Question

the formula and name for the missing product

(a) \\(\text{c}_2\text{h}_6\\) \\(\longrightarrow\\) ? + \\(\text{h}_2\\)
ethane hydrogen

(b) \\(\text{c}_{12}\text{h}_{26}\\) \\(\longrightarrow\\) \\(\text{c}_6\text{h}_{14}\\) + \\(\text{c}_2\text{h}_4\\) + \\(\text{c}_4\\) ?
dodecane hexane ethene

(c) \\(\text{c}_{20}\text{h}_{42}\\) \\(\longrightarrow\\) ? + \\(\text{c}_3\text{h}_6\\) + \\(\text{c}_9\text{h}_{18}\\)
icosane propene nonene

Explanation:

Identify the reaction type

The reactions represent thermal cracking or dehydrogenation of alkanes. We must balance the total number of carbon (\(\text{C}\)) and hydrogen (\(\text{H}\)) atoms on both sides of each chemical equation to find the missing molecular formulas, and then name the products.

Solve part (a)

The reactant is ethane, \(\text{C}_2\text{H}_6\).
The products are a missing substance and hydrogen, \(\text{H}_2\).
Subtracting the atoms of \(\text{H}_2\) from \(\text{C}_2\text{H}_6\):
Carbon: \(2 - 0 = 2\)
Hydrogen: \(6 - 2 = 4\)
The missing formula is \(\text{C}_2\text{H}_4\).
An alkene with two carbon atoms is named ethene.

Solve part (b)

The reactant is dodecane, \(\text{C}_{12}\text{H}_{26}\).
The products are hexane (\(\text{C}_6\text{H}_{14}\)), ethene (\(\text{C}_2\text{H}_4\)), and a missing substance \(\text{C}_4\text{H}_?\).
Subtracting the known products from the reactant:
Carbon: \(12 - (6 + 2) = 4\) (which matches the \(\text{C}_4\) in the prompt)
Hydrogen: \(26 - (14 + 4) = 8\)
The missing formula is \(\text{C}_4\text{H}_8\).
An alkene with four carbon atoms is named butene.

Solve part (c)

The reactant is icosane, \(\text{C}_{20}\text{H}_{42}\).
The products are a missing substance, propene (\(\text{C}_3\text{H}_6\)), and nonene (\(\text{C}_9\text{H}_{18}\)).
Subtracting the known products from the reactant:
Carbon: \(20 - (3 + 9) = 8\)
Hydrogen: \(42 - (6 + 18) = 18\)
The missing formula is \(\text{C}_8\text{H}_{18}\).
An alkane with eight carbon atoms is named octane.

Answer:

Question (a)

• Formula: \(\text{C}_2\text{H}_4\)
• Name: ethene

Question (b)

• Formula: \(\text{C}_4\text{H}_8\)
• Name: butene

Question (c)

• Formula: \(\text{C}_8\text{H}_{18}\)
• Name: octane