Experiment #5 Lab Report 2e144e

Properties of hydrates

Nazarbayev University School of Science & Technology Chemistry I Lab Report: Properties of hydrates ID: 201075648 March 2, 2012 Lab instructor: Eugene Douglass

Properties of hydrates Introduction A hydrate is a chemical compound that contains water as part of its crystal structure. The water is tightly bound to the compound, but it is still a separate and unique compound. The water in a hydrated compound is called the water of hydration. The chemical formula of a typical hydrated compound, such as barium chloride, is written as BaCl2 • 2H2O. The dot in the formula indicates that the two compounds are bound together. Upon heating to about 115oC, the water can be evaporated leaving an anhydrous salt: BaCl2 • 2H2O (s) → BaCl2 (s) + 2H2O (g) This process is reversible. By adding water to the anhydrous salt, the hydrate will reform. Most hydrates are stable at room temperature. However, some spontaneously lose water upon standing in the atmosphere; they are said to be efflorescent. Others can spontaneously absorb water from the surrounding atmosphere; they are said to be hygroscopic. Some hygroscopic compounds, such as P2O5 and CaCl2, are widely used to “dry” liquids and gases; they are referred to as desiccants. Other hygroscopic compounds, such as solid NaOH, absorb so much water from the atmosphere that they dissolve in this water; they are said to be deliquescent. Finally, it should be noted that some compounds, such as carbohydrates, release water upon heating by decomposition of the compound rather than by loss of the water of hydration. These compounds are not considered true hydrates as the process is not reversible. In this experiment some of the properties of hydrates will be studied. In the second part, the percent water in a hydrated compound and the formula of the compound will be determined. Aim 

To identify the hydrates in a group of compounds

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To observe the reversibility of the hydration reaction

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To test some substances for efflorescence and deliquescence

Properties of hydrates 

To determine the amount of water lost by a sample of unknown hydrate on heating

Method Identification of hydrates Compounds to be tested: nickel (II) chloride, potassium chloride, sucrose, calcium carbonate, barium chloride, sodium tetraborate. Compounds were placed into small dry test tubes, one compound to a tube. These tubes were then heated with a burner flame. For only those substances that showed condensation, the residue was allowed to cool down and dissolved in a few cm3 of water. The color of the dissolved residue was noted. Reversibility of hydration A few crystals (0.3g) of hydrated cobalt (II) chloride, CoCl2 • 6H2O, were gently heated in an evaporating dish until any total color change. The residue was dissolved in a few cm3 of water and boiled to dryness. The color of residue was noted. The solution was then left to cool. Deliquescence and Efflorescence Compounds to be tested: potassium aluminum sulfate (alum), calcium chloride, copper (II) sulfate, sodium carbonate (washing soda). On an analytical balance, a few crystals of each of the compounds were placed on separate clean, dry watch glasses. The values as initial masses of glasses and samples were recorded. All samples were labeled and placed next to the dish of. After one hour, the samples were weighed and the masses were recorded as final masses. Percent water in a hydrate The crucible were cleaned with 6M HNO3 and rinsed with distilled water. Then, the mass of crucible was found. A sample of a hydrated salt was obtained and weighed. The crucible (with the salt inside) was placed on the clay triangle in the iron ring stand and heated lightly at first

Properties of hydrates increasing the flame to full intensity. The sample was heated inside the crucible for 10-12 minutes. Once the heating is completed, the flame was turned off and the crucible was allowed to cool completely. After it cooled down, the mass of the crucible and its contents was measured. Results Part A: Identification of Hydrates

Nickel chloride Potassium chloride Sodium tetraborate Sucrose Calcium carbonate

H2O appears + + +

Color of residue orange white white brown white

Water soluble Soluble Soluble Soluble Soluble White precipitate formed

Hydrate + + -

Part B: Reversibility of Hydration The initial color of CoCl2 ˖ 6H2O was dark red, but after heating the color changed to light blue. Then, when water was added and as crystals absorbed water, the color of solution became purple. Eventually, while the sample was heated one more time, again change in color was observed. According to these observations it is clear that dehydration of hydrates is a reversible reaction and hence they can reabsorb water and return to initial phase. Part C: Deliquescence and Efflorescence Mass (sample + glass) Na2CO3 ˑ 10 H2O

Initial 31.09 g

Final 31.10 g

Observations increase in mass

Conclusion absorbed water from atmosphere

CaCl2

2.05 g

2.09 g

increase in mass

absorbed water from atmosphere

CuSO4 CoCl2

2.10 g 46.91 g

2.10 g 46.97 g

constant increase in mass

Neither absorbed water from atmosphere

One of the possible sources of error could be outside temperature of -26˚C. This condition could have influenced the way hydrates behaved, since at normal conditions, for example, Na2CO3 ˑ 10 H2O will lose water. In this case water from air in the room was absorbed, thus, increasing

Properties of hydrates hydrates’ mass. In addition, most of the hydrates gained in mass more or less which showed that air in lab room were humid. Part D: Per Cent Water in a Hydrate Mass (g) crucible + cover 50.97 crucible + cover +solid hydrate 51.12 crucible + cover + residue 51.03 m(solid hydrate) = 51.12 – 50.97 = 0.15 g m( residue) = 51.03 – 50.97 = 0.06 g m( H2O lost) = 0.15 – 0.06 = 0.09 g % H2O in unknown hydrate = 0.09/0.15 ×100 = 60 % The formula mass of anhydrous salt wasn’t given by lab instructor and that’s why mole ratio of 1:1 was used in order to find moles of water per mole of unknown hydrate. The number of moles of water per one mole of unknown hydrate is: n (H2O) : n(un.hydrate) = 1:1 n (H2O) = 0.09/18 = 0.005 moles So, there is 0.005 mole of H2O per one mole of unknown hydrate. The number of unknown that was given is #3. Conclusion Overall, different properties of hydrates, such as efflorescence and deliquescence, were examined. Various solid ionic compounds were observed and determined whether they true hydrates or not. The lab work was performed successfully as we followed properly all the instructions provided.