Section 9 Oceans Flashcards
(22 cards)
Dissolving ionic substances?
Most ionic substances only dissolve in polar solvents.
Non-polar molecules don’t interact strongly enough with ions to pull away from ionic lattice. Because electrostatic forces between ions way stronger than any bond that could form between an ion and a non-polar solvent molecule.
Dissolving covalent substances?
Most covalent substances only dissolve in non-polar solvents.
Intermolecular bonds between covalent molecules tend to be pretty weak, so can he broken by non polar solvent molecules.
Covalent substances tend not to dissolve in polar solvents. E.g. iodine doesn’t dissolve in water because hydrogen bonds between molecules are stronger than bonds that would form between iodine and water.
Standard lattice enthalpy?
Enthalpy change when 1 mole of an ionic lattice is formed from its gaseous ions under standard conditions.
Always negative because bonds are formed and energy is released.
Enthalpy change of hydration?
Enthalpy change when 1 mole of aqueous ions is formed from gaseous ions.
E.g
Na+ (g) > Na+(aq)
Enthalpy change of solution?
Net effect of lattice enthalpy and enthalpy of hydration
Enthalpy change when 1 mole of an ionic substance dissolves in enough solvent to form an infinitely dilute solution.
Measuring enthalpy change of solution?
Put reactants in polystyrene beaker with lid with hole for thermometer and measure temperature change.
How does charge density affect lattice enthalpy?
Ions with a higher charge density are better at attracting each other in ionic lattice than those with lower charge densities
This means the ionic bonds are stronger, so more energy is released when bonds are made giving them a more exothermic lattice enthalpy.
Affect of charge density on enthalpy change of hydration?
Ions with a higher charge density better at attracting water molecules than those with lower charge densities.
This means the intermolecular bonds are stronger, meaning more energy is released when the bonds are made giving them a more exothermic hydration enthalpy.
Entropy?
Entropy, S, is a measure of number of ways that particles can be arranged and the number of ways that the enemy can be shared out between the particles.
Substances more energetically stable when there’s more disorder. So particles move to try and increase the entropy.
Factors affecting entropy?
Physical state: solid, liquid, gas in order of increasing entropy.
Amount of energy more energy higher entropy
Number of particles more particles = more ways to arrange so higher entropy
E.g
N2O4 > 2NO2 entropy increases because number of moles increases.
Total entropy change formula?
ΔtotS = ΔsysS + ΔsurrS
Units of entropy are J K-¹ mol-¹
Entropy change of system formula?
ΔsysS = Sproducts - Sreactants
Entropy change of surroundings
ΔsurrS = -ΔH/T
ΔH = enthalpy change J mol-1
T = temperature (K)
How do you know if a reaction is feasible?
If ΔtotS = 0 or more
Brønsted-Lowry acids?
Proton donors- release hydrogen ions when they’re mixed with water.
Always combined with H2O to form hydroxonium ions, H3O+
Brønsted-Lowry bases?
Proton acceptors
When in solution take hydrogen ions from molecules of water.
Strong and weak acids and bases?
Strong acids dissociate almost completely in water, almost all protons released. E.g. HCl
Weak acids dissociate only slightly in water- so only small numbers of H+ ions are formed. Equilibrium set up which lies well to the left.
Strong bases like NaOH ionise almost completely in water.
Conjugate pairs?
Two species that differ by one hydrogen ion.
One with proton acid
One without proton base
What makes a solution?
[H+] = [OH-]
If H+ is greater acidic
If OH- is greater alkaline
What is pH?
-log10 [H+]
Kw in pure water?
Kw = [H+][OH-]
So in pure water can be written as:
Kw = [H+]²
Can be used to find pH of strong base.
Ka?
Acid dissociation constant
Ka=[H+]² ÷ HA/HF
HA is strong acid
HF is weak acid
