Standard temperature and pressure [STP]
♦ Standard Temperature is °0Celcius.
❖ All gas calculations must be in Kelvin scale
❖ Degree celcius can be converted into Kelvin scale, as K=°C+273
❖ Notice it is K - not °K .
❖ You say "Kelvins"- not "Degrees Kelvin"
♦ Standard Pressure is 1atmosphere (at sea-level)
Concept Quide on Qaseous Properties
A gas has no definite Volume or shape; a gas will fill what{ever} volume is available to it. Co
this to the behaviour of a liquid, which always.has a distinct upper surface when its volume is less than that of the space it occupies.
The other outstanding characteristic of gases is their low densities, compared with those of liquids and solids. One mole of liquid water at 298 K and 1 atm pressure occupies a volume of 18.8 cm3, whereas the same quantity of water vapor at the same temperature and pressure has a volume
of 30200 cm3, more than 1000 times greater.
The most remarkable property of gases, however, is that to a very good approximation, they all behvave the same way in response to changes in temperature and pressure, expanding or contracting by predictable amounts. This is very different from the behaviour of liquids or solids, in which the properties of each particular substance must be determined individually.
We will see later that each of these three macroscopic characteristics of gases follows directly from the microscopic view.
The molecules of a gas, being in continuous motion, frequently strike the inner walls of their container. As they do so, they immediately bounce off without loss of kinetic energy, but the reversal of direction (acceleration) imparts a force to the container walls. This force, divided by the total surface area on which it acts, is the pressure of the gas. The pressure of a gas is observed by measuring the pressure that must be appiled externally in order to keep the gas from expanding or cotnracting. To visualizee this, imagine some gas trapped in a cylinder having one end enclosed by a freely moving piston. In order to keep the gas in the container, a certain amount of weight (more precisely, a force, f) must be placed on the piston so as to exactly balance the force exerted by the gas on the bottom of the piston, and tending to push it up. The pressure of the gas is simply the quotientf/A, where A is the cross-section areaof thepiston.
Gas Laws
The discussions on the need for four properties and corresponding units t(5 describe gases would provide a platform to explore how these properties are related and the statement of gas laws.
Gas laws can be taken up.
1. By recalling the behaviour of gases in some familiar situations. Syringe with needle plugged (Application of pressure decreases volume) Aerated cold drink is heated (With rising temperature volume increases) After these observations are discussed,
2. The students may graphdataon these dimensions and analyse the graphs. Graphs to discuss Boyle Ylaw relationship.
After carrying out the appropriate activities let the students interpret the relevant graphs or data to arrive at a statement of the laws on their own. The data sets below can be used for interpretation or for drawing graphs.
1. Boyle's law -
Variations in Gas Pressure and Volume when Tenperature and Number of Moles of Gas are held constant
State | Pressure (atm) | Volume (L) | Px V(Lxatm) |
1. | 5.31 | 0.377 | ' 2.00 |
2: | 2.67 | 0.75 . | 2.00 . |
3.- | 1.20 | 1.67 | 2.00 |
4. | 0.663 | 3.02 | 2.00 |
2. Charle's Law
•^nation in Gas Volume with change in temperature at constant pressure. ■
.State f | t(°C) | T(K) | Volume (L) | M | |
i 1- | : -23.0 | 250 | 5.50 | 0.0220 | -0.239 |
2. | 27.0 | 300 | 6.60 | 0.0220 | 0.244 |
3. | 77.0 | 350 | 7.70 | 0.0220 | 0.100 |
4. | 152 ; | 425 | 9.35 | 0.0220 | 0.0620 |
Chemical Maths corner
When attempting gas law problems, the student must remember the following.
♦ Boyle's law tell us
so. What happens to the volume when pressure is varied K What happens to pressure when volume is varied.
♦ Charles law applies in situations where pressure and amount of gas are constant.
♦ Gay-Lussac's law is applicable when we-want to know.
<a What happens to the pressure of a gas when temperature is .changed, b. What happens to the temperature of a gas when pressure is changed. Avogadro's law applies when we want to find out
(a. What happens to the volume of a gas when the number of moles changes, and & What number of moles would be contained in a new and different volume.
♦ The combined gas law can be used to find out
■ What is the new pressure of a gas when both temperature and the volume are changed simultaneously?
gf. What Ss the new temperature of a gas when both the pressure and the volume are changed simultaneously?
c. What is the new volume of a gas when both the temperature and the pressure are.
1 ^r I rrT ■
changed simultaneously? ,um
♦ To use Charles law, it is necessary to convert Celsius temperature into Kelvin's.- This conversion uses the relations Kelvins. This conversion uses the relation.
Kelvin temperature = Celsius temperature + 273
Always check whether the gas law problems involves unit - conversion and if conversion factor is a ratio.
Teaching Ttp
The discussions on gas laws should enable the student to comprehend the following concepts.
♦ The chief characteristic of gases is their physical similarity to one another. Gaseous substances composed of widely differing molecules neverthless display virtually identicalphysicdprpge.
♦ " Equal volumes of gases measured at the same termperature and pressure will contain qgual numbers of molecules. vtasc
♦ The volume of, say, one billion molecules of any gas, will be same.