Lithium iodide has a lattice energy of -7.3 * 102 kJ>mol and a heat of hydration of -793 kJ>mol. Find the heat of solution for lithium iodide and determine how much heat is evolved or absorbed when 15.0 g of lithium iodide completely dissolves in water.

The term 'lattice energy' might sound complicated, but it's just a fancy way of talking about the glue that holds ionic compounds together. Think of an ionic compound like a bunch of magnets neatly arranged in a pattern. Lattice energy is like the force needed to pull these magnets apart. When a compound forms from its individual ions, this 'magnetic force', in simple terms, releases energy.

Lattice energy is, therefore, the energy released when one mole of an ionic crystalline compound forms from its gaseous ions. It's measured in kJ/mol and typically has a negative sign because it's energy being given off, not absorbed. The bigger the charge on the ions and the smaller their size, the higher the lattice energy. This is because smaller and more highly charged ions pack more closely together, making their attractive forces stronger.

Let's move on to 'heat of hydration'. Imagine you're soaking in a tub of water, and you feel that comforting warmth. That's a little like what happens on a tiny scale when ions from a salt start swimming around in water. The heat of hydration is the energy change when one mole of ions gets cozy with water molecules.

This process is exothermic for many salts, meaning it releases heat (hence why it feels warm in our imaginary tub!). If you're measuring how much energy this process spews out, that's going to be a negative number since the system (the ions and water) is losing heat as it passes on to the surroundings (kind of like a warm hug from the ions to the water).

Lastly, let's talk about 'enthalpy change', which is a bit like the scorecard of energy for chemical reactions. Imagine you're looking at a seesaw with energy input on one side and energy output on the other—enthalpy change essentially tells you which side is dipping lower.

It's represented as \( \Delta H \) and is the total heat content change of a system during a process. This could be heating, cooling, or a chemical reaction, like dissolving a salt in water. We see it in action when we calculate the heat of solution: add up the lattice energy (pulling the magnets apart) and the heat of hydration (cozying up to water). The result tells us how much heat is moving around: is it being given off into the surroundings, or does the system need to absorb heat from around it to make the process go?