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Propene can be produced by the cracking of some hydrocarbons obtained from crude oil - Edexcel - GCSE Chemistry - Question 5 - 2019 - Paper 1
Question 5
Propene can be produced by the cracking of some hydrocarbons obtained from crude oil. The equation shows the cracking of one molecule of decane to produce one molecu... show full transcript
Worked Solution & Example Answer:Propene can be produced by the cracking of some hydrocarbons obtained from crude oil - Edexcel - GCSE Chemistry - Question 5 - 2019 - Paper 1
Step 1
a) (i) Calculate the values of x and y in CₙHₓ.
Answer
To crack one molecule of decane (C₁₀H₂₂) into one molecule of butene (C₄H₁₀) and another product (C₆H₁₄), we need to balance the carbon and hydrogen atoms:
- From C₁₀H₂₂, we have 10 carbon atoms and 22 hydrogen atoms.
- Butene contributes 4 carbon atoms and 10 hydrogen atoms, while the other product must have 6 carbon atoms and 14 hydrogen atoms.
Thus, we can conclude:
Number of carbon atoms in Cₙ = 10 (from decane) = 4 (from butene) + 6 (from the other product), so y = 14.
- Hydrogen atoms: 22 = 10 (from butene) + 14 (from the other product).
Therefore, x = 6 and y = 14.
Step 2
a) (ii) State the total mass of products formed if 25 g of decane is cracked in this way.
Answer
The molar mass of decane (C₁₀H₂₂) can be calculated as follows:
- C: 12 g/mol x 10 = 120 g
- H: 1 g/mol x 22 = 22 g
- Total molar mass of decane = 120 g + 22 g = 142 g/mol.
Now, to find the moles of decane in 25 g:
Since the cracking of one mole of decane produces one mole of butene and one mole of C₆H₁₄, it results in 2 moles of product.
Therefore, the total mass of products formed is:
- Molar mass of butene (C₄H₁₀) + molar mass of C₆H₁₄: Molar mass of butene = 58 g/mol, C₆H₁₄ = 86 g/mol.
- Total molar mass of products = 58 g/mol + 86 g/mol = 144 g/mol.
- Total mass of products = 2 moles x 144 g/mol = 288 g.
Step 3
b) (i) Complete Figure 9 to show the electrons of the C=C double bond.
Answer
The incomplete dot and cross diagram for the C=C double bond should consist of:
- 4 electrons, 2 from each carbon atom, which can be drawn as pairs of dots between the two carbon atoms to represent the double bond.
- Each carbon atom also has two dots representing the single bonds to the hydrogen atoms.
Step 4
Step 5
c) Explain how the structure of substance X shows that it is an unsaturated hydrocarbon.
Answer
Substance X is identified as an unsaturated hydrocarbon if it contains at least one double or triple bond between carbon atoms. The presence of such bonds allows the molecule to react with hydrogen or undergo addition reactions, indicating unsaturation. In the structure of substance X, the visible double bond between carbon atoms confirms its unsaturated nature.
Step 6
d) Explain these results.
Answer
In the testing of the two hydrocarbons with bromine water:
- Hydrocarbon A, which is an alkane, reacted with bromine water, causing the solution to turn from orange to colourless. This indicates that alkanes can undergo addition reactions due to saturation.
- Hydrocarbon B, known as an alkene, did not cause the colour of the bromine water to change, indicating that it remains unsaturated and does not react with bromine water in this particular test.