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Titration of a Newtown Creek Environmental Water Sample to Determine the Amount of Chloride Ions Competency-Inquiry/Problem Solving, Global Learning and writing communication Learning objectives: To analyze and calculate the amount of chloride ions in an environmental sample by precipitation titration To determine the equivalence point by visual observation To become familiar with using pipettes and burettes for quantitative measurements Introduction: The chloride ion is an inorganic anion that is naturally occurring in environmental waters. Seawater contains 1.94% chloride and can be commonly found in varying forms such as potassium chloride (KCl), sodium chloride (NaCl) and magnesium chloride (MgCl2) with these salts all being water soluble (1). The Environmental Protection Agency has set a standard for the chloride concentration in drinking water to be 250 mg/L (2). Although chloride is non-toxic to humans, high levels of chloride can affect plants and corrode away infrastructure such as roads and pipes. Possible sources that increase the chloride concentration include industrial processes, sewage, road salts, and fertilizers. Titration is a technique whereby adding a measured quantity of known concentration (titrant) is reacted with an unknown substance (analyte). The goal of this process is to determine the concentration of the unknown using the stoichiometry of the chemical reaction between analyte and titrant. In this experiment, a precipitation titration will be conducted where the reaction of the analyte and titrant will give ionic compounds with limited solubility. In today’s experiment, the amount of chloride in water from Newtown Creek will be determined by titrating the chloride analyte with silver nitrate solution and using potassium chromate as an indicator. The titrant is slowly added to the environmental water sample whereby the silver ions react with chloride ions in a 1:1 ratio to form a silver chloride precipitate: +- Ag(aq) +Cl(aq)→AgCl(s) The end point of the titration occurs when all of the chloride ions has precipitated out of solution (as solid AgCl) and the free silver ions begin to react with the chromate ions (CrO4 2-) to give a brown-red precipitate. Knowing the volume/concentration of the titrant used to attain the equivalence point and stoichiometry of the chemical reaction, the amount of chloride ions can be calculated. Ideally, the titration should be carried out at a pH between 7 and 9 as a low pH environment will cause the chromate anion to protonate and lower the chromate concentration where an endpoint cannot be detected. Conversely, a high pH will lead to the formation of a brown silver hydroxide compound which again inhibits the observation of the endpoint. Pre-Laboratory Exercise 1. Define the terms equivalence point, endpoint, indicator, analyte and titrant? 2. What are the chloride ion limits for drinking water secondary standards set-forth by the Environmental Protection Agency? Experimental Procedure 1. Using a 10 mL pipette, place 10 mL of filtered Newtown Creek water sample in an Erlenmeyer flask and measure the pH. The pH value should be in the range of 7-9. If your environmental sample has a pH value outside this range, adjust the acidity with nitric acid or sodium hydroxide. 2. Add 3-4 drops of potassium chromate indicator to the Erlenmeyer flask. A light yellow color solution should appear. 3. Set up a 50 mL burette: a) Obtain a burette, burette tip, burette clamp, accompanying ring stand and assemble. A magnetic stirrer device will be used as a base for the apparatus set-up. b) Clean the burette and burette tip with distilled water then discard. Afterwards, pre-rinse the burette with a few mL portion of standard AgNO3 solution to remove any remaining water droplets. c) Place a funnel at the top of the burette and fill the burette with standard AgNO3 solution to the 0.00 mL mark. Allow a few mL to run (to remove air bubbles) and refill up to the 0.00 mL mark. (NOTE: You should only need to fill up the burette once) 4. Place a stir bar in the Erlenmeyer flask, turn on the magnetic stirrer so that a gentle swirl of solution is reached. 5. Record the concentration of AgNO3 6. Titrate with AgNO3 solution. A white precipitate will initially form. The endpoint will be identified by the first appearance of a permanent brownish-red precipitate (with the accompanying white precipitate). Record the volume of silver nitrate used. (Note: The first trial should be used as a “rough” titration) 7. Repeat the titration until the volume of titrant used for different trials are in agreement. 8. Calculate the concentration chloride ion concentration (Molarity, mg/L and parts per million) Data Sheet: Titration of a Newtown Creek Environmental Water Sample to Determine the Amount of Chloride Ions Data: pH of water sample:________________ Molarity of AgNO3 standard: __________ Trial 1 Trial 2 Trial 3 Volume of water sample added (mL) Initial volume of AgNO3 (mL) Final volume of AgNO3 Volume of AgNO3 used (mL) Average volume of AgNO3 used (mL) Moles of AgNO3 used: Moles of Cl- ions added to Erlenmeyer flask: Volume of analyte sample added (L): Concentration of Chloride Ion (M): Concentration of Chloride Ion (mg/L): Concentration of Chloride Ion (ppm): _________________________ _________________________ _________________________ _________________________ _________________________ _________________________ Questions to Address in Lab Report: Show all calculations to find moles and concentrations of chloride? Write the relevant chemical equation(s) associated with silver ions? What is the purpose of chromate ions in chloride determination? What are some possible sources of error in this lab? (Note- this is not a pure water sample) What are some of the concerns with high levels of chloride ions in environmental waters? Does the measured chloride concentration from Newtown Creek exceed the Secondary Standard limits set-forth by the Environmental Protection Agency? In theory, if the chloride concentration in the titrated water sample is too high, what method(s) could be employed to reduce the amount of chloride ions? What are some factors (natural or artificial) that could influence the amount of chloride in Newtown Creek? Thermochemistry: Heat of Neutralization and Hess’s Law Students will be able to: Determine the heat of neutralization of three separate reactions and manipulate the chemical equations to find the heat of neutralization of a fourth reaction Use their experimental ∆H values and compare it to literature values Identify, handle and react acids/bases in a constructed calorimeter apparatus. Measure temperature changes over extended reaction times, generate mixing curves and extrapolate temperature changes to calculate heats of neutralization Communicate their change of enthalpy findings through the formal lab report format of: Introduction, Materials, Procedure, Results, Sample Calculations, Discussion and References Background: A chemical reaction is accompanied by an energy change described as a change in heat content: energy is absorbed (endothermic reaction) or released (exothermic reaction). In general, the breaking of bonds in reactants requires the consumption of energy and the creation of new bonds in products involves the release of energy. The potential energy that is stored in chemical bonds can be thought of as the heat content of a system, enthalpy and when these chemicals react, a change in energy (absorb or release of energy) will result in a change in enthalpy, ∆H. The overall change in energy will depend on the unique properties of the reactants and products. Calorimetry is the study of heat transferred in a chemical reaction. The amount of heat absorbed or released during this thermochemical process is measured by a change in temperature. The device used to measure the change is called a calorimeter (two nested styrofoam cups), where ideally, the calorimeter would not absorb any heat from its surroundings and at the same time not allow any heat from the reaction to escape. An equation for calculating the heat associated with a temperature change is: (1) q = msΔT where q = heat , m = mass, s = specific heat (J g-1 oC-1) , ΔT = Temperature change The specific heat of a substance, s, is the amount of heat required to raise the temperature of one gram of a substance by one degree Celsius (an intensive property). Enthalpy is a property of a substance that can be applied to determine the heat absorbed or released in a chemical reaction. The relationship between enthalpy change and heat is: (2) ∆H = qp where ∆H is the enthalpy change and q is the heat The “p” in equation 2 denotes that the reaction occurs at constant pressure. This is convenient, as a great deal of reactions are open to a constant atmospheric pressure. Hess’s Law : Unlike mass or temperature, there is no instrument that can measure heat or enthalpy, H. However , an enthalpy change ∆H can be calculated from equation 1. In addition, if a reaction is carried out in a series of steps, ΔH for the overall process is equal to the sum of enthalpy change for each individual step. Let’s look at an example- You would like to know the enthalpy change to transform graphite into diamonds. This is an extremely difficult task (aside- need a high activation energy) in the lab but if you know the enthalpy changes when the different forms of carbon reacts with oxygen to produce CO2, the enthalpy change of graphite to diamond can be calculated: (1) (2) (3) 2 C (s,graphite) + 2O2 (g) → 2 CO2 (g) ΔH1o= -787 kJ C (s,diamond) + O2 (g) → CO2 (g) ΔH2o= -396.5 kJ ___________________________________________________________ C (s,graphite) → C (s, diamond) ΔHoverallo= ?? kJ We simply cannot add equations (1) and (2) to get the ΔHoverallo equation (3) because there would be three oxygen molecules in the reactants and three carbon dioxide molecules in the products and our overall desired reaction (3) does not contain any O2 and/or CO2 molecules. Two possible operations to manipulate the chemical equations so that our overall reaction (3) is attained are to a) multiply/divide the chemical equation by a coefficient and/or b) reverse a chemical equation. When an operation (a or b) is done, the corresponding ΔHo must undergo the same operation. In our case, we can flip reaction (2) (3) appears in the products and we can divide equation (1) by a factor of two. C (s,graphite) + O2 (g) → CO2 (g) ΔH1o= -393.5 kJ (Equation (1) divided by 2) CO2 (g) → O2 (g) + C (s,diamond) ΔH2o= 396.5 kJ ( Equation (2) flipped) ___________________________________________________________ C (s,graphite) → C (s, diamond) ΔHoverallo= 3 kJ (2) so that C (s,diamond) (1) Adding ΔH1o+ ΔH2o= ΔHoverallo = 3 kJ By altering the chemical equations, multiplying the coefficients ( and the corresponding ΔH1ovalue) in equation (1) by 0.5 and by reversing equation (2) (and changing the sign of ΔH2o ) into an endothermic reaction, the oxygen and carbon dioxide molecules cancel when (1) and (2) are added. Prelab questions: 1. You would like to determine how many calories are in 10 grams of chicken that is served in the LaGCC cafeteria. Using your understanding of a bomb calorimeter (Figure 5.18 in your textbook) a) Devise a scheme to find how many calories are in the meat and b) Assuming the specific heat of chicken is 3.68 J/g0C and the change in temperature was 1.900C, how many calories are in 10 grams of chicken? 2. For the following hypothetical reactions: 2A + 3B → C + D E+A →C A +0.5D→ B _________________________________________________ Overall reaction: 3A + B → E ∆H = ? Calculate ∆H for the overall reaction Your task: In this lab, you will measure the enthalpy change that occurs in three separate exothermic acid/base reactions involving a) NaOH and HCl b) NaOH and CH3COOH c) NH3 and HCl and calculate the respective enthalpy change for each reaction. For these experiments, you will assume the specific heat of the each reactions is 4.18 J/g oC and the density of the solutions is 1.0 g/cm3 . Materials Equipment Consumables ∆H = -46 KJ ∆H=50KJ ∆H=23KJ One thermometer Two styrofoam cups (calorimeter) One cup lid 50 mL graduated cylinder 250 mL beakers Stirring Rod Timer 1.0 M hydrochloric acid 1.0 M ammonia 1.0 M acetic acid 1.0 M sodium hydroxide Distilled water Procedure development 1. Write the objective of your experiment 2. State the three acid/base exothermic chemical reactions in your experiment 3. State equation(s) you will use to find the change in enthalpy IN LAB, BEFORE BEGINNING YOUR EXPERIMENT, YOU WILL DEVELOP AND HANDWRITE A PROTOCOL 4. Compose a step by step procedure to determine ΔH for the reaction of HCl and NaOH. In terms of details, your procedure should be clear to the point where a fellow colleague could read your methodology and repeat the experiment without having to consult with you. Ensure that all of the equipment and consumables are included in the procedure. Numerate your steps. Suggestions on writing procedures: -Draw and label your apparatus set-up. -Indicate how much of each reactant you will use? Should they be the same/different amounts? Is there a limiting reagent? -Decide how many trials you will conduct? - The maximum amount of solution in the calorimeter should not exceed 50 mL at any time. -Finish your procedure with, “Repeat steps __ to __ for the reactions between NaOH and HAc “ “Repeat steps __ to ___for the reactions between NH3 and HCl” 5. Create labeled data tables to record your data 6. Write equation(s) and how will you use them to find the moles of product 7. State how will each variable be measured/calculated? With the collected data, using the enthalpy of neutralization values and in conjunction with Hess’s Law, determine ΔH (Joules per mole) for the reaction between ammonia (NH3) and acetic acid (CH3COOH) and compare it to the literature value Notes: 1. Safety measures: Keep in mind all safety rules from the previous experiments and in addition a) Wear safety eye protection at all times b) Be aware of the following details regarding the chemicals: Chemical Hydrochloric Acid Ammonia Acetic acid Sodium hydroxide c) Dispose of all chemicals in the appropriate waste containers DO NOT BEGIN EXPERIMENTING UNTIL YOUR INSTRUCTOR HAS SIGNED YOUR PROTOCOL Results and Calculations: Write the three experimental exothermic reactions and write the balanced chemical equation for the reaction of ammonia with acetic acid (This is the overall ΔH enthalpy change you are determining) Plot three separate “acid/base” mixing graphs (Use Excel or other graphic programs) Extrapolate temperatures, ∆T, from your graph to calculate q for your three neutralizations What is the heat of neutralization ∆H per mole Use Hess’s law to calculate ∆H for the reaction of ammonia and acetic acid Compare your ∆H value with the literature value. What is the percent error? Post lab questions/discussion topics: Was your objective met? How are calorimetry and Hess’s Law used to find the heat of neutralization of ammonia and acetic acid? Why are the heat of neutralizations values negative? Explain why it is it important to know if energy is being released or absorbed during a chemical reaction? What are some sources of error in your set-up and how can they be reduced to achieve a greater accuracy?

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The goal of this experiment is to determine the amount of chloride ions (Cl-) from the Newton Creek superfund site, which is located near LaGuardia community college.

The Newton Creek is tributary of the East River, located between Brooklyn and Queens. The water from Newtown Creek is polluted and probably has a higher concentration of chlorides than normal. This whole water surface is meant to be a protected area for fish survival only, but it does not meet these parameters currently. The chloride ion limit for drinking water secondary standards set-forth by the EPA is 250 mg/L. The water sample from Newton Creek was taken and analyzed in the

The goal of this experiment is to determine an enthalpy change (∆H) of three different reactions using calorimeter and compare it with the literature value. Calorimetry is based on two principles: the Law of Conservation of Energy and that the calorimeter is an isolated system. The determining of heat flow is possible based on the fact that energy is defined as the ability to do work or to transfer heat.
Calorimetry is a measuring method for determining the amount of heat. Calorimetry measures the amount of heat energy that binds to or is released during a physical or chemical process. This method is based on observing the effects that heat produces: raising the temperature of matter by bringing in heat, changing the physical (aggregate) state of matter, and converting chemical, electrical or mechanical energy into heat. Calorimetric determinations are applied in many branches of science and technology; they are important for understanding and interpreting many physical and chemical processes and also serve to determine the heat capacity of matter, the energy value of fuel, food, and more.
Hess's law in thermodynamics refers to thermal changes in chemical reactions. As enthalpy is a function of...
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