Chemistry Friends/Gases

Properties and Patterns

When elements are in their gaseous form, they follow the following rules.

1. They fill their container but don't have a volume of their own.
2. They are very compressible and can be pushed into a space much smaller than usual.
3. They mix through the air without being mixed
4. When they get hotter they expand

Pressure and Volume

Pressure is force per unit of area. If you had a book with a large area and pushed it softly, it might have the same amount of pressure as a pin if you pushed it really hard. The SI unit for pressure is the kilopascal. One kilopascal (kPa) is equal to 1000 newons of force over 1 square meter. Or 1kPa=1000N/1m². We experience pressure every day. When we lay in our bed the blanket pushes us down. When we sit down for breakfast, we up pressure on our chairs. Even the air had pressure. This is called 'atmospheric pressure'. The average Atmospheric Pressure at sea level is 101 kPa. We call this (101 kPa) 'one standard atmosphere' or 'atm'. When the conditions are all normal and the temperature is 0°C and the atmospheric pressure is at atm, we call it 'standard temperate and pressure' or 'STP'. When the pressure is 100 kPa and the temperature is 25°C it is called 'standard ambient temperature and pressure" or 'SATP'. Just remember that STP has less letters and less heat while SATP has more letters and more heat.

Boyle's law

Robert Boyle was an English chemist. He developed the law about pressure and volume. He realized that if you compress a container of gas the volume will decrees with the same proportions. If you doubled the pressure, the volume would be halved. This is the equation.

v₁p₁=v₂p₂

The first side of the equation is how things where initially, (v=Volume and p=Pressure) the second side is how the Volume and pressure were after they changed.

Temperature and Volume

Jacques Charles was a French physicist. He noticed a relationship between the temperature of a gas and its volume. He noticed that as the temperature of a gas increased, the volume increased proportionately. This can be shown in a few different equations.

v=kT

The v represents volume, the T is temperature and k is a constant value. We can also write the equation like this.

v₁/T₁=k and this v₂/T₂=k

Or it can be simply written together like this:

v₁/T₁=v₂/T₂

The Combined Gas Law

Charles and Boyle both made useful formulas to calculate the changes in gasses. If we combine their theories, we can make a very useful formula that can help us calculate anything we need to know about gasses. The formula looks like this.

p1v1/T1=p2v2/T2.

The letters stand for volume (v), pressure (p) and temperature (T). The nice thing about this equation is that we can customize it to fit our question. If the pressure remains constant we can take it out of the equation and simply write:

p₁v₁=p₂v₂

Kelvin Temperature Scale

Scientists use the Kelvin Scale when measuring temperature. It is written in Kelvin and not degrees Kelvin. When converting Celsius to Kelvin, add 273°C. So if we wanted to know how hot it was on a day in the summer, we'd add 273°C to 25°C. That day, it would be 298 K outside! When writing in Kelvin, the temperature is always a positive number. Not even insanely cold temperatures are negative. This is because 0 Kelvin is Absolute Zero. No where in the universe are things that cold or colder. Absolute Zero means the particles themselves have stopped moving. Even in space the stars keep things from getting that cold.

Avogadro's Theory and Molar Volume

Before a theory is accepted, experiments have to be done to prove it. If an idea helps explain lots of unknown phenomena, it is even better. If you walked into you house and saw your grandmother's shoes on the shoe rack and heard her voice coming from the living room, you may come up with a few theories. One, your grandmother came to visit or two your grandmother left her shoes on the shoe rack when she was sneaking through your house last night and the voice in the living room is a parrot that can mimic voices. You would probably got with theory one: If you conclude that your grandmother is over it explicates two things. Also you may have observed that your grandmother drops by a lot in the afternoons. Theory two makes less scene. You have never known your grandmother to be a forgetful person or go any where at night. Also, you don't own a parrot. If a theory explains lots of things, then it usually makes more sense. The kinetic molecular theory was accepted amoung scientists for this reason. It explains why gases are compressible and why there's gas pressure. It also complements Boyle's and Charles's laws.

The Law of Combining Volumes

Joseph Gay-Lussac was a french scientist. He discovered the Law of Combining Volumes. The law says that if you combine gases the ratio of molecules is in simple whole numbers.

Avogadro's Theory

Avogadro found the reason for the simple ratios. His theory states that if you two containers containing equal amounts of gas at the same temperature, they would have the same amount of molecules. (I don't know how he proved this but I doubt he counted them all.)

The Law of Combining Gas Volumes

Because of the Law of Combining Volumes and Avogadro Theory, we have a simple way of figuring out the amount of each molecule in an equation. Say we knew that hydrogen and oxygen make dihydrogen oxide or in other words:

O₂ + H → H₂O

According to the Law of Combining Volumes, the number of dihydrogen oxide molecules should be a simple whole number. Right now, the equation is wrong. It says two oxygen atoms plus one hydrogen atom equals two hydrogen atoms and one oxygen atom. We still need to balance the equation. Let's start with oxygen. There is one oxygen atom needed in dihydrogen oxide but it is in groups of two on the left side of the equation. Therefore, this equation makes two molecules of dihydrogen oxide. Now for the hydrogen. Since we've decided that there are two molecules of H₂O, that means there are four hydrogen atoms needed. So there are four atoms of it on the left side of the equation. It will look like this:

O₂ + 4H → 2H₂O

Molar Volume of Gases

A mole is a number of particles. It's a bit like saying a dozen but it's a more complicated number. The molar volume of a gas is one mole of the gas at a specific temperature. This volume doesn't change between gases. A gas at STP has a molar volume of 22.4 L/mol. Scientists like to use moles a lot. Working with volume is easier than mass. It is also more exact. You can use molar volume to convert between moles and liters.