Oceans (O) Flashcards
Dissolving ionic substances in polar substances:
Most ionic substances only dissolve in polar solvents
Ionic bonds are broken when an ionic substance dissolves
Ion-dipole bonds form between solvent molecules and free ions
The strength of the bonds formed is similar to the strength of the bonds broken
Therefore, the energy released by bond formation is sufficient to compensate for the energy required to break the bond between
Dissolving ionic substances in non-polar substances:
Non-polar solvents don’t interact strongly enough with ions to pull them away from an ionic lattice
The electrostatic forces between the ions are stronger than any bonds that could form between the ions and non-polar solvent molecules
Ion-dipole bonds formed between the solvent molecules and ions are much weaker than the bonds that are broken
So the energy released by bond formation - not sufficient to compensate for the energy required to break the bond between ions
Dissolving covalent substances in non-polar solvents
Most covalent substances only dissolve in non-polar solvents
Intermolecular bonds between covalent molecules = weak
They can be broken by non-polar solvent molecules
Dissolving covalent substances in polar solvents
Covalent substances don’t tend to dissolve in polar substances as
Instantaneous dipole- induced dipole bonds between its molecules and water are weaker than the hydrogen bonds in water
Dissolving ionic compounds in water:
1) H-O bonds in water = polar
2) ion-dipole bonds can be formed between water molecules and the dissolved ions
3) Positive H ions are attracted to negative solute ion/ negative O ions are attracted to positive solute ion
4) Ions separate from ionic lattice + become surrounded by water molecules- hydration (if the solvent isn’t water = solvation) Ionic dipole bonds pull lattice apart
Enthalpy change of hydration definition
Enthalpy change when 1 mole of gaseous ion is hydrated by forming bonds to water molecules
(Gas———— Aq)
Hydrated ion definition
An ion bonded to a water molecule
When will the enthalpy change of hydration be more negative?
- Ionic charge greater
- Ionic radius is smaller
- Greater charge density of ion
Ions with greater charge density = attracts more water molecules + forms stronger ion-dipole bonds. Energy released when forming bonds is greater
Lattice enthalpy definition
Enthalpy change when 1 mole of an ionic solid is formed from gaseous ions
(Gas——-Solid)
When will the lattice enthalpy be more negative?
- Ionic charge greater
- Ionic radius is smaller
- Greater charge density of ions in lattice
Ions with greater charge density = attract each other more strongly- greater electrostatic attraction
Why is lattice enthalpy always negative?
Bonds are broken and energy is released (exothermic)
The more negative the lattice enthalpy, the stronger the bonding
Enthalpy change of solution definition
The enthalpy change when 1 mole of solute dissolves to form a solution
(Solid—— Aq)
Enthalpy change of solution equation
Enthalpy solution = (En hydration (cation) + En hydration (anion)) - lattice enthalpy
Enthalpy change of solution is the overall effect on the enthalpy when something dissolves
It’s the net effect of the lattice enthalpy and enthalpy change of hydration
For a compound to dissolve, the enthalpy change of solution should be….
exothermic (negative) or very slightly endothermic
Techniques and procedures for measuring the enthalpy change of solution
1) Known mass of solid dissolved in a known mass of solvent + temp change measured
2) Energy exchanged with water calculated ( E= mc deltaT )
3) Scale up to find for 1 mole of solute
Overall, the greater the charge density of ions……
- The greater the electrostatic attraction + more exothermic the lattice enthalpy
- The greater the attraction of water molecules + more exothermic the hydration enthalpy
Greenhouse effect:
1) Solar energy reaches Earth mainly as visible and UV
2) Earth absorbs some of this energy, heating up and radiating IR
3) Greenhouse gases (eg carbon dioxide + methane) in troposphere absorb some of this IR in the ‘IR window’
4) Absorption of IR by greenhouse gas molecules increases the vibrational energy of their bonds. The energy is transferred to other molecules by collisions, increasing their kinetic energy and raising the temperature
5) Greenhouse gas molecules also re-emit some of the absorbed IR in all directions, some of which heats up the Earth
Increased concentrations of greenhouse gases—— enhanced greenhouse effect
Greenhouse effect definition
Process in which infrared radiation emitted by the Earth is absorbed by gases in the atmosphere, causing a temperature increase
Greenhouse gasses definition
Gasses that absorb infrared radiation emitted by the Earth
IR window definition
The wavelengths of IR that greenhouse gasses do not absorb