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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rosyposy
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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.
 

FAQ: Question on Hydrates and Reactions

What are hydrates in chemistry?

Hydrates are compounds that contain water molecules integrated into their crystal structure. These water molecules are not just physically trapped but are chemically bonded to the host molecule, typically in a fixed ratio. For example, copper(II) sulfate pentahydrate (CuSO4·5H2O) contains five water molecules for each formula unit of copper(II) sulfate.

How do you determine the formula of a hydrate?

To determine the formula of a hydrate, you typically need to know the mass of the hydrated compound and the mass of the anhydrous compound (after heating to remove the water). By calculating the mass of water lost and comparing it to the molar mass of water, you can determine the number of water molecules per formula unit of the compound. This ratio gives you the formula of the hydrate.

What happens to hydrates when they are heated?

When hydrates are heated, they generally lose their water of hydration through a process called dehydration. This usually results in the formation of an anhydrous compound and water vapor. The temperature at which this occurs can vary depending on the specific hydrate. Some hydrates may lose water gradually, while others may lose all their water at once.

What is the role of hydrates in chemical reactions?

Hydrates can play several roles in chemical reactions. They can act as reactants or products, influence the physical properties of the materials (such as solubility and stability), and affect the reaction pathway. In some cases, the water of hydration can participate directly in the reaction, while in other cases, it may simply be released or absorbed.

How can you distinguish between a hydrate and an anhydrous compound?

There are several ways to distinguish between a hydrate and an anhydrous compound. One common method is to heat the compound and observe any mass loss due to the release of water. Another method is to use techniques like infrared spectroscopy (IR) or X-ray diffraction (XRD) to identify the presence of water molecules in the crystal structure. Additionally, hydrates often have different physical properties, such as color and solubility, compared to their anhydrous counterparts.

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