Determining the limiting reactant and the percent yield i a precipitation reaction free essay sample

One example of a double replacement (metathesis) reaction is the mixing of two solutions resulting in the formation of a precipitate. In solution chemistry, the term precipitate is used to describe a solid that forms when a positive ion (cation) and a negative ion (anion) are strongly attracted to one another. In this experiment, a precipitation reaction will be studied. Stoichiometry will then be used to investigate the amounts of reactants and products that are involved. The word stoichiometry is derived from two Greek words: stoicheion (meaning â€Å"element†) and metron (meaning â€Å"measure†). Stoichiometry is an important field of chemistry that uses calculations to determine the quantities (masses, volumes) of reactants and products involved in chemical reactions. It is a very mathematical part of chemistry. In this experiment, you will react a known amount of sodium carbonate solution with a known amount of calcium chloride solution. The skeletal (unbalanced) equation for the resulting double replacement reaction is: Na2CO3(aq) + CaCl2(aq) ? NaCl(aq) + CaCO3(s) Note that three of the chemicals have their states or phases designated as (aq) and one is designated as (s). The (aq) represents the term aqueous which means that the substance is soluble and dissolved in water. The (s) means that the substance is a solid (in this case, it is a precipitate). Precipitate formation is easily observed as the mixed solutions turn cloudy and, if desired, the precipitate can be easily separated from the solution by filtering. Since your precipitate will be separated and weighed, you will need to dry the precipitate in a drying oven before measuring its mass. Stoichiometry will then be used to determine the amount of precipitate that should be formed in the reaction. It is often difficult as well as impractical to combine just the right amount of each reactant that is required for a particular reactant that is required for a particular reaction to occur. Given this fact, this experiment is designed so that only one of the reactants will be completely used up. This is called the limiting reactant because it limits the amount of products formed. Since the other reactant will have a quantity remaining, it is called the excess reactant. One of your tasks will be to determine which of your reactants is limiting and which is in excess. The two chemical reactants in this experiment have common uses in our lives. In one solid form, sodium carbonate is known as â€Å"washing soda† and is used to enhance the effectiveness of laundry soap. Calcium chloride solid can act as a desiccant (drying agent) and is used by recreational vehicle owners to remove moisture from the air in the vehicle during winter storage. OBJECTIVES 1. to observe the reaction between solutions of sodium carbonate and calcium chloride. 2. to determine which of the reactants is the limiting reactant and which is the excess reactant. To determine the theoretical mass of precipitate that should form. 4. to compare the actual mass with the theoretical mass of precipitate and calculate the percent yield. MATERIALS Equipment Chemicals Balance 0. 70 M sodiuim carbonate solution, Na2CO3 2 graduated cylinders – 25 mL 0. 50 M calcium chloride solution, CaCl2 3 beakers – 250 mL dH2O wash bottle Filtering apparatus (ring with stand, funnel) Filter paper Drying oven Lab apron Goggles PROCEDURE 1. Put on your lab apron and safety goggles. 2. Obtain two clean, dry 25 mL graduated cylinders and two clean, dry 250 mL beakers. In one of the graduated cylinders measure 25 mL of the Na2CO3 solution. In the other graduated cylinder measure 25 mL of the CaCl2 solution. Record these volumes in your lab notebook. 4. Pour the contents of both graduated cylinders into one of the 250 mL beakers and observe the results. Record these qualitative observations in your notebook. Allow the contents of the beaker to sit undisturbed for 5 min. to see what happens to the suspended solid particles. Meanwhile, proceed to step 5. 5. Obtain a piece of filter paper and put your name on it using a pencil. Weigh and record the mass of the filter paper, then use it to set up a filtering apparatus as shown in Figure 6D-1. 6. Use the wash bottle to lightly wet the filter paper in the funnel to keep the filter paper in place. Swirl the beaker and its contents to suspend the precipitate in the solution, then pour it carefully and slowly into the filter funnel. It takes time to complete the filtering process so plan to do it in stages. Use the wash bottle to rinse the remaining precipitate from the beaker. 7. Use the wash bottle one last time to rinse the precipitate in the filter paper. This will remove any residual NaCl(aq) that remains with the precipitate. 8. After the filtering is complete, remove the wet filter paper containing CaCO3 precipitate and suspend it carefully in the third 250 mL beaker. Put your labeled beaker with filter paper + precipitate in the drying oven for 30-45 min. 9. Remove the beaker with filter paper + precipitate from the drying oven and allow to cool for a few minutes. 10. Weigh and record the mass of the dry filter paper containing the CaCO3 precipitate. 11. Clean up all your apparatus. 12. Wash your hands thoroughly with soap and water before leaving the laboratory. Reagent Disposal Rinse all solutions down the sink with copious amounts of water. Any solids should go into the designated containers. Post Lab Considerations The double replacement reaction in this experiment formed two chemicals which are commonly known to you. The NaCl(aq) is salt water and the CaCO3(s) is a component of some classroom chalks. Using the data collected, you will be able to calculate the moles of each of the chemicals that are added together to react. Then using the principles of stoichiometry you will be able to determine which chemical is the limiting reactant and thereby predict how much precipitate should form. This stoichiometric determination will then be compared to the actual mass of CaCO3(s) formed. Chemists are often concerned with optimal yields in manufacturing a certain chemical. One way of measuring this is to calculate the percent yield of that particular chemical by using this formula: Percent yield = actual mass produced (grams)____ x 100% theoretical mass produced (grams) Include the following information in the results section and data tables: Qualitative observations Quantitative observations Volume of 0. 70 M Na2CO3 solution Volume of 0. 50 M CaCl2 solution Mass of filter paper + dry solid CaCO3 Mass of filter paper ANALYSIS OF RESULTS 1. Copy the chemical equation for the reaction from the introduction and balance it. 2. From your Part I results, calculate the moles of Na2CO3 that were added to the beaker. 3. From your Part I results, calculate the moles of CaCl2 that were added to the beaker. Use your answers from Analysis 1-3 above and stoichiometric principles to determine which chemical is the limiting reactant. 5. Use stoichiometric calculations to determine the theoretical mass of CaCO3 precipitate that should have formed. 6. From your Part II results, calculate the actual mass of CaCO3(s) precipitate that formed. 7. Calculate the percent yield of CaCO3(s). FOLLOW-UP QUESTIONS 1. If you were to evaporate the filtered solution to dryness, would you be left with only solid NaCl? Explain. 2. What theoretical volume of the Na2CO3 solution used in this experiment would result in no excess reactant? 3. A precipitation reaction occurs when 50. 0 mL of 0. 50 M BaCl2(aq) is mixed with 75. 0 mL of 0. 75 M Na2CO3(aq). The only precipitate is the BaCO3(s) formed. a) Write the balanced equation that describes this reaction. b) Which chemical is the limiting reactant? c) Predict the theoretical mass of BaCO3(s) that should form. d) This experiment was conducted and the percent yield was found to be 82%. What was the actual mass of BaCO3(s) that formed?