lab techniques

MCAT lab techniques

what do boiling chips do in distillation?

provide a site for gas bubbles to form (nucleation site), preventing the liquid or solution from superheating by ensuring smooth boiling and preventing bumping.

what is a vacuum’s use in distillation?

It lowers pressure inside flask, lowering the boiling point of liquids, allowing for distillation at reduced temperatures and minimizing thermal decomposition.

If gibbs free energy is negative, Keq is (less than or greater than) 1?

Keq is greater than 1

Means that at equilibrium, the products of the reaction predominate over the reactants

Keq > 1 according to dG=-RTln(Keq)

how do you know if an atom can have an expanded octet?

if they are at period 3 or beyond of the periodic table

Ex: phosphorus, sulfur, or chlorine, xenon, bromine, iodine

accommodate more than eight electrons in their valence shell due to having a d-orbital

count total number of valence electrons

subtract bonds

subtract the remaining lone pairs for substituents

remaining electrons/2 are the number of lone pairs

explain hydrophobi molecules in water vs hydrophilic molecules in terms of entropy and energetic favorability

Type	Water Behavior	Entropy (ΔS)	Energy Favorability
Hydrophobic	Water forms ordered shell	↓ (decreases)	Unfavorable
Hydrophilic	Water bonds freely, stays fluid	↑ (increases)	Favorable

hydrophobic effect

Locally around each individual nonpolar molecule → entropy ↓ (because of the ordered shell)

When nonpolar molecules aggregate (hydrophobic effect) → overall system entropy ↑ (because water molecules are freed up and become more disordered)

The phenomenon where nonpolar molecules cluster together in aqueous environments, leading to an increase in overall entropy as water molecules are released from their ordered state around individual nonpolar molecules.

what is the relationship between Keq, Ka, and Kd?

Keq: equilibrium constant, which is Ka or Kd based on the direction.

Ka: association constant; 1/Kd

Kd: dissociation constant; 1/Ka

electrolytic cells vs. galvanic cells vs. capacitors

Feature	Electrolytic Cells	Galvanic Cells	Capacitors
Function	Convert electrical energy into chemical energy consume electrical energy to drive chemical rxns	Generate electrical energy from spontaneous chemical reactions chemical rxn generates electrical energy	Store electrical energy as charge
Reaction	Chemical reactions (oxidation/reduction) driven by an external power source	Spontaneous redox reactions generating electricity	No chemical reactions, just charge accumulation
Energy Source	Requires external power to drive reaction	No external power needed; energy is generated spontaneously	Does not need external power once charged
Energy Flow	Electrical energy is consumed to drive chemical reactions	Electrical energy is produced by spontaneous reactions	Energy is stored and released when needed
Uses	Electroplating, electrolysis, chemical synthesis	Batteries, fuel cells, galvanic batteries	Smoothing circuits, power backup, storing energy for flashes
Example	Electrolysis of water into hydrogen and oxygen	Zinc-copper galvanic cell (e.g., battery)	Storing energy for a flash in a camera

forces on ramp physics questions

mgcosθ: crushing force; perpendicular to plane

mgsinθ : sliding force; parallel to plane

what does heating the distillation flask at a slower rate do during fractional distillation?

the temperature raises more slowly. A slower heating rate ensures that heat is transferred effectively to the mixture, allowing the more volatile components to vaporize at their respective boiling points. This prevents the less volatile components from prematurely vaporizing, which can contaminate the distillate. 

what does creating a lower pressure inside the distilling apparatus do in fractional distillation

lowers the boiling points of both liquids and NARROWS their difference in boiling point making it necessary to increase the length of the fractionating column to achieve the same degree of separation

what are the IR values for OH, C=O, C=C, sp3, and aromatics?

3300 (OH)

1700 (C=O)

1600 (C=C)

2800-3000 (SP3)

below 1600 (aromatics)

Gel filtration chromatography

Size exclusion chromatography

• Smaller cells are trapped by pores and elute last

• Bigger cells elute first (faster)