CHAPTER 6: HOW SUBSTANCES DISSOLVE Flashcards
what is common between all solutions
- Cannot see any solute visually
- Solute and solvent cannot be distinguished from each other visually
- Amount of dissolved solute can vary from one solution to another
- Solution is HOMOGENEOUS (uniform throughout)
- The particles are not evenly distributed throughout the solution, so this is not a homogeneous mixture
like substances dissolve like
polar and non-polar
dissolution process
- Solute particles are attracted to some of the solvent particles.
- The particles of the solute are separated from one another.
- Some of the solvent particles are separated from one another to allow the solute particles to disperse throughout the liquid.
- Solvent particles not attached to solute particles will still be attracted to other solvent particles.
- The solute particles are surrounded by solvent molecules and carried throughout the solution.
forces in dissolution process
- force of attraction holding particles of solid substance togeter
- force of attraction between water molecules and particles of the solid substance
- force of attraction between molecules
for successful dissolution: forces of attraction between water molecules and particles of solid substnace need to be greater than or equal to the other forces
ways commpunds dissolve in water
- forming hydrogen bonds with water (polar molecular compounds w OH group)
- no chemical reaction occurs
- molecules dissolve by forming hydrogen bonds w water - forming ions (highly polar compounds can ionise)
- dissociation of covalent compounds involves formation of new products
- becomes ionised - dissolution (only for ionic compounds)
- dissociation of ionic compounds does no involved formation of new products
- lattice start to break apart and ions start to separate
insoluble ionic compounds
- Not all ionic compounds are soluble in water.
- Remember forces of attraction between the solute-solvent > solute-solute and solvent-solvent
- In this case, the forces of attraction between the ionic compounds and water are not stronger than the strong electrostatic forces between the ions to break the ionic lattice
- Ionic compounds that are ‘insoluble’ often are soluble to varying degrees in water
what is chromatograohy
- a technique used to separate the components in a mixture
paper chromatography
- The pattern of bands or spots is called a chromatogram
- All methods of chromatography have a
❖ Stationary phase
❖ Mobile (moving) phase
E.g. components in ink are carried up over the paper (stationary phase) by the solvent (mobile phase) - Components in plant pigments and food dyes are usually easily seen.
However, many organic compounds fluoresce and appear blue under UV light. - Amino acids can be sprayed with ninhydrin to give blue- and brown- coloured compounds
how paper chromatography works
- The rate of movement of each ink component depends on:
❖ How strongly the component adsorbs onto the stationary phase
❖ How readily the component desorbs from the stationary phase and dissolves in the mobile phase
- This depends on the polarity of substances
- As the components in the ink are swept upwards over the stationary phase by the solvent, they undergo a continual process of adsorption onto the solid stationary phase, followed by desorption and dissolving into the mobile phase.
MOBILE PHASE: POLAR
STATIONARY PHASE: NON-POLAR
paper vs thin layer chromatography (TLC)
TLC is very similar to paper chromatography but does not always use paper
(may use a thin layer of aluminium oxide (Al2O3) or silicon dioxide(SiO2) on a glass plate)
benefits of paper chromatography vs thin layer chromatography
paper:
* cheaper
* little prep
* more efficient for polar/water soluble compounds
* easy to handle and store
thin layer:
* detects smaller ampounts
* better separation of less polar compounds
* corrosive materials can be used
* a wide range of stationary phases is available
how do we analyse chromatograms
- components of a mixture can be identified from a chromatogram by:
- introducing standard of known chemicals on the same chromatogram
- calculating retardation facter Rf
THESE ARE QUALITATIVE
Rf values
distance component travelled from origin / distance solvent travelled from origi
● Each component has a characteristic Rf Value for the conditions that a chromatogram was produced
● Rf Values of unknown components can be compared to Rf values of common substances
will always be less than one and the component most strongly adsorbed onto the stationary phase moves the shortest distance and has the lowest Rf value
when sodium chloride dissolves in water
When sodium chloride dissolves in water:
Ionic bonds within the sodium chloride lattice are broken.
Hydrogen bonds between water molecules are broken.
Ion–dipole attractions form between ions and polar water molecules.
adsorption
the attraction of one substance to the surface of another
