There are gas laws but solid laws or liquid laws.
increasing the temperature of a solid at constant pressure cause phase changes
solid --> liquid --> gas (gas- small molecules like CO2, H2S, CH4)
( solid and liquid are considered condensed phases and incompressible)
Weak intermolecular forces leads to a low density (10-3g/mL), compressibility, and formation of homogenous mixture. Also, there is lots of empty space between molecules.
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macroscopic variables:
Volume (space) : measure in liters (L) : V
Temperature (speed) : measured in Kelvin (K) : T
Pressure (intermoleculare forces): measured in atmospheres (atm) : P
Amount: measured in moles: n
Molecules (or atoms) in the gas phase have collisions with the walls of a container, this creates pressure. There are two factors are influence the amount of pressure exerted by the gas molecules on the walls of the container: # of collisions per unit area and the force at which the molecules hit the wall. Pressure can be expressed in several different units:
Boyle's Law : V is proportional to 1/P (T, n constant) P1V1 = P2V2
Charles' Law: V is proportional to T ( P, n constatnt) V1T2 = V2T1
Avogadro's Law: V is proportional to n (T,P const) V1n2 = V2n1
Since V is proportional to nT/P, we can remove the proportional sign if we substitute a constant, k or in this case R. We can arrange the variables to obtain
the ideal gas law: PV = nRT
Demonstration : soda can with water is heated and fills with steam. When it cools, the pressure inside becomes less than that outside. The can is crushed because the pressure outside is greater than the pressure inside.
Picture diagram to appear here soon
Demonstration: Boyle's Law apparatus. Keeping the temperature constant. When the volume in a syringe or piston is decreased, the pressure increases. Why?
If the system's number of molecules does not change, then the pressure is greater when the volume decreases because there are more collisions per unit area in a given unit of time with the walls of the container. Since the temperature is the same, the molecules are moving with the same average speed compared to before.
At a constant temperature, the pressure of a gas is inversely proportional to the volume occupied by the gas.
Gay -Lussac's Law of Combining Gas Volumes:
When measured at the same pressure and temperature, the volume of a gas reactants and products are in ratios of simple whole numbers.
H2(g) + Cl2(g) --> 2 HCl(g) stoich ratio ,volume ratio 2.5 L 2.5 L 5.0 L 1:1:2, 1:1:2 3 H2(g) + N2(g) ---> 2NH3(g) stoich ratio ,volume ratio 7.5 L 2.5 L 5.0 L 3:1:2, 3:1:2
Why do gases combine in simple whole numbers? Because when gases react the molecules react in simple whole numbers.
6 molecules of H2 react with 6 molecules of Cl2 to produce 12 molecules of HCl. You cannot have half a molecule reacting or two-thirds of a molecule reacting.
The volume of a gas is proportional to the number of molecules of gas in the sample. The proportion of reacting gases by volume are the same as the proportion of gas molecules reacting.
If we react 2.5 L of H2 gas with 2.5 L of Cl2 gas, each at 1.00 atm pressure and 25.0°C, how many molecules of H2 and Cl2 react? The HCl gas collected is at 1.00 atm pressure and 25.0°C.
Calculate the number of molecules of H2 gas present before the reaction.
Calculate the number of molecules of Cl2 gas present before the reaction.
For different gases at the same volume, temperature, and pressure, there are equal moles and equal number of gas molecules present. This is known as Avogadro's Hypothesis.
Also, this illustrates a point about the ideal gas law equation. The ideal gas law equation treats all gases the same. It does not matter what the gas is, since the assumption is that the gases will behave the ideal gas law. Now, this is not always true. Gases are real and under certain conditions of low T and high P, gases do not obey the ideal gas law.
Let's put this in context of numbers we can more easily work with. Let' draw a picture diagram and assign the circle to represent a small volume of 1.0 X 10-21 L. How many molecules of H2 and Cl2 will be present to begin with and to end with?
When we allow these two gases to mix and react, how many molecules of HCl will be formed? Consider what volume the gas will be confined to if we want to follow Gay -Lussac's Law of Combining Gas Volumes. Remember the final volume must be twice as large as the initial volume of each gas. You can see that if we keep the volume at 1.0 X 10-21 L there will be twice as molecules present, and the pressure would increase, say double. The final volume of the circle representing the new volume must be 2.0 X 10-21 L in order to keep the pressure constant.
Problem 2: An aerosol can at 1.5 atm, 25oC. What is the pressure when heated to 450oC ??
Use the equation, P1V1T2 = P2V2T1
Because V1 = V2, you can set up the problem like this......
(1.5 atm) (723K) = (P2)(298K)
P2= 3.6 atm
Problem 3. Calculate the volume of a 0.500 mole sample of Helium gas at 1.00 atm presure and 298 K.
PV = nRT