Exprt D: Hydrates Flashcards
Efflorescent
Hydrates that spontaneously lose hydrated water to the atmosphere are classified as efflorescent. 
For efflorescent compounds, the water vapour pressure exerted by the hydrated water is greater than the water vapour pressure in the surrounding atmosphere, meaning that the efflorescent compounds lose water to the atmosphere.
Hygroscopic 
Compounds that spontaneously absorb water from the atmosphere are classified as hygroscopic. May absorb so much water from the atmosphere that they dissolve in their own water of hydration.
For hygroscopic compounds, the water vapour pressure exerted by the hydrated water is less than the water vapour pressure in the surrounding atmosphere, hence hygroscopic compounds absorb water from the atmosphere. 
Deliquescent
Hygroscopic compounds may absorb so much water from the atmosphere that they dissolve in their own water of hydration, these are called deliquescent.
Desiccants
Industrially, some hygroscopic compounds, such as CaCl2 or P2O5, are used to dry liquids or gases.
Anhydrous
Heating a hydrated compound increases its water vapour pressure and drives off hydrated water. The temperature required to remove water from hydrate varies from hydrate to hydrate, depending on the strength of the interactions holding water within the hydrate.
At a sufficiently high temperature, all of the water is gone, and the compound is said to be anhydrous.
What solutions will reaction with what anions table
Anion: CO3^2-
A1- add 1 mL of 1.0 mol/L HNO3:
Colourless gas evolved
A2- add 9 mL of H2O; 1 mL of 0.25 mol/L Ba(NO^3)2:
No reaction
A3- add 1 mL of 0.10 mol/L AgNO3:
Colourless gas & white ppt (precipitate)
Anion: SO4^-2
A1- add 1 mL of 1.0 mol/L HNO3:
No reaction
A2- add 9 mL of H2O; 1 mL of 0.25 mol/L Ba(NO^3)2:
White ppt
A3- add 1 mL of 0.10 mol/L AgNO3:
White ppt
Anion: Cl^-
A1- add 1 mL of 1.0 mol/L HNO3:
No reaction
A2- add 9 mL of H2O; 1 mL of 0.25 mol/L Ba(NO^3)2:
No reaction
A3- add 1 mL of 0.10 mol/L AgNO3:
White curdy ppt
Anion: Br^-
A1- add 1 mL of 1.0 mol/L HNO3:
No reaction
A2- add 9 mL of H2O; 1 mL of 0.25 mol/L Ba(NO^3)2:
No reaction
A3- add 1 mL of 0.10 mol/L AgNO3:
Cream- coloured ppt
Hydrates
Most solid chemicals contain some water if they have been exposed to the atmosphere. In some cases, the water present is absorbed only on the surface of the compound and is easily removed by gentle heating. In other cases, water is an integral part of the crystal structure of the compound. The water present in such cases is called ‘hydration water’ or ‘water of crystallization.’
Hydrate: compounds are in quasi-equilibrium with the ambient water vapour pressure (humidity).
Mass % of water formula
Mass % water
= water mass/hydrate mass x100
Mole ratio formula
Mole ratio =
moles of water/moles of anhydrous salt
Example of hydrate and it’s anhydrous salt
Hydrate:
CuSO4 • 5H20
Anhydrous salt:
CuSO4
Cation is Cu
Anion is SO4
Example
Mass of unknown Hydrate
|
|dehydrate by heating
V
Mass of anhydrous salt
Initial
5.768g Na2CO3 • n H2O
Heated
3.248g —> mass of anhydrous salt
Mass of H2O = 5.768g-3.248g = 2.520g
Mass % = 2.520g/5.768g x100 = 43.69%
Mol H2O = 2.520g • 1 mol H2O/ 18.01g = 0.1399222654 mol
Mol H2O = 0.140 mol
Mol Na2CO3 = 3.248g • 1 mol/ 105.988g = 0.0306 mol
Mole ratio = 0.140 mol/0.0306 mol = 4.57
Na2CO3 • 4.5 H2O
As a hydrated compound is heated, the compound may undergo one or more of the following changes as water is lost:
(1) It may undergo a colour change. The colour of the hydrate often depends on the number of water molecules associated with the cation.
(2) The form of the Crystal may change. If it does, a dry powdery solid usually results.
(3) It may dissolve in its own water of crystallization as the crystal structure breaks down. I continued healing, this water will evaporate, leaving the anhydrous solid behind.
(4) It may decompose giving a noticeable colour change or emitting coloured vapors. Decomposition is an undesirable change. 
Is CaCl2 efflorescent or hygroscopic? Explain why, in terms of vapour pressure of water in the atmosphere, and the vapour pressure of water exerted by the waters of hydration In the solid.
CaCl2 in closed is white chunks.
CaCl2 in open is a liquid.
This would suggest that CaCl2 is hygroscopic. This means the water vapour pressure exerted by the water in CaCl2 is lower than the water vapour pressure in the atmosphere. Therefore, CaCl2 absorbs water from the surrounding atmosphere.
Was CdCl2 crystalline in the open or closed bottle? What happened to the water in the other bottle? Explain why, in terms of vapour pressure of water in the atmosphere, and the vapour pressure exerted by the waters of hydration in the solid.
CdCl2 was crystalline in the closed bottle.
CdCl2 was a dry white powder in the open bottle.
The crystal structure of CdCl2 has changed; ergo broken down. This would suggest that CdCl2 is an efflorescent, meaning the water vapour pressure exerted by the waters in CdCl2 is greater than the water vapour pressure in the atmosphere. Therefore, CdCl2 is going to lose water of hydration to the atmosphere.
Compare and explain the observations made when heating copper (||) sulfate and sugar (sucrose), as well as, the observations for the addition of water to the anhydrous residues of these two compounds.
Copper (||) sulfate before heating was a bright blue colour. While heating condensation was observed at the top of the tube. After heating, it had become a pale blue. When the addition of water was made, some of the colour had been returned to the solid, forming a medium blue colour. This suggests it was a hydrate as it was capable of reabsorb inch some of the water. However, I believe this also suggests that it is an efflorescent.
Sugar was white before heating. While heating the sugar became a brown thick liquid then black. During this time condensation was observed on the top of the tube along with the produce of a cream coloured smoke. When the addition of water was made, it only produced muddy looking water, suggesting that sucrose it not a pure hydrate, as it wasn’t able to reabsorb water. OR IN FACT, it had decomposed and therefore unable to reabsorb water.
