Aqueous Solution Chemistry of Acids and Bases, ph, and the Use of Indicators PRELIMINARY QUESTIONS

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1 CHM 1041 J. Bieber Aqueous Solution Chemistry of Acids and Bases, ph, and the Use of Indicators PRELIMINARY QUESTIONS Name(s): Section: Date: INTENT In this experiment you will review qualitative acid/base chemistry and be introduced to simple quantitative methods for the investigation of the chemistry of acids and bases in aqueous (water is solvent) solution. PRELIMINARY QUESTIONS. (To be answered BEFORE coming to lab.) 1. When HCl is added to H 2 O which reactant is......the Brönsted Acid.? _...the Brönsted Base? 2. Since this transformation goes to K100% can one write a meaningful equilibrium expression for this system under these conditions? Explain. _ 3. Based on the extent of this reaction, what can you say about the relative basicities of H 2 O and Cl 1-? Explain. _

2 2 4. Based on the extent of the reaction between HCl and water, which is the stronger B.A., HCl or H 3 O 1+? Explain. _ 5. Nitric acid (HNO 3 ) and sulfuric acid (H 2 SO 4 ) are also B.A. s stronger than H 3 O 1+. Write the equations for the dissolving of appreciable amounts (approximately 1 mole) of each of these acids in separate 1 L portions of pure H 2 O. _ 6. Now, using square bracket notation {e.g. for 0.5 M Na 2 CO 3 : [Na 1+ ] = 1.0 M (or 1 M); [CO 3 2> ] = 0.5 M} quantitatively inventory: (a) 0.1 M HCl (b) 0.2 M HNO 3 (c) 1 M H 2 SO 4 (d) M H 2 SO 4 (e) 0.5 M NaOH (f) M Ba(OH) 2 7. Give the ph (to 1 sig. fig.) of the solutions you inventoried in Question 6: (a) (c) (e) (b) (d) (e) 8. Give the ph (to 1 sig. fig.) of: (a) pure H 2 O (c) 2 M HCl (b) 1 M HCl _ (d) 2 M NaOH

3 3 CHM 1041 J. Bieber Aqueous Solution Chemistry of Acids and Bases, ph, and the Use of Indicators Name(s): Section: Date: INTENT In this experiment you will review qualitative acid/base chemistry and be introduced to simple quantitative methods for the investigation of the chemistry of acids and bases in aqueous (water is solvent) solution. DISCUSSION ALL OF THE PRELIMINARY QUESTIONS ARE TO BE ANSWERED BEFORE COMING TO LAB. Recall from CHM 1040 the functional definition of acids and bases according to the Brönsted- Lowry) Theory. Brönsted Acid (B.A.) Brönsted Base (B.B.) any chemical substance which can act as a proton (H 1+ ) donor. any chemical substance which can act as a proton (H 1+ ) acceptor. Hydrolysis Hydrolysis is a reaction with water where a proton is transferred. It is just a special case of an acid/base reaction (where water is either the acid or the base). An acidic molecule or ion may transfer a proton to water to produce H 3 O 1+ and the conjugate base of the original acid; alternatively, a basic molecule or ion may accept a proton from water to produce OH 1- and the conjugate acid of the original base. Therefore, when you are considering whether an aqueous solution will be acidic, neutral, or basic you need to consider what ions will be present in the solution when the solute dissolves. If the solute is a strong acid (or strong base) you will form H 3 O 1+ (or OH 1- ). Then you must consider whether any of the ions which may have formed on dissolution can act as either a weak acid or a weak base in an aqueous solution.

4 4 Strong Acids and Bases Gaseous hydrogen chloride [HCl(g)] reacts essentially completely with excess H 2 O according to the equation: HCl(g) + H 2 O(l) S H 3 O 1+ (aq) + Cl 1- (aq) That is, if we bubble in approximately 1 mole of HCl(g) into approximately 1 L of H 2 O this reaction proceeds to the right as written to an extent of K100% to produce H 3 O 1+, and the conjugate base of the original acid (Cl 1-, in this case). Answer Preliminary Questions 1-3 Now, since we will be using water as the solvent we shall classify H 3 O 1+ (the conjugate acid of the solvent) and any B.A. stronger than H 3 O 1+ as a strong acid. And, we ll also see that OH 1- (the conjugate base of the solvent), and any B.B. stronger than OH 1- is equally well classified as a strong base. While most strong acids will react K100% with water to produce H 3 O 1+, a strong acid is defined as any acid that reacts to a large extent (> 50%) with water. Answer Preliminary Questions 4 and 5 The species produced by the reaction of H 2 SO 4 with one mole of water, HSO 4 1> does not lose its proton to an appreciable extent. The concept of extent will become more meaningful as you deal with chemical equilibrium quantitatively. But at this point, for the purpose of calculating [H 3 O 1+ ] in a H 2 SO 4 solution we will assume that the second proton of H 2 SO 4 is either not lost at all (at high concentration) or totally lost (at very low concentration). Hence, H 2 SO 4 is a diprotic acid; i.e., is can donate two protons. Answer Preliminary Question 6 The ph of an Aqueous Solution. The Ion Product (K w ) of Water The term ph is related to the [H 3 O 1+ ] of a water solution. It is defined as: ph = >log 10 [ H 3 O 1+ ] (Note: log 10 = log to base 10) Or, this can be expressed as: [H 3 O 1+ ] = 10 -ph. Thus, if [ H 3 O 1+ ] = 1M., ph = 0.0. If [ H 3 O 1+ ] = 1. x 10 >1 M, ph = 1.0. (Note: Only decimal places to the right carry significant figures, those figures to the left are decimal point locators, and are not significant figures)

5 5 But, if we have [H 3 O 1+ ] = 2. x 10 >3 M, then it is more convenient to use the logarithmic form of our definition, e.g.: ph = 1/log[H 3 O 1+ ] = > log (2 x 10 >3 M) = >(log 2) + (>log 10 >3 ) = (> 0.3) + (3 exactly ) = 2.7 (Note: This ph value of 2.7 has one sig. fig., just as the concentration does.) Since H 3 O 1+ is a strong acid and OH 1- is a strong base, they react with each other to a very large extent to form H 2 O. H 3 O 1+ + OH > H 2 O + H 2 O = 2 H 2 O ( B.A. + B.B > Conj. Acid Conj. Base ) However, let us now examine the reverse of this reaction: H 2 O + H 2 O -----> H 3 O 1+ + OH 1- Since this reaction proceeds to a measurable extent we recognize that H 2 O acts as both an acid and a base in pure water. Therefore, in pure H 2 O, few H 3 O 1+ ions do coexist with a few OH 1- ions. Even though this number is very small, at room temperature (25 C) the following relationship is satisfied for every aqueous solution: [H 3 O 1+ ]`[OH 1- ] = 1.0 x = ion product of H 2 O = K w Therefore, in pure water [H 3 O 1+ ] = [OH 1- ] = 1.0 x 10 >7 ; and all water solutions for which this is true are said to be neutral (i.e., ph = 7.00). If [H 3 O 1+ ] > [OH 1- ], the solution is a acidic (i.e., ph < 7.00); whereas if [OH > ] > [H 3 O 1+ ], the solution is basic (i.e., ph > 7.00). Answer Preliminary Questions 7 and 8 Weak Acids and Bases, Percent Hydrolysis. When a weak acid is dissolved in water, all of it must be present as either unreacted acid, or as H 3 O 1+, and its conjugate base (through reaction with H 2 O). By using indicators you can determine [H 3 O 1+ ] and then calculate the fraction of acid molecules which have reacted with H 2 O in order to achieve equilibrium. You can then calculate the fraction of acid molecules which has not reacted with water. (Note: The sum of these two fractions must equal one.) To illustrate, if, in 0.1 M HX (a hypothetical weak acid) the [H 3 O 1+ ] is estimated to be 2. x 10-3, the fraction HX reacted is (2. x 10 >3 /0.1), or The fraction of HX unreacted is (1 > 0.02), or The fraction of the HX which has reacted with the H 2 O is the fraction hydrolyzed. If you multiply the fraction hydrolyzed by 100% you get the percent hydrolysis.

6 6 Measuring ph with Indicators By comparing acid solutions with fixed and known [H 3 O 1+ ] (called standards) which contain an appropriate indicator, you will be able to make an approximate determination of [H 3 O 1+ ] in any solution. For example, when the hypothetical indicator Jerry Bieber Blue is placed in a solution which is ph = 4 the indicator turns blue, at a higher ph, say 5, the color appears green, and at a still higher ph (when the solution has ph = 6) the solution remains yellow since the indicator Jerry Bieber Blue has only one proton to lose. If we represent the acidic form of the indicator as HJBB (which is blue) and the form of the indicator as JBB 1- (which is yellow) then we can represent the change occurring with increasing ph as: HJBB (aq) + H 2 O (l) > H 3 O 1+ (aq) + JBB 1- (aq) (blue) (yellow) A green color would then correspond to an incomplete reaction, where both HJBB and JBB 1- are present in the solution in approximately equal amounts. Weak Acids and Percent Hydrolysis Only a few acids are strong enough to react K100% with water. Most acids, weak acids, react with water to a much smaller extent to produce some H 3 O 1+ and the conjugate base of the original acid. When a sufficient amount of the weak acid acetic acid, HC 2 H 3 O 2, reacts with water to produce a 1 M HC 2 H 3 O 2 solution, the reaction: HC 2 H 3 O 2 (aq) + H 2 O (l) > H 3 O 1+ (aq) + C 2 H 3 O 2 1- (aq) occurs, but to an extent Π100%. After this net reaction has occurred then there will be an equilibrium established. Write the principal acid/base equilibrium in a 1 M HC 2 H 3 O 2 solution. Therefore, one can see that all of the acetic acid in the 1 M solution is present in the solution as either unreacted HC 2 H 3 O 2, or as H 3 O 1+ and C 2 H 3 O 2 1- (from the reaction of acetic acid and water).

7 EXPERIMENTAL Observation of Various Indicator Color Ranges: 7 Observe the solutions that are on display. They contain solutions of various ph s (as indicated) with just enough of various indicators added to produce a detectable color. Enter the colors observed in the accompanying table as shown for HJBB. ph Indicator color color color color color color Jerry Bieber Blue Methyl Violet Methyl Orange Bromophenol Blue Bromocresol Green Bromothymol Blue Thymol Blue Alizarine Yellow Indigo Carmine B B Y Y Y Y Summary of Indicator Observations Indicator ph Range of Color Change lower ph/color middle ph/color upper ph/color Jerry Bieber Blue Methyl Violet Methyl Orange Bromophenol Blue Bromocresol Green Bromothymol Blue Thymol Blue 4 (B) < 2 (Y?) 5? (G) * 2 (O) 6 (Y) Alizarine Yellow Indigo Carmine * The hypothetical indicator Jerry Bieber Blue changed color between ph = 4 and ph = 6. Therefore, while the color corresponding to an equal mixture of HJBB and JBB 1- will be green, without a ph = 5 solution one can only speculate as to exactly where the color change would occur.

8 8 Thymol blue can lose 2 protons over the ph range of Therefore, it is different from your other indicators you can observe two color changes. Predict the color of the various aqueous solutions below, if a drop of these various indicator solutions was place in them. Solution HMV/MV 1- HMO/MO 1- HBpB.BpB 1> HBcG/BcG 1- HTB/TB 1- HIC/IC 1- ph 1 M H 2 SO M HCl 1 x 10 >4 M HNO 3 ph = 6.0 sol n. ph = 8.0 sol n ph = 10.0 sol n ph = 12.0 sol n What is the ph (to 1 sig. fig.) of a solution which is (1)blue when bromocresol green is added, (2) blue bromothymol blue is added, or (3) yellow when thymol blue is added? ph = _

9 9 Measuring the ph of Dilute Aqueous Solutions, and Calculating Their Percent Hydrolysis Acid Solutions Use a ph meter to measure the ph of 10. ml of each of the solutions listed in the table and record these values in the table. Calculate the [H 3 O 1+ ] in each of these solutions. Dividing [H 3 O 1+ ] by the original concentration gives the fraction of the acetic acid that hydrolyzed. If you then multiply the fraction by 100% you get the percent hydrolysis. Conc. of Acetic Acid 1 M HC 2 H 3 O M HC 2 H 3 O M HC 2 H 3 O 2 Measured ph Calc. [H 3 O + ] % Hydrolysis What happens to the percent hydrolysis of a weak acid as it is diluted? Defend your answer based on your calculations. _ Is 1 M HCl or 1 M HC 2 H 3 O 2 a better conductor of electricity? Why? _ Strong and Weak Base Solutions Use a ph meter to measure the ph of 10. ml of each of the following solutions: Solution 0.10 M NaOH M NaOH 0.10 M NH M NH 3 Measured ph

10 10 An aqueous NaOH solution contains a strong/weak (circle one) base at high concentrations because _ An aqueous NH 3 solution contains a strong/weak (circle one) base at high concentrations because _