energy changes in solution formation Flashcards
(15 cards)
what are the 3 steps of dissolution? describe each step as endo or exothermic.
- solute particles separate from each other/lattice is broken: endothermic
- solvent particles separate from each other: endothermic
- solute and solvent particles mix to form a stable, homogenous solution: exothermic
describe why ∆H is either positive or negative for each of the 3 steps of dissolution
step 1 (solute particles separate): breaking bonds requires energy, ∆H positive
step 2 (solvent particles separate): breaking bonds requires energy, ∆H positive
step 3 (solute/solvent particles mix): energy is released (which is more favorable) as molecules return to a lower energy/more favorable state, ∆H negative
what is the heat of solution equation? describe each component
∆H soln = ∆H solute + ∆H solvent + ∆H mix
∆H soln: energy absorbed/released when dissolving a solute in solvent
∆H solute: energy/heat need to separate solute particles
∆H solvent: energy needed to separate solvent particles
∆H mix: energy needed to mix solute/solvent particles
what is ∆H?
enthalpy aka heat flow
in what cases is ∆H soln exothermic? endothermic?
when the sum of the endothermic processes (∆H solute + ∆H solvent) is higher than the value of exothermic processes (∆H mix), ∆H soln = endothermic
when the sum of the endothermic processes (∆H solute + ∆H solvent) is less than the value of exothermic processes (∆H mix), then ∆H soln = exothermic
what is solvation? what is hydration?
solvation= process by which a solute particle becomes surrounded by solvent particles
hydration= when solute becomes surrounded by water as the solvent particles
what is analogous to ∆H solvation (or ∆H hydration if in water)? why is this?
∆H solvation/hydration = ∆H solvent + ∆H mix
we combine ∆H solvent and ∆H mix into 1 term because they are difficult to measure individually!
accounting for ∆H solvation/hydration, what is the “new” equation for the heat of solution?
∆H soln = ∆H solute + ∆H solvation/hydration
the hydration of an ion is always ____. why?
always exothermic! ∆H always negative, energy released as new bonds are formed with polar water molecules and ions.
ion dipole forces always very strong!
what is the main factor affecting the enthalpy of hydration (∆H hydration) of an ion?
charge density!
what is charge density?
ratio of charge to volume
what is the relationship between CD and ∆H hydr?
inversely proportional relationship. as charge density goes up, ∆H hydr goes down
a higher ion charge and smaller radius indicates a ____ attraction
stronger attraction!
down a group, does CD increase or decrease? why?
CD density DECREASES down groups!
charge stays the same, while atomic radius increases, leading to lower charge to volume ratio
does CD increase or decrease across a period for metals? what about nonmetals? why?
metals: CD INCREASES! charge increases by + 1 increments, while the change in volume across a period is small
nonmetals: CD DECREASES! charge decreases by -1 increments, while the change in volume across period stays small
