of the indicator. 2. Begin the titration by draining a few ml s of HCl out of the buret into the flask containing the antacid sample. Stop and swirl the flask vigorously to mix the solution. As you added HCl you may have seen the appearance of yellow or peach/orange color. However, after swirling the flask, the color should have reverted to green. In the next step you will continue to slowly add HCl in this manner while swirling the flask. As you add more and more HCl, the peach/orange color will appear brighter (less yellow) and last longer. A bright peach/orange color signals the endpoint of the reaction, the point at which the base in the antacid sample has been neutralized by the acid from the buret. It is a bit tricky to identify the exact endpoint for this particular reaction because the peach/orange color appears but then disappears after swirling the flask. Therefore, when the bright peach/orange color remains for about 10 seconds before fading back to yellow or green, you should stop the titration (the endpoint has been reached). 3. Complete the titration by slowly adding more HCl while swirling the flask vigorously. When you near the endpoint, the HCl should be added very slowly only a couple drops at a time so as not to go past the endpoint. When the endpoint has been reached, take a final volume reading from the buret and record it on the report sheet. Use the initial and final volume readings to calculate the total volume of HCl used in the titration; record this total volume on the second page of the report sheet (p. 9). Discard the contents of the 250 ml flask down the drain and rinse the flask clean. 4. Prepare for the titration of the other antacid by refilling the buret with 0.3 M HCl so that the liquid level is just below, or exactly on, the 0.00 ml mark. Take an initial volume reading and record it on the report sheet. Record the name of the other antacid on the report sheet. Repeat steps 1 3 of the titration procedure for this antacid. 5. When both titrations are complete, discard any unused HCl from the buret down the drain and rinse the buret thoroughly with tap water followed by distilled water. Complete the calculations on the report sheet to determine which antacid is the better buy. To do the calculations, you will need to obtain information from the labels on the antacid containers.

8 8 Comparison of Antacids Report Sheet Name Data First Titration Second Titration 1. Name of antacid 2. Exact mass of antacid sample g g 3. Initial volume reading of HCl in buret ml ml 4. Final volume reading of HCl in buret ml ml

9 9 Comparison of Antacids Report Sheet Name Calculations First Titration Second Titration 5. Total volume of HCl used ml ml 6. Volume of HCl neutralized per dose of antacid ml/dose ml/dose Formula to use: (vol HCl used / mass antacid sample) x mass of dose Show work below: 7. Number of doses per container doses doses Formula to use: number of tablets per container / tablets per dose Show work below: 8. Cost per dose $ $ Formula to use: cost of container / number of doses per container Show work below: 9. Cost effectiveness: Volume of ml per $ ml per $ HCl neutralized per unit cost Formula to use: vol HCl neutralized per dose / cost per dose Show work below:

10 10 Comparison of Antacids Postlab Name 1. Which antacid is the better buy? In other words, which one neutralized more HCl per dollar spent? 2. What two quantities will be equal at the endpoint of the titration carried out in this experiment? How did you know when you had reached the endpoint (what did you see)? Why is it important not to add too much HCl and go past the endpoint? 3. Show the products of the neutralization reaction below: Be sure your reaction is balanced. H 2 SO 4 + Mg(OH) 2 Label the acid, the base, and the salt in your reaction above.