Answer key · Unit 11

Organic chemistry.

Each lesson’s practice questions, their correct answers and the mistakes they spot. Answers read off the Scene use the same setting the learner sees.

Misconception catalogue

Answer key 4

Lesson 1 of 4 · About 20 minutes

Hydrocarbons

Why carbon builds chains, how to count the hydrogens of an alkane, what the first alkanes are called, and what forms when they burn.

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The 5 steps and their answers

Step 1 of 5
Open this step
Chains of carbon
Practice · Choose
Question
How many covalent bonds does each carbon atom in the chain make?

Correct answer
- Correct Four
Setup, choices and feedback
The Scene at this step
Carbon atoms
6
End group
–H (alkane)
Compare boiling points
One molecule
Choices
Shown in a random order.
- Correct Four
- Wrong Two
  Feedback
  Two is oxygen’s number of bonds. Carbon has four outer electrons, and it shares each of them.
- Wrong Six
  Feedback
  Carbon has six electrons in all, but only four of them are outer electrons that take part in bonds.
Feedback for a right answer
Carbon has four outer electrons, and each takes part in one bond. The four bonds point to the corners of a tetrahedron, about $109.5^\circ$ ⁠ apart, which is why a carbon chain zigzags instead of running straight.

Worked solution
1. Carbon’s atomic number is 6: 2 electrons in the first shell and 4 in the outer shell.
2. To complete an octet it needs 4 more, which it gets by sharing.
3. So carbon makes 4 covalent bonds.
Step 2 of 5
Open this step
Counting hydrogens
Practice · Calculate
Question
The Scene shows hexane: a chain of 6 carbons, all single-bonded. How many hydrogen atoms does the molecule have?

Correct answer
14 ± 0.1
Setup, choices and feedback
The Scene at this step
Carbon atoms
6fixed
End group
–H (alkane)fixed
Compare boiling points
One molecule
Accepted within
± 0.1
Computed from the Scene at this step’s setting
Hydrogen atoms in the molecule/nodes/chain/outputs/hydrogens
Wrong answers it recognises
- 12 ± 0.1
  Misconception: Writes an alkane as CₙH₂ₙ, forgetting the extra hydrogen at each end of the chain.
  Feedback from the catalogue
  In an alkane each carbon in the middle of the chain has two hydrogens, but the two carbons at the ends have one more each, since they have only one carbon neighbour. So the formula is $\mathrm{C_nH_{2n+2}}$ ⁠: hexane has 14 hydrogens, not 12. $\mathrm{C_nH_{2n}}$ ⁠ is the formula of alkenes, which have a double bond.
Feedback for a right answer
14: $\mathrm{C_6H_{14}}$ ⁠. The four middle carbons carry two hydrogens each, and the two end carbons three each: $4 \times 2 + 2 \times 3 = 14$ ⁠.

Hint
Each carbon has four bonds. How many of them go to neighbouring carbons?

Worked solution
1. An alkane’s formula is $\mathrm{C_nH_{2n+2}}$ ⁠.
2. With $n = 6$ ⁠: $2 \times 6 + 2 = 14$ ⁠.
3. Check: 4 inner carbons × 2 hydrogens, plus 2 end carbons × 3 hydrogens, $8 + 6 = 14$ ⁠.
If you answered 12, you left out the extra hydrogen at each end.
Step 3 of 5
Open this step
The formula
Practice · Reach the target
Question
End group is fixed at alkane. Change Carbon atoms until the molecule has exactly 8 hydrogen atoms, then press Check.

Goal
- 8 ± 0.1
  At the start of the step: 4
Setup, choices and feedback
The Scene at this step
Carbon atoms
1
End group
–H (alkane)fixed
Compare boiling points
One molecule
Read from the Scene at
Hydrogen atoms in the molecule/nodes/chain/outputs/hydrogens
One setting that reaches it
Carbon atoms
3
Feedback for a right answer
Three carbons: $\mathrm{C_3H_8}$ ⁠, propane, the gas in bottled cooking gas.

Feedback for a wrong answer
Not yet. Solve $2n + 2 = 8$ ⁠.

Hint
$2n + 2 = 8$ ⁠.

Worked solution
1. In an alkane the number of hydrogens is $2n + 2$ ⁠.
2. $2n + 2 = 8$ ⁠, so $2n = 6$ ⁠ and $n = 3$ ⁠.
3. Set Carbon atoms to 3: propane.
Step 4 of 5
Open this step
Names
Practice · Choose
Question
What is the name of the alkane $\mathrm{C_4H_{10}}$ ⁠, the gas in a lighter?

Correct answer
- Correct Butane
Setup, choices and feedback
The Scene at this step
Carbon atoms
4
End group
–H (alkane)
Compare boiling points
One molecule
Choices
Shown in a random order.
- Correct Butane
- Wrong Propane
  Feedback
  Propane has three carbons: $\mathrm{C_3H_8}$ ⁠.
- Wrong Pentane
  Feedback
  “Pent-” means five, as in pentagon: $\mathrm{C_5H_{12}}$ ⁠.
- Wrong Butene
  Misconception: Writes an alkane as CₙH₂ₙ, forgetting the extra hydrogen at each end of the chain.
  Feedback
  Butene, $\mathrm{C_4H_8}$ ⁠, is an alkene: it has a double bond and two fewer hydrogens.
Feedback for a right answer
The names are a prefix that counts carbons plus the alkanes’ ending -ane: meth- (1), eth- (2), prop- (3), but- (4), and from five on the Greek numbers: pent-, hex-, hept-, oct-.

Worked solution
1. 4 carbons: the prefix but-.
2. $\mathrm{C_4H_{10}}$ ⁠ fits $\mathrm{C_nH_{2n+2}}$ ⁠, so it is an alkane: the ending -ane.
3. The name: butane.
Step 5 of 5
Open this step
Fuels
Practice · Choose
Question
What forms when methane, $\mathrm{CH_4}$ ⁠, burns in air with plenty of oxygen?

Correct answer
- Correct Carbon dioxide and water
Setup, choices and feedback
The Scene at this step
Carbon atoms
1
End group
–H (alkane)
Compare boiling points
One molecule
Choices
Shown in a random order.
- Correct Carbon dioxide and water
- Wrong Nothing: the gas burns away and is gone
  Feedback
  Burning does not destroy atoms: the methane’s carbon and hydrogen combine with oxygen, and the products are colourless gases that are hard to see.
- Wrong Carbon and hydrogen
  Feedback
  Methane does not split into its elements; it reacts with oxygen. Soot, which is carbon, forms only when oxygen runs short.
Feedback for a right answer
$\mathrm{CH_4} + 2\mathrm{O_2} \rightarrow \mathrm{CO_2} + 2\mathrm{H_2O}$ ⁠. Every carbon becomes $\mathrm{CO_2}$ ⁠ and every two hydrogens a water molecule. That is why hydrocarbons are fuels: the reaction releases a lot of energy.

Worked solution
1. Methane is made of carbon and hydrogen only.
2. In complete combustion every atom combines with oxygen: carbon to $\mathrm{CO_2}$ ⁠ and hydrogen to $\mathrm{H_2O}$ ⁠.
3. Balanced: $\mathrm{CH_4} + 2\mathrm{O_2} \rightarrow \mathrm{CO_2} + 2\mathrm{H_2O}$ ⁠.

Lesson 2 of 4 · About 25 minutes

Homologous series

A family of compounds, each one CH₂ longer than the last: how the molar mass climbs in equal steps, why the boiling point climbs too, and which alkane is the first to be liquid at room temperature.

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The 5 steps and their answers

Step 1 of 5
Open this step
A series
Practice · Calculate
Question
What is the molar mass of octane, the alkane with 8 carbons? $\mathrm{C} = 12.01$ ⁠, $\mathrm{H} = 1.008$ ⁠.

Correct answer
114.2 g/mol ± 0.2 g/mol
Setup, choices and feedback
The Scene at this step
Carbon atoms
8fixed
End group
–H (alkane)fixed
Compare boiling points
One molecule
Accepted within
± 0.2 g/mol
Computed from the Scene at this step’s setting
Molar mass of the molecule/nodes/chain/outputs/molarMass
Any unit of the same quantity is accepted; it is converted before checking.
Wrong answers it recognises
- 112.2 g/mol ± 0.2 g/mol
  Misconception: Writes an alkane as CₙH₂ₙ, forgetting the extra hydrogen at each end of the chain.
  Feedback from the catalogue
  In an alkane each carbon in the middle of the chain has two hydrogens, but the two carbons at the ends have one more each, since they have only one carbon neighbour. So the formula is $\mathrm{C_nH_{2n+2}}$ ⁠: hexane has 14 hydrogens, not 12. $\mathrm{C_nH_{2n}}$ ⁠ is the formula of alkenes, which have a double bond.
Feedback for a right answer
$\mathrm{C_8H_{18}}$ ⁠: $8 \times 12.01 + 18 \times 1.008 = 114.2\ \mathrm{g/mol}$ ⁠. Octane is one of the main components of petrol.

Hint
The formula first: $\mathrm{C_8H_{?}}$ ⁠.

Worked solution
1. The formula: $\mathrm{C_nH_{2n+2}}$ ⁠ with $n = 8$ ⁠, that is $\mathrm{C_8H_{18}}$ ⁠.
2. Carbon: $8 \times 12.01 = 96.08$ ⁠.
3. Hydrogen: $18 \times 1.008 = 18.14$ ⁠.
4. Total: $96.08 + 18.14 = 114.2$ ⁠.
If you got 112.2, you counted only 16 hydrogens.
Step 2 of 5
Open this step
One step
Practice · Calculate
Question
Each member of the alkanes’ homologous series is one $\mathrm{CH_2}$ ⁠ group longer than the one before. By how much does the molar mass grow at each step? $\mathrm{C} = 12.01$ ⁠, $\mathrm{H} = 1.008$ ⁠.

Correct answer
14.03 g/mol ± 0.05 g/mol
Setup, choices and feedback
The Scene at this step
Carbon atoms
7
End group
–H (alkane)
Compare boiling points
One molecule
Accepted within
± 0.05 g/mol
A fixed value in the question
Any unit of the same quantity is accepted; it is converted before checking.
Wrong answers it recognises
- 13.02 g/mol ± 0.05 g/mol
  Misconception: Writes an alkane as CₙH₂ₙ, forgetting the extra hydrogen at each end of the chain.
  Feedback
  $\mathrm{CH}$ ⁠ is a carbon with one hydrogen. Each extra carbon in the chain brings two hydrogens with it.
Feedback for a right answer
$12.01 + 2 \times 1.008 = 14.03$ ⁠. In the Scene: octane is 114.23 and heptane 100.20, a difference of 14.03.

Worked solution
1. The step is $\mathrm{CH_2}$ ⁠: one carbon and two hydrogens.
2. $12.01 + 2 \times 1.008 = 14.03$ ⁠.
3. The answer: 14.03 g/mol.
Step 3 of 5
Open this step
Boiling points
Practice · Predict, then look
Question
Methane, $\mathrm{CH_4}$ ⁠, boils at −162 °C. What happens to the boiling point as the chain grows, carbon by carbon?

Correct answer
- Correct It rises with every carbon
Setup, choices and feedback
The Scene at this step
Carbon atoms
1
End group
–H (alkane)
Compare boiling points
One moleculefixed
Choices
- Correct It rises with every carbon
- Wrong It falls: a big molecule is easier to break up
  Misconception: Thinks boiling breaks the covalent bonds inside the molecules.
  Feedback from the catalogue
  When a liquid boils, the molecules come apart from each other, but each molecule stays whole. The covalent bonds inside a molecule are tens of times stronger than the pull between molecules, and breaking them takes hundreds of degrees more. Hexane vapour is whole hexane molecules. The boiling point measures the attraction between molecules.
- Wrong It hardly changes: they are all hydrocarbons
  Feedback
  They are alike in their chemical reactions, but not in their physical properties: those change step by step along the series.
Once the prediction is locked in, the Scene changes
Compare boiling points
Boiling pointsfixed
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
From −162 °C for methane up to 216 °C for dodecane, with 12 carbons. Each $\mathrm{CH_2}$ ⁠ adds points of contact with neighbouring molecules, and the attraction between them grows.

Worked solution
1. To boil, molecules have to come apart from each other.
2. The longer the chain, the more places it touches its neighbours, and the stronger the attraction between them.
3. So more energy, a higher temperature, is needed to pull them apart.
Step 4 of 5
Open this step
Side by side
Practice · Choose
Question
With Compare boiling points at Side by side, a neighbouring molecule appears, and the hydrogens touching it light up. Why does octane boil higher than butane?

Correct answer
- Correct A longer molecule has more points of contact with its neighbours, so it is pulled to them more strongly
Setup, choices and feedback
The Scene at this step
Carbon atoms
8
End group
–H (alkane)
Compare boiling points
Side by side
Choices
Shown in a random order.
- Correct A longer molecule has more points of contact with its neighbours, so it is pulled to them more strongly
- Wrong A longer chain has more covalent bonds to break
  Misconception: Thinks boiling breaks the covalent bonds inside the molecules.
  Feedback from the catalogue
  When a liquid boils, the molecules come apart from each other, but each molecule stays whole. The covalent bonds inside a molecule are tens of times stronger than the pull between molecules, and breaking them takes hundreds of degrees more. Hexane vapour is whole hexane molecules. The boiling point measures the attraction between molecules.
- Wrong The long molecules tangle like threads and get tied together by bonds
  Feedback
  Chains of 8 carbons are too short to tangle, and there are no covalent bonds between them. The attraction between them is weak, but it grows with every point of contact.
Feedback for a right answer
Between alkane molecules act van der Waals forces: a weak pull between momentary charges, working only where the molecules come close. The more points of contact, the bigger the total. Octane boils at 126 °C, and butane already at −0.5 °C.

Worked solution
1. In boiling the molecules stay whole; only the attraction between them is overcome.
2. The attraction between alkanes is van der Waals forces, acting at every point of contact.
3. Octane has almost twice as many points of contact as butane, so separating its molecules takes more energy.
Step 5 of 5
Open this step
At room temperature
Practice · Reach the target
Question
Methane, ethane, propane and butane are gases at room temperature. End group is fixed at alkane. Change Carbon atoms until the sample beside the molecule is liquid at 20 °C with a boiling point below 40 °C, then press Check.

Goal
- 1 ± 0.1
  At the start of the step: 0
- 298.1 K ± 15 K
  At the start of the step: 111.6 K
Setup, choices and feedback
The Scene at this step
Carbon atoms
1
End group
–H (alkane)fixed
Compare boiling points
One molecule
Read from the Scene at
At 20 °C: 0 gas, 1 liquid, 2 solid/nodes/chain/outputs/state
Read from the Scene at
Boiling point at 1 atm/nodes/chain/outputs/boilingPoint
One setting that reaches it
Carbon atoms
5
Feedback for a right answer
Pentane, $\mathrm{C_5H_{12}}$ ⁠: it boils at 36 °C, so it is liquid at 20 °C, but evaporates very fast off your palm. From here on the alkanes are liquids, and from about 17 carbons solids, like paraffin wax.

Feedback for a wrong answer
Not yet. Look at the sample: gas, liquid or solid?

Hint
Butane boils at −0.5 °C. How many degrees does each carbon add?

Worked solution
1. An alkane is liquid at 20 °C if its boiling point is above 20 °C.
2. Butane boils at −0.5 °C: a gas. The next carbon adds about 35 degrees.
3. Pentane boils at 36 °C: a liquid, and below 40 °C.
4. Set Carbon atoms to 5.

Lesson 3 of 4 · About 25 minutes

Functional groups

What happens when one hydrogen is swapped for OH or COOH: alcohols and carboxylic acids, hydrogen bonds that send the boiling point leaping, and why ethanol is not a base.

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The 5 steps and their answers

Step 1 of 5
Open this step
The hydroxyl group
Practice · Predict, then look
Question
Ethane, $\mathrm{C_2H_6}$ ⁠, is a gas at room temperature. What will happen to the sample when you swap the hydrogen on the end for $\mathrm{OH}$ ⁠, giving ethanol?

Correct answer
- Correct It will become a liquid
Setup, choices and feedback
The Scene at this step
Carbon atoms
2fixed
End group
–H (alkane)fixed
Compare boiling points
One molecule
Choices
- Correct It will become a liquid
- Wrong It will stay a gas: only two atoms were added
  Misconception: Thinks the heavier molecule always boils higher, whatever groups it carries.
  Feedback from the catalogue
  Mass is not the point: what matters is how strongly the molecules pull on each other. Ethanol, $\mathrm{C_2H_5OH}$ ⁠ (46 g/mol), and propane, $\mathrm{C_3H_8}$ ⁠ (44 g/mol), have almost the same mass. But ethanol’s $\mathrm{OH}$ ⁠ makes hydrogen bonds with its neighbours, so it boils at 78 °C, and propane at −42 °C.
- Wrong It will become a solid, like sodium hydroxide
  Misconception: Thinks the –OH of an alcohol is a hydroxide ion, so alcohols are bases.
  Feedback from the catalogue
  In $\mathrm{NaOH}$ ⁠ the $\mathrm{OH^-}$ ⁠ is an ion, which comes free in water and makes the solution basic. In an alcohol the $\mathrm{OH}$ ⁠ group is joined to a carbon by a covalent bond, and does not come free. So ethanol in water is neutral: a solution of ethanol has a pH of about 7.
Once the prediction is locked in, the Scene changes
End group
–OH (alcohol)fixed
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
Ethanol boils at 78 °C, more than 160 degrees above ethane (−89 °C). The $\mathrm{OH}$ ⁠ makes a hydrogen bond: its hydrogen is pulled hard towards the oxygen of a neighbouring molecule.

Worked solution
1. In ethane only weak van der Waals forces act between the molecules.
2. In $\mathrm{O-H}$ ⁠ the oxygen pulls the electrons, leaving the hydrogen slightly positive.
3. That hydrogen is pulled hard to a neighbour’s oxygen: a hydrogen bond, much stronger than van der Waals forces.
4. So the boiling point jumps, and ethanol is a liquid at room temperature.
Step 2 of 5
Open this step
The same mass
Practice · Reach the target
Question
Ethanol has a molar mass of 46 g/mol and boils at 78 °C. End group is fixed at alkane. Find, with Carbon atoms, the alkane whose molar mass is closest to ethanol’s, then press Check.

Goal
- 46 g/mol ± 2.5 g/mol
  At the start of the step: 16.04 g/mol
Setup, choices and feedback
The Scene at this step
Carbon atoms
1
End group
–H (alkane)fixed
Compare boiling points
One molecule
Read from the Scene at
Molar mass of the molecule/nodes/chain/outputs/molarMass
One setting that reaches it
Carbon atoms
3
Feedback for a right answer
Propane, 44 g/mol: almost the same mass as ethanol. But propane boils at −42 °C, 120 degrees below ethanol. The whole difference comes from the $\mathrm{OH}$ ⁠’s hydrogen bonds.

Feedback for a wrong answer
Not yet. $14n + 2 \approx 46$ ⁠.

Hint
Each carbon in an alkane adds about 14 g/mol, and methane weighs 16.

Worked solution
1. An alkane’s molar mass: $12.01n + 1.008\,(2n + 2) \approx 14n + 2$ ⁠.
2. $14n + 2 = 46$ ⁠ gives $n \approx 3.1$ ⁠.
3. $n = 3$ ⁠: propane, 44.1 g/mol.
4. Set Carbon atoms to 3.
Step 3 of 5
Open this step
Ethanol
Practice · Calculate
Question
What is the molar mass of ethanol, $\mathrm{C_2H_5OH}$ ⁠? $\mathrm{C} = 12.01$ ⁠, $\mathrm{H} = 1.008$ ⁠, $\mathrm{O} = 16.00$ ⁠.

Correct answer
46.07 g/mol ± 0.2 g/mol
Setup, choices and feedback
The Scene at this step
Carbon atoms
2fixed
End group
–OH (alcohol)fixed
Compare boiling points
Side by side
Accepted within
± 0.2 g/mol
Computed from the Scene at this step’s setting
Molar mass of the molecule/nodes/chain/outputs/molarMass
Any unit of the same quantity is accepted; it is converted before checking.
Wrong answers it recognises
- 45.06 g/mol ± 0.2 g/mol
  Misconception: Writes an alkane as CₙH₂ₙ, forgetting the extra hydrogen at each end of the chain.
  Feedback
  Count every hydrogen: five on the carbons and one more on the oxygen, six in all.
Feedback for a right answer
$2 \times 12.01 + 6 \times 1.008 + 16.00 = 46.07\ \mathrm{g/mol}$ ⁠.

Hint
Count atoms: 2 carbons, 6 hydrogens, one oxygen.

Worked solution
1. In $\mathrm{C_2H_5OH}$ ⁠: 2 carbons, $5 + 1 = 6$ ⁠ hydrogens, and one oxygen.
2. $2 \times 12.01 = 24.02$ ⁠; $6 \times 1.008 = 6.05$ ⁠; $1 \times 16.00 = 16.00$ ⁠.
3. Total: $24.02 + 6.05 + 16.00 = 46.07$ ⁠.
Step 4 of 5
Open this step
Not a base
Practice · Choose
Question
Both sodium hydroxide, $\mathrm{NaOH}$ ⁠, and ethanol, $\mathrm{C_2H_5OH}$ ⁠, have $\mathrm{OH}$ ⁠ in their formula. Is a solution of ethanol in water basic?

Correct answer
- Correct No: it is neutral, because the $\mathrm{OH}$ ⁠ is held to carbon by a covalent bond
Setup, choices and feedback
The Scene at this step
Carbon atoms
2
End group
–OH (alcohol)
Compare boiling points
Side by side
Choices
Shown in a random order.
- Correct No: it is neutral, because the $\mathrm{OH}$ ⁠ is held to carbon by a covalent bond
- Wrong Yes: anything with $\mathrm{OH}$ ⁠ releases hydroxide ions
  Misconception: Thinks the –OH of an alcohol is a hydroxide ion, so alcohols are bases.
  Feedback from the catalogue
  In $\mathrm{NaOH}$ ⁠ the $\mathrm{OH^-}$ ⁠ is an ion, which comes free in water and makes the solution basic. In an alcohol the $\mathrm{OH}$ ⁠ group is joined to a carbon by a covalent bond, and does not come free. So ethanol in water is neutral: a solution of ethanol has a pH of about 7.
- Wrong No: it is acidic, because the $\mathrm{OH}$ ⁠ releases $\mathrm{H^+}$ ⁠
  Feedback
  The hydrogen of ethanol’s $\mathrm{OH}$ ⁠ is held too tightly to come free in water. An organic acid needs a $\mathrm{COOH}$ ⁠ group, where the extra oxygen helps the hydrogen come free.
Feedback for a right answer
In $\mathrm{NaOH}$ ⁠ there is an ionic bond between $\mathrm{Na^+}$ ⁠ and $\mathrm{OH^-}$ ⁠, and they separate in water. In ethanol the $\mathrm{OH}$ ⁠ is bonded covalently to carbon and stays put. It is a functional group, hydroxyl, not an ion.

Worked solution
1. A base releases $\mathrm{OH^-}$ ⁠ ions in water.
2. $\mathrm{NaOH}$ ⁠ is an ionic compound, so its ions separate in water.
3. In ethanol every bond is covalent, and the $\mathrm{OH}$ ⁠ does not come free.
4. So an ethanol solution is neutral.
Step 5 of 5
Open this step
Acids
Practice · Choose
Question
Ethanoic acid, $\mathrm{CH_3COOH}$ ⁠, boils at 118 °C, and ethanol at 78 °C. Why does the acid boil higher?

Correct answer
- Correct The $\mathrm{COOH}$ ⁠ group has two oxygens, and two molecules hold together by two hydrogen bonds
Setup, choices and feedback
The Scene at this step
Carbon atoms
2
End group
–COOH (carboxylic acid)
Compare boiling points
Side by side
Choices
Shown in a random order.
- Correct The $\mathrm{COOH}$ ⁠ group has two oxygens, and two molecules hold together by two hydrogen bonds
- Wrong Only because it is heavier
  Misconception: Thinks the heavier molecule always boils higher, whatever groups it carries.
  Feedback from the catalogue
  Mass is not the point: what matters is how strongly the molecules pull on each other. Ethanol, $\mathrm{C_2H_5OH}$ ⁠ (46 g/mol), and propane, $\mathrm{C_3H_8}$ ⁠ (44 g/mol), have almost the same mass. But ethanol’s $\mathrm{OH}$ ⁠ makes hydrogen bonds with its neighbours, so it boils at 78 °C, and propane at −42 °C.
- Wrong The acid splits into ions in the liquid, and ions are hard to boil
  Feedback
  Pure ethanoic acid hardly splits into ions at all; it is a weak acid even in water. What holds it together is hydrogen bonds.
Feedback for a right answer
In a carboxylic acid the $\mathrm{OH}$ ⁠ of one molecule bonds to the $\mathrm{C=O}$ ⁠ of the other, and the other way round: two hydrogen bonds hold a pair of molecules as one unit. That lifts the boiling point higher still.

Worked solution
1. Ethanol and the acid both have $\mathrm{OH}$ ⁠, so both make hydrogen bonds.
2. The acid also has $\mathrm{C=O}$ ⁠, an oxygen that takes one more hydrogen bond.
3. So each two acid molecules hold on to each other by two hydrogen bonds.
4. A stronger pull between the molecules: a higher boiling point.

Lesson 4 of 4 · About 25 minutes

Polymers

Thousands of monomers in one chain: addition polymerisation, and why a plastic bag stretches and stays long, a rubber band springs back, and a saucepan handle does not melt.

Open the lesson

The 5 steps and their answers

Step 1 of 5
Open this step
Monomers
Practice · Choose
Question
Ethene, $\mathrm{CH_2{=}CH_2}$ ⁠, polymerises into poly(ethene). What happens to the double bond?

Correct answer
- Correct One of its two bonds opens, and each carbon bonds to a carbon of the next monomer
Setup, choices and feedback
The Scene at this step
Chain length
60 beads
Cross-links
0 %
Grips
Hold
Stretch
Relaxed
Choices
Shown in a random order.
- Correct One of its two bonds opens, and each carbon bonds to a carbon of the next monomer
- Wrong It stays, and the monomers join through their hydrogens
  Misconception: Thinks the monomer’s C=C double bond survives in an addition polymer.
  Feedback from the catalogue
  In addition polymerisation the double bond opens: one of the two bonds between the carbons breaks, and each carbon uses the freed electron to bond to the next monomer. So in poly(ethene) every bond along the chain is single, and it is really a very long alkane.
- Wrong The monomers do not bond: they are only packed together
  Feedback
  A polymer is one long molecule, held together by covalent bonds all along its length. That is why it is so strong.
Feedback for a right answer
This is addition polymerisation: thousands of monomers join into one chain, losing no atoms. $n\,\mathrm{CH_2{=}CH_2} \rightarrow (\mathrm{-CH_2-CH_2-})_n$ ⁠. What comes out is, in effect, a very long alkane.

Worked solution
1. A double bond is two shared pairs of electrons.
2. In polymerisation one pair opens, leaving each carbon a spare electron.
3. Each carbon uses it for a single bond to a carbon of a neighbouring monomer.
4. In the polymer every bond along the chain is single.
Step 2 of 5
Open this step
Long chains
Practice · Calculate
Question
One chain of poly(ethene) has a molar mass of 280 000 g/mol. Ethene’s molar mass is 28 g/mol. How many monomers make up the chain?

Correct answer
10000 ± 1%
Setup, choices and feedback
The Scene at this step
Chain length
120 beads
Cross-links
0 %
Grips
Hold
Stretch
Relaxed
Accepted within
± 1%
A fixed value in the question
Wrong answers it recognises
- 5000 ± 1%
  Misconception: Thinks the monomer’s C=C double bond survives in an addition polymer.
  Feedback
  Each monomer gives the chain all of its 28 g/mol: addition polymerisation loses no atoms. There is no need to halve it.
Feedback for a right answer
$\dfrac{280\,000}{28} = 10\,000$ ⁠ monomers, and 20 000 carbons in one chain. In the Scene each bead stands for a stretch of tens of monomers.

Worked solution
1. In addition polymerisation the chain’s mass is the sum of the monomers’ masses.
2. Number of monomers: $\dfrac{280\,000}{28}$ ⁠.
3. $= 10\,000$ ⁠.
Step 3 of 5
Open this step
Thermoplastic
Practice · Predict, then look
Question
The block is long chains with no cross-links: a thermoplastic, like a plastic bag. It is pulled to double its length and let go. What will happen?

Correct answer
- Correct It will stay long
Setup, choices and feedback
The Scene at this step
Chain length
100 beadsfixed
Cross-links
0 %fixed
Grips
Holdfixed
Stretch
Relaxedfixed
Choices
- Correct It will stay long
- Wrong It will spring back to its length, like a rubber band
  Misconception: Thinks a plastic or rubber stretches because the covalent bonds along its chains stretch.
  Feedback from the catalogue
  The covalent bonds along a chain hardly lengthen at all. When rubber is stretched, its coiled chains straighten between the cross-links; when a thermoplastic is stretched, its chains slide past each other. That is why rubber stretches to several times its length with a small force.
- Wrong It will snap before it reaches double
  Feedback
  Short chains would snap like that, as wax does. Long chains tangle with each other and draw out without breaking.
Once the prediction is locked in, the Scene changes
Grips
Let gofixed
Stretch
Pulled to doublefixed
The Scene then shows: 0.9805 · Stretch kept for good, as a share of the relaxed length (0.5 is half as long again)/nodes/tangle/outputs/kept
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
The chains slid past each other and stayed where they ended up: the block stays almost twice as long. The number after the test is how much of the stretch was kept, as a share of the original length. That is how a plastic bag stretches and does not come back.

Worked solution
1. Between the chains of a thermoplastic there are no covalent bonds, only a weak attraction and tangling.
2. When it is pulled, the chains slide past each other.
3. When it is let go, nothing pulls them back, and the block stays stretched.
Step 4 of 5
Open this step
Rubber
Practice · Reach the target
Question
The block has been pulled to double its length and let go, and it stayed long. Change Cross-links until the block goes all the way back to its original length without breaking, then press Check.

Goal
- 0 ± 0.005
  At the start of the step: 0.9805
- 0 ± 0.1
  At the start of the step: 0
Setup, choices and feedback
The Scene at this step
Chain length
60 beadsfixed
Cross-links
0 %
Grips
Let gofixed
Stretch
Pulled to doublefixed
Read from the Scene at
Stretch kept for good, as a share of the relaxed length (0.5 is half as long again)/nodes/tangle/outputs/kept
Read from the Scene at
1 once the block has broken, else 0/nodes/tangle/outputs/broken
One setting that reaches it
Cross-links
0.5 %
Feedback for a right answer
A few cross-links per hundred beads turn the thermoplastic into a rubber: the chains cannot slide, only straighten between the tie points, and when they are let go they coil up again. That is what vulcanising rubber with sulfur does.

Feedback for a wrong answer
Not yet. With no cross-links the chains slide; with too many, the block cracks.

Hint
A few cross-links hold the chains together; too many make the block stiff and brittle.

Worked solution
1. With no cross-links the chains slide and do not come back.
2. With a few cross-links, 0.5% to 5%, they are tied into a loose network that returns to its shape.
3. From 5.5% on the network is too tight and tears before it reaches double length; at 10% it cracks within a few percent.
4. Set Cross-links to 2%, for example.
Step 5 of 5
Open this step
Thermoset
Practice · Choose
Question
A saucepan handle is made of a thermoset: a block with many cross-links. What will happen to it if it gets very hot?

Correct answer
- Correct It will not soften; in extreme heat it will char
Setup, choices and feedback
The Scene at this step
Chain length
60 beads
Cross-links
10 %
Grips
Hold
Stretch
0.3 (Relaxed → Pulled to double)
Choices
Shown in a random order.
- Correct It will not soften; in extreme heat it will char
- Wrong It will soften and melt, like any plastic
  Misconception: Thinks every plastic softens and melts when heated.
  Feedback from the catalogue
  Only thermoplastics, whose chains are separate, melt: with heat the chains slide past each other. In a thermoset the chains are tied to each other by covalent cross-links into one network, and no amount of heat makes them slide. Heated hard enough, a thermoset chars and burns, but it does not melt.
- Wrong It will turn flexible like rubber
  Feedback
  Rubber is flexible because it has few cross-links. A thermoset has so many that the chains cannot even straighten, and heat does not change that.
Feedback for a right answer
In a thermoset all the chains are tied into one network by covalent bonds. To melt, the chains would have to slide past each other, which means breaking those bonds, at a heat where the material is already falling apart. That is why handles, electric sockets and strong glues are made of thermosets, and why they cannot be recycled by melting.

Worked solution
1. A plastic melts when its chains slide past each other.
2. In a thermoset the chains are tied by covalent cross-links, and cannot slide.
3. So it does not soften with heat; hot enough, its bonds break, and it chars.

Across the unit · 7 misconceptions

Misconception catalogue

Each common mistake this unit recognises, in a few words. Open one for what the learner reads when an answer matches it, and where it is asked.

Index

Hydrocarbons

Chains of carbon

Question

Correct answer

The Scene at this step

Choices

Worked solution

Counting hydrogens

Question

Correct answer

The Scene at this step

Wrong answers it recognises

Hint

Worked solution

The formula

Question

Goal

The Scene at this step

One setting that reaches it

Hint

Worked solution

Names

Question

Correct answer

The Scene at this step

Choices

Worked solution

Fuels

Question

Correct answer

The Scene at this step

Choices

Worked solution

Homologous series

A series

Question

Correct answer

The Scene at this step

Wrong answers it recognises

Hint

Worked solution

One step

Question

Correct answer

The Scene at this step

Wrong answers it recognises

Worked solution

Boiling points

Question

Correct answer

The Scene at this step

Choices

Once the prediction is locked in, the Scene changes

Worked solution

Side by side

Question

Correct answer

The Scene at this step

Choices

Worked solution

At room temperature

Question

Goal

The Scene at this step

One setting that reaches it

Hint

Worked solution

Functional groups

The hydroxyl group

Question

Correct answer

The Scene at this step

Choices

Once the prediction is locked in, the Scene changes

Worked solution

The same mass

Question

Goal

The Scene at this step

One setting that reaches it