Copyright

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.

Thermodynamics was developed to more safely and efficiently use cannons.
Thermodynamics

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:

null

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:

null

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:

null

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:

null

The equation for the sum of these processes is:

null

Let's do a sample problem.

If

null

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

To unlock this lesson you must be a Study.com Member.
Create your account

Register to view this lesson

Are you a student or a teacher?

Unlock Your Education

See for yourself why 30 million people use Study.com

Become a Study.com member and start learning now.
Become a Member  Back
What teachers are saying about Study.com
Try it risk-free for 30 days

Earning College Credit

Did you know… We have over 200 college courses that prepare you to earn credit by exam that is accepted by over 1,500 colleges and universities. You can test out of the first two years of college and save thousands off your degree. Anyone can earn credit-by-exam regardless of age or education level.

To learn more, visit our Earning Credit Page

Transferring credit to the school of your choice

Not sure what college you want to attend yet? Study.com has thousands of articles about every imaginable degree, area of study and career path that can help you find the school that's right for you.

Create an account to start this course today
Try it risk-free for 30 days!
Create an account
Support