A reversible reaction reaches equilibrium when the rates of the forward and reverse reactions are equal. The concentrations stop changing, but the reaction hasn't stopped — molecules keep reacting in both directions.
Three Factors That Determine Equilibrium
🌡️ Temperature
Raising temp favors the endothermic direction; lowering favors exothermic
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🧪 Concentration
Adding a substance shifts equilibrium to the opposite side to "use it up"
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📦 Volume / Pressure
Decreasing volume (↑ pressure) shifts toward the side with fewer gas moles
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🔄 Le Chatelier's Principle Interactive
When a stress is applied to a system at equilibrium, the system shifts to partially counteract that stress.
CH₄ + H₂O ⇌ CH₃OH + H₂ + heat
Click a stress to see which way the equilibrium shifts:
💡 Memory Trick
"The system fights back"
Add something → equilibrium shifts AWAY from it. Remove something → shifts TOWARD it. Think of it like a seesaw rebalancing itself.
🔗 Alkanes Organic Chemistry
Alkanes are the simplest hydrocarbons — compounds containing only C and H atoms. Every carbon forms 4 single bonds, and they follow the general formula:
CnH2n+2
where n = number of carbon atoms
Key Characteristics
All bonds are single bonds (no double or triple bonds)
Also called saturated hydrocarbons — saturated with hydrogen
Ending: -ane
Nonpolar → don't dissolve in water
The Prefix System (You Must Memorize These!)
Prefix
# Carbons
Name
Formula
meth-
1
Methane
CH₄
eth-
2
Ethane
C₂H₆
prop-
3
Propane
C₃H₈
but-
4
Butane
C₄H₁₀
pent-
5
Pentane
C₅H₁₂
hex-
6
Hexane
C₆H₁₄
hept-
7
Heptane
C₇H₁₆
oct-
8
Octane
C₈H₁₈
non-
9
Nonane
C₉H₂₀
dec-
10
Decane
C₁₀H₂₂
💡 Mnemonic for 5–10
Pent · Hex · Hept · Oct · Non · Dec
Please Help Happy Octopuses Name Dogs — or just think: pentagon (5), hexagon (6), heptagon (7), octopus (8), November was month 9, December was month 10 in the old Roman calendar!
Quick-Check: How Many Carbons?
Propane?
3 carbons
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Heptane?
7 carbons
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Octane?
8 carbons
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Hexane?
6 carbons
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📝 IUPAC Naming Rules Must Know
The IUPAC system gives every organic compound a systematic name. Follow these 5 rules in order:
1
Find the longest carbon chain — this gives the parent name (e.g., 6 carbons = hexane)
2
Number the chain so substituents get the lowest possible numbers
3
Identify & locate substituents — name them as alkyl groups (methyl, ethyl, etc.) with their position number
4
Alphabetical order — list different substituents alphabetically (ethyl before methyl, because e < m)
5
Use multiplier prefixes for repeated groups — di- (2), tri- (3), tetra- (4). These do NOT affect alphabetical order!
⚠️ Common Mistake
The prefixes di-, tri-, tetra- are NOT counted for alphabetical ordering. So "dimethyl" still files under M, not D. "Ethyl" (E) comes before "dimethyl" (M).
Number from right so substituents are at positions 2 and 4 (not 3 and 5)
3
Position 4: ethyl group (–CH₂CH₃) | Position 2: two methyl groups (–CH₃)
4
Ethyl before Methyl alphabetically
5
Two methyls at same position → 2,2-dimethyl
4-ethyl-2,2-dimethylhexane
🔵 Cycloalkanes Ring Structures
When the two end carbons of an alkane chain bond together, you get a ring. Add the prefix cyclo- to the alkane name.
CnH2n
Cycloalkane formula — 2 fewer H's than the open-chain alkane
△ 3 C ring
Cyclopropane C₃H₆
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◇ 4 C ring
Cyclobutane C₄H₈
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⬠ 5 C ring
Cyclopentane C₅H₁₀
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⬡ 6 C ring
Cyclohexane C₆H₁₂
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Naming Substituted Cycloalkanes
One substituent: no number needed → methylcyclopentane
Two+ substituents: number the ring starting with the group that comes first alphabetically → 1-ethyl-2-methylcyclopropane
✅ Quick Check
Study Guide Q④ asks: What is the name of a 5-membered ring with no substituents?
→ Cyclopentane
🌿 Alkyl Groups Substituents
Remove one H from an alkane → you get an alkyl group (a substituent). Change the ending from -ane to -yl.
Parent Alkane
Alkyl Group
Structure
Methane
Methyl
CH₃–
Ethane
Ethyl
CH₃CH₂–
Propane
Propyl
CH₃CH₂CH₂–
Propane
Isopropyl
(CH₃)₂CH–
Butane
Butyl
CH₃CH₂CH₂CH₂–
Butane
tert-Butyl
(CH₃)₃C–
💡 Key Insight
-ane → -yl
Methane → Methyl, Ethane → Ethyl, Propane → Propyl. Just swap the ending! The "yl" means "it's a branch, not the main chain."
Special Branched Alkyl Groups
When H can be removed from different positions, you get structural isomers:
Isopropyl
CH₃–CH– | CH₃ H removed from middle C
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tert-Butyl
(CH₃)₃C– 3 methyls on one carbon
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sec-Butyl
CH₃CH₂–CH– | CH₃ Branch at 2nd carbon
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Isobutyl
CH₃–CH–CH₂– | CH₃ Branch at end
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⚡ Alkenes & Cis/Trans Isomers Double Bonds
Alkenes contain at least one C=C double bond. The ending changes from -ane to -ene. The position of the double bond is indicated by its lowest-numbered carbon.
CnH2n
Same formula as cycloalkanes — both have one degree of unsaturation
Cis vs. Trans Isomers
A C=C double bond cannot rotate (unlike single bonds). When each double-bond carbon has two different groups attached, two arrangements are possible:
🤝
CIS
H H
\ /
C=C
/ \
CH₃ CH₃
Same groups on the SAME side
↔️
TRANS
H CH₃
\ /
C=C
/ \
CH₃ H
Same groups on OPPOSITE sides
💡 Memory Trick
"Cis = Same Side" · "Trans = acTross"
Think: cisters (sisters) sit together on the same side. Transatlantic = going across.
✅ Study Guide Q⑤ Answer
cis-2-butene: Both CH₃ groups on the SAME side of the double bond, both H's on the same side.
That's choice B in the study guide — H₃C and CH₃ both on top, H and H both on bottom.
Naming Alkenes — Examples
Formula
Name
Why
CH₃CH=CHCH₂CH₃
2-pentene
5 C's, double bond starts at C2
CH₂=CHCH₂CH₂CH₂CH₃
1-hexene
6 C's, double bond starts at C1
CH₃CH=CHCH₃
2-butene
4 C's, double bond starts at C2
🧪 Key Functional Groups Exam Focus
A functional group is the reactive part of an organic molecule. Knowing the functional group tells you how the molecule behaves.
Functional Group
Structure
Ending
Example
Alcohol
–OH
-ol
Ethanol (CH₃CH₂OH)
Aldehyde
–CHO (C=O at end)
-al
Pentanal (CH₃CH₂CH₂CH₂CHO)
Ketone
C=O (middle)
-one
Propanone / Acetone
Carboxylic Acid
–COOH
-oic acid
Ethanoic acid (Acetic acid)
Ester
–COO–
-oate
Methyl ethanoate
Amine
–NH₂
-amine
Ethylamine
✅ Study Guide Q⑩
The structure H–C–C–C–C–C(=O)–H (5 carbons + aldehyde at the end) is an aldehyde.
Name: Pentanal (5C = pent- + aldehyde = -al).
The carbonyl group (C=O) at the end of the chain makes it an aldehyde.
Naming Alcohols (Study Guide Q⑧)
An alcohol has an –OH group. Change the -e of the alkane name to -ol and number to give –OH the lowest position.
CH₃ — CH₂ — CH — CH₂ — CH₃
|
OH
3-pentanol (–OH on carbon 3 of a 5-carbon chain)
Diols (Study Guide Q⑨c)
The compound HOCH₂CH₂CH₂CH₂OH has –OH groups on both ends of a 4-carbon chain.
1,4-butanediol
✏️ Practice Questions Test Yourself
Try each question, then check your answer. These match the study guide format.
1
What 3 factors determine equilibrium conditions?
A Heat, light, catalyst
B Temperature, concentration, volume
C Pressure, density, moles
D Mass, energy, time
Temperature, concentration, and volume (pressure) are the three stresses that shift equilibrium. A catalyst speeds up both directions equally and does NOT shift equilibrium.
2
Adding more CH₄ to: CH₄ + H₂O ⇌ CH₃OH + H₂ + heat shifts the equilibrium…
A To the RIGHT (forward)
B To the LEFT (reverse)
C No change
Adding more reactant (CH₄) causes the system to shift right (forward) to consume the added reactant and re-establish equilibrium.
3
What is the main characteristic of alkanes?
A They contain at least one double bond
B They contain only oxygen and hydrogen
C Each carbon is bonded to 4 other atoms — all single bonds
D They are always cyclic structures
Alkanes are saturated hydrocarbons — every carbon forms 4 single bonds. No double bonds, no triple bonds. General formula: CnH2n+2.
4
What is cis-2-butene?
A CH₃ groups on opposite sides of the double bond
B CH₃ groups on the same side of the double bond
C A cyclic 4-carbon compound
D A 2-carbon compound with a triple bond
Cis = same side. In cis-2-butene, both CH₃ groups are on the same side of the C=C double bond. Trans-2-butene would have them on opposite sides.
5
An isopropyl group has the structure:
A CH₃CH₂CH₂–
B (CH₃)₂CH–
C (CH₃)₃C–
D CH₃CH₂–
Isopropyl = (CH₃)₂CH– (two methyl groups attached to a central CH). Choice A is propyl (straight chain), C is tert-butyl, and D is ethyl.
6
The carbonyl group C=O at the END of a carbon chain indicates which functional group?
A Ketone (-one)
B Aldehyde (-al)
C Alcohol (-ol)
D Carboxylic acid (-oic acid)
C=O at the end of a chain (bonded to an H) = aldehyde (ending: -al). If C=O is in the middle, it's a ketone. If C=O + OH → carboxylic acid.
7
Name this compound (type your answer):
CH₃ — CH₂ — CH₂ — CH₂ — C(=O) — H (5-carbon chain with C=O at the end)
8
Name this compound:
HOCH₂CH₂CH₂CH₂OH (–OH on both ends of a 4-carbon chain)
9
Which correctly names CH₂=CH–CH₂–CH₂–CH₂–CH₃?
A 5-hexene
B 1-hexene
C Hexane
D 1-hexyne
6 carbons = hex-. Double bond at C1 = 1-. Alkene ending = -ene. → 1-hexene. Always give the double bond the lowest possible number.
10
Raising the temperature of this exothermic reaction shifts equilibrium which way? CH₄ + H₂O ⇌ CH₃OH + H₂ + heat
A RIGHT — more products
B LEFT — more reactants
C No change
Heat is a product in this reaction (exothermic). Raising temperature = adding more "product" → system shifts LEFT to consume the added heat.