STUDYING CHEMICAL REACTIONS BY TITRATION ANALYSIS

Transcription

1 STUDYING CHEMICAL REACTIONS BY TITRATION ANALYSIS OBJECTIVES: Study the relationship of reactants & products in solution phase chemical reactions, Learn how to prepare solutions from solid and liquid stock, Use titration procedures to standardize a Sodium Hydroxide (NaOH) solution, & Use the standardized NaOH solution to titrate and determine the concentration of a Hydrochloric Acid ( HCl ) in aqueous solution. BACKGROUND: Quantitative relationships in chemical reactions include: Moles of reactant and product used in a reaction process, Mass values ( Equation Mass ) of reaction components, Numbers of particles of each reactant and product in the reaction process, Gas Volumes of gas phase components of the reaction, and Energy values associated with the process under study. These quantitative relationships can be studied by controlled solution phase reactions called titrations. Many reactions are not evaluated directly by measuring moles, mass, particles, gas volumes, and/or energy values. The most common format for a chemical reaction analysis is based upon solutions of the reacting reagents. When studying solution phase reactions, the components of a solution are measured in terms of concentration and are defined by the following relationship: Concentration = Mass of Solute Volume of Solution One such concentration term is Molarity (M). Empirically, Molarity (M) = Moles of Solute Volume of Solution ( Liters ) From this expression, moles of solute can be calculated and related to the reaction coefficients of a chemical equation. That is, Moles of Solute (n) = Molarity (M) X Volume (L) This experiment consists of two parts. In Part I an acid solution, Potassium Hydrogen Phthlate ( KHPh ), having a known concentration will be prepared and used to determine the concentration of a Sodium Hydroxide base solution ( NaOH ) having and unknown concentration. The base solution will then be used to determine the concentration of a Hydrochloric Acid solution of unknown concentration. The Part I: Standardization of NaOH solution The reaction used is: Reaction 1: KHPh(aq) + NaOH(aq) NaKPh(aq) + H 2 O(l) KHPh(aq) is Potassium Hydrogen Phthlate and functions as a control or standard acid solution preparation. The reaction is a 1:1 ratio between the KHPh and NaOH. That is, the moles of KHPh used is equal to the moles of base (NaOH) neutralized. 1

2 Moles KHPh = Moles NaOH ( M x V ) \KHPh = ( M x V ) NaOH M NaOH = ( M x V ) KHPh Used ( Vol NaOH Titrated ) Part II: Determination of the concentration of HCl using the standardized solution of NaOH. Reaction 2: NaOH (aq ) + HCl (aq ) NaCl (aq ) + H 2 O (l) Reaction 2 is also a 1:1 reaction ratio between NaOH and HCl. Moles NaOH used equals moles of HCl neutralized. That is, Moles NaOH = Moles HCl ( M x V ) NaOH = ( M x V ) HCl M HCl = ( M x V ) NaOH Used ( Vol HCl Titrated ) PROCEDURES: PART I: Calibration of NaOH Solution PREPARATION OF KHPh SOLUTION: Calculate the mass (grams) of KHPh needed to prepare 100-mL of 0.10M KHPh in deionized water. Assume the purity factor to be 100%. Ask instructor for formula weight. Measure the calculated amount of KHPh into a 150-mL beaker and add deionized water up to but not to exceed 100-mL. Insert a magnetic stirring bar into the mix and stir until all of the KHPh is dissolved. Transfer to a polyethylene storage bottle, label and store for calibration of NaOH preparation. PREPARATION OF NaOH SOLUTION: Obtain 20-mL of stock 1-Molar NaOH (aq) solution from instructor and add to a 250-mL beaker. Using the Dilution Equation, calculate the volume of solution when the given NaOH stock solution is diluted to about 0.10-M. Add deionized water up to but not to exceed the calculated volume. Transfer preparation into a polyethylene storage bottle, label and store for calibration. TITRATION OF NaOH USING KHPh: Set-up a titration apparatus using a 50-mL burette and a 100-mL or 150-ml beaker. Transfer ( by pipette ) 20-mL of the NaOH prep, into the 100-mL beaker. Add KHPh into the burette until reaching the 50-mL mark. Into an empty beaker or flask, open the burette stop-cock to purge the tip of the burette. Add additional KHPh solution to the burette to reestablish the 50-mL volume of titrant. Add 5 or 6 drops of Bromothymol Blue (BTB) indicator into the base solution. The solution will turn blue. Mix the NaOH/BTB solution to obtain a homogeneous solution. Position the KHPh solution ( in the burette ) over the NaOH solution ( in the beaker ). Record the initial volume of the KHPh solution before starting additions into the NaOH solution. Open the burette stopcock and add the KHPh until the NaOH solution turns to a pale green. Record the volume of KHPh used. Using Table-1, calculate the the NaOH solution using the following relationship: Molarity ( KHPh ) x Volume (ml) KHPh used = Molarity ( NaOH ) x Volume (ml) NaOH titrated Record Calculated & Experimental NaOH Molarity in Table 1 of data tables. 2

3 Part II: Titration of HCl with Standardized NaOH Base Solution Set-up a titration apparatus using a 50-mL burette and a 100-mL or 150-mL beaker. Transfer ( by pipette ) 20-mL of the HCl solution (supplies by instructor ) into a 100-mL beaker. Add the standardized NaOH solution into the burette until reaching the 50-mL mark. Into an empty beaker or flask, open the burette stop-cock just long enough to purge the tip of the burette. Add additional NaOH solution to the burette to reestablish the 50-mL volume of titrant. Add 5 or 6 drops of Bromothymol Blue (BTB) indicator into the acid solution. The solution will turn yellow. Mix the HCl/BTB solution to obtain a homogeneous solution. Position the NaOH solution ( in the burette ) over the HCl solution ( in the beaker ). Record the initial volume of the NaOH solution before starting addition of NaOH into the HCl solution. Open the burette stop-cock and add the NaOH until the HCl solution turns to a pale green. Record the volume of NaOH used. Using Table-2 calculate the the HCl solution using the following relationship: Molarity ( NaOH ) x Volume (ml) NaOH used = Molarity ( HCl ) x Volume (ml) HCl titrated Record calculated and experimental HCl Molarity in Table-2 for your group. 3

4 DATA TABLES: Transfer Group Results to the Following Data Tables Table-1: NaOH STANDARDIZATION GROUP Molarity KHPh (moles/liter) Volume of KHPh Sol n Used (ml) Volume of Used (ml) Experimental Theoretical Gp-1 Gp-2 Gp-3 Gp-4 Gp-5 Gp-6 CLASS AVERAGE STDEV Conf Intvl RR (Low) RR (High) 4

5 DATA TABLES: Transfer Group Results to the Following Data Tables Table-2: HCl Standardization GROUP Molarity NaOH (moles/liter) Volume of Used (ml) Volume of HCl Sol n Used (ml) Experimental HCl Sol n Theoretical HCl Sol n Gp-1 Gp-2 Gp-3 Gp-4 Gp-5 Gp-6 CLASS AVERAGE STDEV Conf Intvl RR (Low) RR (High) 5

6 CONCLUSIONS: The Class average concentration of NaOH(aq) was found to be ± M. The Class average concentration of HCl(aq) was found to be ±.M. DISCUSSION: Assuming a sufficient amount of HCl(aq) solution remained for further testing, explain how you would set-up a titration process and use it to determine the concentration of sodium sulfate in the following reaction. Assume BTB to be your end-point indicator. Na 2 SO 3 (aq) + 2HCl(aq) 2 NaCl(aq) + SO 2 (g) + H 2 O(l) NOTE: Neither Na 2 SO 3 (aq) or it s products will show color in the presence of the BTB Indicator. Only the HCl solution will show color. ( yellow if the solution is acidic ). 6