CHMA11H3 Lecture 3: CHMA11 Lecture 3

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CHMA11 Lecture 3: Solutions part 2
- Heat of Hydration: enthalpy change that occurs when 1 mol of a gaseous solute’s ions
are dissolved in water
o The ions-dipole interactions between a dissolved ion and surrounding water
molecules are much stronger than the hydrogen bonds in the water
This means that the heat od hydration value tends to always be very
negative
o Represented by the following symbol
Hhyd
o Hsolute : in ionic compounds, refers to the energy needed to separate the solute
into its constituent particles
o Enthalpy of ionic aqueous solutions
For ionic aqueous solutions, the overall enthalpy of solution value
depends mostly on the relative magnitudes of the heat of hydration and
Hsolute values with 3 possible outcomes
1: The heat of hydration is greater than the Hsolute
o In this case, the amount of energy needed to separate the
solute into its constituent ions is less than the energy given
off when the ions are hydrated
o The enthalpy change of the solution will be negative
This process is exothermic
o When these solutes dissolve in water, the solution formed
will feel warm
2: The heat of hydration is less than the Hsolute
o In this case, the amount of energy needed to separate the
solute into its constituent ions is greater than the energy
given off when the ions are hydrated
o The enthalpy change of the solution will be positive
This process is endothermic
o When these solutes dissolve in water, the solution formed
will feel cool
3: The values of the heat of hydration and Hsolute are similar
o In this case, the amount of energy needed to separate the
solute into its constituent ions is very close to the amount
of energy given off when the ions are hydrated
o The enthalpy change of the solution will be approximately
zero in this case
The solution process is thus neither endothermic
nor exothermic
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