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# enthalpies of hydration watch

1. Hello there, needed help on the following question.

When calcium chloride dissolves in water, the process can be represented by the following equation:

Ca2+(Cl-)2 +aq > Ca2+ + 2Cl-

the enthalpy change for this process is called the enthalpy change of solution. Its value can be calculated from an energy cycle using the following data.

Lattice energy of calcium chloride= -2258 kJ mol-1

Enthalpy change of hydration of Ca2+= -1657 kJ mol-1

Enthalpy change of hydration of Cl-= -340 kJ mol-1

a) Draw and label the cycle linking the enthalpy change of solution of calcium chloride with the enthalpy changes in the data above.
b) Use your cycle to calculate the enthalpy change of solution of calcium chloride.
c) what factors affect the size of the enthalpy change of hydration of Ca2+ compared with that of Li+?
d) why are the hydration enthalpies of both anions and cations negative?

I've answered parts a,b and c correctly. It's just part d) I'm stuck on.

Any help would be greatly appreciated.

Many thanks
2. (Original post by ajudd12)
Hello there, needed help on the following question.

When calcium chloride dissolves in water, the process can be represented by the following equation:

Ca2+(Cl-)2 +aq > Ca2+ + 2Cl-

the enthalpy change for this process is called the enthalpy change of solution. Its value can be calculated from an energy cycle using the following data.

Lattice energy of calcium chloride= -2258 kJ mol-1

Enthalpy change of hydration of Ca2+= -1657 kJ mol-1

Enthalpy change of hydration of Cl-= -340 kJ mol-1

a) Draw and label the cycle linking the enthalpy change of solution of calcium chloride with the enthalpy changes in the data above.
b) Use your cycle to calculate the enthalpy change of solution of calcium chloride.
c) what factors affect the size of the enthalpy change of hydration of Ca2+ compared with that of Li+?
d) why are the hydration enthalpies of both anions and cations negative?

I've answered parts a,b and c correctly. It's just part d) I'm stuck on.

Any help would be greatly appreciated.

Many thanks
what is hydration?

does it involve "bond" formation or breaking?

is that exothermic or endothermic? why?
3. (Original post by ajudd12)
d) why are the hydration enthalpies of both anions and cations negative?
I've answered parts a,b and c correctly. It's just part d) I'm stuck on.
Any help would be greatly appreciated.
Many thanks
You're forming bonds between solute and solvent when you dissolve something, and dissolution can be either endo or exothermic depending on whether it took more energy to break bonds in the process, or took more to form them. When you dissolve something in aqueous solution, you're dissolving it in water.
So it's more vigorous because opposite charges attract and water has a load of H+ floating around, and OH-, so whether it's an anion or a cation, water has both types of ion which will form ionic bonds (between oppositely charged ions) in solution.

Definition of the enthalpy of hydration is when 1 mole of gaseous ions are dissolved in an infinite amount of water solution forming one mole of aqueous ions under standard conditions (298K and pressure of 1 atmosphere)

When you dissolve cations/anions in water, you used more energy to form the bonds between the water ions and the cations/anions; hence the amount of bonds formed releases more energy than the amount of bonds broken in the solution, so the overall enthalpy change is negative, as bond forming is an exothermic process.
4. Ah I see. Thank you
5. To be precise, there is no bond breaking at all - by definition of the enthalpy of hydration you use "scattered" gaseous ions (i.e. gaseous ions in theory far enough apart that there are no forces of attraction holding them together and thus no bonds to be broken) whereas when the ions are then dissolved in water you form bonds between the ions and the water, meaning that your reaction will be exothermic overall.

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Updated: April 11, 2013
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