6. EDD VSEPR ad IMFs Flashcards
(73 cards)
1
Q
VSEPR Theory
A
- Stands for: Valence Shell Electron Pair Repulsion
- Electron pairs repel each other → arrange as far apart as possible
- Applies to bonding & lone pairs
- Affects molecular shape
- Most atoms aim for 8 valence electrons (Octet Rule)
- Hydrogen follows the Duet Rule (2 electrons)
2
Q
Liner Molecule Shape
A
Examples: HCL, N2, CO2, HCN
- No valence electrons spare on central atom
3
Q
Tetrahedral Molecular Shape
A
Examples: CH4, NH4^+, PO4^3-
4
Q
Trigonal Pyramidal Shape
A
- Valence electrons on central atoms
Examples: NH3, H3O^+, PCl3
5
Q
V-Shaped / Bent Shape
A
-Valence electrons on central atom
Examples: H20, OF2, SCl2
6
Q
Trigonal Planar Shape
A
Examples: CO3^2-, BF3 (non-octet), CH2)
- No valence electrons on central atom
7
Q
What is a Chemical Bond
A
A chemical bond is a connection between atoms involving shared or transferred electrons.
In covalent bonds, electrons are shared between atoms.
8
Q
Electronegativity (EN) - EDD VSEPR and IMFs
A
Definition: Ability of an atom to attract shared electrons in a bond.
The difference in EN between atoms determines bond type.
9
Q
Polar Bond
A
Formed between two different atoms
Electrons are shared unequally
Creates partial charges (dipoles)
One end becomes slightly negative (δ⁻)
Other end becomes slightly positive (δ⁺)
Examples:
H–Cl
O–H
N–H
10
Q
Non-Polar Bonds
A
Formed between two of the same atom
Electrons are shared equally
No charge difference across the bond
Examples:
H–H
O=O
Cl–Cl
N≡N
11
Q
Simplified Polar/Non-Polar Bond Rule
A
Same atoms = Non-polar bond
Different atoms = Polar bond
12
Q
What are Polar Molecules
A
- Molecules with either valence electrons on the central atoms, or different types atoms bonded to the central atom
- They have an uneven charge distribution.
They have a positive and negative end (dipole).
Example: H₂O, NH₃
13
Q
What are Non-Polar Molcules
A
Molecules where charge is evenly distributed. This happens when bonds are non-polar or dipoles cancel due to symmetry.
Example: O₂, CH₄, CO₂
14
Q
Difference between Polar and Non-Polar Molecules
A
Polar: Have polar bonds and an asymmetrical shape → uneven charge
Non-Polar: Have non-polar bonds or symmetrical shape → dipoles cancel out
Tip: Shape matters as much as bond type!
15
Q
What Are Dispersion Forces?
A
- London Dispersion Forces are the weakest type of intermolecular force, present in all molecules, but they are the only forces acting between nonpolar atoms and molecules (e.g. helium, noble gases, nonpolar hydrocarbons).
- They are the weakest type of intermolecular force, but they increase in strength by: Increasing Electrons, Increasing Molecular Mass, Increasing Molecular Size
16
Q
How do Dispersion Forces Work?
A
- Arise from the random motion of electrons that creates a temporary dipole within a molecule.
- This temporary dipole can induce a dipole in a nearby molecule.
- The resulting interaction between the temporary dipole and the induced dipole forms the dispersion force.
- These forces are: Weak electrostatic attractions, and temporary – constantly forming and disappearing
- Occur in Non-Polar Molecules
17
Q
Factors That Increase Dispersion Forces (and Boiling Point):
A
Increasing number of electrons
→ More electrons mean a larger, more polarizable electron cloud, which increases the likelihood of forming temporary dipoles.
Increasing molecular mass
→ Heavier molecules tend to have more electrons, enhancing the strength of dispersion forces.
Increasing molecular size
→ Larger molecules have more surface area for intermolecular contact, allowing stronger attractions between molecules.
Greater surface area (linear shape over branched)
→ Linear molecules can pack more closely together, increasing the area over which dispersion forces can act.
18
Q
Liner vs. Branched Molecules and Dispersion Forces
A
- Linear Molecules have a higher MPt and BPt compared to branching molecules of the same molecular weight
- Linear molecules are able to align better with each other so the dispersion forces can act over a large area
19
Q
How can Polar Molecules have Dispersion Forces?
A
Even though polar molecules have permanent dipoles, their electrons are still moving. This creates temporary dipoles, causing weak dispersion forces in addition to their stronger intermolecular forces.
20
Q
Key Factors Affecting Dispersion Force Strength?
A
1. Number of Electrons (Molecular Size/Mass):
- More electrons = greater polarizability = stronger dispersion forces
- Noble gases and alkanes show this trend (e.g. fluorine to iodine)
- Boiling point increases as dispersion forces increase
(Example: CH₄ < C₂H₆ < C₃H₈ < C₄H₁₀ < C₅H₁₂)
2. Shape of the Molecule
- Linear molecules can pack more closely and have greater surface area for contact → stronger dispersion forces
- Branched/spherical molecules have less surface area, resulting in weaker dispersion forces
- Example:
- Pentane (linear) → boiling point 36.1°C
- Methylbutane (branched) → 27.9°C
- Dimethylpropane (spherical) → 9.5°C
21
Q
How Boiling Point Increases with Stronger Dispersion Forces?
A
Boiling point reflects how much energy is needed to separate molecules from each other into the gas phase. Stronger intermolecular forces = higher boiling point, because more energy is required to overcome those attractions.
22
Q
What is Vapour Pressure?
A
- Vapour pressure is a measure of how easily a liquid’s molecules escape into the gas phase. It shows how likely a substance is to evaporate.
23
Q
Vapour Pressure & Boiling Point
A
- Boiling Point occurs when the vapour pressure of a substance equals atmospheric pressure.
- A substance with a high vapour pressure at a given temperature will reach this point faster → lower boiling point.
- Higher vapour pressure = weaker intermolecular forces (IMF) → easier for molecules to escape into gas phase.
- So: Low IMF → High Vapour Pressure → Low Boiling Point
24
Q
Difference between evaporation and boiling?
A
- Evaporation is a slow surface process that can happen at any temperature. Boiling occurs throughout the liquid at a specific temperature (the boiling point), when the liquid’s vapour pressure equals the surrounding atmospheric pressure.
25
How do IMFs affect vapour pressure?
- Weaker intermolecular forces allow molecules to escape more easily, resulting in higher vapour pressure. Stronger IMFs hold the molecules in the liquid more tightly, so vapour pressure is lower.
26
What are Dipole-Dipole Forces
These are attractive forces between the positive end of one polar molecule and the negative end of another.
The electrostatic attraction between oppositely charged dipoles is what causes these forces
- Occur in polar molecules (with polar bonds)
27
Effects of Dipole–Dipole Forces:
Increase melting and boiling points of polar substances.
Polar molecules are more strongly attracted to each other than nonpolar ones of similar size.
This extra attraction raises the energy needed to separate them (i.e., boil or melt)
28
Solubility and Dipole-Dipole Forces
"Like dissolves like": Polar solvents dissolve polar solutes; non-polar solvents dissolve non-polar solutes.
Polar molecules attract each other via dipole–dipole forces, promoting solubility.
Ethanol (polar) dissolves in water (polar).
Mineral turpentine (non-polar) does not dissolve in water.
29
What determines if a solute will dissolve in a solvent?
- The intermolecular forces between the solute and solvent must be as strong as or stronger than the forces between solute particles and between solvent particles. If this condition is met, the solute will dissolve.
30
What is a Solute?
- A solute is the substance that dissolves in a solvent. It becomes uniformly dispersed throughout the solvent and is usually present in a smaller amount. Examples include sugar, salt, and gases.
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What is a Solvent?
- A solvent is the substance that dissolves the solute. It is usually present in a larger amount. Examples include water, alcohol, and even molten metals like zinc (in brass).
32
What are the energy changes when a substance dissolves
- Breaking solute-solute and solvent-solvent bonds is endothermic (requires energy), while forming solute-solvent bonds is exothermic (releases energy). Dissolving will occur if the energy released is equal to or greater than the energy absorbed.
33
What are solute-solute, solvent-solvent, and solute-solvent interactions?
- **Solute–solute interactions:** Attractions between particles of the solute. These resist mixing.
- **Solvent–solvent interactions:** Attractions between particles of the solvent. These also resist mixing.
- **Solute–solvent interactions:** Attractions between solute and solvent particles. These assist mixing.
34
What is the "Like Dissolves Like" rule?
- Substances with similar intermolecular forces dissolve in each other. Polar solutes dissolve in polar solvents; non-polar in non-polar. This is a general rule, not a full explanation.
35
What determines whether solubility actually occurs?
- Solubility happens when the assistance to mixing (from solute-solvent interactions) is greater than or equal to the resistance (from solute-solute and solvent-solvent interactions).
36
Does copper sulfate (CuSO₄) dissolve in petrol? Why or why not?
- No. CuSO₄ has strong ionic bonds (electrostatic forces), resisting mixing. Petrol is non-polar and has only weak dispersion forces, also resisting mixing. Since there is no significant solute–solvent interaction (they are chemically incompatible), resistance outweighs assistance, and they will not mix.
37
Does copper sulfate (CuSO₄) dissolve in methanol? Why?
- Yes. CuSO₄ has strong ionic bonds, which resist mixing. Methanol is polar and can form hydrogen bonds, giving it moderate resistance. However, methanol and CuSO₄ can interact through ion–dipole interactions, which provide strong assistance to mixing. Assistance outweighs resistance, so a solution forms.
38
Does ethanol dissolve in water? Why?
- Yes. Both ethanol and water are polar and capable of hydrogen bonding. Although both have internal cohesion, their ability to form hydrogen bonds with each other provides strong solute-solvent interaction. Assistance outweighs resistance, so ethanol and water are miscible.
39
Does petrol dissolve in water? Why not?
- No. Petrol is non-polar and has only dispersion forces, while water is polar and forms hydrogen bonds. There is no strong solute–solvent interaction. The internal resistance in both substances outweighs any potential assistance, so petrol and water are immiscible.
40
Ion–Dipole Forces
Not the same as dipole–dipole forces.
Occur when an ion is attracted to the dipole of a polar molecule. (Strongest IMF)
Important in dissolving ionic substances (e.g., salts in water).
41
What is Hydrogen Bonding?
Hydrogen bonds are directional intermolecular forces (meaning they occur in a specific alignment).
They can be as strong as 12% of a covalent carbon–carbon bond — much stronger than other intermolecular forces.
Hydrogen bonding only occurs in molecules containing:
- H–F
- H–O
- H–N
- (Hydrogen must be directly bonded to fluorine, oxygen, or nitrogen)
42
How Hydrogen Bonding Works:
A hydrogen bond forms due to electrostatic attraction between:
- A lone pair of electrons on F, O, or N
- And a hydrogen atom already covalently bonded to F, O, or N
This is a form of extreme dipole-dipole attraction because:
- The positive dipole on hydrogen is strongly attracted to the negative dipole of the lone pair on F, O, or N, as they are extremely electronegative.
43
Effects of Hydrogen Bonding
1. Boiling and Melting Points
- Substances capable of hydrogen bonding have significantly higher melting and boiling points than similar-sized molecules without it.
- This is due to strong intermolecular attraction requiring more energy to break.
2. Solubility
- Molecules that hydrogen bond are often extremely soluble in other hydrogen-bonding solvents (e.g., water).
- Solubility pattern: like dissolves like (e.g., alcohols dissolve well in water).
44
H2O Hydrogen Bonding Structure
**Water (H₂O):**
- Donor H atoms: 2 (each H can donate 1 H bond)
- Lone pairs (acceptors): 2
- Can form up to 4 hydrogen bonds (2 donate, 2 accept)
**Why not more?**
- Oxygen only has 2 hydrogens to donate.
- Only 2 lone pairs are available for accepting H bonds.
45
NH3 Hydrogen Bonding Structure
**Ammonia (NH₃)**
- Donor H atoms: 3
- Lone pairs (acceptors): 1
- Can typically form 2 hydrogen bonds (1 donate, 1 accept)
**Why not more?**
- Only 1 lone pair to accept hydrogen bonds.
- Although NH₃ has 3 hydrogens, only one hydrogen bond is typically donated due to:
- Molecular shape (trigonal pyramidal → steric hindrance)
- Weaker N–H polarity compared to O–H or F–H
- So, only 1 donor and 1 acceptor hydrogen bond commonly occur.
46
HF Hydrogen Bonding Structure
**Hydrogen Fluoride:**
- Donor H atoms: 1
- Lone pairs (acceptors): 3
- Can typically form 2 hydrogen bonds (1 donate, 1 accept)
**Why not more?**
- Only 1 hydrogen to donate.
- Although fluorine has 3 lone pairs, in reality only 1 is commonly used:
- HF is a small, linear molecule with strong repulsion between lone pairs.
- Steric hindrance and repulsion prevent full use of all 3 lone pairs in bonding.
47
48
What is Gas Chromatography?
- Gas Chromatography is a separation technique used to analyze mixtures of gases or volatile liquids
- It separates the mixture into individual components (analytes) and tells us:
* What substances are present (qualitative)
* How much of each analyte is present (quantitative)
49
What are the Main Components of a Gas Chromatograph?
**Injection Port:** Heater chamber that vaporizes the liquid sample immediately after injection.
**Carrier Gas (Mobile Phase):** An inert gas (generally not reactive gas like helium or nitrogen) moves the sample through the column without reacting with it.
**Column:** Coiled tube (packed or capillary - types) housed in an oven. Contains the stationary phase.
**Stationary Phase:** Non-volatile liquid coating inside the column that temporarily holds onto components based on their volatility (how easily it evaporates).
**Detector:** Measures when and how much of each analyte exits the column. Common
50
How does Gas Chromatography work?
**1) Injection:**
- Sample dissolved in a volatile solvent is injected into a heated port then instantly vaporised.
**2) Transport:**
- Carrier gas pushes the vaporises sample into the column.
**3) Separation:**
- Contents separated based on ease of vaporising (volatility), and interaction with stationary phase (e.g. intermolecular forces.)
**4) Detection:**
- Components exit the column at different times (retention times) --> detector records signal --> creates chromatogram
51
Liquid Coating (Stationary Phase), and how it works
**What does the liquid coating (stationary phase) do in Gas Chromatography:**
- The liquid coating is called the stationary phase because it does not move.
- It is a non-volatile, high boiling point liquid.
- It is either: Coated onto solid particles (In packed columns), or, coated directly on the inside wall of the tube (in capillary columns).
**Function of the Stationary Phase:**
- It interacts with each analyte (component) in the vaporised sample.
- The strength of these interactions determines how long each analyte is retained (slowed down) in the column.
**How separation occurs:**
- As the vaporized sample is carried through the column by an inert gas (mobile phase), each component:
- Moves quickly when in the mobile phase (gas)
- Slows down when it dissolves in and interacts with the stationary phase (liquid coating)
**Key Idea:**
- Each compound alternates between the gas phase and the liquid stationary phase as it moves through the column.
- When in the gas phase, the compound moves rapidly along with the carrier gas.
- When interacting with the stationary phase, the compound is temporarily held up (slowed down) by molecular interactions with the liquid coating.
- The amount of time each compound spends interacting with the stationary phase versus remaining in the gas phase determines how quickly it travels through the column.
- More time interacting with the stationary phase: Moves slower → longer retention time
- More time in the gas phase: Moves faster → shorter retention time
**Factors affecting interaction with stationary phase:**
**1) Boiling Point / Volatility:**
- High boiling point = low volatility = spends more time in liquid --> longer retention time.
- Low boiling point = high volatility = prefers gas phase --> shorter retention phase.
**2) Intermolecular Forces:**
- Stronger forces (dipole-dipole, hydrogen bonding, dispersion) = stronger attraction to liquid = longer retention
**3) Polarity (like dissolves like)**
- Polar analytes interact more with polar stationary phases.
- Non-polar analytes interact more with polar stationary phases.
- Non-polar analytes interact more with non-polar stationary phases.
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What is an Analyte
An analyte is a specific substance or chemical constituent that is being analyzed or measured in a sample.
53
What is a Chromatogram and what does it show?
- A chromatogram is a graph of detector signal vs time
- Each peak represents a different component (analyte) in the sample.
- Retention time (tR) = time an analyte takes to travel through the column. This is used to identify it (qualitative)
- Peak area or height is proportional to the amount of analyte, which is used to quantify it (quantitative).
54
What detectors are used in Gas Chromotography?
**Flame Ionization Detector (FID):**
- Burns analytes in a hydrogen-air flame.
- Produces ions → current measured → peak generated.
- Signal strength ∝ amount of analyte.
**Mass Spectrometer (MS):**
- Breaks analytes into fragments.
- Measures mass-to-charge ratio (m/z).
- Provides precise ID → useful in forensics, doping control.
55
What Affects Retention Time (tR) in Gas Chromatography
**1. Boiling Point / Volatility**
Low BP = High volatility:
Prefers gas/mobile phase → interacts less with stationary phase → short retention time.
High BP = Low volatility:
Prefers liquid/stationary phase → sticks longer → longer retention time.
**2. Intermolecular Forces (IMFs) with Stationary Phase**
Stronger IMFs = more attraction = longer time in column.
Types of forces:
Dispersion forces (increase with molar mass/surface area)
Dipole–dipole interactions (polar molecules)
Hydrogen bonding (if functional groups allow it)
The nature of the stationary phase (polar or non-polar) matters:
Polar stationary phase interacts more with polar analytes.
Non-polar phase interacts more with non-polar analytes.
“Like dissolves like” applies.
**3. Polarity**
Affects how well an analyte dissolves/interacts with the stationary phase.
If both the analyte and stationary phase are polar or both non-polar → stronger interaction → longer tR.
**4. Molar Mass**
Higher molar mass = more electrons = stronger dispersion forces = more interaction with stationary phase = longer retention time.
**5. Column Temperature**
Higher temp → analytes spend less time in the stationary phase → shorter tR.
Used to speed up separation or elute high-boiling components faster.
Temperature programming (ramping) helps separate compounds with a wide range of BPs.
**6. Stationary Phase Composition**
Affects which types of molecules are retained more strongly.
E.g., polar vs non-polar, or specific functional group interactions.
56
How is Gas Chromotography Calibrated and used to identify unknowns.
- Known standard substances are run first to determine their retention times.
- The unknown sample is compared:
- Match retention time to identify analyte
- Compare peak areas to determine concentration
Note that retention time alone isn't always definitive --> identification should be supported by standards or mass spectrometry.
57
Real world applications of Gas Chromatography
Forensic analysis (e.g., drugs, poisons, arson investigation)
Environmental testing (e.g., air/water pollutants, CO detection)
Pharmaceutical quality control
Petroleum and food industry (e.g., flavour/fragrance analysis)
58
What is HPLC
HPLC stands for High Performance Liquid Chromatography.
It is a powerful and precise technique used to separate, identify, and quantify components in a liquid sample, especially when:
- The substances can't be vaporized (unlike in gas chromatography)
- Or they decompose under heat
59
Key Components of HPLC
**1) High-Pressure Pump**
- Pushes the mobile phase (solvent) through the system at high pressure (up to 40-50 MPa)
**2) Solvent Reservoir**
- Hold the mobile phase (can be a mix of polar or non-polar solvents depending on the sample)
**3) Injector**
- A microsyringe injects the liquid sample into the stream of solvent before it enters the column.
**4) Chromatography Column**
- A short, stainless steel tube packed with tiny solid particles coated in stationary phase.
**5) Stationary Phase**
- Fine particles of silica (SiO2), sometimes chemically modified (e.g., C18, benzene rings)
**6) Detector**
- Detects analytes as they elute (exit) from the column.
- Types: UV, Mass Spectrometry, Fluorescence, Refractive Index
- Sends signals to a computer which generates a chromatogram (retention time vs. signal strength
60
How Does HPLC Work?
1) The mobile phase is pumped through the column under high pressure.
2) The sample is injected into this flowing solvent.
3) The mixture moves through the stationary phase inside the column.
4) Each compound interacts with the stationary phase differently:
- Some move slower (stronger interaction),
- Others move faster (weaker interaction).
5) As compounds exit the column, the detector records:
- Retention time (used to identify compounds),
- Peak size/area (used to measure concentration).
61
Chromatograph Interpretation
A chromatogram shows peaks:
- Position (x-axis) = Retention time.
- Height/area (y-axis) = Amount (concentration).
To find concentration:
- Create a standard curve (e.g. peak absorbance vs concentration).
- Compare the unknown sample’s peak to this curve to determine its concentration.
62
Factors Affecting Retention Time in HPLC
**1) Polarity**
**In normal-phase HPLC:**
- Stationary phase is polar, mobile phase is non-polar.
- Polar compounds interact more and elute (exit) slower.
**- In reverse-phase HPLC:**
- Stationary phase is non-polar, mobile phase is polar
- Non-polar analytes are retained longer.
- More polar analytes elute first
**Stationary Phase Particle Size**
- The stationary phase is made of tiny silica particles packed inside the column.
- Smaller silica particles = more surface area for analytes to interact with.
This leads to better separation:
- Components in the mixture come out at different times.
- Producing clear, sharp, non-overlapping peaks on the chromatogram
**Solvent (Mobile Phase) Polarity:**
- The mobile phase is the solvent (or mixture of solvents) that flows through the column and carries the sample.
- Its polarity affects how strongly analytes stay dissolved in the mobile phase vs. how much they interact with the stationary phase. - As like dissolves like
- By adjusting the polarity of the solvent, you can:
Control retention times of analytes
Improve separation between similar compounds,
Change elution order in reverse-phase or normal-phase HPLC.
**Molecular Properties:**
- A molecule's physical and chemical properties affect how it moves through the column.
_Polarity / Hydrophobicity_:
- In reverse phase HPLC: Polar molecules prefer the polar mobile phase --> elute faster.
- Non-Polar molecules interact more with the non-polar stationary phase --> elute slower
_Charge (Ion-Exchange HPLC)_
- Stationary phase is charged.
- Analytes with the opposite charge bind more strongly --> lower retention time.
- Used for amino acids, proteins, and nucleotides,
_Size (Size-Exclusion HPLC)_:
- Stationary phase has porous particles.
- Small molecules enter the pores → move slower.
- Large molecules cannot enter pores → move faster.
- Used for separating large biomolecules or polymers.
63
Types of HPLC
**🔹 Normal Phase HPLC**
Stationary phase: polar (e.g., unmodified silica)
Mobile phase: non-polar (e.g., hexane)
Polar analytes are retained longer.
**🔹 Reverse Phase HPLC (most common)**
Stationary phase: non-polar (e.g., C18 hydrocarbon chains)
Mobile phase: polar (e.g., water with methanol)
Non-polar analytes are retained longer.
**🔹 Size Exclusion HPLC**
Stationary phase is porous.
Small molecules enter pores → slower.
Large molecules bypass pores → elute faster.
**🔹 Ion Exchange HPLC**
Stationary phase is charged.
Separation based on electrostatic attraction:
Positively charged analytes retained on negatively charged stationary phase (and vice versa).
64
What is TLC (Thin Layer Chromatography)
Thin Layer Chromatography is a simple, quick, and inexpensive separation technique used to:
- Identify compounds in a mixture,
- Check purity,
- Compare substances by their movement up a plate.
TLC is similar to paper chromatography, but is uses a glass or plastic plate coated with a thin layer of adsorbent solid, such as:
Silica (SiO2) or Alumiona (Al2O3) - Both are polar
Note: Adsorbent is a material that can accumulate another substance onto its surface without causing any chemical changes to either the adsorbent or the substance being adsorbed.
65
TLC Components
**Stationary Phase:**
- A thin layer of polar solid (silica or alumina) coated on a glass or plastic slide.
- It does not move during the experiment.
- Interacts with molecules in the sample via polarity and adsorption (capability of all solid substances to attract to their surfaces molecules of gases or solutions with which they are in contact.)
**Mobile Phase (Solvent):**
- A solvent or solvent mixture that moves up the plate by capillary action.
- Solvent polarity can be adjusted to improve separation.
- The choice of mobile phase determines how far different compounds travel .
66
How Does TLC Work?
- A small spot of the sample is applied near the bottom of the TLC plate.
- The plate is placed in a developing chamber with a shallow layer of solvent (mobile phase)
_- As the solvent travels up the plate:_
- It carries the components of the sample with it.
- Each component moves at a different rate, depending on its polarity, and its solubility in the solvent vs. its adsorption to the stationary phase.
- Once the solvent reaches near the top, the plate is removed and dried.
- The separated spots can be visulaised and Rf values calculated.
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Polarity and Separation in TLC
Polar compounds interact strongly with the polar stationary phase (silica or alumina) → move slower.
Non-polar compounds are less attracted to the plate and move faster with the solvent front → travel further.
So, the more polar the compound, the lower its Rf (retardation factor).
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What is the Retardation Factor (Rf)
- Used to compare how far each compound travels relative to the solvent.
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Which Substances move up faster in Paper / Thin Layer chromatography
In thin layer chromatography (TLC) or paper chromatography, the substance that moves higher up the paper is the one that is:
More soluble in the mobile phase (solvent)
Less attracted to the stationary phase (paper or TLC plate)
So:
Non-polar substances move further in non-polar solvents.
Polar substances move further in polar solvents.
Example (with a polar stationary phase like paper):
A non-polar substance is less attracted to the paper → moves further up.
A polar substance is more attracted to the paper → moves less.
✅ The higher up it goes, the more soluble it is in the solvent and the less it sticks to the stationary phase.
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Uses of TLC
Purity checks for chemicals and natural products.
Screening tests for:
Dyes, plant extracts, food colorings.
Drugs of abuse: opioids, amphetamines, cocaine, marijuana.
Quality control in industries like pharmaceuticals, cosmetics, and food.
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TLC Chromatography Advantages and Disadvantages
**✅ Advantages:**
- Simple and inexpensive — no advanced equipment needed.
- Fast results — separation usually takes just a few minutes.
- Good for comparing samples — useful in purity testing and detecting presence of substances.
- Many compounds can be tested simultaneously on the same plate.
**❌ Disadvantages:**
- Not quantitative — can’t accurately measure concentrations.
- Lower resolution — not suitable for complex mixtures.
- Rf values depend on conditions (solvent, temp, etc.) → hard to reproduce.
- Some compounds are colorless → need UV light or stains to see them.
- Prone to false positives — confirmation needed with GC/MS.
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Gas Chromatography Advantages and Disadvantages
**✅ Advantages:**
High resolution — excellent for separating volatile compounds.
Quantitative and qualitative — gives both retention time and concentration (peak area).
Very sensitive — detects trace amounts of substances.
Fast analysis for volatile samples (minutes).
**❌ Disadvantages:**
Only works with volatile or thermally stable compounds — cannot use for large or heat-sensitive molecules.
Sample must be vaporised — limits its application.
Expensive equipment and requires carrier gas (e.g. helium).
Needs skilled operation and proper calibration.
73
HPLC Advantages and Disadvantages
**✅ Advantages:**
Works with non-volatile and heat-sensitive compounds — ideal for large biomolecules.
Very high resolution and precision — separates complex mixtures.
Quantitative and reproducible — accurate measurement of concentration.
Wide variety of column types → customisable for different substances.
**❌ Disadvantages:**
More expensive and complex than TLC.
Requires high pressure pumps and careful maintenance.
Solvent use can be costly and sometimes hazardous.
Longer analysis time compared to GC for small molecules.
