If a reversible reaction is exothermic in one direction, it is endothermic…

The idea

A reversible reaction can proceed in both directions, and the two directions have opposite energy changes. If the forward reaction releases energy to the surroundings, it is exothermic; the reverse reaction must then absorb the same quantity of energy from the surroundings and is endothermic. The energy transferred is equal in magnitude but opposite in sign, so the energy change for the reverse direction is the negative of that for the forward direction. For example, heating hydrated copper(II) sulfate drives off water to form anhydrous copper(II) sulfate, an endothermic forward change; adding water to the anhydrous solid regenerates the hydrated salt and releases energy, so the reverse change is exothermic. The same amount of energy is transferred in each case, meaning no energy is created or destroyed overall.

What earns the marks

• States that a reversible reaction can occur in both the forward and reverse directions.
• Identifies that if one direction is exothermic, the opposite direction is endothermic.
• Explains that the energy transferred in the forward direction is equal in magnitude to the energy transferred in the reverse direction.
• Uses the sign convention correctly: an exothermic change has a negative energy change and an endothermic change has a positive energy change.
• Applies the idea to a named example, such as hydrated copper(II) sulfate losing water when heated and anhydrous copper(II) sulfate releasing energy when water is added.
• Recognises that the equality of energy transfer follows from the principle of conservation of energy.

Common mistakes and how to put them right

• Thinking that both directions of a reversible reaction can be exothermic.

  Put it right: The directions have opposite energy changes, one exothermic and one endothermic.

• Assuming the reverse reaction transfers a different amount of energy.

  Put it right: The same amount of energy is transferred in each direction.

• Confusing the terms exothermic and endothermic when describing the reverse direction.

  Put it right: Link exothermic to energy release and endothermic to energy absorption, then reverse the label for the opposite direction.

Exam tips

• When asked about a reversible reaction, state the energy change for one direction and immediately deduce the opposite sign for the other direction.
• Use a concrete example such as hydrated copper(II) sulfate and anhydrous copper(II) sulfate to show both directions and their energy changes.
• Check that any numerical energy value quoted for the reverse direction has the same magnitude but the opposite sign to the forward value.