To link structure with a transformation, balance the atoms, decide whether the structure is saturated or unsaturated, then use the observation to support your decision. A formula gives you a clue, and a test gives you evidence.
This lesson comes after explaining electrochemical observations in integrated chemical reasoning, and it uses the same skill of linking an observation to the particles behind it.
What does the transformation tell you?
Two transformations come up repeatedly. Cracking breaks a large alkane into a smaller alkane and an alkene. Addition happens when an alkene reacts across its double bond, as with bromine.
- Alkane formula: CₙH₂ₙ₊₂ (saturated, only single bonds).
- Alkene formula: CₙH₂ₙ (unsaturated, one C=C double bond).
- Bromine water test: decolourised by an alkene, unchanged by an alkane.
Cracking is carried out industrially at high temperature, often with a catalyst. At school level you are asked to understand the idea and the equation, not to carry it out.
Worked example
The details below are invented for practice. Decane, C₁₀H₂₂, is cracked.
One product is octane, C₈H₁₈. A second product, a gas, decolourises bromine water.
Step 1, find the second product by balancing: C₁₀H₂₂ → C₈H₁₈ + ?
Carbon: 10 − 8 = 2. Hydrogen: 22 − 18 = 4. So the missing product is C₂H₄, which is ethene.
Step 2, check the structure: C₂H₄ fits CₙH₂ₙ with n = 2, so it is an alkene with a C=C double bond.
Step 3, link to the observation: an alkene is unsaturated, so it adds bromine and the bromine water turns from orange-brown to colourless.
Step 4, write the addition equation: C₂H₄ + Br₂ → C₂H₄Br₂
Step 5, check by mass: Mr of C₁₀H₂₂ = (10 × 12) + 22 = 142. Mr of C₈H₁₈ = 96 + 18 = 114. Mr of C₂H₄ = 24 + 4 = 28. And 114 + 28 = 142, so mass is conserved.
The mistake to watch for
Mistaken answer: C₁₀H₂₂ → C₈H₁₈ + C₂H₆
Counting hydrogen shows the problem: 18 + 6 = 24, but the left side has 22. The equation is not balanced.
Also, C₂H₆ is ethane, an alkane, which would not decolourise bromine water, so it clashes with the observation. The correction is to balance first, get C₂H₄, and then confirm with the test. If the formula and the observation disagree, one of them has been misread.
Check yourself
The formulae below are invented as practice questions.
1. Hydrocarbon P has formula C₅H₁₀ and decolourises bromine water. Hydrocarbon Q has formula C₅H₁₂. Which is the alkene, and what does Q do with bromine water?
Show answer
P is the alkene, since C₅H₁₀ fits CₙH₂ₙ. Q fits CₙH₂ₙ₊₂, so it is an alkane. Q is saturated and does not decolourise bromine water, so the mixture stays orange-brown.
2. Complete the cracking equation: C₁₂H₂₆ → C₈H₁₈ + ?
Show answer
Carbon: 12 − 8 = 4. Hydrogen: 26 − 18 = 8. The missing product is C₄H₈, an alkene.
3. Write the equation for the addition of bromine to C₃H₆.
Show answer
C₃H₆ + Br₂ → C₃H₆Br₂. One molecule of bromine adds across the double bond.
Where this leads next
The final lesson of the module uses evidence from two tests together, in comparing evidence from two analysis methods. Then use the mixed practice set. The scientific investigation critic helps you check whether one observation is enough to support a structure.
Students who rush into explanations without counting atoms often lose marks on questions they understand in principle. Our teachers in online one-to-one Co-ordinated Sciences tuition can show you how to build a short checking routine into every chemistry answer.