Ex 4. Patterns of Reactivity

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San Francisco State University *

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Feb 20, 2024

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Exercise 4, Page 1 of 6 Exercise 4 Patterns of Reactivity Chemical Classification and Potential Hazards of Chemicals Used in Experiment CHEM 180 Ex 4 Patterns of Reactivity of Ions in Solution Chemical Classification Possibility of: NFPA Codes Poison A Flammable Gas Flammable Liquid Combustible Liquid Flammable Solid Reacts with Water Oxidizer Organic Peroxide Poison B Corrosive Acid Corrosive Base Irritating or Harmful Misc. Hazard No Hazard Fire Sudden Release of Pressure Reactive Immediate (Acute) Health Hazard Delayed (Chronic) Health Hazard Fire Health Reactivity Special Precautions Hydrochloric Acid (HCl),1M X X X 0 3 2 COR Sulfuric Acid (H 2 SO 4 ),1M X X X 0 2 1 COR Sodium Carbonate (Na 2 CO 3 ),1M 0 0 0 Strontium nitrate (Sr(NO 3 ) 2 ), 0.1M X 0 1 1 Zinc nitrate (Zn(NO 3 ) 2 ), 0.1M X X 0 2 1 Requirements for this exercise: Mode: Inquiry, groups of 2 and individual work Grading: Notes and Lab Report Safety: Goggles, closed-toe shoes, long pants/skirt and lab coat required, Latex or nitrile gloves recommended; wash hands after lab. Research Question Can the pattern of reactivity of an ion be used to identity an unknown solution? An important part of being a scientist is developing good observation skills and learning how to use observations in combination with deductive reasoning to draw conclusions about events in the physical world. For chemists this often means devising a contrived scheme, called an experiment, which can be used in the laboratory to investigate a particular question, referred to as a hypothesis. If the experiment involves the mixing together of two or more substances, then careful observation can determine if any new substances formed, how long it took for the new substances to form, or how much energy was transferred as heat. Experiments are done for a variety of reasons, from the discovery of new knowledge to the characterization of a sample. In this lab, you will explore the question about whether or not you can use observations of the reactivity of ions in solution to identify the type of cation present in an unknown.

Exercise 4, Page 2 of 6 Background Information Solutions: A solution is a homogenous mixture of substances. The substance present in the largest quantity is called the solvent, and the substance(s) present in the smaller quantity if called the solute. By far the most common solvent on the Earth is water, H 2 O. Most beverages you drink, the cells in your body and the ocean are all examples of solutions with water as the solvent. A solution can have a single solute, like vinegar (contains acetic acid and water), or a solution can have multiple solutes, like the ocean (contains dozens of ions along with small organic molecules dissolved in water). Solutions don’t have to be liquid. Steel is a solid solution of carbon dissolved in iron. When water is a solvent, the solution is often referred to as being aqueous. The amount of solute in a solution is expressed as a concentration. We will learn more about concentrations later, but a very convenient unit of concentration is molar concentration, abbreviated with M. This stands for moles of solute per liter of solution (M = mol/L). A mole is a number of particles, much like a dozen is a number of items. As you know, a dozen always represents 12 of something. A mole always represents 6.022 x 10 23 of something. A solution that is 1M HCl has 1 mole of HCl molecules in a 1L volume of solution. Ionic Compounds: Ionic compounds consist of a positively charged cation and a negatively charged anion. An example is sodium chloride (NaCl), table salt, which consists of an equal number of Na + ions (sodium) and Cl − ions (chloride). Water is generally a very good solvent for ionic compounds, dissolving and releasing them from each other, so that there are ions dispersed in the solution. Such dissolution is depicted in the following chemical equation: NaCl(s) → Na + (aq) + Cl − (aq) The chemical equation for the dissolution of a water-soluble ionic compound (commonly called a salt). The symbol (s) means solid and (aq) means aqueous; (aq) is used specifically to communicate that a substance is dissolved in water. This equation is read to mean that solid sodium chloride dissolves in water to give aqueous sodium ions and aqueous chloride ions. The bonds between the sodium and chloride ions in the solid phase are broken by water molecules that liberate the ions from the solid and pull them into the solution. Dissolution represents a chemical change. An experimentalist would observe a white solid disappearing into a clear liquid. Confirmation that ions are present in the solution comes from conductivity measurements or from selective precipitation of ions using solubility rules. Not all ionic compounds are soluble in water – these ionic compounds are classified as insoluble. The solubility of an ionic compound depends on the strength of the ionic bond in the solid – if that bond is stronger than the ability of water molecules to separate the ions, the solid remains a solid in water and very few, if any, ions are released to the solution. When dissolved ions encounter each other in solution, they will interact with each other based on electrostatic forces of attraction and repulsion – ions of like charge will repel and move away from each other in solution, while ions of opposite charge will be attracted to each other. If there are a variety of cations and anions present in solution, chemical reactions can occur between ions of opposite charge, resulting in the formation of a solid ionic compound in a precipitation reaction. For example, when Ca 2+ (aq) ions encounter carbonate, CO 3 2- (aq) ions in water, the ionic compound calcium carbonate CaCO 3 will form under neutral pH conditions – this precipitation reaction is responsible for the formation of stalagmites, stalactites and the other awesome features of caves. Ca 2+ (aq) + CO 3 2− (aq) → CaCO 3 (s) The chemical equation for the precipitation of an insoluble ionic compound from ions in solution.

Exercise 4, Page 3 of 6 Safety goggles must be worn by everyone as soon as the first group starts this part of the lab. Discover the Pattern of Reactivity You will be observing whether or not a cation and an anion form an insoluble ionic compound, a precipitate (ppt) . You will be observing if there is a unique pattern of reactivity of each cation with the anions, and predict if you can use that pattern to identify the presence of that cation in a solution. Your goal is to systematically investigate how each cation interacts with each anion. There are several design parameters to keep in mind including experimental controls: minimization of error, minimization of contamination, and reproducibility of results . Gather stock solutions and supplies needed • Clean and label test tubes for each cation solution, control, and anion solutions. It is recommended that you rinse test tubes with distilled water before using them. Tap excess water out of the test tubes, but complete drying of the test tubes is not necessary. • Get a large beaker for the bench and label it as WASTE. Collect your waste here as you proceed through the experiment. • Get the other equipment or supplies that you have decided to use for your investigation. • Label things as needed using the tape provided in lab. Before you begin, read these experimental tips : • A dark background, such as the bench surface, might be best to observe any reaction that occurs. • Record your observations in your lab notebook; be very detailed! Do not simply record that the solution became cloudy; record instead, for example, that a white, cloudy, gelatinous substance was seen in the solution after the addition of 3 drops of anion solution. • If your procedure differs from your plan (for example if you decide to add three more drops of HCl or three fewer drops of HCl), make a note of it in your lab notebook. Copy the data table into your laboratory notebook. Be sure the table is large for you to write detailed observations for each cation and anion reaction. Data Table of Cation and Anion Reactions Cation being tested Observation/result of Cl − test Observation/result of SO 2− test 4 Observation/result of CO 2− test 3 Sr 2+ Zn 2+ (control)* (unknown number)** * Replace these words with the identity of your control and your unknown number. Discuss with your lab partner and your lab instructor the identity of the control. ** Leave the unknown blank until you receive the coded solution from your lab instructor for the “Identifying an Unknown Solution” section.

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Exercise 4, Page 4 of 6 Test each cation solution and the control with each anion • Each person in the partnership needs to be actively involved in the investigation. It is not appropriate for one person to carry out the experiment while the other person watches. There are two cation solutions and one control that need to be investigated. • Divide the solutions between the two people in the team, and share the data. If you don’t have a partner, then you will do the experiment on your own. • Fill each labeled test tube about halfway with the appropriate cation reagent solution. You can always get more solution if you need it. • Add the anion solution 1 drop at a time, and swirl or agitate the solution between each drop, observing what happens with each drop. • Record results in your data table. • If no reaction is observed after adding 10 drops of anion solution, then record in your lab notebook “no reaction after 10 drops” as your experimental result. The cation solutions were prepared by dissolving solid nitrate salts (NO 3 − ) of the cations in distilled water: Ionic compound Cation under investigation Sr(NO 3 ) 2 (s) → Sr 2+ (aq) + 2 NO 3 − (aq) strontium (II), Sr 2+ (aq) Zn(NO 3 ) 2 (s) → Zn 2+ (aq) + 2 NO 3 − (aq) zinc (II), Zn 2+ (aq) The anion samples are hydrochloric acid (HCl), sulfuric acid (H 2 SO 4 ) and sodium carbonate (Na 2 CO 3 ). These substances dissociate in water to give cations (H + or Na + for these examples) and anions: Ionic compound Anion provided for investigation HCl (aq) → H + (aq) + Cl − (aq) chloride, Cl − (aq) H 2 SO 4 (aq) → 2 H + (aq) + SO 4 2 − (aq) sulfate, SO 4 2 − (aq) Na 2 CO 3 (s) → 2 Na + (aq) + CO 3 2 − (aq) carbonate, CO 3 2 − (aq) Data share and reproducibility check Share any data you collected with your partner and add their data to your lab notebook. Be sure to indicate in your lab notebook which data you collected and which your partner collected. If you need to repeat any reactions to confirm results, do that now, and make a note in your lab notebook about what needed to be repeated, why it needed to be repeated and the results of the repeated experiment. Review your results and discuss with your lab partner if you can make the following predictions: • What is the pattern of reactivity of Sr 2+ with Cl − , SO 2 − and CO 2 − ? Is the pattern aq, ppt, aq, or something else? • How will a sample containing Zn 2+ react in the presence of Cl − , SO 2 − and CO 2 − ? • How will a sample containing H 2 O react in the presence of Cl − , SO 2 − and CO 2 − ? Identifying an Unknown Solution Each student obtains a coded solution from the lab instructor. Write the code of the unknown in your data table. The unknown may contain one of the cations or it may contain distilled water .

Exercise 4, Page 5 of 6 Each student will do an analysis of their unknown independently. You can discuss your results with your partner, but everyone does their own investigation. Plan how you are going to investigate the reactivity pattern of your unknown with Cl − , SO 4 2− and CO 3 2− . You will have no more unknown than the amount provided to you in the test tube. Dispense half of your unknown into a fresh test tube to avoid contamination of your unknown; you may have to repeat the test. Carry out the investigation. Record your procedure and your observations for each step in your lab notebook. Interpreting Evidence What is your conclusion about the identity of the cation in your unknown solution? Confirm the identity of your unknown with your lab instructor. If your interpretation is incorrect, your lab instructor may ask you to repeat the tests . Your lab instructor will initial your lab notebook to certify your result and conclusion. FINAL CLEAN UP Discard any remaining stock solutions of cations and anions in the waste beaker. Use the squeeze bottle of distilled water to rinse the test tubes with water into the waste beaker. Return any remaining unknown to your lab instructor. Remove all tape labels from all test tubes, pipettes or 24-well plates. The labels can be discarded in the regular trash can. Use the squeeze bottle of distilled water to thoroughly rinse the contents of the 24-well plate or the test tubes you used for the reactions into the waste beaker. The 24-well plates can be returned to the original container wet. Pour the contents of the waste beaker into the waste bottle in the hood – use the funnel provided to minimize chemical spills, and put the cap back on the bottle when done. If the waste bottle is full, check for an empty waste bottle behind the active one, and switch the empty one to the front. If both waste bottles are full, bring the beaker back to your bench and tell the instructor a waste bottle is needed. Pipettes should be discarded in the yellow solid hazardous waste container under the hood. Any test tubes that need to be discarded should be rinsed and discarded in the broken glass waste container. Return all stock bottles to the containers on the bench or in the hood, wherever they were when you started the lab. Wipe down your workspace on the bench with a damp paper towel and put that paper towel in the yellow solid hazardous waste bin. Wash your hands before you leave the lab. Nitrates can cause allergic reactions, so you do not want to transfer that to your cell phone, computer key pad or to anything you are eating or drinking. Turn in a copy of your lab notes before you leave for the day.

Exercise 4, Page 6 of 6 Laboratory report – Check lab schedule for due date Each student writes their own lab report. Your report must include the title of the experiment, the date, your name and the name of your lab partner. Your lab report should be written starting on a new page in your lab notebook. Include these components in your lab report: Research Question – state the research question for this exercise. Materials and Methods – what did you do to carry out the investigation? When writing this part of the lab, do not include every detail (such as “washed 12 test tubes and put them in a rack”), but you do need to communicate the overall experimental plan including: • Solutions used, including amounts, concentrations, and identity of the compound within each solution. Be sure to include the experimental control . • A brief procedure for testing your unknown The information for materials and methods can be prepared as a series of bullet points or as a clear and concise narrative. Data and Observations – What did you measure or observe during the investigation? How did you manipulate any of the data collected? Include the following in this part of the lab report: • Your observations for all reactions presented in a single, organized table. The table should have a title to communicate the purpose of the table. • The code number and identity of your unknown. Discussion – Write your answers in complete sentences. Use evidence from the experiment to explain your answers. • What is your answer to the research question of this experiment? • How did you reach a decision about the identity of your unknown? Reflection – Answer the following questions in your lab report. You do not need to write out the question, but number the answers and write your answers in complete sentences. 1. What was the identity of your control, and why was it chosen as the control? 2. Imagine that you are a supervisor of a laboratory. Your group is analyzing a sample of stream water that has high levels of an anion contaminant. You know that the sample contains chloride, sulfate or carbonate. You have a new student intern do the analysis of the sample using solutions of Sr 2+ and Zn 2+ . The intern carries out a similar investigation as the one you did in this lab. The result of the reactions between each of the cations with each of the anions is the same as your results. Here is an excerpt from the intern’s report about the test on the stream sample. 15 drops of stream water was placed in a test tube using a 1 mL plastic pipette. 5 drops of 0.10M Sr 2+ solution was added to the stream water, one drop at a time, with shaking between each drop to mix the solutions. I observed a white, cloudy precipitate form in the solution. This reaction was the same as the reaction I saw when I did a control reaction between a solution of Sr 2+ with a solution of SO 4 2- . Therefore, I conclude the anion in the stream sample is SO 4 2- . Do you agree or disagree with the intern’s conclusion about the identity of the anion in the stream? Provide some constructive criticism of the intern’s work, explaining why you either agree or disagree with the intern’s conclusion. Explain what further tests would be needed to convince you of the identity of the anion, or, explain why you are convinced that the intern is correct.

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