ACS Cram

ACS General Chemistry Crash Course — “Podcast Notes” Edition

Think of this as an ACS exam review podcast where each episode connects the entire course together instead of teaching isolated facts. Your missed questions are emphasized with deeper explanations and “ACS trap alerts.”

Episode 1: Atomic Structure — The Architecture of Matter

The Core Story

Chemistry starts with electrons.

Everything:

  • bonding

  • periodic trends

  • polarity

  • reactivity

comes from where electrons are and how tightly atoms hold them.

Quantum Numbers

Principal Quantum Number

nnn

Determines:

  • energy level

  • orbital size

Higher nnn:

  • farther from nucleus

  • higher energy

Angular Momentum Quantum Number

lll

Determines orbital SHAPE.

l=0→s,l=1→p,l=2→dl=0\rightarrow s,\quad l=1\rightarrow p,\quad l=2\rightarrow dl=0→s,l=1→p,l=2→d

Orbital Counting

ACS favorite:
“How many orbitals are in n=3n=3n=3?”

Remember:

  • s = 1 orbital

  • p = 3 orbitals

  • d = 5 orbitals

Total:

1+3+5=91+3+5=91+3+5=9

Episode 2: Electron Configurations & Periodicity

Aufbau Principle

Electrons fill LOWEST energy orbitals first.

1s→2s→2p→3s→3p→4s→3d1s\rightarrow2s\rightarrow2p\rightarrow3s\rightarrow3p\rightarrow4s\rightarrow3d1s→2s→2p→3s→3p→4s→3d

ACS Trap: Excited States

Ground state = lowest energy arrangement.

Excited state:
an electron jumps upward.

Example:

1s22s22p53s11s^22s^22p^53s^11s22s22p53s1

Periodic Trends

Atomic Radius

Across a period:
radius decreases.

WHY?
More protons pull electrons inward harder.

Electronegativity

Across a period:
increases.

Fluorine is highest.

Ionization Energy

Energy required to remove electrons.

Higher:

  • across periods

  • upward on periodic table

Episode 3: Bonding & Molecular Structure

Sigma vs Pi Bonds

Sigma Bond

Head-on overlap.

Strongest bond type.

Pi Bond

Sideways overlap.

Found in:

  • double bonds

  • triple bonds

Methane Bonding

Hydrogen 1s overlaps with carbon hybrid orbital.

That’s:

s−ps-ps−p

overlap.

Resonance

Electrons delocalize across multiple atoms.

Classic ACS examples:

  • benzene

  • nitrate

  • sulfate

  • sulfur dioxide

Episode 4: Electron-Withdrawing Groups (HIGH PRIORITY)

This was one of your weak areas.

Nitro Group

−NO2-NO_2−NO2​

is strongly electron-withdrawing.

Two Mechanisms

Induction

Electronegative atoms pull electron density through sigma bonds.

Resonance Withdrawal

Nitro stabilizes negative charge through resonance.

Result:
benzene ring becomes electron-poor.

ACS Pattern Recognition

Electron Donors

Usually:

  • OH

  • NH2

  • OR

Electron Withdrawers

Usually:

  • NO2

  • carbonyls

  • CN

  • halogens

Episode 5: Stoichiometry — The ACS Backbone

Most ACS questions are secretly stoichiometry.

Universal Workflow

grams→moles→mole ratio→desired units\text{grams} \rightarrow \text{moles} \rightarrow \text{mole ratio} \rightarrow \text{desired units}grams→moles→mole ratio→desired units

Memorize this process.

Episode 6: Balancing Equations

ACS combustion favorite:

Hydrocarbon + oxygen:

CO2+H2OCO_2 + H_2OCO2​+H2​O

Example:

C3H8+5O2→3CO2+4H2OC_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2OC3​H8​+5O2​→3CO2​+4H2​O

Episode 7: Mole Conversions (HIGH PRIORITY)

You missed several because the ACS expects AUTOMATIC mole reflexes.

Core Formula

n=mMn=\frac{m}{M}n=Mm​

Where:

  • nnn = moles

  • mmm = mass

  • MMM = molar mass

ACS Survival Rule

If you see grams:
CONVERT TO MOLES IMMEDIATELY.

Episode 8: Limiting Reagents (HIGH PRIORITY)

Think:
Which ingredient runs out first?

Steps

  1. Convert all reactants to moles

  2. Divide by coefficients

  3. Smallest adjusted value = limiting reagent

Ammonia Example

N2+3H2→2NH3N_2 + 3H_2 \rightarrow 2NH_3N2​+3H2​→2NH3​

Nitrogen often limits because hydrogen is usually in excess.

Episode 9: Gas Stoichiometry & STP (HIGH PRIORITY)

At STP:

1 mol gas=22.4 L1\text{ mol gas}=22.4\text{ L}1 mol gas=22.4 L

ACS Shortcut

Moles gas ↔ liters gas at STP instantly.

Butane Combustion

2C4H10+13O2→8CO2+10H2O2C_4H_{10}+13O_2\rightarrow8CO_2+10H_2O2C4​H10​+13O2​→8CO2​+10H2​O

Key ratio:

1 mol butane→4 mol CO21 \text{ mol butane} \rightarrow 4 \text{ mol CO}_21 mol butane→4 mol CO2​

Then:

moles×22.4\text{moles}\times22.4moles×22.4

Episode 10: States of Matter & Solutions

Gas vs Liquid

Gas molecules:

  • move faster

  • farther apart

  • weaker intermolecular forces

Episode 11: Intermolecular Forces

Strength order:

dispersion<dipole-dipole<hydrogen bonding\text{dispersion} < \text{dipole-dipole} < \text{hydrogen bonding}dispersion<dipole-dipole<hydrogen bonding

Hydrogen bonding explains:

  • water’s high boiling point

  • surface tension

  • ice density anomaly

Episode 12: Raoult’s Law & Colligative Properties (HIGH PRIORITY)

The Big Idea

Adding solute makes it harder for solvent molecules to escape.

Raoult’s Law

Psolution=XsolventPsolvent∘P_{solution}=X_{solvent}P^\circ_{solvent}Psolution​=Xsolvent​Psolvent∘​

Lower solvent mole fraction:
→ lower vapor pressure.

Boiling Point Elevation

Lower vapor pressure means:
more heat needed to boil.

Therefore:
boiling point rises.

Freezing Point Depression

Solute disrupts crystal formation.

Therefore:
freezing point decreases.

Episode 13: Phase Diagrams & Critical Point

Triple Point

All 3 phases coexist.

Critical Point (HIGH PRIORITY)

Tc,PcT_c,P_cTc​,Pc​

Above critical point:
liquid and gas become indistinguishable.

Supercritical fluid forms.

Episode 14: Thermochemistry

First Law (HIGH PRIORITY)

ΔU=q+w\Delta U=q+wΔU=q+w

ACS LOVES sign conventions.

Signs

Heat absorbed:

q>0q>0q>0

Work done BY system:

w<0w<0w<0

Episode 15: Gibbs Free Energy & Spontaneity (HIGH PRIORITY)

Master equation:

ΔG=ΔH−TΔS\Delta G=\Delta H-T\Delta SΔG=ΔH−TΔS

Meaning

Negative ΔG

Spontaneous

Positive ΔG

Nonspontaneous

ACS Temperature Logic

ΔH

ΔS

Spontaneous?

-

+

Always

+

-

Never

-

-

Low T

+

+

High T

This table is EXTREMELY high yield.

Episode 16: Kinetics (HIGH PRIORITY)

Arrhenius Concept

Higher temperature:
→ faster molecules
→ more effective collisions
→ faster reactions

Half-Life

First-order:

t1/2=0.693kt_{1/2}=\frac{0.693}{k}t1/2​=k0.693​

Higher temperature:
→ larger kkk
→ smaller half-life

Episode 17: Catalysts

Catalysts:

  • lower activation energy

  • speed BOTH forward and reverse reactions

  • do NOT change equilibrium constant

Episode 18: Equilibrium & ICE Tables (HIGH PRIORITY)

ICE Tables

I = Initial
C = Change
E = Equilibrium

Equilibrium Constant

K=productsreactantsK=\frac{products}{reactants}K=reactantsproducts​

Large KKK:
products favored.

Small KKK:
reactants favored.

Episode 19: Le Châtelier’s Principle (HIGH PRIORITY)

System shifts to oppose stress.

Exothermic Reactions

Treat heat as product:

A+BrightleftharpoonsC+heatA+B\\rightleftharpoons C+heatA+BrightleftharpoonsC+heat

Decrease temperature:
→ shift RIGHT.

Inert Gas Trap

Constant Volume

NO equilibrium shift.

Because partial pressures stay unchanged.

This is one of the ACS’s favorite conceptual tricks.

Episode 20: Electrochemistry

Oxidation vs Reduction

OIL RIG

  • Oxidation Is Loss

  • Reduction Is Gain

Galvanic Cells

Anode:
oxidation

Cathode:
reduction

Electrons flow:
anode → cathode

Cell Potential

Ecell∘=Ecathode∘−Eanode∘E^\circ_{cell}=E^\circ_{cathode}-E^\circ_{anode}Ecell∘​=Ecathode∘​−Eanode∘​

Positive voltage:
spontaneous.

Episode 21: Nernst Equation (HIGH PRIORITY)

Real systems aren’t standard.

Nernst accounts for concentration effects.

Main Idea

Changing concentration changes voltage.

More reactants:
stronger driving force.

Episode 22: Descriptive Chemistry

Nitrogen Triple Bond (HIGH PRIORITY)

NequivNN\\equiv NNequivN

Triple bonds:

  • short

  • strong

  • stable

Explains why nitrogen gas is relatively unreactive.

Group 13

Valence configuration:

ns2np1ns^2np^1ns2np1

Three valence electrons.

Episode 23: Laboratory Chemistry (HIGH PRIORITY)

Distillation Air Leak

Vapor escapes.
Thermometer reads too LOW.

Ice Baths

Purpose:
control exothermic reactions.

Not just “cooling.”

pH Meter Calibration

Multiple buffers:
verify linear response.

Blank Titration

Measures background contamination.

Subtracts impurity effects from real trial.

Huge analytical chemistry principle:
always account for background signal.

Final ACS Exam Strategy

Questions ACS LOVES

  • mole conversions

  • limiting reagents

  • gas stoichiometry

  • Gibbs free energy logic

  • equilibrium shifts

  • periodic trends

  • intermolecular forces

  • electrochemistry

  • lab setup errors

Fast ACS Memory Triggers

If you see:

grams

→ convert to moles

STP

→ use 22.4 L/mol

ΔG

→ think spontaneity

ΔH negative

→ exothermic

catalyst

→ lowers activation energy

inert gas at constant volume

→ NO equilibrium shift

nonvolatile solute

→ vapor pressure DOWN
→ boiling point UP

Group 13

→ ns2np1ns^2np^1ns2np1

triple bond

→ short + strong