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#1
“MgCl2 (s) + 4H20(l) —> MgCl2.4H2O. Calculate the enthalpy change for the absorption of water by MgCl2 (s) to form MgCl2.4H2O (s)”
The question gives you enthalpy of solution values (MgCl2 = -155, MgCl2.4H2O = -39). The markscheme says “-155-(-39)”. I don’t teally understand, how is this equation showing enthalpy of solution? I’m confused please help
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11 months ago
#2
(Original post by anactualmess)
“MgCl2 (s) + 4H20(l) —> MgCl2.4H2O. Calculate the enthalpy change for the absorption of water by MgCl2 (s) to form MgCl2.4H2O (s)”
The question gives you enthalpy of solution values (MgCl2 = -155, MgCl2.4H2O = -39). The markscheme says “-155-(-39)”. I don’t teally understand, how is this equation showing enthalpy of solution? I’m confused please help
The equation is NOT showing enthalpy of solution. It is showing a specific enthalpy change from anyhydrous to hydrated solid.

However, you can construct a Hess' cycle to use the enthalpy of solutions of both species to determine the required enthalpy change.

anhydrous salt == (aq) ==> solution <== (aq) == hydrated salt

So what is the enthalpy change from anhydrous salt to hydrated salt?
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#3
(Original post by charco)
The equation is NOT showing enthalpy of solution. It is showing a specific enthalpy change from anyhydrous to hydrated solid.

However, you can construct a Hess' cycle to use the enthalpy of solutions of both species to determine the required enthalpy change.

anhydrous salt == (aq) ==> solution <== (aq) == hydrated salt

So what is the enthalpy change from anhydrous salt to hydrated salt?
Thank you so much. Also? Why could the enthalpy change not be determined directly by calorimetry?
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11 months ago
#4
(Original post by anactualmess)
Thank you so much. Also? Why could the enthalpy change not be determined directly by calorimetry?
How can you add exactly 4 moles of water molecules to 1 mole of anhydrous salt and expect the ionic structure to take up exactly four molecules of water per mole? And if it could happen, what would you measure?

It's an impossible situation.
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