Energy changes and reversible reactions: If a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction. The same amount of energy is transferred in each case. For example:

The idea

A reversible reaction can proceed in both directions, and its energy change depends on direction. 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, so it is endothermic. Energy is conserved: the magnitude of the energy transfer is identical in both directions, only the sign of the overall enthalpy change is reversed. For example, heating hydrated copper(II) sulfate crystals drives off water to form white anhydrous copper(II) sulfate; this forward dehydration is endothermic. Adding water to the white solid regenerates the blue hydrated crystals and releases energy, so the reverse hydration is exothermic. The same amount of energy is transferred in each case, meaning the energy change for the reverse direction is equal in size but opposite in sign.

What earns the marks

• Identifies that a reversible reaction has both a forward and a reverse direction, each with its own energy change.
• States that if one direction is exothermic, the opposite direction is endothermic.
• Explains that the energy transferred in the reverse direction is equal in magnitude to that in the forward direction, but with the opposite sign.
• Applies the principle to a named example, such as hydrated copper(II) sulfate ⇌ anhydrous copper(II) sulfate + water, linking each direction to its observed temperature change.
• Uses the conservation of energy to justify why the two energy changes must be equal in size.
• Interprets an energy level diagram or data table to identify which direction is exothermic and which is endothermic.

Common mistakes and how to put them right

• Saying both directions are exothermic or both are endothermic.

  Put it right: In a reversible reaction, one direction is exothermic and the opposite direction is endothermic.

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

  Put it right: The same amount of energy is transferred in each case; only the direction of transfer reverses.

• Confusing the sign of the energy change with the size of the energy change.

  Put it right: The magnitude is the same, but an exothermic change has a negative sign and an endothermic change has a positive sign when expressed as enthalpy change.

• Assuming a reversible reaction always reaches completion in one direction.

  Put it right: Reversibility means both directions can occur, and the observed direction depends on conditions.

Exam tips

• When asked to explain energy changes in a reversible reaction, always name the direction you are discussing and state whether it is exothermic or endothermic.
• Use the phrase 'equal in magnitude but opposite in sign' to make the energy relationship clear.
• If given a practical example, link the observed temperature change to the direction of the reaction, for example heating hydrated copper(II) sulfate causes an endothermic colour change from blue to white.
• Check that any energy value you quote for the reverse reaction has the same number but the opposite sign compared with the forward reaction.