The Synthesis of Zinc Iodide (ZnI2)
Published 31 Oct 2017
Zinc iodide, ZnI2, is a while (hydrated) or yellow (dry), granular, odorless, hygroscopic (absorbs water in the atmosphere) solid compound that is soluble in water (1g:0.3 mL water). It turns to brown when exposed to heat or sunlight, showing decomposition with release of iodine gas (DeMeo, 1995, p. 836). It is extensively used in the laboratory to illustrate the Law of Definite Proportions and Law of Conservation of Mass, two of which are the foundations of chemical reactions and basis of stoichiometry.
A balanced chemical equation best describes how many atoms, molecules, or charged species come in contact to produce certain product, but does not illustrate the mechanism of the reactions. Reactant are driven by e.g. exposure to heat, air, or by just mixing them up, etc. to proceed reaction. The amount of product formed is based on the quantity of the reactants and how well the reaction was taken out. One of the reactants can be the limiting reactant, which is consumed in the course of the reaction, and the other one is the excess.
Reactants are of known initial quantity and measured for the change in amount after a successful reaction has been performed. The masses of the reacted species are relative amounts required for complete reaction met. The chemical or physical properties of the reactants should no longer be observed in the product, which can be further verified by physical, chemical and electrochemical tests.
0.5g sample of iodine (I2), 2mL H2O and four drops of CH3COOH were mixed in a large, clean test tube. H2O was added for the immediate addition of the same mass of the Zn metal, whereas CH3COOH (a weak acid) was used to prevent the formation of Zn(OH)2. ZnI2 reacts with H2O via a double displacement reaction mechanism (Demeo, 2003, p.797):
ZnI2(aq) + 2H20(l) -> Zn(OH)2(s) + 2H3O+(aq)+ 2I-(aq)
Careful weighing of the reactants was done to avoid errors. The mass of the Zn weighed before the reaction was above 0.5g, which is 0.510g. Observed in the reaction mixture upon agitation was the gradual loss of color of iodine (reddish-brown) as the reaction progressed. Since the Zn metal was used as the excess reagent, iodine was expected to be totally used up in the reaction. To obtain the un-reacted Zn metal, a gravity filtration was performed.
Quantitative transfer was employed by washing with very minimal amount of water to facilitate faster evaporation as the dry, un-reacted Zn was weighed in a watch glass later on in the procedure. It was noted that all the washings should also be quantitatively obtained since there were dissolved Zn species in it. The amount of Zn left was measured to be 0.348 g, giving the amount of Zn consumed to be 0.162g. On the other hand, the amount of I2 consumed was 0.503g. These values were used in the determination of mole equivalents and the mole ratio as shown in the computations in the results portion.
ZnI2 was recovered through medium heat evaporation which prevented losses due to splattering as excessive heat is applied. Cooling down the weighing vessels was first done to avoid also the lowering of the mass to be measured as the air below the weighing vessel may push it upwards because heat can push the air up. The moles of Zn and I2 were used in the computation of the theoretical yield. The percent yields from Zn and I2 were found to be 39.8% and 47.2% respectively.
A positive error that was also avoided was the entrapment of water during evaporation. It might seem confusing but as crystallization happens during the evaporation, water may be trapped underneath the bulk of the ZnI2 crystals. It was also confirmed that dehydration has properly taken place by heating back the crystals and checking if the weight has lowered. Although the crystals may look dry, water of hydration may still be present, making the species be called hydrates (salts with water molecules in their crystalline structure).
A chemical equation that can be proposed based on the experimental results would be:
Zn(s) + I2(s) – Znl2(s)
Since both used approximately 0.002 moles each, which would give a ratio of 1:1, thus we can write the balanced equation as such. An error that cannot be avoided in the experiment was the absence of the previous knowledge on the quality of reactants that may affect its reactivity. Factor such as contamination may lead to the wrong computation of the theoretical yield. Also, while weighing the I2 crystals, a negative error might occur because the I2 crystals sublime, making the amount of the limiting reactant less than what was weighed, since the some of the crystals have already been converted to gas.
- DeMeo, S. (1995). Synthesis and Decomposition of Zinc Iodide: Model Reactions for Investigating Chemical Change in the Introductory Laboratory. J. Chem Educ., 72(9), 836.
- Demeo, S. (2003). Synthesis and Decomposition of Zinc Iodide Revisited. J. Chem Educ., 80(7), 796.