Objectives To prepare a dilute solution of a weak acid. To prepare a buffer of a specific ph value.

Transcription

1 E x p e r i m e n t Chemistry Is phun! Objectives To prepare a dilute solution of a weak acid. To prepare a buffer of a specific ph value. To observe the effects of adding acid and base to a buffer solution. To understand the concept of conjugate acids and bases. In the Lab Get your assigned ph from the Blackboard grade book. Students work in pairs. Waste Disposal methods will vary among the acids and salts. Check bottles for disposal information. Check the label of the bottle or ask your TA where to dispose of each chemical. Safety Concentrated acids and bases can cause serious burns. Use caution and wear gloves during this experiment. 85

2 E x p e r i m e n t 1 6 Chemistry Is phun! Buffers are used to maintain a constant ph by responding to changes in the system. Blood contains buffers 1 which are critical for survival. A change in the blood ph (either up or down) can result in significant health problems and a substantial change (~0.5 ph units) can result in death. In this experiment, you will need to determine how to prepare a buffer of a specified ph and then measure its buffering capacity. To do this, we need to understand some concepts. A buffer is made up of an acid and its conjugate base or a base and its conjugate acid. An example of an acidconjugate base pair is acetic acid acetate ion. An acid and a conjugate base must differ by only one proton. The choice of the acid-conjugate base or base-conjugate acid pair depends on the ph you need. Generally, a solution can act as a buffer within ± one of its pk a or pk b value. ph The ph of a solution is related to its acidity or alkalinity, that is the concentration of H and OH, respectively. Remember that ph and poh are related to H and OH by the following equations : ph log [H ] or [H ] 10 ph poh log [OH ] or [OH ] 10 poh it gives a very broad range. Indicators work in a similar method to litmus paper in that they give us a range of ph values for a solution. However, the use of well-chosen indicators can narrow that range. ph paper, like litmus paper, has indicators impregnated into the paper and can estimate the ph within 1 or 2 ph units based on the color change of the paper which is to be compared with a color key that is supplied with the ph paper. The most accurate method of determining the ph is the use of a calibrated ph sensor. The actual method used to check the ph of a solution will be determined by how accurate the results need to be. For example, it may be sufficient to know that a solution is an acid or base, in which case litmus paper would be adequate. For a titration to determine the concentration of an unknown, a ph sensor would be the best choice since it accurately determines the ph value of the unknown solution. pk a and pk b Dissociation Constants for Weak Acids Strong acids, such as sulfuric acid (H 2 SO 4 ), hydrochloric acid (HCl) and nitric acid (HNO 3 ) are 100% dissociated (ionized) in aqueous solution. Weak acids, on the other hand, are only partially dissociated. A solution of a weak acid will consist of the acid, plus its conjugate base: ph poh 14 Acidic solutions have a ph less than 7.0, and basic solutions have a ph of more than 7.0. In a neutral solution, ph 7.0, and [H ] [OH ]. Because the ph scale is logarithmic, every one ph unit change will be equal to a 10 times change in [H ]. The amount of dissociation at equilibrium is given by the acid-dissociation constant, or K a for the acid: HA(aq) H 2 O(l) H 3 O (aq) A (aq) weak acid also written as: Measuring ph There are many ways to measure the ph of a solution. From the least to the most accurate method, we can use litmus paper, indicators, ph paper, or ph sensors. Litmus paper shows us only if the solution is acidic (turns red when ph < 7) or basic (turns blue when ph > 7). This is the least accurate method of determining the ph since HA(aq) H (aq) A (aq) conjugate base of acid The amount of dissociation at equilibrium is given by the acid-dissociation constant, or K a for the acid: K a [H ][A ] [HA] 1 LabTutorials/Buffer/Buffer.html (Accessed May 2005) 86

3 E x p e r i m e n t 1 6 Chemistry Is phun! where: K a acid-dissociation constant [A ] equilibrium concentration of the conjugate base [H ] equilibrium concentration of H ions [HA] equilibrium concentration of the acid The concentration of [H ] can be found directly from the ph. Since one molecule of [HA] dissociates into one molecule of [H ] and one molecule of [A ], the concentration of the conjugate base must equal the concentration of [H ]. The equilibrium concentration of HA is equal to the initial concentration of the acid, minus whatever dissociates: The concentration of [OH ] can be found from the ph. Analogous to the case for the weak acid, one molecule of [BOH] dissociates into one molecule of [B ] and one molecule of [OH ], so [B ] [OH ]. The equilibrium concentration of the base BOH is equal to the initial concentration of the base, minus whatever dissociates: [BOH] initial concentration of the base [OH ] Many weak bases are called amines, derivatives of ammonia, NH 3, where the hydrogen atoms are replaced by other groups. Weak bases have K b values < 1, furthermore, the larger the value of K b the stronger the base. In many cases the K b of the acid is expressed as the pk b : [HA] initial concentration of the acid [H ] If the initial concentration is large in relation to the dissociation, we can assume that the amount dissociated is negligible, so that [HA] initial concentration of the acid. Weak acids have K a values < 1, furthermore, the larger the value of K a the stronger the acid. For indicators that are weak acids, the smaller the value of K a, the higher the ph range for the color change. In many cases the K a of the acid is expressed as the pk a (pk a log K a ). Dissociation Constants for Weak Bases For weak bases: BOH(aq) OH (aq) B (aq) weak base conjugate acid of the base The amount of equilibrium dissociation is given by the base-dissociation constant, or K b for the base: where: K b K b [B ][OH ] [BOH] base-dissociation constant [OH ] equilibrium concentration of OH ions [B ] equilibrium concentration of the conjugate acid [BOH] equilibrium concentration of the base pk b log K b The relationship between K a and K b for all conjugate acid base pairs is: (K a for the acid) (K b for the base) K W Henderson-Hasselbalch A buffer is a solution that is capable of maintaining its ph at a constant value even when small amounts of acids or bases are added. Two species are required in a buffer solution: an acid component to react with hydroxide ions that may be added to the solution and a base component to react with added hydrogen ions. The equilibrium concentrations of both the acid and the conjugate base are assumed to be the same as the starting conditions. In general, the buffer system is represented as the ratio of the conjugate base/acid; for example the acetate acetic acid buffer can be written as CH 3 COO /CH 3 COOH. The Henderson-Hasselbalch equation shows that the ph of a solution of a weak acid and its conjugate base is dependent primarily on the pk a of the acid. Minor changes in the ph are given by the amounts of acid and conjugate base present in the solution: [conjugate base] ph pka log [acid] 87

4 E x p e r i m e n t 1 6 Chemistry Is phun! 88 Choosing a Buffer System A buffer solution is prepared from a conjugate acid base pair with concentrations in the range 0.10 to 1.0 M. Since the ph of a buffer is primarily determined by the pk a, you should always choose a weak acid (or base) with a pk a close to the ph you want to work with. A solution can buffer changes in ph that are within one ph unit of the pk a value. Buffer Capacity A buffer has a limited amount of acid or base that it can absorb without a change in ph. Buffer capacity is the amount of acid or base that can be added before the buffer is no longer effective and a change of ph greater than 1 is seen. - + ^molesof OH orh addedh Buffer Capacity = ( ph change)( volumeofbuffer inl) The moles of acid or base added can be determined from the molarity of the acid or base solution used and the volume added to the buffer solution to change the buffer s ph by 1 ph unit. Prepare a Dilute Solution (see Figure.1) 1. Add approximately ml of water to the volumetric flask. The exact value is not important for this step. 2. Use a funnel to add the needed mass of the salt (i.e., the conjugate base) to prepare a buffer with your assigned ph. If the salt will not dissolve, warm gently on the hot plate. Do not boil the solution. Record the exact mass of salt used. 3. Add the required volume of acid needed to prepare 100 ml of your M solution. Swirl to mix completely. Record the exact amount used. 4. Stopper the flask and mix well. 5. Add enough distilled water to the volumetric flask so that the bottom of the meniscus is even with the calibration line on the volumetric flask. To avoid adding too much, use a dropper to add water dropwise so that you don t add too much. If you add too much, you will need to remake your solution. 6. Stopper the flask and mix well. Pre-Lab Exercise You must complete some calculations on Chem21 before coming to lab for this experiment. There is a PowerPoint on Blackboard under Experiments that guides you through the calculations for this experiment. You will not be allowed to start the experiment until the calculations are completed correctly. Tips for Procedure Complete calculations before the lab session. Record all digits on the balance display when determining the mass of a substance. Titration with HCl should have the file name 001 and with the NaOH should be named 002. For each titration, use a graduated cylinder to measure out ~10 ml of your buffer solution into a beaker and add ~15 ml of distilled water. Record the exact volume of buffer used. Titrate the buffer until the ph value changes by more than one ph unit. Carry your titration out to the equivalence point. This may mean you have to refill your buret to continue titrating. Make sure that you record how much solution you add on the first and second buret full. Materials volumetric flask with stopper ph paper semi-micro test tubes two burets ring stand buret clamp 250 ml beaker distilled water MeasureNet ph sensor drop counter funnel iron clamp clamp holder

5 E x p e r i m e n t 1 6 Chemistry Is phun! Procedure You must write your procedure and prepare your lab notebook to record data before coming to lab to do the experiment. Make sure that you will collect the data necessary to complete the data analysis questions. It s better to have too much information and not need it, than to need something and not have it. Instructions on using burets can be found in Chapter 3. Steps for using the ph sensor and drop counter with the MeasureNet workstation are in Chapter 4. Data Analysis 1. Based on the actual volumes used, what is the concentration of the acid in your buffer solution? The actual volume used is rarely exactly the same as the amount determined before the lab. 2. Based on the actual volumes used, what is the concentration of the base (salt) in your buffer solution? The actual volume used is rarely exactly the same as the amount determined before the lab. 3. What is the pk a of the acid you used? 4. Using the concentrations calculated in #1 and 2, what is the theoretical ph of your buffer solution? 5. What is the percent error between your measured ph and your theoretical ph? 6. Using your graph, what is the volume in ml of acid added to change the ph by one unit? 7. What is the buffer capacity with the addition of acid? 8. Using your graph, what is the volume in ml of base added to change the ph by one unit? 9. What is the buffer capacity with the addition of base? 10. Is your buffer more resistant to the addition of acid or base? Explain your response. 89

6 E x p e r i m e n t 1 6 Chemistry Is phun! Making a Dilution Dispense a known amount of concentrated solution from a buret into a clean volumetric flask. 2. Using a wash bottle, rinse the neck of the flask with distilled water. 3. Fill flask ~ 1 /2 full with distilled water and swirl to mix. (slow swirling) 5. Stopper the flask and turn end over end several times to mix. Meniscus Etched line Hayden-McNeil, LLC 4. Dilute the solution until the meniscus is level with the etched line on the neck of the flask. Figure.1. Preparing a dilute solution. 90