The enthalpy for the decomposition of hydrogen peroxide into water and oxygen is -98.2 kj.

The enthalpy change for the formation of hydrogen peroxide from water and oxygen is +98.2 kJ·mol⁻¹.

The negative sign tells you that the reaction releases energy.

To make the reaction go in the reverse direction, you must put the same amount of energy back into the system.

Thus, you simply reverse the sign of the enthalpy change for the forward direction.

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