Recognise alcohol combustion and reaction with sodium
Balance complete combustion and use sodium's reaction with the O–H proton to form hydrogen gas.
Section 16 · Hydroxy compounds
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. State reagents and conditions, follow the stated reaction pathway, and distinguish each alcohol class and oxidation product. Written practice is self-checked
Before you begin
- Use functional-group formulae and reaction types from Topic 13, alkene reactions from Topic 14, and halogenoalkane substitution from Topic 15.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What are the products of complete combustion of an alcohol?
What you will learn
- Write complete-combustion products
- Describe reaction with sodium
- Balance hydrogen and sodium coefficients
Alcohols burn in oxygen
Complete combustion of an alcohol forms carbon dioxide and water. Balance carbon and hydrogen first, then oxygen; check the oxygen already present in the alcohol.
For ethanol: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O. The equation conserves all atoms.
Sodium replaces the O–H hydrogen
Alcohols react with sodium metal to form a sodium alkoxide and hydrogen gas. Two alcohol molecules supply two O–H hydrogens: 2ROH + 2Na → 2RONa + H₂.
This shows the O–H bond's acidic hydrogen can be replaced by a metal. Do not write sodium oxide or water as the organic reaction product.
Pause and explain
What gas forms when alcohol reacts with sodium?
Worked example
Write balanced equations for complete combustion of propanol and its reaction with sodium.
Show the worked solution
- For combustion, form CO₂ and H₂O, then balance oxygen.
- For sodium, replace the O–H hydrogen with Na.
- Use two alcohol molecules to form one H₂ molecule.
Answer C₃H₇OH + 9/2 O₂ → 3CO₂ + 4H₂O (or double all coefficients); 2C₃H₇OH + 2Na → 2C₃H₇ONa + H₂.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Balance the complete combustion equation for methanol: CH₃OH + O₂ → CO₂ + H₂O.
Give me a hint
Balance C and H before oxygen.
Compare my reasoning
- One carbon gives one CO₂.
- Four H atoms give two H₂O.
- The products contain four oxygen atoms; one is in methanol, so 3/2 O₂ is needed.
CH₃OH + 3/2O₂ → CO₂ + 2H₂O; or 2CH₃OH + 3O₂ → 2CO₂ + 4H₂O.
Look for these in your work
- Carbon balanced
- Hydrogen balanced
- Oxygen balanced
Write the sodium reaction for ethanol and identify the salt formed.
Give me a hint
Use 2ROH + 2Na → 2RONa + H₂.
Compare my reasoning
- Replace each O–H hydrogen with sodium.
- The organic product is sodium ethoxide.
- Two O–H hydrogens form one H₂.
2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂; sodium ethoxide forms.
Look for these in your work
- Alkoxide named
- Coefficients correct
- Hydrogen shown
Why does the sodium equation need two alcohol molecules for one H₂ molecule?
Give me a hint
Count hydrogen atoms in the gas product.
Compare my reasoning
- H₂ contains two hydrogen atoms.
- Each alcohol contributes one O–H hydrogen.
- Two molecules therefore supply the pair.
Two alcohol molecules each supply one O–H hydrogen, so one H₂ forms.
Look for these in your work
- H₂ atom count used
- O–H source identified
- Ratio explained
Common mistakes
- Forgetting the alcohol oxygen when balancing combustion
- Making one H₂ from one alcohol molecule without balancing
- Confusing alkoxide with hydroxide
What can you explain now?
What are the products of complete combustion of an alcohol?
Compare with the explanation
Carbon dioxide and water
Complete oxidation forms CO₂ and H₂O.
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Make it stick
What structural hydrogen is replaced when an alcohol reacts with sodium?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.