Фізичні властивості газів

As we have studied the phases of matter, we have looked at the physical properties of solids and liquids. Now, we turn our attention to gases. Gases have the most translational energy. So, we assume they don't experience Van der Waal's interactions. We also assume that the particles don't have a volume. These two assumptions come from the kinetic molecular theory. In solids and liquids, electrostatic interactions cause packing. The atoms or molecules are close together. But between gas particles, there is empty space. Remember, at the atomic level, there is nothing between molecules. So, gases are free to bounce around in their container. Sometimes, they collide with their container. Sometimes they collide with each other. Their movement is random.

Compressibility and Expandability

Because of the empty space between gas particles, they can compress or expand. If we increased a container's volume, the internal gas would fill that space. There would be more space between the gas particles. If we shrunk the container, the gas would compress. There would be less space between the particles. So, they would collide more frequently.

Volume

Gases fill their containers. That's because they are expandable and compressible. So, the container's volume is the gas's volume. It does not matter if we have one mol of gas or two. If there are two moles of gas, there are more collisions. But if we have a rigid container, the volume stays fixed.

So, how do we measure the volume of a gas? We have to use a eudiometer tube. A eudiometer is a long test tube. It has volumetric markings for recording a gas's volume. To use a eudiometer, we start by filling it with water. Next, we plug the open side and submerge it in a water-filled beaker. Underwater, we undo the plug and run a plastic tube into the eudiometer. The other end of the plastic tube connects to our gas canister. When we open the canister, the gas bubbles through the water into the eudiometer. The gas pushes the water down into the beaker. So, we can read the volumetric marks.

We can also connect the plastic tube to a reaction vessel. For example, let's say we wanted to react sodium metal with water. Sodium is an alkali metal so, it's very reactive. It rips apart the water and makes hydrogen gas. We would place water and sodium in a volumetric flask. We would then plug the flask and have the plastic tube inserted through the plug. As they reacted, the hydrogen gas would collect in the eudiometer.

Pressure

A gas's pressure is the force per area it exerts on its container. We use the SI units, pascals. A pascal is equal to a newton per meter squared ( $\frac{N}{m^2}$ ). A gas's collisions are what generate its pressure. The more the gas molecules hit their container, the greater the pressure. If they collide less often, you have less pressure. So, anything that increases the number of collisions increases pressure. Anything that lessens collisions decreases pressure.

Gas Laws

Now that we've examined volume and, pressure let's turn our attention to the gas laws. Each gas law explains how a gasses physical properties relate.

Boyle's Law: Unsurprisingly, Robert Boyle developed Boyle's law. It relates a gas's volume and pressure. When we keep a gas's temperature constant, pressure and volume are inversely related. So, if you increase volume, pressure decreases. But, when you decrease volume, pressure increases. Because gases are expandable, they fill their container. So, if the gas has to fill more space, it will collide less. But, if the gas has less space, it will impact its container more. It makes sense!
Charles' Law: Charles' law relates a gas's volume and temperature at constant pressure. Charles found that they are directly correlated. When you heat a gas, it has more kinetic energy. So, it will collide more. But, Charles was looking at constant pressures. For the gas to have more kinetic energy, it had to increase its volume. For Charles' law, you need a container that has a moveable volume.
Gay-Lussac's Law: The Gay-Lussac law is similar to Charles' Law. But, instead of keeping the pressure constant, they fixed the volume. As the temperature increased, the gas had more translational energy. So, it collided with the container more. Consequently, the pressure increased. They concluded that pressure and temperature were directly related.
Combined Gas Law: What if multiple factors change? We use the combined gas law. It combines all three of the above gas laws. Let's imagine that you increased the container's volume, which lowered the gas's pressure. If you want to find the new temperature, use the combined gas law.
Avagadro's Law: The above laws assumed that we had a set amount of gas. Avagadro's law asks, "what happens if we change the number of moles?" Under constant temperature and pressure, moles of gas and volume are directly proportional. The more gas we fill our container with, the greater its volume. Imagine filling blowing up a balloon. The more air you add, the bigger it gets.
Ideal Gas Law: The Ideal gas law combines all the above gas laws. It is the most user-friendly. In one equation, we use pressure, volume, moles, and temperature. So, you don't have to remember what's kept constant. When you know three of the terms, you can solve for the other.

Real Gasses vs. Ideal Gasses:

The ideal gas law needs clarification. We assume that most gases particles have neither volume nor electrostatic interactions. In reality, we know that's not true. Even the smallest noble gases or diatomic elements have some volume. And they experience dispersion forces. But, the ideal gas law is a good approximation.

We call non-ideal gases "real gases." For them, we use Van der Waal's equation. It accounts for a gas's volume and Van der Waal's forces. But, the calculations are more complex. Generally, you can assume most gases behave ideally.

Chemistry in the Real World: The Internal Combustion Engine

Did you know that the physical properties of gases move your car? In a car, the fuel injector sprays gasoline into the engine. The spark plugs then ignite it. When gasoline ignites, it initiates a combustion reaction. The reaction's product is carbon dioxide gas and water vapor. We increased the moles of gas. So, according to Avogadro's law, our volume increases. The gas pushes the engine's piston. The piston attaches to the crankshaft. Through a series of gears, the crankshaft rotates your tires. So, your car moves! As the gases cool, the piston retracts. The retraction pushes the gas into the exhaust valve.

Next, the exhaust enters the catalytic converter. Your car's catalytic converter uses the transition metals platinum ( $Pt$ ), rhodium ( $Rh$ ), and palladium ( $Pd$ ). The metals combine to catalyze a series of oxidation-reduction reactions. The main goal is to convert nitrogen oxides into nitrogen and oxygen gas.

Nitrogen oxides are undesirable because they produce smog. You can look up pictures of Los Angeles covered in a thick brown gas. That gas is smog. Los Angeles has had a smog problem since 1943. When it first appeared, it was during World War II. Los Angeles' residents thought that Japan had launched a chemical attack. But, it was just terrible pollution.