The Freezing Point of Acetic Acid / Understanding the kinetic molecular theory

  1. Background:

Both the nature of a pure substance and its temperature determine whether that substance will be found primarily as a solid, liquid, or gas. Today you change the state of “glacial acetic acid” from liquid to solid.

Fyi: acetic acid is the main ingredient in vinegar… But, pure glacial acetic acid is about 20x more concentrated than vinegar.

The theory called the “kinetic molecular theory” states that:

- All matter is composed of tiny “particles” (atoms and molecules)that are constantly moving (thus all particles possess “kinetic energy” or “energy of motion”).

-What we measure as a substance’s temperature is a measure of the substance’s “average kinetic energy”. As you observe a change in temperature, you observe a change in the average kinetic energy of the sample.

-There is empty space between all particles of matter.

-There are attractive forces between all particles of matter (these forces are “negligible” in the gas state).

-As state is changed by adding or removing heat energy: the kinetic energy, the amount of space between the particles, the density, the strength of the attractive forces between the particles, the type of motion, and the organization of the particles will change. It is your task to be able to (by using any resource available, including this handout) discuss how these 6 concepts change as a substance changes from solid, to liquid, to gas (or vice versa).

Kinetic energy:

Particles in the gas phase have the most kinetic energy, they move randomly,very rapidly, and achieve a great distance of separation from each other; there are no attractions between the particles in this state.

Particles in the solid phase have the least kinetic energy, their motion is only vibrational (back-and-forth), the particles remain in their original places, tightly packed and orderly. Very strong interparticular attractions.

Particles in the liquid phase have an amount of kinetic energy that falls in between that of gases and solids; the particles flow around and pass by the other particles as original attractions are broken and new attractions temporarily formwith different particles.

The kinetic molecular theory explains what is occurring during a phase transition (a change of state) as heat energy is either added or removed.

-As heat energy is added to a pure substance: Solids melt to form liquids, liquids vaporize to form gases, and solids sublime to form gases.

-As heat energyis removed from a pure substance: Gases condense to form liquids, liquids freeze to form solids, and gases deposit to form solids.

-At the moments during which a phase transition (change of state) is occurring, either:

-The heat energy that is being added is being used to break“bonds” (the forces of attraction between the particles) rather than to change the kinetic energy (thus the temperature does not change during a phase change).

-OR-The heat energy that is being removed comes from the internal bond energy rather than from the kinetic energy (thus, the temperature does not change during a phase change).

-Important Point: Temperature changes before and after, but not during, phase changes.

-EverydayPhase transitions (changes of state):

Phase transitions are an important part of the world around us. For example, our bodies’ perspiration evaporates from our skin by absorbing heat energy from our body, causing our body to cool down.

And, by adding a solute (such as salt) to a solvent (such as water), we effectively decrease the solvent’s freezing point. Applying this knowledge, communitiescommonly salt roads when the temperature falls to around 32 oF the freezing point of water. H2O will stay in the liquid state at lower temperatures when salt is with it.

  1. Cooling and Heating Curves: A graph of Temperature (y-axis) vs Time (x-axis)

Today you will observe a sample of glacial acetic acid as you remove its heat and decrease its temperature from ~20 0C to ~10 oC. By analyzing your data you be able to determine its “freezing point” (the temperature at which freezing occurs). When a pure liquid substance is cooled too quickly it may exhibit a phenomenon called “supercooling”, wherein the liquid continues to cool below its freezing point. This happens if heat is removed too quickly (and if you aren’t stirring). Ex of a super-cooled liquid that we are all familiar with: glass!

You will need to understand the cooling curve from * to * below: Don’t worry about supercooling (- - -)

Cooling Curve:Heating Curve:

II. Safety: Glacial acetic acid isvery damaging to eyes, skin, and clothing. Use eye wash immediately. Avoid vapors.Wash hands.

  1. Goals: Remove heat from the sample of acetic acid by placing the test tube of acetic acid into a beaker containing icy salt water. Monitor the temperature of the acetic acid continually until the entire sample has frozen. Use the time points found in the chart below – careful, the first several time points go by 5 second intervals, and the temperature will drop quickly. Whoever is reading the thermometer must be both quick and correct; whoever is writing down the data must be ready. Then, use your data to determine the freezing point of acetic acid.
  1. Gather supplies: Assembly consisting of a “stand”, “utility clamp”, “thermometer clamp”, large test tube with glacial acetic acid (stored in the fume hood), glass thermometer, 1 plainglass stirring rod, 1 glass stirring rod with a rubberized tip, timer, 400-mL beaker, NaCl.

V. Procedure:

-Using the estimation lines on the beaker, add ~100 mL distilled water (from one of the large distilled water containers). To this add ~35 grams NaCl. Stir it with the rubber-tipped stirring rod until dissolved. Chill down the salt water mixture by adding lots of ice.

- Using the utility clamp, clamp the test tube with glacial acetic acid to the stand, so that the tube’s bottom is a “thumb’s width” above the tabletop. Using the thermometer clamp, clamp the thermometer so that its bulb is submersed entirely in the acetic acid. Carefully put the plain glass stirring rod into the test tube, along side of the thermometer. Record the temperature now… this is the “0min 0sec” data.

- Plan before you start! You will shortly need to lift the entire ring stand assembly (with test tube and acetic acid and thermometer) and then lower the test tube into the beaker with the ice/salt/water. When it hits the ice/salt/water….Immediately begin timing, stirring the acetic acid, and recording temperaturesevery 5 SECONDS. Follow the times in the data table below. The bulb of the thermometer must stay in the acetic acid; and, the top of the acetic acid in the test tube must stay below the top of the ice/salt/water mixture. When things slow down, set up a graph whose y-axis goes from “0” to “25”0C by consistent intervals; and, whose x-axis goes from 0 to 16 minutes by consistent intervals. You may stop data collection when10oC is reached.

VII. Clean-up: -Leave your lab tableas you first found it! Return everything! Be sure to dry your table.

VIII. Experimental Data:

Time (m’s”) / Temp. (oC) / Time (m’s”) / Temp. (oC) / Time (m’s”) / Temp. (oC) / Time (m’s”) / Temp. (oC)
0’0” / 0’30” / 3’00” / 8
0’5” / 0’45” / 3’30” / 9
0’10” / 1’00” / 4’00” / 10
0’15” / 1’30” / 5’00” / 12
0’20” / 2’00” / 6’00” / 14
0’25” / 2’30” / 7’00” / 16

Usethis handout, your data, your graph, along with outside resources as needed, in order to answer the questions below:

1. What is the freezing point of your sample of acetic acid? ______

  1. What does the temperature of a substance do during a phase transition (during a change in state) over the time period in which the substance is actually undergoing the transition?

______

Building Your Understanding:

Today we determined the “freezing point” of a pure liquidby removing heat, so that it entered the solid state. This same temperature would have been called the “melting point” had we started with a solid sample and added heat until it melted. The temperature of freezing = the temperature of melting for any given substance because freezing is the opposite of melting. Pure liquid water will freeze if its temperature is lowered to 0 0C; pure solid water (ice) will melt if its temperature is raised to 0 oC.

The “boiling point” is the temperature at which a substance boils. The freezing point (melting point) are always less than the boiling point of a sample, because the solid state of a substance is always present at a lower temperature than the same substance in the gaseous state. On a horizontal temperature scale going from low to high temperatures, the solid phase of a given substance would occur to the left, the liquid phase in the middle, and the gaseous phase to the right. Along a horizontaltemperature scale, the MP/FP is found to the left (between the solid and liquid phases); and the BP is found to the right (between the liquid and gaseous phases).

Knowing that room temperature is about 20 0C, we can use our understanding of given phase change temperatures (freezing/melting point and boiling point) in order to determine whether a substance will be found in the solid, liquid, or gaseous state at room temperature.

An easy way to think through this is to use the horizontal line (see below)… in which MP represents melting point, BP represents boiling point, S = solid, L= liquid, G=gas. Write the substance’s actual MP (or FP) and BP along the horizontal line at the spots indicated. Then determine whether room temperature (20 0C) falls numerically to the left of the MP, or in between MP and BP, or to the right of BP.

[Ex: If 20 oC is less than the given MP, it falls to the left of MP. Etc.] If 20 oC falls to the left of MP, the S reminds you that you have a substance that will be in the solid state at room temperature. If20 oC falls to the right of BP – you have a gas; and if 20 oC falls in between MP and BP – you have a liquid. Example: FP = 0 0C; BP = 100 0C. Is this substance going to be a solid, liquid, or gas at room temperature? The answer is worked through for you below: This substance is a Liquid at room temperature (20 oC).

Determine the state of the following 6 substances at room temperature; and at 100 oC; and at –100 0C:

Substance / MP (oC) / BP (oC) / State at 20 0C / State at 100 0C / State at –100 0C
Neon / -248.7 / -245.8
Lithium / 97.5 / 899.0
Bromine / -7.2 / 58.78
Chlorine / -100.98 / -34.6
Mercury / -38.87 / 356.58
Iodine / 113.5 / 184.35

Feel free to google “kinetic molecular theory states of matter” if you have any difficulty with this assignment. Place answer all questions on a sheet of notebook paper (or computer paper).

  1. Given 20 mL of a hypothetical newly discovered substance with melting point 50 0C and boiling point 250 0C, predict graphically what a heating curve and freezing curve would look like for the pure substance….This means, sketch two graphs; label the x-and y-axis; include appropriate y-axis values.
  1. Fill in the chart below. It could be filled in satisfactorily by using each of the following words at least once: high, medium, low, strong, weak, rapid, vibrational, flowing, orderly, disorderly. Or, you may choose your own descriptive words to use.

Strength of interparticular attractions / Amount of kinetic energy / Amount of space between particles / Type of motion / Density / Organization of particles
Solid
Liquid
Gas

You may now choose to answer only 1 of the following – either answer 3A, 3B, or 3C!

In this manner you will show that you understand the concepts taught during today’s acetic acid lab and the pre-lab demonstration with water and salt water, and can expand from them on your own.

3A. Compare and contrast the results of adding energy to and removing energy from a sample of water. Your answer should include as much as you understand about the kinetic molecular theory (an explanation of phase changes). Hint - incorporate the information you placed into the columns in the chart located above.

3B. Using the heating curve and cooling curve that is provided earlier in this lab handout, differentiate between the two curves by discussing what is happening regarding the strength of interparticular attractions, the amount of kinetic energy, thetype of motion, density, amount of space between the particles, and organization of the particlesin the sample as you move along each curve from left to right.

3C. Create a novel tune (compose a rap) or compile a set of imagesin which you describe the differences between solids, liquids, and gases in terms of the strength of interparticular attractions, amount of kinetic energy, type of motion, amount of space between the partricles, density, and organization of particles.