ChemistryChapter 18: Alcohol
Combustion and Dehydration
Alcohols burn: CO2 + H2O if complete (ethanol fuel slogan, not a still design).
Learning objectives
- Explain combustion and dehydration in simple chemical language.
- Connect 1. combustion with 2. dehydration using equations, observations, or trends where needed.
- Answer a short Federal Board question with the correct terms and a clear reason.
Definition
Alcohols burn: CO2 + H2O if complete (ethanol fuel slogan, not a still design). Dehydration: hot conc H2SO4 or Al2O3 named, lose water, make an alkene (ethanol -> ethene). Meaning: elimination of water, reverse of hydration of alkenes. Conditions are exam names, not a petrochemical unit. Complete combustion CO2 + H2O; dehydration with conc H2SO4 heat gives alkene. Primary alcohols need distillation to stop at aldehyde; dichromate colour change should be stated.
Important terms
- COMBUSTION AND DEHYDRATION
- Alcohols burn: CO2 + H2O if complete (ethanol fuel slogan, not a still design).
- 1. COMBUSTION
- C2H5OH + 3O2 -> 2CO2 + 3H2O.
- 2. DEHYDRATION
- Need a neighbouring H.
Overview
Burn, then knock out water:
Combustion
C2H5OH + 3O2 -> 2CO2 + 3H2O. Incomplete: CO. Bioethanol named as a fuel slogan. No engine-tuning. Elimination mechanism E1/E2 named lightly — product alkene from alcohol.
Examples Blue flame · Water from H and OH · Carbon footprint later
Dehydration
Need a neighbouring H. Zaitsev if two alkenes. Conc H2SO4 heat named. Reverse: steam / acid on ethene (hydration named). Excess O2 for complete combustion exam equation.
Examples Ethene from ethanol · Al2O3 named · Not a plant
WHAT — COMBUSTION AND DEHYDRATION
Alcohols burn: CO2 + H2O if complete (ethanol fuel slogan, not a still design). Dehydration: hot conc H2SO4 or Al2O3 named, lose water, make an alkene (ethanol -> ethene). Meaning: elimination of water, reverse of hydration of alkenes. Conditions are exam names, not a petrochemical unit. Complete combustion CO2 + H2O; dehydration with conc H2SO4 heat gives alkene. Primary alcohols need distillation to stop at aldehyde; dichromate colour change should be stated.
WHY — Why this topic matters
COMBUSTION AND DEHYDRATION is an independent FBISE learning unit because students must use its definitions, conditions, equations, or trends separately in explanations and calculations.
HOW — How it works
COMBUSTION: C2H5OH + 3O2 -> 2CO2 + 3H2O. Incomplete: CO. Bioethanol named as a fuel slogan. No engine-tuning. Elimination mechanism E1/E2 named lightly — product alkene from alcohol. Examples: Blue flame; Water from H and OH; Carbon footprint later. DEHYDRATION: Need a neighbouring H. Zaitsev if two alkenes. Conc H2SO4 heat named. Reverse: steam / acid on ethene (hydration named). Excess O2 for complete combustion exam equation. Examples: Ethene from ethanol; Al2O3 named; Not a plant. Exam ethanol dehydration: hot conc H2SO4 (named) C2H5OH -> C2H4 + H2O alkene; reverse is hydration of ethene
In simple words
Named conditions. No fuel-still or acid-plant design.
Key points
- burn like other organics
- conc acid / Al2O3 -> alkene
- complete vs not
- alkene + water reverse
Exam preparation
Exam Focus
Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.Combustion and Dehydration
Sticky Notes · Quick Revision
Core Idea
- Alcohols burn: CO2 + H2O if complete (ethanol fuel slogan, not a still design). Dehydration: hot conc H2SO4 or Al2O3 named, lose water, make an alkene (ethanol -> ethene). Meaning: elimination of water, reverse of hydration of alkenes. Conditions are exam names, not a petrochemical unit. Complete combustion CO2 + H2O; dehydration with conc H2SO4 heat gives alkene. Primary alcohols need distillation to stop at aldehyde; dichromate colour change should be stated.
1. COMBUSTION
- C2H5OH + 3O2 -> 2CO2 + 3H2O. Incomplete: CO. Bioethanol named as a fuel slogan. No engine-tuning. Elimination mechanism E1/E2 named lightly — product alkene from alcohol.
- Blue flame
- Water from H and OH
- Carbon footprint later
2. DEHYDRATION
- Need a neighbouring H. Zaitsev if two alkenes. Conc H2SO4 heat named. Reverse: steam / acid on ethene (hydration named). Excess O2 for complete combustion exam equation.
- Ethene from ethanol
- Al2O3 named
- Not a plant
Exam Example
- Exam ethanol dehydration:
- hot conc H2SO4 (named)
- C2H5OH -> C2H4 + H2O
- alkene; reverse is hydration of ethene
Common Misconceptions
- Do not confuse 1. COMBUSTION with 2. DEHYDRATION; state the difference before giving an example.
- Writing only “burn like other organics” is not enough. Explain what it means in a complete sentence.
FBISE Exam Check
- Explain combustion and dehydration, compare its two main parts, and include the correct chemical evidence.
- Named conditions. No fuel-still or acid-plant design.
60-second revision
- Alcohols burn: CO2 + H2O if complete (ethanol fuel slogan, not a still design).
- 1. COMBUSTION: C2H5OH + 3O2 -> 2CO2 + 3H2O.
- 2. DEHYDRATION: Need a neighbouring H.
- Exam reminder: Exam ethanol dehydration: hot conc H2SO4 (named) C2H5OH -> C2H4 + H2O alkene; reverse is hydration of ethene
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