The shape of the lesson
By the end of the lesson, learners can
- Four kinds of formula: Build a molecule in the simulator and switch between its molecular, structural and displayed formulae.
- Homologous series: Members of a homologous series behave alike chemically because they share a functional group.
- Molecular C2H6O, structural CH3CH2OH, displayed every bond.
- Functional groups: C=C alkene, −OH alcohol, −COOH carboxylic acid.
- Homologous series: same general formula and functional group, similar chemistry, trend in physical properties.
- Saturated = C−C single bonds only; unsaturated = contains C=C.
One picture every idea returns to
A molecular formula gives the number and type of each atom (C2H6O); a structural formula shows how the atoms are arranged without drawing bonds (CH3CH2OH); a displayed formula shows every atom and every bond. A functional group is the atom or group of atoms that decides the chemical properties of a compound: C=C in alkenes, −OH in alcohols, −COOH in carboxylic acids. A homologous series is a family of compounds with the same general formula and same functional group, so they have similar chemical properties and a trend in physical properties; neighbouring members differ by CH2. A saturated compound has only C−C single bonds; an unsaturated compound contains at least one C=C double (or C≡C triple) bond.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, One molecule, four formulae.. Organic chemistry is the chemistry of carbon compounds, and there are millions of them. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Four kinds of formula | Build a molecule in the simulator and switch between its molecular, structural and displayed formulae. Project tab 1 of the lab (Organic chemistry lab, formulae) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Homologous series | Members of a homologous series behave alike chemically because they share a functional group. | Cooperative Concept Mapping |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 14 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Homologous series, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Cooperative Concept Mapping
Teams receive the key terms on cards and arrange them on a large sheet, drawing and labelling the links. Compare maps between teams before showing the model map.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| A homologous series has similar chemical properties but a trend in physical properties: do not swap the two words. "Saturated" means only single carbon-carbon bonds, not "contains as much hydrogen as possible dissolved in it". | And the −OH in an alcohol is a covalently bonded group, not a hydroxide ion: alcohols are not alkalis. |
Formulae, functional groups and homologous series: worksheet
6 questions, 14 marks. Written in the style of the papers; no past paper question is reproduced.
State three characteristics of a homologous series.
Butane has the structural formula CH3CH2CH2CH3. Give its molecular formula and show that it fits the general formula of the alkanes.
Draw the displayed formula of ethanoic acid, CH3COOH, and circle its functional group.
Classify each compound by homologous series: (a) C3H6, a compound that decolourises bromine water; (b) C2H5OH; (c) C3H8.
Explain why ethane is described as saturated and ethene as unsaturated.
The boiling points of methane, ethane and propane are −162 °C, −89 °C and −42 °C. Describe the trend and predict whether butane is a gas at room temperature (20 °C).
Answer key and marking notes
- Members have the same general formula.
- They have the same functional group, so similar chemical properties.
- They show a trend (gradual change) in physical properties such as boiling point; consecutive members differ by CH2.
- C4H10.
- CnH2n+2 with n = 4 gives 2(4) + 2 = 10 hydrogen atoms.
- Three H atoms on the first carbon; the second carbon bonded to it, with a double bond to one O and a single bond to an O−H.
- Circle the whole −COOH group (C, the =O, and the O−H).
- (a) alkene (CnH2n, contains C=C): propene.
- (b) alcohol (−OH group): ethanol.
- (c) alkane (CnH2n+2): propane.
- Ethane contains only single carbon-carbon bonds.
- Ethene contains a carbon-carbon double bond, C=C.
- Boiling point increases as the chain gets longer (each extra CH2).
- The increases are getting smaller, so butane boils a little below 0 °C (actual value about −1 °C): it is a gas at 20 °C.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which statement describes a homologous series?
- Aa family of compounds with the same functional group and the same general formula, with similar chemical properties
- Ba family of compounds with the same molecular formula but different structures
- Ca family of compounds that all have the same boiling point
- Da family of compounds that contain only carbon and hydrogen
Which compound is unsaturated?
- AC3H6
- BC3H8
- CC2H6
- DCH4
Which functional group is found in all carboxylic acids?
- A−COOH
- B−OH on its own
- CC=C
- D−COO− joining two carbon chains
Butane has the structural formula CH3CH2CH2CH3. What is its molecular formula?
- AC4H10
- BC4H8
- CC2H5
- DC4H12
The first three members of a homologous series are CH3OH, C2H5OH and C3H7OH. What is the formula of the fifth member?
- AC5H11OH
- BC5H10OH
- CC5H9OH
- DC4H9OH
Which pair of compounds are structural isomers of each other?
- ACH3CH2CH2CH3 and CH3CH(CH3)CH3
- BCH3CH3 and CH2=CH2
- CCH3OH and CH3CH2OH
- DCH4 and CH3Cl
Which is a structural isomer of but-1-ene, CH2=CHCH2CH3?
- ACH3CH=CHCH3
- BCH3CH2CH2CH3
- CCH2=CHCH3
- DCH2=CHCH2CH2CH3
How many different structural isomers have the molecular formula C5H12?
- A3
- B2
- C4
- D5
Quiz key
Members of a homologous series share a functional group and a general formula, differ by CH2 from one member to the next, have similar chemical properties and show a trend in physical properties. Same molecular formula with different structures describes structural isomers. Boiling points are not the same: they rise as the chain gets longer. Containing only carbon and hydrogen defines a hydrocarbon, and series such as the alcohols contain oxygen too.
C3H6 fits CnH2n, the alkene formula, so it is propene and has a C=C double bond, which makes it unsaturated. C3H8, C2H6 and CH4 all fit CnH2n+2: they are the alkanes propane, ethane and methane, which have only single bonds and are saturated.
Carboxylic acids contain the carboxyl group, −COOH. An −OH group on its own (attached to a carbon chain) is the alcohol functional group. C=C is the alkene functional group. −COO− joining two carbon chains is the ester linkage, found in esters, not acids.
There are 4 carbon atoms and 3 + 2 + 2 + 3 = 10 hydrogen atoms, so the molecular formula is C4H10, matching CnH2n+2. C4H8 is butene. C2H5 is the empirical formula (simplest ratio), not the molecular formula. C4H12 is impossible because each carbon can form only four bonds.
These are alcohols, CnH2n+1OH. For n = 5 the formula is C5H11OH (pentanol). C5H10OH and C5H9OH do not follow the CH2 step and have too few hydrogens. C4H9OH is the fourth member, butanol, not the fifth.
Butane and methylpropane both have the molecular formula C4H10, but one has a straight chain and the other a branched chain, so they are structural isomers. Ethane (C2H6) and ethene (C2H4) have different numbers of hydrogens. Methanol and ethanol differ by CH2, so they are neighbouring members of a series. Methane and chloromethane contain different atoms.
But-1-ene is C4H8. But-2-ene, CH3CH=CHCH3, is also C4H8 but has the double bond in a different position, so it is a structural isomer. Butane is C4H10, two hydrogens more. Propene (C3H6) and pent-1-ene (C5H10) have a different number of carbon atoms.
There are three: pentane (straight chain of 5), methylbutane (chain of 4 with a methyl branch) and dimethylpropane (chain of 3 with two methyl branches on the middle carbon). Answering 2 misses dimethylpropane. Answers of 4 or 5 usually come from counting the same molecule twice, for example putting a methyl group on carbon 3 of butane, which is just methylbutane drawn from the other end, or bending the chain, which does not change the structure.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.