AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Cracking and alkenes: Students should be able to balance chemical equations as examples of cracking given the formulae of the reactants and products.
Cracking breaks a long-chain alkane into a shorter, more useful alkane and an alkene.
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Cracking breaks a long-chain alkane into a shorter, more useful alkane and an alkene. A word equation such as decane → octane + ethene becomes a balanced symbol equation by writing correct formulae and then adjusting coefficients so every element has the same number of atoms on each side. For example, C₁₀H₂₂ → C₈H₁₈ + C₂H₄ is already balanced: carbon 10 = 8 + 2 and hydrogen 22 = 18 + 4. If the products given do not balance, only the large numbers in front of formulae may be changed; formulae themselves must not be altered. Check each element in turn, starting with carbon and hydrogen, and remember cracking must also produce at least one alkene.
What earns the marks
- Write the correct formulae of the reactant alkane and the product alkane and alkene before attempting to balance.
- Balance by placing whole-number coefficients in front of formulae; never change a formula's subscripts to make an equation balance.
- Check carbon atoms first, then hydrogen atoms, and confirm the totals are equal on both sides.
- Recognise that a valid cracking equation produces a shorter alkane and an alkene, so the products must include a C=C-containing molecule.
- For C₁₀H₂₂ → C₈H₁₈ + C₂H₄, verify carbon: 10 = 8 + 2, and hydrogen: 22 = 18 + 4.
- If coefficients are needed, for example 2C₈H₁₈, multiply every atom in that formula by the coefficient when checking.
Common mistakes and how to put them right
Changing a subscript in a formula, such as writing C₈H₂₀ instead of C₈H₁₈, to force the equation to balance.
Put it right: Only coefficients may be changed.
Forgetting to multiply all atoms by the coefficient when a product has a number in front of it.
Producing only alkanes as products.
Put it right: A cracking equation must include an alkene such as ethene or propene.
Exam tips
- Count and write the number of each type of atom on both sides before and after balancing so your working is clear.
- Balance carbon first, then hydrogen, and recheck both at the end.
- If the equation is already balanced, say so and show the atom counts rather than adding unnecessary coefficients.
Try the lesson
The start of the Cracking and alkenes: Students should be able to balance chemical equations as examples of cracking given the formulae of the reactants and products. lesson, made from checked teaching notes for this specification point.
Have a go first
Cracking produces ethene and hydrogen from ethane. Which balanced equation for this cracking reaction is correct?
Listen
Cracking breaks a long-chain alkane into a shorter, more useful alkane and an alkene. A word equation such as decane → octane + ethene becomes a balanced symbol equation by writing correct formulae and then adjusting coefficients so every element has the same number of atoms on each side. For example, C₁₀H₂₂ → C₈H₁₈ + C₂H₄ is already balanced: carbon 10 = 8 + 2 and hydrogen 22 = 18 + 4. If the products given do not balance, only the large numbers in front of formulae may be changed; formulae themselves must not be altered. Check each element in turn, starting with carbon and hydrogen, and remember cracking must also produce at least one alkene. A strong answer earns marks for these points. Write the correct formulae of the reactant alkane and the product alkane and alkene before attempting to balance. Balance by placing whole-number coefficients in front of formulae; never change a formula's subscripts to make an equation balance. Check carbon atoms first, then hydrogen atoms, and confirm the totals are equal on both sides. Recognise that a valid cracking equation produces a shorter alkane and an alkene, so the products must include a C=C-containing molecule. Exam tip: Count and write the number of each type of atom on both sides before and after balancing so your working is clear.
Key words
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Balancing symbol equations using coefficients
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Cracking of long-chain alkanes
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Products of cracking: shorter alkanes and alkenes
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See every definition
- Balancing symbol equations using coefficients
- Writing correct formulae and then adjusting coefficients so every element has the same number of atoms on each side.
- Cracking of long-chain alkanes
- Breaking a long-chain alkane into a shorter, more useful alkane and an alkene.
- Products of cracking: shorter alkanes and alkenes
- A valid cracking equation produces a shorter alkane and an alkene, so the products must include a C=C-containing molecule.
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