Propene, CH2=CHCH3 (molecular formula C3H6), is reacted with hydrogen gas in the presence of a nickel catalyst. This addition reaction forms a single saturated organic product. What is the molecular formula of this organic product?

IGCSE Chemistry 0620: Hydrogenation of Propene with a Nickel Catalyst

Step 1: Recall what hydrogenation does to an alkene

Propene contains a carbon–carbon double bond, $CH_2=CHCH_3$ . In the presence of a nickel catalyst, hydrogen gas, $H_2$ , adds across this double bond: both hydrogen atoms of the $H_2$ molecule attach to the two carbon atoms that were joined by the double bond, which becomes a single bond. This is an addition reaction, so only one product is formed and the atoms of both reactants end up in that one product.

Step 2: Add the atoms of propene and hydrogen together

$C_3H_6 + H_2 \rightarrow C_3H_8$

Propene, $C_3H_6$ , has 3 carbon atoms and 6 hydrogen atoms. Adding the 2 hydrogen atoms from one molecule of $H_2$ gives $3$ carbon atoms and $6+2=8$ hydrogen atoms, i.e. $C_3H_8$ (propane, the saturated alkane with 3 carbon atoms).

Step 3: Check why the other options are wrong

A ( $C_3H_6$ ): this is propene’s own formula. It wrongly assumes no reaction occurs.
C ( $C_3H_4$ ): this comes from subtracting hydrogen atoms instead of adding them.
D ( $C_6H_{12}$ ): this wrongly assumes two propene molecules combine with each other, rather than one propene molecule reacting with one hydrogen molecule.

Final answers

Hydrogenating propene gives propane, $\boxed{C_3H_8}$ , option B.