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Determination of the Cations and Anions

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Experiment 11 Title: Determination of the Cations and Anions via their Precipitation and Solubility Properties. Objectives: > To classify a number of ions on the basis of their solubility. > To identify two unknown ionic compounds on the basic of their solubility. > To draw a conclusion from observations of a number of precipitation reaction. > To identify the cations and anions in an unknown solution based on its reactivity. > To study the solubility properties of the compounds. Introduction: Precipitation is the formation of a solid in a solution during a chemical reaction. When the chemical reaction occurs the solid formed is called the precipitate. This can occur when an insoluble substance, the precipitate, is formed in the solution due to a reaction or when the solution has been supersaturated by a compound. The formation of a precipitate is a sign of a chemical change. In most situations, the solid forms ("falls") out of the solute phase, and sinks to the bottom of the solution (though it will float if it is less dense than the solvent, or form a suspension). An important stage of the precipitation process is the onset of nucleation. The creation of a hypothetical solid particle includes the formation of an interface, which requires some energy based on the relative surface energy of the solid and the solution. If this energy is not available, and no suitable nucleation surface is available, supersaturation occurs. An example of a precipitation reaction is when aqueous silver nitrate (AgNO3) is added to a solution containing potassium chloride (KCl) and the precipitation of a white solid, silver chloride is observed. AgNO3(aq) + KCl(aq) � AgCl(s) + KNO3(aq) The silver chloride(AgCl) has formed a solid, which is observed as a precipitate. This reaction can be written emphasizing the dissociated ions in a combined solution. Ag+(aq) + NO3-(aq) + K+(aq) + Cl-(aq) � AgCl(s) + K+(aq) ...read more.


From Table 2, we could observe that not every product formed a precipitate. When observing precipitates, it was noted whether the precipitate thick or thin, its colour and how long the precipitate took to show up. All the reaction produced precipitate except for the reaction between AW and BY. Following were the products formed in this experiment: AW + BX --> AX + BW (green precipitate formed) CW + BX --> CX + BW (white precipitate formed) AW + BY --> AY + BW (no precipitate formed) AZ + BX --> AX + BZ (light green precipitate formed) The reaction of AW and BX produced an insoluble green product known as precipitation reaction. A precipitate is an insoluble solid formed by a reaction in solution. Precipitation reactions occur when certain pairs of oppositely charged ions attract each other so strongly that they form an insoluble solid. Precipitates often appear as milky in the solution, or they may form only a slight cloudiness, or may be black or brown in colour. This precipitation reaction also occur when CW with BX and AZ with BX were mixed together. Notice the reaction above that the cations in the two reactants exchange anions A ends up with X and B ends up with W. The chemical formulas of the products are based on the charges of the ions. One A ion was needed to give a neutral compound with X, as well as the formation of BW. It is only after the chemical formulas of the products are determined that the equation can be balanced. Such reactions are known as exchange reactions or metathesis reactions. First, we have to identify the precipitate between the two product formed in a reaction. After certain comparison, we can conclude that the precipitates formed in this whole experiment are AX and CX. AZ was chosen as the precipitate instead of BW or BZ in each reaction because all the reaction that produced AZ has precipitate as well. ...read more.


This was because, according to the solubility rule table provided all salts of NO3- are soluble compounds. Therefore, either three option solution which were Pb(NO3)2, Mg(NO3)2 or NaNO3 when react with Na(NO3)2 would form the same products. The products that formed were soluble compound that not forming any precipitate in the reaction. On the other hand, unknown Y was NiCO3. This was because when unknown Y was added into MgCl2 solution, precipitate with white in colour was formed. According to the solubility rules, only CO32- ion would form precipitate with Mg2+ ion. Thus, the unknown Y was NiCO3. Precaution steps: 1. Handle the sodium hydroxide (NaOH) carefully as it is very caustic and may damage the skin. 2. Advisable to hold the test tube properly when placing it in test tube rack as to prevent the bottom of test tube to break. 3. Make sure that the test tube is totally dry and doesn't contain any form of water because tap water contains a wide variety of ions and could result in misleading observations. 4. It is advisable not to put or to use the wrong dropper into a bottle because the entire contents of the bottle will become contaminated. 5. When mixing the solution in a test tube, it is very important that the solutions are stirred using a thin stirring rod so that the contents of the test tube do not overflow and the reactions can proceed to completion. Conclusions: 1. Precipitation reaction is a reaction between 2 different solutions which produces an insoluble product known as precipitate. 2. Na+ is type 1 cation, where all the salts forms from this ion are always soluble. 3. Mg2+ and Ni2+ are type 2 cation, where the solubility of the salt formed from these ions are depend on its anions. 4. NO3-, CI- and SO4- are type 1 anion, where the salts formed from these ions are generally soluble. 5. OH- is type 2 anion, where the salts formed from these ions are generally insoluble. 6. Compound X is Pb(NO3)2 and compound Y is NiCO3. ...read more.

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