Cracking: smaller molecules, conditions and demand
Account for every atom when larger alkanes split, including hydrogen formation, and connect the products with fuels and chemical feedstocks.
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Distinguish an alkane and alkene formula and balance atoms.
- Explain petroleum fractionation as a physical separation.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How does cracking differ from fractional distillation?
What you will learn
- Describe high-temperature catalytic cracking of larger alkanes to make alkenes and hydrogen.
- Explain why demand for smaller fuels and alkene feedstocks motivates cracking.
Break bonds rather than separate an existing mixture
Cracking breaks larger alkane molecules into smaller molecules using a high temperature and a catalyst. It is a chemical change, because bonds break and new substances form. Fractional distillation instead separates an existing petroleum mixture by different boiling points; it does not create new shorter molecules by breaking their bonds.
A possible balanced model is C₁₀H₂₂→C₅H₁₂+C₅H₁₀: decane gives pentane and an alkene with five carbons. Another possible overall split is C₁₀H₂₂→C₅H₁₀+C₃H₆+C₂H₄+H₂, showing alkene and hydrogen production while conserving ten C and twenty-two H atoms. Real cracking can give a mixture of products; the formula of the starting alkane does not determine one unique product set.
Explain why the products are wanted
Petroleum supply and demand do not necessarily match: longer-chain fractions may be available beyond demand, while smaller molecules useful as fuels and alkene chemical feedstocks are wanted. Cracking converts some longer alkanes into smaller, more useful molecules and provides alkenes for making other chemicals, including polymers. State a use for the products rather than saying only that cracking is useful.
The syllabus also requires hydrogen as a possible cracking product. In a balanced proposed equation, all carbon and hydrogen must remain somewhere: check both elements before identifying the products. The equation C₂H₆→C₂H₄+H₂ is a simple hydrogen-accounting illustration, not a claim that ethane is the larger petroleum fraction in the industrial example. Conditions remain high temperature and a catalyst; a catalyst speeds the reaction and is not a source of new C or H atoms.
Sources for this lesson
Pause and explain
Which equation includes a balanced hydrogen-producing split?
Worked example
Check C₁₀H₂₂→C₅H₁₀+C₃H₆+C₂H₄+H₂, state the conditions and explain one fuel/feedstock reason for cracking.
Show the worked solution
- Product carbons total 5+3+2=10; product hydrogens total 10+6+4+2=22.
- The balanced model gives smaller alkenes and hydrogen under high temperature with a catalyst.
- Cracking supplies smaller useful molecules and alkene feedstocks when demand differs from the available longer-chain fractions.
Answer C₁₀H₂₂→C₅H₁₀+C₃H₆+C₂H₄+H₂ balances C₁₀H₂₂; use high temperature and a catalyst to obtain useful smaller molecules and alkene feedstocks.
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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.
For C₁₀H₂₂→C₅H₁₂+C₅H₁₀, check atom conservation and identify the two smaller hydrocarbon families.
Give me a hint
Count totals, then use each family's stated general formula.
Compare my reasoning
- Products contain 5+5=10 C and 12+10=22 H.
- C₅H₁₂ matches the simple alkane formula CₙH₂ₙ₊₂.
- The stated C₅H₁₀ product is an alkene in this cracking model; high temperature and a catalyst are required.
C₁₀H₂₂→C₅H₁₂+C₅H₁₀ is balanced; smaller alkane and stated alkene, at high temperature with a catalyst.
Look for these in your work
- Both atom totals are checked.
- Families are assigned within the stated product model.
Complete the specified split C₈H₁₈→C₃H₈+____, given that the remaining product is a simple alkene. State the conditions.
Give me a hint
Subtract the supplied propane atoms before using the stated alkene family.
Compare my reasoning
- Propane uses three C and eight H atoms, leaving five C and ten H.
- C₅H₁₀ matches the stated simple alkene product: C₈H₁₈→C₃H₈+C₅H₁₀.
- State high temperature and a catalyst; a molecular formula alone does not identify the alkene's exact structural isomer.
C₅H₁₀; C₈H₁₈→C₃H₈+C₅H₁₀ at high temperature with a catalyst, without a unique structural-isomer claim.
Look for these in your work
- The atom remainder and stated family agree.
- Conditions and the molecular-versus-structural limit are included.
A refinery has excess longer-chain fractions but wants smaller fuels, alkene feedstocks and some hydrogen. Evaluate using fractionation alone versus cracking; give a balanced hydrogen-containing decane model.
Give me a hint
Separation cannot create molecules that were absent; a conversion can.
Compare my reasoning
- Fractionation separates existing molecules and cannot alone convert a larger alkane into the required new smaller molecules.
- High-temperature catalytic cracking can supply smaller products and alkene feedstocks, with hydrogen possible.
- C₁₀H₂₂→C₅H₁₀+C₃H₆+C₂H₄+H₂ conserves C₁₀H₂₂, but real products need not be this unique set.
Use high-temperature catalytic cracking for conversion; C₁₀H₂₂→C₅H₁₀+C₃H₆+C₂H₄+H₂ is one balanced model, not a unique real product mixture.
Look for these in your work
- Product demand is linked to conversion.
- The balanced model and non-unique product limitation are both stated.
Common mistakes
- Calling cracking a physical separation by boiling point.
- Omitting the catalyst or high-temperature condition.
- Forgetting hydrogen is a possible product.
- Assuming the initial formula fixes one unique cracking product mixture.
What can you explain now?
Which two conditions are required for the stated catalytic cracking?
Compare with the explanation
High temperature and a catalyst.
Breaking larger molecules is a chemical conversion, unlike fractional distillation.
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Make it stick
State cracking's conditions and two demand reasons. Balance an alkane/alkene split and a hydrogen-producing model, then distinguish both from fractionation.
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.