Endothermic Reactions

In an endothermic reaction energy is taken in from the surroundings, so the temperature of the surroundings falls. The products have more enthalpy than the reactants and ΔH is positive.On a reaction profile the products are drawn higher than the reactants. The enthalpy change ΔH is the vertical difference between the reactants and the products, shown by an arrow pointing upwards.The activation energy E_a is measured from the energy of the reactants up to the top of the curve, which is the transition state. For an endothermic reaction the activation energy is always greater than ΔH, because the peak must lie above the products.The activation energy of the reverse reaction is E_a minus ΔH, so it is smaller than that of the forward reaction.Examples of endothermic reactions include the thermal decomposition of calcium carbonate, CaCO₃ → CaO + CO₂, photosynthesis and dissolving ammonium nitrate in water. Many endothermic reactions need continuous heating to keep going.