Question on Hydrates and Reactions

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In summary, you would need to include the water of hydration in the formula for the reaction if you want to weigh the copper recovered from the reaction.
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Hey! I am currently working on a project where I have to react copper (II) chloride dihydrate (dissolved in water) and aluminum. When writing the equation and the ionic equation for this, would you include the dihydrate in the formula since it’s dissolved in water and no longer in its crystalline form or would you still write it as CuCl2.2H2O, can you also provide an explanation for the correct way to write since I am really confused as some sites write it with the H2O and some don’t. Thank you!
 
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  • #2
The water of hydration - .2H20 - adds mass to the molecule. So, if are going to get a dry weight to assay some reaction in a solution you have to include it. Is that what you are asking? You can write the reaction ignoring the extra mass but I do not see any immediate benefit other than teaching an introduction.
Practical aspect is what you have dictates what you do:
Copper chloride appears as a yellowish-brown powder (the anhydrous form) or a green crystalline solid (the dihydrate). The green stuff is usually what you have. If have the brown stuff you would measure the mass of copper chloride with no water to worry about.
 
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  • #3
Hey, so I am only weighing the copper retrieved from the reaction to find the concentration of the original solution using stoichiometry, but I also need to write the formula for the whole reaction with states so I was just asking if I would write the copper (II) chloride dihydrate as a dihydrate since it is dissolved in water and I’m not sure if you are supposed to write dihydrates as aqueous since they exist in crystalline form.
 
  • #5
There is no simple yes/no answer to your question.

A lot depends and what you want to show. Every way of writing the reaction has its own applications and makes sense in different contexts.
 

What is a hydrate in chemistry?

In chemistry, a hydrate is a compound that has water molecules physically bound to its structure. These water molecules are typically integrated into the crystal lattice of the compound. Hydrates can be formed in several ways, often by direct combination of the host compound with water under specific conditions of temperature and pressure.

How can you determine the formula of a hydrate?

The formula of a hydrate can be determined through a process known as hydrate analysis. This involves heating a known mass of the hydrate to remove the water content, and then measuring the mass of the anhydrous (water-free) compound left behind. By comparing these masses, the number of moles of water per mole of anhydrous compound can be calculated, giving the stoichiometry of the hydrate in its chemical formula.

What happens during the dehydration of a hydrate?

During dehydration, a hydrate is heated, causing the water molecules bound in the crystal lattice to be released as water vapor. This process results in a change in the physical structure and weight of the compound. The remaining anhydrous substance often has different chemical and physical properties compared to the original hydrate.

Can a hydrate be rehydrated after dehydration?

Yes, many hydrates can be rehydrated after dehydration. This involves exposing the anhydrous compound to water under conditions suitable for reformation of the hydrate. The ability to rehydrate depends on the chemical nature of the substance and the environmental conditions such as humidity and temperature.

What are some common examples of hydrates and their uses?

Common examples of hydrates include copper(II) sulfate pentahydrate (CuSO4·5H2O), which is used in Fehling's solution for reducing sugars tests, and calcium chloride dihydrate (CaCl2·2H2O), which is used for ice and dust control on roads. Gypsum, or calcium sulfate dihydrate (CaSO4·2H2O), is used in the manufacture of plaster and drywall. Hydrates are important in various industrial and laboratory applications due to their unique properties.

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