Burhan Riaz10/25/2011

How does the addition of solute affect the cooling curve of cyclohexane?

Beginning Ideas:

Using the equation deltaTf=Kfm, the molality is directly proportional to the change in the

freezing point. So compounds that form into more particles than other compounds should have a

greater impact on Tf. That is why I believe that the electrolytic compounds that will be used will

lower the freezing point while nonelectrolytic compounds will have a lesser affect. Naphthalene

should be soluble and therefore lower the freezing point.

Tests:

We first obtained 10.0g or 12.8mL of cyclohexane and placed it into a 50.0mL beaker. Then we

measured .5g of our solute, naphthalene, and added the it into the cyclohexane and used a

stirring rod to fully dissolve the naphthalene. We then filled a 500mL beaker with ice and water

and used it as an ice bath. The 50mL beaker along with a thermometer was then placed into the

500mL beaker. As the temperature decreased in the 50mL beaker it finally reached 0C. To

decrease the temperature even more, we added 6.3 grams of NaCl into the ice bath. The

temperature decreased to -2.0C and we recorded our results.

Observations:

Naphthalene + Cyclohexane Temperature Change

Minutes Temperature (C)0 5.0

1 1.82 0.53 0.04 0.05 -1.06 -2.0

The mixture started to freeze after 6 minutes at -2.0C. Naphthalene smelled like moth balls.

Evidence:

The molality of the mixture was 0.6m.

The deltaT was calculated by subtracting the freezing point of the solvent by the mixture. 7C-(-

2C) = 9C.

The Kf constant was calculated by the equation Kf=deltaT/m. Kf=9C/0.6m. Kf=15C/m.

Claim:

Naphthalene will lower the freezing point of cyclohexanol . This is because naphthalene is

soluble and will increase entropy which according to colligative properties, is a factor that lowers

the freezing point.

Reading:

Every year, in cold environments, roads become slippery due to ice sheets forming on

the roads. Local governments have organized a routine salting of the roads to lower the freezing

point. In this experiment, the class is given the solvent cyclohexane and 4 solutes. These solutes

will decrease the freezing point to some extent. To predict how these solutes will affect the

freezing point one can use colligative properties1. Colligative properties are concentration

dependent and depend on molality which can be calculated by dividing moles of solute over the

mass(kg) of the solvent1. The more particles in the solvent, the greater change in freezing point

temperature. This is due to the fact that adding a compound to the solvent increases entropy2.

Electrolytes or ionic compounds can dissociate into ions and are excellent to decrease Tf1. A non-

disassociating compound like glucose will not significantly decrease the freezing point2. The

change in the freezing point temperature can be calculated by the equation deltaTf=Kfm1. The Kf

property of the solvent signifies the temperature change according to molality, Kf=deltaT/m1.

This means the greater the Kf of the solvent, the more impact a solute can have on the freezing

point.

Nonvolatile compounds are used to lower the freezing point because volatile compounds

have a tendency to form vapors2. The freezing point does not necessarily mean the liquid will be

a solid at this point. Instead an equilibrium is formed between the two states until one of them

completely transitions into the other2.

Reflection:

The outcome of the solutes' effects on cyclohexanol make perfect sense when applying

colligative properties. Steryl alcohol for example is a large compound with 16 carbons and an

alcohol at the end. The compound does not easily dissociate and therefore the change in the

freezing point is minimal. Lauric acid and stearic acid, both saturated fatty acids, also have a low

tendency to dissociate. Our solute, Naphthalene had a higher solubility than the other solutes

mentioned. This in accordance with deltaTf=Kfm gave us a larger deltaTf than the rest of the

class. The freezing point for our mixture was -2 and was achieved this by adding 6.3 grams of

NaCl to lower the temperature of the ice bath. Assuming that the freezing point for cyclohexanol

is 7C(from the lab instructor), the deltaTf is 9C. This gives us a Kf=15C/m. If this procedure was

repeated, I would first find the experimental freezing point of cyclohexanol to confirm if its

freezing point is actually 7C.

News Brief:

During winter, you may have noticed that salting the roads helps keep ice from forming on it.

What is the cause behind this phenomenon? You can't just pour anything onto the roads to get

the same effect. Salts, when mixed with water or snow will separate into smaller parts. These

smaller parts are the key to understanding why the freezing point of water is lowered. The more

a compound can disassociate into smaller parts, the more it can lower the freezing point. Salts, or

ionic compounds, are best known for their ability to dissociate. These compounds are called

electrolytic compounds. Compounds that cannot disassociate easily are nonelectrolytic and they

would not work on the roads to keep ice away. The same concept applies to adding antifreeze to

your car and making ice cream quickly.

Citations:

1Chemistry 2046L Laboratory Manual for Chemistry Fundamentals II

2Tro, Nivaldo J. Chemistry: a Molecular Approach. Upper Saddle River, NJ: Pearson Prentice

Hall, 2011. Print.

Burhan Riaz

News Brief:

During winter, you may have noticed that salting the roads helps keep ice from forming on it.

What is the cause behind this phenomenon? You can't just pour anything onto the roads to get

the same effect. Salts, when mixed with water or snow will separate into smaller parts. These

smaller parts are the key to understanding why the freezing point of water is lowered. The more

a compound can disassociate into smaller parts, the more it can lower the freezing point. Salts, or

ionic compounds, are best known for their ability to dissociate. These compounds are called

electrolytic compounds. Compounds that cannot disassociate easily are nonelectrolytic and they

would not work on the roads to keep ice away. The same concept applies to adding antifreeze to

your car and making ice cream quickly.