Consider the reaction represented by the following equation: C$_2$H$_2$ + yH$_2$ → C$_2$H$_6$. The value of y in the reaction is

Analyze the question: We need to balance the equation C₂H₂ + yH₂ → C₂H₆. This is the hydrogenation of ethyne (acetylene) to give ethane.

Concept: To balance an equation, the number of atoms of each element must be the same on both sides (law of conservation of mass).

Solution:

• Carbon: 2 on the left, 2 on the right, so carbon is balanced.
• Hydrogen on the left: 2 (from C₂H₂) + 2y (from yH₂)
• Hydrogen on the right: 6 (from C₂H₆)

Set them equal:

2 + 2y = 6 2y = 4 y = 2

The balanced equation is:

C₂H₂ + 2H₂ → C₂H₆

Check: The left has 2 C and 2 + 4 = 6 H. The right has 2 C and 6 H. Both sides match.

Tip: Going from an alkyne to an alkane means adding 2 moles of H₂ across the triple bond: one mole turns the triple bond into a double bond (alkene), and the second turns the double bond into a single bond (alkane). So alkyne → alkane always needs 2 H₂, while alkene → alkane needs 1 H₂.