# Enthalpy of Solutions

Instructor: Stephanie Gorski

Steph has a PhD in Entomology and teaches college biology and ecology.

In this lesson, we will discuss thermodynamics and enthalpy. Specifically, we will discuss enthalpy of solutions and the equation for determining the enthalpy of a solution.

## Thermodynamics

Many scientific disciplines evolved from what were once practical considerations. Botany, the study of plants, began as early people searched for plants to use as food and medicine.

Thermochemistry, on the other hand, was born of war. Thermochemistry is pretty much what it sounds like - the study of the relationship between chemical reactions and energy changes involving heat. Early studies of thermochemistry were mostly used for making cannons. As you can surely imagine, nobody wants to fire off a cannon unless they're pretty confident of how these heat and energy changes will occur!

According to the First Law of Thermodynamics, energy can neither be created nor destroyed. Energy can, however, change form. The sun doesn't create energy when it burns. The sun merely converts stored energy in hydrogen atoms into light and heat energy. When you fire a cannon, you are taking chemical energy stored in the gunpowder and converting it into other forms of energy - the expanding gases, the light from the flash, and the kinetic energy of the cannonball.

## Enthalpy

Enthalpy is the heat transferred during a process of constant pressure. It comes from the Greek roots en, meaning put into, and thalpein, meaning to heat. So, literally, enthalpy means putting heat into something.

The formula for enthalpy is:

H = E + PV

The H stands for enthalpy. The E stands for internal energy, which you can think of as heat capacity or heat energy. The PV is pressure times volume.

But in general, chemistry doesn't like absolutes. Chemistry cares more about how things change. We normally represent change as the Greek letter Delta, which looks like a triangle. So our equation would be:

## Enthalpy of Solutions

The enthalpy of solutions refers to the total amount of heat absorbed or released when two substances go into solution. This total can be either positive or negative. A positive enthalpy of solution results in an endothermic reaction, which takes in heat and feels cold to the touch. A negative enthalpy of solution results in an exothermic reaction, which gives off heat and feels hot to the touch.

Why would just dissolving a substance require it to take in or give off heat? Three separate processes have to take place, and if their change in enthalpy doesn't equal exactly zero, there will be some loss or gain of heat.

The first process that has to take place is that the solute, the substance that is being dissolved, has to separate. Perhaps you have some table salt, NaCl. NaCl is a molecule because the positive sodium ion is attracted to the negative chloride ion, and so they stick together. So, to dissolve NaCl, you will have to exert a certain amount of energy. We call the enthalpy of this process:

The second process that has to take place is that the molecules in solvent, the substance that is doing the dissolving, have to separate. Perhaps you want to dissolve your table salt in water. Water molecules are polar, so their partially negative poles stick to the partially positive poles of neighboring water molecules. It takes some energy to separate them. We call the enthalpy of this process:

The third process that has to take place is that the solute and solvent have to mix together. We call the enthalpy of this process:

The equation for the sum of these processes is:

Let's do a sample problem.

If

I'll bet that was easier than what you were expecting!

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