Introduction
The laboratory work aimed to investigate the chemical properties of metallic zinc and calcium in reactions with hydrochloric acid and water, determine the acidity of the products, and perform an acid-base titration to determine the empirical formula. Given the multiplicity of objectives, the generalized objective of this work can be reduced to the practice of laboratory skills with respect to chemical interactions. The goal was fully achieved, and the results from each step demonstrated their overall success.
Discussion
Firstly, it was shown that the splint test was performed correctly and, as expected, the hydrogen gas collected reacted with oxygen with a slam (reaction 1). To obtain hydrogen, metallic zinc and calcium reacted with hydrochloric acid, as shown in reactions 2 and 3. Since both metals are more active than hydrogen, the reaction was successful.
Zinc was more active with the acid, as more gas was collected and the popping was louder, as expected from its properties. Regarding the reaction with water, as expected, zinc showed no interaction, whereas calcium underwent a reaction with water similar to that with acid (reaction 4). Zinc was unable to produce products in this reaction because the theoretically formed zinc hydroxide is insoluble, indicating the reactants lacked the energy to complete the reaction.
The present work also included acid-base interaction and acidity studies of the solutions. When phenolphthalein was added to the reaction mixture shown in Equation 4, the indicator turned pink or purple. This was because the calcium hydroxide formed dissociates in water to form hydroxide ions, as shown in reaction 5. The presence of hydroxide ions increases the pH of the medium, making it more basic — in such a medium, phenolphthalein turns pink.
At the same time, acid-base titration with hydrochloric acid and calcium hydroxide led to the correct result because the color of the solution was changed to green by drop-wise addition of acid to alkali in two tests at once (Fig. 1). Since the universal indicator has a green color in neutral medium, the result indicated that the pH equivalence point had been reached. The titration reaction is shown in number 6 below.
The titration, schematically shown in Figure 1, was carried out to obtain data for the empirical formula of calcium hydroxide. The results of two tests simultaneously showed that the formula for calcium hydroxide is Ca(OH)3 rather than the theoretically expected Ca(OH)2. Several problems may have led to the formula determination being off, including an excess of metallic calcium at the start of the reaction, which accounted for more moles of hydroxide ions; potential errors due to haste and inattention; and misreading the titrant concentration.
Overall, the results indicated that titrimetric analysis can be used to determine the empirical formula, but more research is needed. Some options to correct errors included taking only the required number of moles of analyte, using uncrowded burettes with HCl, and observing the color change more closely to reach the first equivalence point.
Conclusion
The present laboratory work aimed to carry out a series of chemical tests to determine the properties of metallic zinc and calcium, and to determine the empirical formula of calcium hydroxide in titrimetric analysis. The work was based on theoretical expectations and the hypothesis that the results obtained would be consistent with the concepts.
The interactions of zinc and calcium with hydrochloric acid and water were correct, as their outcomes showed consistency. In the titration experiment, errors were made, leading to the determination of the wrong empirical formula for calcium hydroxide. This indicates that the hypothesis was only partially satisfied. In the future, it is suggested that titrimetric analysis be adjusted to reduce the risk of errors and achieve more accurate results.