Pressure, Volume, and Temperature
Thermodynamics is the study of the conversion of heat energy into other forms of energy, and vice versa. There are three major parameters that we often study when experimenting in thermodynamics.
Pressure is the force per unit area applied to the surface of an object. Its unit is one Newton per square meter, also called the pascal (Pa). The standard atmosphere (atm) is a constant 1.01 X 105 Pa (this is the average atmospheric pressure at sea level). In Boone, atmosopheric pressure is 0.900 X 105 Pa. This is called absolute pressure, which is a measurement that refers to zero as the pressure in an absolute vaccuum. Another way to describe pressure is using gauge pressure, which refers to zero as the actual atmospheric pressure (at sea level 1 atm, in Boone 0.9 atm).
Volume is the amount of three dimensional space an object occupies. The SI unit of volume is the cubic meter (m3), but often it is more practical to use the cubic centimeter (cm3), which is equal to milliliter (mL). There are 1,000,000 cubic centimeters in a cubic meter. There is an inverse relationship between volume and pressure, which means that at constant temperature, as one goes up, the other goes down.
Temperature is an indicator of how hot or cold an object is. It is measured by your thermometer on the Celsius scale (ºC). However, when a value of absolute temperature is needed, you should convert to Kelvin (K). When two objects are brought into thermal contact, temperature difference (ΔT) determines the amount of the energy transfer from one object to the other. Isolating the energy contained within the two-object system, the direction of heat flow will always be from hotter object to the cooler object. The transfer will continue until the objects have reached thermal equilibrium, which means that they are the same temperature. The greater the temperature difference, the greater the rate of change. Because of this, the transfer occurs at an inverse exponential rate, changing quickly at first then more slowly as the temperature difference between the two objects becomes smaller.
The relationship between pressure, volume, and temperature is called the Ideal Gas Law. This law states that:
where p is the absolute pressure of the gas in pascals; V is the volume in m3; n is the amount of substance in moles; R is the Regnault Constant (better known as universal gas constant, 8.314 J/mol·K); and T is the absolute temperature in degrees kelvin.
One extension of the Ideal Gas Law when (n) the number of moles is constant is called the Combined Gas Law. This law states that the ratio of pressure and volume to temperature is constant for all states of a gas.
Useful constants and Conversions:
1.01 X 105 Pa = 14.7 lb/in2 = 1 atm at sea level
Tk = Tc + 273.15
R = 8.314 J/mol·K
By the end of this lab period, you should have gained a working knowledge of the relationship between pressure, volume, and temperature. After carefully recording your observations of various relevant phenomena, you should be able to develop a physical explanation for what you observed. You should also be able to describe what takes place during a phase change from solid to liquid, or liquid to gas, and knowledgeably discuss other applications of thermodynamics as well.
Another objective of this lab is to practice being a good observer. Put detailed observations into your laboratory manual, and that will make your job easier when you begin turning to outside sources in order to explain observed phenomena. For each activity, write a complete explanation into your laboratory manual of the physics behind the effect, and carefully reference each outside source.
SAFTEY: THIS LAB INCORPORATES THE USE OF FIRE AND LIQUID NITROGEN. GOGGLES MUST BE WORN AT THESE STATIONS, AND MITTS WHEN NEEDED! BE SURE TO CLEAR THE AISLES OF ALL LOOSE ITEMS!
For each of the activities below, carefully record your observations. As you move from station to station, please be courteous and allow the group in front of you to finish before moving to their station. Only one group at a time should be at each station! Carefully record your observations of each physical phenomena. Once you have completed the activity, use information in the introduction, your book, or an online source to explain the physics behind what you have observed. Cite your references clearly. Draw diagrams and print graphs if needed. Make sure you are able to fully explain what is happening!
Activity 1. Syringe and Gas Pressure Sensor
Logger Pro, Syringe, and Gas pressure sensor at each station
At your table is a syringe and a Vernier Gas Pressure Sensor. This is plastic so do NOT over-tighten! Before connecting the syringe to the sensor, position the plunger about half-way in. This will allow you to either increase or decrease the pressure.
Record the the Create a plot of pressure vs. volume in Logger Pro. To do this, go to Experiment > Data Collection, and change the time-based data collection to Events with Entry. You will be prompted to enter information about the new events column. Name it "volume" with a short name of "V" and units of "mL".
Now you can gather data that will allow you to plot a graph of pressure vs. volume. When you click "Collect", you will see a floating data point. The first point should be taken with the plunger at its initial position. Click "Keep" and manually type in the volume. Logger Pro will plot a pressure vs. volume point. Gather a good data set by pushing and pulling on the plunger, clicking "Keep" each time you have moved the plunger to a new position. Keep doing this until you have plotted enough points to clearly define your PV curve.
Print this graph. Make sure that the data is recorded in your notebook for later use.
What is the observed relationship between pressure and volume?
Activity 2: Pressure and Temperature (an experimental determination of absolute zero)
- Slow cooker for very hot water, set at 250o F
- Hot water from the sink
- Ice water
- Liquid nitrogen
- Vernier thermometer and gas pressure sensor
- Constant-volume bulb
- Thermal glove and goggles
Activity 2 consists of a constant-volume bulb, several different ways to heat and cool the bulb, and a thermometer and pressure sensor to measure the pressure and temperature. The sphere with the handle on it is called a constant-volume bulb because its volume doesn't change when heated or cooled. However, as the bulb is heated or cooled, the pressure inside the bulb will change. The pressure is measured using the vernier gas pressure sensor.
Examine the Ideal Gas Law formula. If pressure and temperature are varied while volume is held constant, what type of equation is this? (Linear? Quadratic? Exponential? Something else?)
Go to the Activity 2 station and make measurements of temperature and pressure using Logger Pro. Record these in your notebook. You can dip the bulb and the thermometer into a ice water, hot water from the sink, boiling water in the cooker (be careful, it is really hot!), and liquid nitrogen (be careful, it is really cold!). *** NOTE: Please do NOT use the thermometer to measure the temperature of the liquid nitrogen! This will damage the thermometer! *** Instead, use the given boiling point of liquid nitrogen: -196 ºC. Remember that others are waiting to use the apparatus, so record your data efficiently.
Return to your table and create a temperature vs. pressure graph using Logger Pro or Excel. If you are using Logger Pro, remember that you must unplug any sensors before creating a manual graph. Be sure to enter the temperature in degrees Celsius, not Kelvin.
This graph should appear linear, according to the ideal gas law formula. What does this indicate about the relationship between pressure and temperature when volume is held constant? Put this into your own words.
With this particular graph, what is the meaning of the Y-intercept? Again, put this into your own words.
Using percent error, compare your value of the Y-intercept to the accepted value of -273 oC.
Print this graph for your notebook.
Activity 3. Phases: Liquid to gas
- Liquid nitrogen
- Can with lid
- Thermal glove and goggles
Pour a small amount of liquid nitrogen into a coffee can. Put the lid on the can. Be careful! Record your observations.
Pour one tablespoon of liquid nitrogen into a bottle, and quickly slip a balloon over the mouth of the bottle.
Explain what is going on in terms of the Ideal Gas Law.
Activity 4. Balloon in Liquid Nitrogen
- Liquid nitrogen
Place an inflated balloon into liquid nitrogen, then remove. Explain how this experiment demonstrates the relationship between pressure, volume, and temperature. Has the amount of air inside the balloon changed?
Has the density of the balloon changed? Explain.
A ping-pong ball that has been dented can often be restored by placing it in hot water. Explain why this works.
Activity 5. Balloon in a vacuum
Vacuum chamber with pump and ballon
Place a small, partially-inflated balloon into a vacuum chamber and pump out the air. You will need to pump several times. Be careful -- everything is plastic! Record your observations. Explain what is happening in terms of the Ideal Gas Law.
Has the amount of air inside the balloon changed? Has the density of the balloon changed? Explain.
When you are finished, loosen the tube fastener at the top of the vacuum chamber to let the air back in, then gently tighten it back on.
Activity 6. Collapse the can (omit if in CAP 247)
- Heat source
- Tongs, a
- Aluminum can
- Dish of water
Using the tongs, your instructor will heat an aluminum can containing about an inch of water over a burner until the inside of the can is totally filled with steam. You'll see the steam coming out the top of the can when it is ready. The can is quickly inverted into a bowl of water so that the mouth of the can is totally submerged. Record your observations. Discuss the experiment with your partners and your lab instructor in order to formulate an explanation of what is happening in terms of the Ideal Gas Law. Note that a change of phase is occuring inside the can! Describe your conclusions in paragraph form.
Activity 7. Determining the number of molecules of air in a syringe
The ideal gas law predicts that, if temperature is held constant, there is a linear relationship between pressure (p) and the inverse of volume (1/V). Be sure to convert to SI units of Pascals, degrees Kelvin, and cubic meters first!
p = (nRT)(1/V)
If we were to plot p vs. 1/V, the above relationship predicts that the data will describe a line which has a slope of nRT.
Using your data from activity one, and the value of room temperature, create a graph that will allow you determine "n", the number of moles contained in your syringe.
You may do this in either Excel or Logger Pro. Remember, before you can create a manual graph in Logger Pro, you must first disconnect the USB cable connecting the sensors to the computer.
Does this value of "n" seem reasonable? (Mathmatically show your reasoning. At STP, one mole of air will have a volume of 22.4 liters).
Using Avagadro's number, calculate the number of molecules of air in the syringe.