Question

What are the balanced thermochemical equations for the dissolving of calcium chloride In water?

Answer 1

Start by writing out your reactants: `CaCl_2 + H_2O`. We now need to determine what kind of reaction this is. Is it a...

Synthesis reaction? No, you can't have `CaCl_2H_2O` since we're not dealing with polyatomic ions.

Decomposition reaction? No, there are two reactants.
Combustion reaction? No, there's no carbon.

Single replacement? No, there are not 3 elements in total.

Double replacement? Seems like it; there are 4 elements making up two compounds.

We now predict products: `CaCl_2+H_2O\rightarrow CaO+HCl`. Now balance: `CaCl_2+H_2O\rightarrow CaO+2HCl`. Since `CaO` is a solid, our reaction works. Hopefully this is what you're asking for, shoot me a PM if not.

Answer 2

Warning! Long Answer. The balanced equation is `"CaCl"_2"(s)"stackrelcolor(blue)("H"_2"O"color(white)(mm))(→) "Ca"^"2+""(aq)" + "2Cl"^"-""(aq)"`

Explanation:

There are three steps in the process of dissolving `"CaCl"_2`,

Step 1. The ions must separate from each other

`"CaCl"_2"(s)" → "Ca"^"2+" + "2Cl"^"-"; ΔH_2`

It takes energy to separate the ions from each other, so this step is endothermic. `ΔH_2 > 0`.

Step 2. The water molecules must separate from each other to make room for the ions

It takes energy to separate the water molecules from each other, so this step is also endothermic. `ΔH_1 > 0`.

Step 3. The ions interact with the water molecules and form hydrated ions.

Some of the water becomes water of hydration.

`"Ca"^"2+" + "2Cl"^"-" + "H"_2"O(l)" stackrelcolor(blue)("H"_2"O"color(white)(mm))(→) "Ca"^"2+""(aq)" + "2Cl"^"-""(aq)" + "H"_2"O(hyd)"; ΔH_3`

This step is exothermic, so `ΔH_3 < 0`.

The overall process

The overall process is the sum of the three steps.

`"CaCl"_2"(s)" → color(red)(cancel(color(black)("Ca"^"2+"))) + color(red)(cancel(color(black)("2Cl"^"-"))); color(white)(mmmmmmmmmmmmmmmmmmmm)ΔH_1`
`color(red)(cancel(color(black)("H"_2"O(l)"))) → color(red)(cancel(color(black)("H"_2"O(hyd)"))); color(white)(mmmmmmmmmmmmmmmmmmmmmm)ΔH_2`

`ul(color(red)(cancel(color(black)("Ca"^"2+"))) + color(red)(cancel(color(black)("2Cl"^"-"))) + color(red)(cancel(color(black)("H"_2"O"("hyd"))))stackrelcolor(blue)("H"_2"O"color(white)(mm))(→) "Ca"^"2+""(aq)" + "2Cl"^"-""(aq)" + color(red)(cancel(color(black)("H"_2"O(l)"))); ΔH_3`
`"CaCl"_2"(s)"stackrelcolor(blue)("H"_2"O"color(white)(mm))(→) "Ca"^"2+""(aq)" + "2Cl"^"-""(aq)"; Δ_text(soln)H = ΔH_1 + ΔH_2 + ΔH_3`

When calcium chloride dissolves, the energy released by hydrating the ions
(`ΔH_3`) is greater than the energy required to separate the ions and the water molecules (`ΔH_1 + ΔH_2`).

`Δ_text(soln)H = underbrace(ΔH_1 + ΔH_2)_color(red)("positive") + underbrace(ΔH_3)_color(red)("negative") <0`

Thus, the dissolving of calcium chloride in water is an exothermic process, and the heat evolved goes into heating the solution in the heating pack