Double Bonds Are Stronger Bonds, Not Stronger Molecules

What students typically write

“Ethene is more stable than ethane because it has a double bond, which is stronger.” Or: “The double bond in alkenes makes them harder to react.”

Students learn that double bonds have higher bond energies than single bonds and conclude that molecules with double bonds must be more stable or less reactive. This confuses individual bond strength with overall molecular reactivity.

Why it is wrong

Bond strength and molecular reactivity are different concepts. A C=C double bond does require more energy to break completely (bond energy approximately 614 kJ/mol) compared to a C-C single bond (approximately 348 kJ/mol). But reactivity depends on how easily a bond participates in a reaction, not just how much energy it takes to break it in isolation.

The C=C double bond has a high electron density concentrated between two carbon atoms. This makes it a target for electrophilic addition reactions. Bromine, hydrogen and other reagents readily attack the double bond, breaking it and forming two new single bonds. Alkenes are therefore more reactive than alkanes, despite having a “stronger” bond.

At IGCSE level, the key distinction is: alkanes are saturated (only single bonds) and relatively unreactive, while alkenes are unsaturated (contain C=C double bonds) and undergo addition reactions. The double bond is what makes alkenes reactive, not what makes them stable.

What the mark scheme actually wants

A typical question: “Explain why ethene reacts with bromine water but ethane does not.” [2]

Marking points:

1. Ethene is unsaturated / has a C=C double bond (1 mark)
2. The double bond allows addition reactions / bromine adds across the double bond (1 mark)

There is no mark for saying the double bond is “stronger”. The mark goes to identifying the double bond as the feature that enables the reaction.

Worked example: wrong vs right

Question: Compare the reactivity of ethane (C2H6) and ethene (C2H4). [2]

Wrong answer: “Ethene is less reactive because it has a double bond which is stronger and harder to break.” This scores 0/2. It gets the reactivity comparison backwards and misapplies bond strength.

Right answer: “Ethene is more reactive than ethane (1) because the C=C double bond allows addition reactions with reagents such as bromine (1).” This scores 2/2.

How to avoid this mistake

Keep two separate ideas in your mind:

1. Bond energy tells you how much energy is needed to break one mole of a specific bond in the gas phase. A double bond has a higher bond energy than a single bond.
2. Reactivity tells you how readily a substance undergoes chemical reactions. This depends on the type of reactions available, not just bond energy.

For alkanes vs alkenes at IGCSE, the rule is simple: the C=C double bond makes alkenes reactive (addition reactions), while alkanes with only C-C single bonds are relatively unreactive (mainly combustion and substitution under UV light).

Never write that a double bond makes a molecule “more stable” or “harder to react”. Instead, identify the double bond as the reactive site.

For the full comparison of alkanes and alkenes, see simple molecules and covalent bonds. Giant covalent structures, where strong bonds throughout the structure genuinely do create stability, are covered at giant covalent structures.