Problem 13.18

The schematic diagram of the solution process as the net sum of three stpes in Fig 13.4 does not show the relative magnitudes of the three components because they will vary from case to case. For the dissolution of NH₄NO₃ in water, which of the three enthalpy changes would you expect to be much smaller than the other two? Explain.

Here's Fig 13.4 reproduced for your shopping convenience:

Recall from your reading that the dissolution of ammonium nitrate is endothermic (that is, the energy of the solution is higher than that of the solvent and solute.) Looking at the diagram, we note that if an arrow points upwards, energy is required, and if an arrow points down, energy is released. In step 1 (with energy change DH₁), solute particles are being moved apart. Here, the solute is ammonium nitrate, an ionic compound. The amount of energy required to move ammonium ions and nitrate ions apart is considerable. Step 2, with energy change DH₂, represents the amount of energy required to separate the solvent particles (in this case, we have to overcome H bonding between water molecules). Step 3, the interaction between solvent and solute, represents the formation of ion-dipole interactions. Since ionic bonding and ion-dipole interactions are stronger than H bonding interactions, we would say that DH₂ is the smallest energy change.

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