22.2 Enthalpy Changes in Solution
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What is enthalpy change of solution?
enthalpy change 1 mole of SOLUTE DISSOLVED completely in WATER under standard conditions
1 mol Ionic lattice (s) ⟶ Cation (aq) + anion (aq)
enthalpy change of solution
What events occur during dissolving an ionic lattice?
• Polar H2O ATTRACTED to SURFACE of ionic lattice • H𝛅+ to anions, O𝛅- to cations • Electrostatic attraction between opposite ions decreases, WEAKENS ionic bonds. • Ionic bonds BREAK, forming gaseous ions. • Water molecules SURROUND ions, now aqueous.
What is hydration enthalpy?
enthalpy change 1 MOL GASEOUS IONS hydrated.
What occurs during enthalpy of hydration?
formation of permanent dipoles between gaseous ions + water molecules forming aqueous ions
How do you determine the enthalpy change of solution?
• weigh sample of solid ionic lattice • measure volume + starting temperature of water. • add solid ionic compound into water + stir with thermometer until fully dissolved. • measure END TEMPERATURE. Q = mcΔT to find energy change, then divide by number of moles + change sign.
What are the units for Q in Q = mcΔT?
joules, J
What are the units for m in Q = mcΔT?
kilograms, Kg
What are the units for c in Q = mcΔT?
Joules per gram per degree Kelvin, J/gK
What are the units for ΔT in Q = mcΔT?
does not matter since a change of 1 kelvin is equal to 1 degree celsius.
What is Q = mcΔT used to calculate?
the change in heat energy. (usually of the surroundings)
During enthalpy change of solution, energy is either absorbed or released by the ions of the _____ when _____ + becoming aqueous. This energy is either absorbed by or taken from the _____. so, calculating the temperature change of a mass of water, + the energy required to cause that temperature change calculates the overall energy change of the surroundings. The increase in the energy of the surroundings = the energy _____ by the dissolving ionic lattice.
solid ionic lattice, dissociating, water, released
What changes of state occur during enthalpy change of solution?
solid ionic compound ⟶ gaseous ions ⟶ aqueous ions
1 mol gaseous ions + aq ⟶ 1 mol aqueous ions
enthalpy of hydration
What occurs during hydration of gaseous ions?
formation of INTERMOLECULAR forces (permanent dipole) between GASEOUS IONS + WATER MOLECULES (formation of intermolecular forces is exothermic)
Solution enthalpy = _____ + _____
-LE + Hyd (of both cation & anion)
Is solution enthalpy endothermic or exotherimic?
can be EITHER, depending on the relative magnitude of lattice enthalpy + hydration enthalpy. sol = -LE + Hyd
Why does solution enthalpy consist of -LE + Hyd?
Solid ionic lattice ⟶ gaseous ions - LE gaseous ions ⟶ aqueous ions + Hyd