Lab 6-Water Of Hydration Flashcards
What is the introduction of this lab?
Most solid chemical compounds will contain some water if they have been exposed to the atmosphere for any length of time. In most cases water is present in very small amounts and is merely adsorbed onto the surface of the crystals. Other solid compounds contain larger amounts of water that is chemically bound to the crystal. These compounds are usually ionic salts. The water that is present in these is called water of hydration and is usually bound to the cation of the salt.
The water molecules in a hydrate are removed relatively easily. In most cases, simply heating the hydrate to a temperature somewhat above the boiling point of water will drive off the water of hydration. Hydrated copper (II) chloride is typical in this regard: it is converted to anhydrous CuCl2 if heated to about 110 degrees Celsius
CuCl2. 2H20—> CuCl2 (s) +2H2O (g)
In the dehydration reaction the crystal structure of the solid will change and the color of the salt may also change. On heating CuCl2. 2H2O, the green hydrated crystals are converted to a brownish-yellow powder.
What is the formula for the hygrometer reactions?
CoCl2. 6H20—> RED 6R
CoCl2. 2H2O—-> Violet. 2V
CoCl2—> Blue. 6R-2V-1B
Define Efflorescence
When a hydrate loses water upon standing.
Define Desiccant
An anhydrous compound that will absorb water from the air or other source.
Define Carbohydrate
An example of a substance that is NOT a true hydrate.
Define Deliquescence
When a substance takes so much water from the atmosphere that is dissolves.
What is the amount of water most in efflorescence dependent upon?
The amount of water in the air, as measured by its relative humidity.
How does CoCl2 change in different weather conditions?
In moist warm air, CoCl2 is fully hydrated and red. MW-HR
In dry, cold air, CoCl2 loses most of its hydration and is blue. DC-LB
At intermediate humidities, CoCl2 exists as a dihydrate and is violet. I-DV
Define Hygroscopic
Some anhydrous ionic compounds will tend to absorb water from the air or other sources. They are said ti be HYGROSCOPIC.
What is an example of a compound that can be involved in Deliquescence
NaOH
Define Anhydrous
A compound or substance containing no water.
What is the last portion of the lab explaining?
Some compounds give off water on being heated but are not true hydrates. The water is produced by decomposition of the compound rather than by loss of water of hydration. Organic compounds, particularly carbohydrates, behave this way. Decompositions of this sort are NOT reversible; adding water to the product will not regenerate the compound. True hydrates typically undergo reversible dehydration. Adding water to anhydrous CuCl2 will cause formation of CuCl2. 2H2O or, if enough water is added, will lead t the formation of a solution containing hydrated Cu2+ ions. All ionic hydrates are soluble in water, and are usually prepared by crystallization from water solution. The amount of bound water may depend upon the way the hydrate is prepared, but in general the number of moles of water per mole of ionic compound is either an integer or a multiple of 1/2.
In cases that some substances do give off water when heated, why are they NOT hydrates?
The water is produced by decomposition rather than by loss of water of hydration.
Define Hydration.
A hydration reaction is a chemical reaction in which a substance combines with water.
What is an example of a substance that gives off water upon being heated but is NOT a true hydrate.
Carbohydrate.
