Did you know that methanoic acid and ethanol both have relative molecular mass 46, yet methanoic acid boils at about 101 °C while ethanol boils at about 78 °C at ordinary atmospheric pressure? The difference is a useful H2 Chemistry lesson in intermolecular hydrogen bonding and carboxylic-acid association. Both molecules contain an O–H group and can form hydrogen bonds. But the carboxyl group in methanoic acid contains both an O–H donor and a carbonyl oxygen acceptor arranged in a way that can favour strong paired hydrogen-bonded structures, including cyclic dimers.
For students looking for an H2 Chemistry tutor in Hougang, carboxylic acid boiling points, hydrogen bonding carboxylic acid dimers, methanoic acid versus ethanol, 9476 intermolecular forces or JC Chemistry comparison questions, the first question is this: If two liquids have the same molar mass, must they have the same boiling point? Of course not. Molar mass is one useful clue about dispersion interactions, but molecular structure determines the range and strength of interactions between molecules.
I would teach this by comparing the functional groups before listing the boiling temperatures. Which atoms can donate hydrogen bonds? Which atoms can accept them? Can the molecules form a strongly associated pair? And how much energy is required for molecules to enter the gas phase?
The larger lesson is not to memorise “acid higher than alcohol” as a universal rule. It is to identify all relevant intermolecular forces, then explain which structural feature matters most for the pair being compared. That method transfers to esters, aldehydes, ketones and even molecules whose formulae are different.
At eduKate, our small three-student class format is designed to keep explanations short enough to test each learner independently on a changed structure. Families enquiring from Hougang should verify the current JC Chemistry class venue, timing and availability directly; a Hougang-focused guide is not evidence of a physical classroom operating in Hougang.
Hougang Chemistry tutor library · Official A-Level H2 Chemistry syllabus
First compare the actual chemical formulae
Methanoic acid, also called formic acid, has formula HCOOH, or CH₂O₂.
It contains one carbon atom, two hydrogen atoms and two oxygen atoms.
Its approximate relative molecular mass is:
Mr(HCOOH) = 12 + 2(1) + 2(16) = 46.
Ethanol has the formula CH₃CH₂OH, or C₂H₆O.
It contains two carbon atoms, six hydrogen atoms and one oxygen atom.
Its approximate relative molecular mass is:
Mr(ethanol) = 2(12) + 6(1) + 16 = 46.
They have the same Mr but different molecular formulae. Therefore they are not structural isomers of each other. This is an important boundary to the comparison.
Molecular mass equality does not mean the molecules contain the same functional groups or form the same intermolecular structures.
The normal boiling points are different
At ordinary atmospheric pressure, representative boiling temperatures are approximately:
| Compound | Condensed structure | Mr | Approximate normal boiling point |
|---|---|---|---|
| Methanoic acid | H–C(=O)–OH | 46 | 100.8 °C |
| Ethanol | CH₃–CH₂–OH | 46 | 78.4 °C |
The difference is approximately 22.4 °C using those rounded values.
This does not mean every individual methanoic acid molecule has 22.4 °C more thermal energy. Temperature is a property of the bulk system, and boiling occurs when the liquid’s vapour pressure reaches the external pressure.
The measured difference reflects the total thermodynamic behaviour of the two liquids, including intermolecular interactions and the relevant vapour-phase species.
H2 students should describe the structural reasons for the property trend, not turn the difference in Celsius into an imaginary energy per molecule.
What does boiling actually involve?
When ethanol boils under ordinary conditions, it becomes ethanol vapour. When methanoic acid boils, it enters the vapour phase as methanoic-acid-containing species, which may include associated structures under certain conditions.
Normal boiling does not require every ethanol C–O or O–H covalent bond to be broken. Likewise, it does not require chemical decomposition of methanoic acid into carbon dioxide and hydrogen.
The main physical issue is the intermolecular attraction associated with the liquid and its vapour.
Energy must be supplied for molecules to separate and redistribute between liquid and gas phases. How much energy is needed depends on the condensed-phase network, the gas-phase species and the entropy changes associated with vaporisation.
This distinction prevents a common error: explaining the boiling-point difference solely with the covalent bond enthalpy of C–O inside each molecule. Those are not the bonds ordinarily broken in a physical change of state.
What hydrogen bonding means here
A hydrogen bond is a particularly important directional intermolecular interaction involving an appropriate hydrogen-bond donor and acceptor.
In the familiar H2 model, an O–H group can donate a hydrogen bond because oxygen draws electron density toward itself, leaving its attached hydrogen partially positive.
An oxygen atom with suitable available electron density can act as a hydrogen-bond acceptor.
Both ethanol and methanoic acid have O–H groups, so both can participate in intermolecular hydrogen bonding.
Therefore the weak explanation “methanoic acid boils higher because it hydrogen-bonds, whereas ethanol does not” is false.
The stronger explanation compares the ways the molecules can organise their hydrogen bonds and the resulting attraction patterns.
Why ethanol hydrogen-bonds with itself
Ethanol has structure CH₃CH₂OH.
Its hydroxyl group contains an O–H hydrogen-bond donor and an oxygen atom bearing lone pairs that can accept hydrogen bonds.
In liquid ethanol, neighbouring molecules can form an extended and dynamic network of intermolecular hydrogen bonds. There are also ordinary London dispersion interactions and permanent dipole–dipole contributions.
Ethanol’s O–H hydrogen-bonding ability helps explain why its normal boiling point is much higher than that of methoxymethane, CH₃OCH₃, an ether with the same molecular formula C₂H₆O but without an O–H donor.
Methoxymethane boils near −25 °C, much lower than ethanol.
That pair teaches the simple donor-versus-acceptor distinction. Methanoic acid versus ethanol requires one step more: both can donate and accept hydrogen bonds, so their detailed association must be considered.
The carboxyl group has two distinct oxygen environments
Methanoic acid contains –C(=O)–OH, the carboxyl group.
The carbonyl oxygen has electron density that can act as an effective hydrogen-bond acceptor.
The O–H group provides a hydrogen-bond donor. The surrounding carboxyl structure creates a favourable geometry for two carboxylic-acid molecules to associate.
The hydrogen in the O–H group of one molecule can form a hydrogen bond with the carbonyl oxygen of a neighbouring molecule, while the neighbouring molecule can make the corresponding interaction back.
This can create a cyclic hydrogen-bonded dimer, a pair of molecules held together by two intermolecular hydrogen bonds.
Carboxylic-acid association is especially prominent in many nonpolar solvents and in suitable vapour-phase or condensed-phase conditions. In a polar hydrogen-bonding solvent such as water, interactions with solvent compete and the distribution of associated species can differ.
The dimer picture is therefore a useful model of strong association, not a claim that every carboxylic-acid molecule remains permanently paired in every environment.
What a cyclic carboxylic-acid dimer is
A dimer is an associated pair of molecules. For two methanoic acid molecules, imagine two carboxyl groups facing each other so that each donates an O–H hydrogen bond toward the other’s carbonyl oxygen.
A simplified association equilibrium is:
2HCOOH ⇌ (HCOOH)₂,
where the right-hand side represents an associated pair rather than a new covalent molecule formed through an ordinary condensation reaction.
The two hydrogen bonds form a ring-like geometry in the familiar structural representation.
The dimer can dissociate and reform dynamically. Its abundance depends on temperature, pressure and solvent conditions.
Because two strong directional intermolecular hydrogen bonds can link the pair, a carboxylic acid can show substantial association compared with many molecules of similar size.
When comparing methanoic acid and ethanol, the possibility of these strong pairwise associations contributes to the explanation for the higher boiling temperature.
A caution: a dimer is not a covalent ester
Students sometimes see two carboxylic acid structures drawn close together and assume they have chemically condensed to form an ester or an anhydride.
That is not the meaning of a hydrogen-bonded dimer.
In a hydrogen-bonded dimer, the original molecules retain their normal covalent structures. The connection is primarily intermolecular hydrogen bonding, not formation of a new C–O covalent bond that changes molecular identity.
No water molecule must be eliminated for two carboxylic acid molecules to associate through hydrogen bonds.
By contrast, ester formation from a carboxylic acid and an alcohol is a chemical transformation with a new covalent ester linkage and, in the usual equation, water as a product.
This distinction is essential for accurately explaining a physical property.
Is every liquid methanoic acid molecule in a cyclic dimer?
No. Real condensed systems contain dynamic molecular interactions and distributions of association patterns.
Carboxylic-acid dimers are useful and well-established structures, especially for describing association in nonpolar solvents and some gas-phase conditions. But pure liquid methanoic acid may also display extended hydrogen-bond networks and other transient arrangements.
A fully quantitative explanation of its normal boiling point requires the free energies of the relevant liquid and vapour states, not merely counting two lines in a single diagram.
For a typical H2 comparison, the defensible conclusion is:
Methanoic acid’s carboxyl group can give strong intermolecular hydrogen-bond associations, including paired interactions, which contribute to the higher boiling point compared with a similar-mass alcohol.
That is scientifically stronger than claiming all acid molecules exist permanently as dimers and every hydrogen bond must be broken on boiling.
Why the same molar mass is useful in this comparison
Molar mass influences several aspects of intermolecular attraction, especially through electron count and polarizability, but it is not a complete descriptor.
Both methanoic acid and ethanol have Mr 46. This controls for one obvious difference that might otherwise dominate a careless comparison.
The molecules still have different electronic structures: methanoic acid contains two oxygens, while ethanol contains one oxygen and an extra carbon. The shapes, polar group arrangements and dispersion contributions are not identical.
So the matched molar mass helps make the comparison meaningful, but it does not remove every other variable.
An H2 answer should identify hydrogen-bond association as an important structural reason without pretending that Mr equality proves all other attractions are equal.
Why carboxylic acids usually have relatively high boiling points
The carboxyl group contains both an O–H donor and a carbonyl oxygen acceptor. That combination permits strong hydrogen-bonded interactions in many carboxylic-acid systems.
For molecules of broadly comparable molar mass and shape, carboxylic acids often have high boiling points relative to many aldehydes, ketones, ethers and sometimes comparable alcohols.
But there is no universal rule that every carboxylic acid boils above every alcohol. A much larger alcohol with greater dispersion attractions might boil above a small carboxylic acid.
The proper comparison is between specifically named compounds with appropriate structural information.
In an exam, always use the given substances and experimental values rather than turning a useful trend into an absolute claim.
Compare with a carboxylic acid and an ether
Methanoic acid, ethanol and methoxymethane offer a three-way comparison:
| Feature | Methanoic acid | Ethanol | Methoxymethane |
|---|---|---|---|
| Formula | CH₂O₂ | C₂H₆O | C₂H₆O |
| Mr | 46 | 46 | 46 |
| Functional group | Carboxylic acid | Alcohol | Ether |
| O–H donor present? | Yes | Yes | No |
| Characteristic intermolecular interaction | Strong carboxyl associations, including dimer-like hydrogen bonding | O–H hydrogen-bond network | Dipole and dispersion interactions without comparable self-H-bond donor |
| Approximate normal boiling point | 100.8 °C | 78.4 °C | −24.8 °C |
All three have Mr 46, yet their boiling points span more than 125 °C. That is a vivid reminder that molecular connectivity and intermolecular forces matter.
A student should also recognise that ethanol and methoxymethane are structural isomers of one another, whereas methanoic acid has a different molecular formula and is not their isomer.
A worked exam explanation
Imagine a question asks:
Explain why methanoic acid has a higher boiling point than ethanol although the compounds have the same relative molecular mass.
A strong H2-level explanation could be:
Both compounds can form intermolecular hydrogen bonds. However, methanoic acid’s carboxyl groups can participate in particularly strong hydrogen-bonded associations, including cyclic dimers where two molecules are linked by two hydrogen bonds. These strong interactions contribute to more energy being required for vaporisation and a higher boiling point than ethanol.
This includes the shared feature, the decisive structural difference and the connection to the measured physical property.
An answer that says only “methanoic acid is heavier” is contradicted by the identical Mr.
An answer that says “ethanol cannot hydrogen-bond” is chemically false.
How many hydrogen bonds are in the familiar dimer sketch?
The conventional cyclic dimer diagram of two carboxylic acid molecules contains two hydrogen bonds between the molecules.
Each acid has an O–H donor and a carbonyl acceptor site, arranged so the pair interacts reciprocally.
If a student draws one hydrogen bond, that is not the complete familiar cyclic dimer.
But do not use “two hydrogen bonds per dimer” as proof of an exact numerical vapourisation enthalpy. The energy of separating a liquid into vapour depends on the broader structure and actual gas-phase association.
A diagram is an explanatory representation, not a direct calorimeter reading.
Why dilution into water changes the association picture
Water is both a hydrogen-bond donor and acceptor. When methanoic acid is dissolved in water, surrounding water molecules can form hydrogen bonds with the acid.
Aqueous methanoic acid also undergoes acid dissociation:
HCOOH + H₂O ⇌ H₃O⁺ + HCOO⁻.
As a result, the solution contains a changing distribution of molecular and ionic species depending on concentration, pH and temperature.
It would be an error to imagine every aqueous methanoic-acid molecule remains in exactly the same cyclic dimer structure as in a nonpolar solvent or an idealised gas-phase sketch.
The problem’s solvent and physical state matter. When asked about pure-liquid boiling point, discuss the liquid and vapour states; when asked about aqueous acid strength, discuss proton-transfer equilibrium and solvation.
This is why chemical language should name the system.
Why carboxylic-acid acidity is a different property
Methanoic acid is more acidic than ethanol in the usual aqueous comparison. That is related to the stability of the conjugate base, formate, HCOO⁻, compared with ethoxide, CH₃CH₂O⁻.
But aqueous acidity is not the same thing as boiling point.
Acid strength refers to proton donation in an equilibrium. Boiling point concerns the liquid-vapour phase transition. Both depend on molecular structure, but they involve different physical processes and different surrounding environments.
Do not explain methanoic acid’s higher boiling point simply by saying “it is the stronger acid.” State the relevant intermolecular interactions instead.
The same caution applies to the difference between ethanol and dimethyl ether: their boiling temperatures are not determined by whether they are stronger or weaker Brønsted acids in water.
Misconceptions worth correcting
| Student statement | Why it fails | Repair question |
|---|---|---|
| Methanoic acid boils higher because it has higher Mr | Both have Mr 46 | Which functional groups differ? |
| Ethanol cannot hydrogen-bond | Ethanol has an O–H donor and oxygen acceptor | Where is ethanol’s hydroxyl group? |
| A hydrogen-bonded dimer is a new covalent ester | Confused intermolecular association with reaction | Did any covalent bond or atom count change? |
| All carboxylic acid molecules are permanent dimers in water | Ignored solvent competition and dynamic equilibria | Can water also hydrogen-bond with the acid? |
| Methanoic acid and ethanol are structural isomers | Ignored different molecular formulae | Do they contain equal numbers of each atom? |
| Boiling breaks all O–H covalent bonds | Confused intermolecular forces with intramolecular bonds | Does ethanol vapour still contain ethanol molecules? |
Independent practice
- Calculate the approximate Mr of methanoic acid and ethanol.
- Are they structural isomers? Explain.
- What functional group in methanoic acid can form a cyclic hydrogen-bonded dimer?
- How many intermolecular hydrogen bonds are in the familiar two-molecule cyclic carboxylic-acid dimer sketch?
- Explain why ethanol cannot be described as a liquid that has only London dispersion interactions.
- Is dimethyl ether able to accept a hydrogen bond from water even though it cannot strongly self-hydrogen-bond like ethanol?
- Why does the presence of carboxylic-acid dimers in a nonpolar solvent not prove the same association fraction in an aqueous solution?
Answers
1. Both approximately 46. 2. No: methanoic acid is CH₂O₂ while ethanol is C₂H₆O. 3. The –C(=O)–OH carboxyl group, with donor and acceptor oxygen sites. 4. Two hydrogen bonds in the standard cyclic-dimer representation. 5. Ethanol contains an O–H group and forms intermolecular hydrogen bonds. 6. Yes, its oxygen lone pairs can act as acceptors to water’s O–H donors. 7. Water competes as hydrogen-bond donor and acceptor and changes solution speciation.
If a student gets Question 1 right but still asserts the liquids must have identical boiling points, the missing link is structure-dependent intermolecular force, not arithmetic.
A compact JC1–JC2 revision route
- Session 1: draw methanoic acid, ethanol and methoxymethane from their names.
- Session 2: identify hydrogen-bond donors and acceptors, and separate covalent from intermolecular bonding.
- Session 3: sketch two methanoic-acid molecules connected by a cyclic pair of hydrogen bonds.
- Session 4: explain why pure-liquid boiling differs at the same Mr without saying ethanol has no hydrogen bonds.
- Session 5: distinguish structural isomers from different-formula compounds.
- Session 6: explain how aqueous solvent interactions may change acid association and why acidity is a separate question.
- Session 7: answer an unfamiliar comparison among alcohol, ether, ketone and carboxylic acid structures, naming the relevant forces.
Each session can be short. The important measure is whether the student can explain a new pair of molecules, not just repeat the methanoic-acid example.
What parents seeking H2 Chemistry support in Hougang can ask
Bring a marked intermolecular-force question and ask the tutor to identify whether the student ignored hydrogen-bond donors, confused a dimer with a new covalent product, assumed all molecules of the same Mr have the same boiling temperature or relied on acid strength to explain a physical state change.
In a small group of three, each student should compare a different molecular pair independently before discussing the mechanisms. A private tutorial should finish with a changed structure solved without hints. Confirm current lesson availability, actual teaching venue and travel arrangements; the Hougang article itself does not prove a physical class operates in Hougang.
Why this matters beyond the A Levels
Hydrogen-bond networks and molecular association influence solubility, liquid properties, solvent selection and chemical separation. Chemists must know when two molecules of the same mass behave differently because their functional groups create different interactions.
The transferable scientific method is simple: draw the structure, identify possible interactions, state the relevant physical process, and avoid claiming more than the evidence supports.
Frequently asked questions
Why is methanoic acid’s boiling point higher than ethanol’s?
Strong intermolecular interactions involving its carboxyl group, including hydrogen-bonded association and cyclic dimers, contribute to its higher boiling point.
Do methanoic acid and ethanol have the same relative molecular mass?
Yes, approximately 46, but their molecular formulae differ.
Can ethanol hydrogen-bond?
Yes. Ethanol can form intermolecular hydrogen bonds through its hydroxyl group.
What is a carboxylic-acid dimer?
An association of two acid molecules, often illustrated as a cyclic pair connected by two hydrogen bonds rather than by newly formed covalent bonds.
Does a dimer exist permanently in every aqueous acid solution?
No. Association is dynamic and depends on the solvent, temperature, concentration and equilibria involved.
Are methanoic acid and ethanol structural isomers?
No. Their molecular formulae are CH₂O₂ and C₂H₆O respectively.
Continue learning
Why ethanol boils much higher than dimethyl ether · Hydrogen bonding, intermolecular forces and boiling points · Why polar bonds can still form a non-polar molecule · Phenol and carboxylic acid comparisons · Hougang Chemistry library · eduKate Singapore Chemistry enquiries.
Boiling temperatures are approximate values at ordinary atmospheric pressure. The molecular-association explanations are educational and do not replace detailed experimental thermodynamic models. Methanoic acid and ethanol can be hazardous and should not be used for unsupervised tests.