Qualitative Analysis: Identifying Unknown Ions In A Solution
Carlie Haeffner
Qualitative Analysis
March 20, 2015
The purpose of this lab is to identify unknown ions in a solution by using a type of chemical reaction called precipitate reactions. The key to finding which ions are present in a solution is to form a precipitate which makes the ions “come out” in a reaction (McNeil, 2013-2014). Water is used in these experiments to act as a solvent. Since water is a polar molecule, the slightly positive and negative charges will sometimes pull apart molecules and surround them based on charges. In other words, water can dissolve ionic compounds and create an aqueous (aq) solution. This lab uses no quantitative analysis, but is entirely based on qualitative analysis which is the observed result instead …show more content…
of the measured result. Looking for a precipitate is qualitative in that physical changes are what is being observed and noted. Precipitates generally have a solid, cloudy or milky substance that does not dissolve easily and occurs when a cation and anion combine in an aqueous solution, but reacts to become insoluble (Precipitation Reactions).
This lab uses two methods in testing for precipitates: spot testing and a flowchart.
Spot testing requires two ionic compounds that are dissolved into a solution to be added together. When the ions separate and reform new bonds to form a precipitate, the physical changes are recorded. Some substances are not soluble, thus will not dissolve in water which is called insoluble. The solubility rules are located on page 67 to 68 in lab manual and explain when a precipitate will occur. Unknown mixtures of compounds are hard to separate and individually identify, thus the flowchart is used. A flowchart is an outline that separates cations from anions where they can then be tested through spot testing. A real world example of qualitative analysis could be used in athletics. Many coaches will say, “Quality over quantity,” when referring to drills. Coaches are looking for concentration and complete effort in doing a drill correctly a few times rather than doing a drill many times without full effort and …show more content…
attention.
In using the solubility rules, the given Flow Chart and spot testing it was found that the unknown #4 consisted of Ag+, Ni2+ and Ba2+. When taking the initial mixture, the white solid was formed as well as a liquid. The solid went into further testing by adding DI water then heated while stirring. A solid was still present so the solution was centrifuged for one minute. Then the liquid remaining was tested for lead by spot testing with K2CrO4. There was no reaction or precipitate that formed. The liquid formed from the first step was taken and mixed with NH4OH and tested with litmus paper to ensure that the solution was basic, turning the paper blue, then centrifuged again. Only liquid was left over and tested with K2CrO4 as well as DMG. Both of these spot test showed a precipitate reaction.
The purpose of this lab was to separate two unknown compounds by using the solubility properties.
In order to make an ion “come out,” using what we know about the solubility rules we are able to take advantage of the compounds and separate them as well as see how new ones are chemically bonded together. The ions are separated by being dissolved in a solution, usually water. These water molecules completely surround the ion which allows them to flow freely in the solution. Some compounds have a stronger bond then the substance that the compound is being put in and will not dissolve remaining to be soluble. When mixing two different ionic compounds together that have been dissolved in a solution, there is a greater chance of a reaction to occur because the compounds are already broken apart (McNeil, 2013-2014). An anion particle will then easily react with a cation particle which causes a precipitate, but some anions and cations in a solution may not react and remain dissolved in the solution. An example of this would be silver has a 1+ charge while a chlorine ion has a 1- charge. These two elements are then attracted together because of opposite charge and form a precipitate because it is not soluble according to the solubility rule #4. A type of reaction where two different compounds are dissolved in the same solution and the cations in each compound switch places is called a double displacement reaction (Wicks, 2014). This further explains why ions form
precipitates because of the oppositely charged particles. The precipitates are a form of qualitative data in that there is a physical change such as color or physical state. The substances that did not change in the chemical reaction are called spectator ions, but must still remain in the equation because of The Law of Conservation of Mass which states that matter cannot be created nor destroyed.
Using the method of spot testing, it becomes possible to identify the individual metal cation present in the mixed solution. In this experiment using the flowchart, cations were only tested and separated, then confirmed by using spot testing. When finding the first solid of the mixture of Pb2+ and Ag+, water was added then heated in attempts to dissolve the solid. After centrifuging the solid remained and the remaining liquid was spot tested for lead with K2CrO4. There was no reaction or precipitate formed therefore no Pb2+ was present. Since no Pb2+ was in the liquid solution the liquid was water while it was concluded that AgCl was the lingering solid. The liquid formed from the first step was taken and mixed with NH4OH and only produced a liquid. The liquid was tested with K2CrO4 and DMG in efforts to identify if Ni2+ and Ba2+ were in the solution. Both caused a precipitate inclining that nickel and barium were present in this compound. The nickel reaction with DMG had turned a bright pink that signifies an apparent precipitate while when testing for barium a yellow color was expected, but almost looked like a faint white precipitate when added with H2SO4. As a check K2CrO4 was spot tested with barium as well which showed very small chunky precipitates. According to the Solubility Rules #5 and #6, a precipitate would show for both of these tests concluding that Ag+, Ni2+ and Ba2+ were in Unknown #4. All chemical reactions from spot testing and the flowchart are shown on the back of Worksheet 4 as well as the explanations of Solubility Rules.
Works Cited
McNeil, H. (2013-2014). Chemistry 141 Labratory Manual. Department of Chemistry and Biochemistry, Montana State University-Bozeman.
Precipitation Reactions. (n.d.). Retrieved from Chem Wiki: http://chemwiki.ucdavis.edu/Inorganic_Chemistry/Reactions_in_Aqueous_Solutions/Precipitation_Reactions
Wicks, K. M. (2014). Double Displacement Reactions. Retrieved from Chemistry Lecture Notes: http://www.chemistrylecturenotes.com/html/double_displacement_reactions.html
Qualitative Analysis
March 20, 2015
The purpose of this lab is to identify unknown ions in a solution by using a type of chemical reaction called precipitate reactions. The key to finding which ions are present in a solution is to form a precipitate which makes the ions “come out” in a reaction (McNeil, 2013-2014). Water is used in these experiments to act as a solvent. Since water is a polar molecule, the slightly positive and negative charges will sometimes pull apart molecules and surround them based on charges. In other words, water can dissolve ionic compounds and create an aqueous (aq) solution. This lab uses no quantitative analysis, but is entirely based on qualitative analysis which is the observed result instead …show more content…
of the measured result. Looking for a precipitate is qualitative in that physical changes are what is being observed and noted. Precipitates generally have a solid, cloudy or milky substance that does not dissolve easily and occurs when a cation and anion combine in an aqueous solution, but reacts to become insoluble (Precipitation Reactions).
This lab uses two methods in testing for precipitates: spot testing and a flowchart.
Spot testing requires two ionic compounds that are dissolved into a solution to be added together. When the ions separate and reform new bonds to form a precipitate, the physical changes are recorded. Some substances are not soluble, thus will not dissolve in water which is called insoluble. The solubility rules are located on page 67 to 68 in lab manual and explain when a precipitate will occur. Unknown mixtures of compounds are hard to separate and individually identify, thus the flowchart is used. A flowchart is an outline that separates cations from anions where they can then be tested through spot testing. A real world example of qualitative analysis could be used in athletics. Many coaches will say, “Quality over quantity,” when referring to drills. Coaches are looking for concentration and complete effort in doing a drill correctly a few times rather than doing a drill many times without full effort and …show more content…
attention.
In using the solubility rules, the given Flow Chart and spot testing it was found that the unknown #4 consisted of Ag+, Ni2+ and Ba2+. When taking the initial mixture, the white solid was formed as well as a liquid. The solid went into further testing by adding DI water then heated while stirring. A solid was still present so the solution was centrifuged for one minute. Then the liquid remaining was tested for lead by spot testing with K2CrO4. There was no reaction or precipitate that formed. The liquid formed from the first step was taken and mixed with NH4OH and tested with litmus paper to ensure that the solution was basic, turning the paper blue, then centrifuged again. Only liquid was left over and tested with K2CrO4 as well as DMG. Both of these spot test showed a precipitate reaction.
The purpose of this lab was to separate two unknown compounds by using the solubility properties.
In order to make an ion “come out,” using what we know about the solubility rules we are able to take advantage of the compounds and separate them as well as see how new ones are chemically bonded together. The ions are separated by being dissolved in a solution, usually water. These water molecules completely surround the ion which allows them to flow freely in the solution. Some compounds have a stronger bond then the substance that the compound is being put in and will not dissolve remaining to be soluble. When mixing two different ionic compounds together that have been dissolved in a solution, there is a greater chance of a reaction to occur because the compounds are already broken apart (McNeil, 2013-2014). An anion particle will then easily react with a cation particle which causes a precipitate, but some anions and cations in a solution may not react and remain dissolved in the solution. An example of this would be silver has a 1+ charge while a chlorine ion has a 1- charge. These two elements are then attracted together because of opposite charge and form a precipitate because it is not soluble according to the solubility rule #4. A type of reaction where two different compounds are dissolved in the same solution and the cations in each compound switch places is called a double displacement reaction (Wicks, 2014). This further explains why ions form
precipitates because of the oppositely charged particles. The precipitates are a form of qualitative data in that there is a physical change such as color or physical state. The substances that did not change in the chemical reaction are called spectator ions, but must still remain in the equation because of The Law of Conservation of Mass which states that matter cannot be created nor destroyed.
Using the method of spot testing, it becomes possible to identify the individual metal cation present in the mixed solution. In this experiment using the flowchart, cations were only tested and separated, then confirmed by using spot testing. When finding the first solid of the mixture of Pb2+ and Ag+, water was added then heated in attempts to dissolve the solid. After centrifuging the solid remained and the remaining liquid was spot tested for lead with K2CrO4. There was no reaction or precipitate formed therefore no Pb2+ was present. Since no Pb2+ was in the liquid solution the liquid was water while it was concluded that AgCl was the lingering solid. The liquid formed from the first step was taken and mixed with NH4OH and only produced a liquid. The liquid was tested with K2CrO4 and DMG in efforts to identify if Ni2+ and Ba2+ were in the solution. Both caused a precipitate inclining that nickel and barium were present in this compound. The nickel reaction with DMG had turned a bright pink that signifies an apparent precipitate while when testing for barium a yellow color was expected, but almost looked like a faint white precipitate when added with H2SO4. As a check K2CrO4 was spot tested with barium as well which showed very small chunky precipitates. According to the Solubility Rules #5 and #6, a precipitate would show for both of these tests concluding that Ag+, Ni2+ and Ba2+ were in Unknown #4. All chemical reactions from spot testing and the flowchart are shown on the back of Worksheet 4 as well as the explanations of Solubility Rules.
Works Cited
McNeil, H. (2013-2014). Chemistry 141 Labratory Manual. Department of Chemistry and Biochemistry, Montana State University-Bozeman.
Precipitation Reactions. (n.d.). Retrieved from Chem Wiki: http://chemwiki.ucdavis.edu/Inorganic_Chemistry/Reactions_in_Aqueous_Solutions/Precipitation_Reactions
Wicks, K. M. (2014). Double Displacement Reactions. Retrieved from Chemistry Lecture Notes: http://www.chemistrylecturenotes.com/html/double_displacement_reactions.html