Oceans Flashcards
Why do solutes dissolve best in solvents with similar intermolecular forces?
When something dissolves, bonds within the solvent and solute break and new bonds form between them
Generally speaking a substance won’t dissolve if the bonds broken are much stronger than those that will be formed
What solvent will ionic substances dissolve in?
Only in polar solvents
Non-polar molecules don’t interact strongly enough with the ions to pull them away from the ionic lattice
The electrostatic forces between the ions are stronger than bonds that could form between the ions and non-polar solvent molecules
The ion-dipole bonds formed between ionic substances and polar solvent molecules are strong enough to dissolve the solid
What solvent will covalent substances dissolve in?
Only non-polar solvents
The intermolecular bonds between covalent molecules tend to be weak so easily broken by non-polar solvent molecules
Example - iodine doesn’t dissolve well in water as the hydrogen bonds between water molecules are stronger than the bonds that would form between water molecules and iodine molecules
How do ionic solids dissolve in water?
They dissolve in water to make an aqueous solution
The H - O bonds are very polar so they have partial charges
Ion-dipole bonds can form between water molecules and dissolved ions
The delta positive on the H are attracted to the negative ions and vice versa
The ions separate from the lattice and become surrounded by water molecules
This process is called hydration and the ions are called hydrated ions
If the solvent isn’t water this process is called solvation
Unless the ionic bonds in the lattice are really strong the ion-dipole bonds will be strong enough to pull the lattice apart
What is the standard lattice enthalpy
The enthalpy change when 1 mole of an ionic lattice is formed from its gaseous ions under standard conditions
Lattice enthalpy is always negative because bonds are broken and energy is released
It’s a measure of ionic bond strength
The more negative the lattice enthalpy, the stronger the bonding
What is the enthalpy change of hydration of ions
The enthalpy change when 1 mole of aqueous ions is formed from gaseous ions
What is the enthalpy change of solution
The enthalpy change when 1 mole of an ionic substance dissolves in enough solvent to form an infinitely dilute solution
What is an infinitely dilute solution?
When you add more solvent there is no change in concentration because there is already so much solvent
Calculating pH
pH = -log10 [H+]
What is charge density?
Ions with a higher charge density are better at attracting ions of the opposite charge
The electrostatic attraction is much stronger
Charge density affects lattice enthalpy and enthalpy of hydration
Lattice enthalpy and charge density
Ions with a higher charge density are better at attracting each other in ionic lattices than those with a lower charge density
This means the ionic bonds are stronger so more energy is released when the bonds are made
More exothermic lattice enthalpy
Enthalpy of hydration and charge density
Ions with a higher charge density are better at attracting water molecules than those with lower charge densities
This means intermolecular bonds are stronger
More energy is released when the bonds are made
More exothermic hydration enthalpy
What is entropy?
A measure of the number of ways particles can be arranged and the number of ways that the energy can be shared out between the particles
Substances are more energetically stable when there’s more disorder
Particles move to increase entropy
What 3 things affect entropy?
Physical state
Solid lowest entropy, very ordered
The amount of energy a substance has - energy can be measured in quanta, the more quant a substance has, the more ways they can be arranged and the greater the entropy
More particles means more entropy as there’s more ways their energy can be arranged
TOTAL entropy change =
sum of the entropy changes of the system + the surroundings
Reactions with a positive value of total entropy change are more likely to occur spontaneously
Entropy change of system =
Sproducts - Sreactants
Entropy change of surroundings =
- delta H (enthalpy change in J)
divided by
Temperature in K
Why wont a reaction be feasible unless total entropy change is positive?
Positive or 0
If the entropy change in the system is negative, the entropy change in the surroundings must be positive and at least as large as delta sys for the reaction to be feasible
What is a feasible reaction?
One that, once started, will carry on to completion without any energy being supplied to it
Calculating solubility
solubility / Mr
Solubility often has gdm-3 units
It changes with temperature so is always given at a specified temp
When calculating Ksp you have to make the assumption that the volume of solution does not change when the solid dissolves in it
What is a Bronsted Lowry acid?
Proton donors
They release H+ ions when mixed with water
the H+ ions always combine with H2O to form hydroxonium ions
H3O+
What is a Bronsted Lowry base?
Proton acceptors
When in solution they grab hydrogen ions from water molecules
B + H2O -> BH+ + OH-
Strong and weak acids
Strong acids dissociate or ionise almost completely in water
Weak acids dissociate only slightly so only a small number of H+ ions are formed
Strong bases
Ionise almost completely in water too
Why are ionic substances soluble in water
Ionic bonds break
Hydrogen bonds in water break
Ion-dipole break
Similar strength between bonds broken and bonds made
