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12.1 Characteristics of Gases

- Gases are unique because the distance between particles is greater than the distances in liquids and solids
- Gases are considered fluids because they can flow
- Because gas particles are far apart they are able to move past one another

- Gases have lower densities the liquids and solids
- Because most of the volume of gas is empty space gases have low densities

- Gases are very compressible because the total space the particles take up is much smaller than the total volume
- Gases completely fill their containers
- Air pressure is the collision of gas molecules
- Pressure is forced divided by area, meaning the amount of force exerted per surface area unit
- The unit of force is called a Newton
- 1 Newton = 1 kg x 1 m/s
^{2}= 1 N

- 1 Newton = 1 kg x 1 m/s
- The unit of pressure is called a pascal
- 1 Pa = 1 N / 1m
^{2}

- 1 Pa = 1 N / 1m
- There are also other units of pressure
- Atmosphere/atm: 1 atm = 101,325 Pa
- Bar: 1 bar = 100,025 Pa
- Millimeter of mercury/mm Hg: 1 mm Hg = 133.322 Pa
- Pounds per square inch/psi: 1 psi = 6.89286 x 10
^{3}Pa - Torr: 1 torr = 133.322 Pa

- Standard temperature and pressure, STP, is equal to 0°C and 1 atm
- The kinetic molecular theory is used to predict gas behavior and states
- Gas particles are in constant rapid, random motion
- Particles are far apart relative to their size
- Pressure exerted is a result of collisions of molecules against the container

- The average kinetic energy is proportional to the temperature in kelvin

12.2 The Gas Laws

- P = pressure exerted by the gas
- T = temperature of the gas in kelvin
- V = total volume occupied by the gas
- n = number of moles of the gas
- k = constant
- Robert Boyle discovered the gas pressure of a gas increased, the volume decreased but the product remained constant
- His law states that there is an inverse relationship between pressure and volume. As one increases the other must decrease
- PV = k
- P
_{1}V_{1}= P_{2}V_{2}

- There is a direct relationship between temperature and volume. This is called Charle’s Law
- It states that as the temperature of a gas increases, the volume also increases. And I’d the temperature decreases, the volume will also decrease
- V/T = k
- V
_{1}/T_{1}= V_{2}/T_{2}

- There is a direct relationship between pressure and temperature known as Gay-Lussac’s law
- It states that the pressure of a gas is proportional to the absolute temperature
- P = kT
- P/T = k
- P
_{1}/T_{1}= P_{2}/T_{2}

- Avogadro’s law states equal volumes of gases at the same temperature and pressure have an equal number of molecules
- V =kn

12.3 Molecular Composition of Gases

- No gas perfectly follows the four laws from section 2. The model of a gas that would follow these laws is called an ideal gas
- The ideal gas is imaginary and has particles that are infinitely small and do not interact with one another
- The relationship between all four variables(P, V, T, and n) can be combined into one equation. This equation is called the ideal gas law
- PV = nRT where R is constant
- R = 8.314 L x kPa / mol K
- R = 0.0821 L atm / mol K

- Diffusion is the movement of particles from regions of high density to areas of lower density
- For example the smell of cookies spreading from the kitchen to the living room

- Gases diffuse at rapid rates
- Diffusion causes an increase in entropy
- Effusion is the passage of gas through a small opening
- For example a hole in a tire

- Thomas Graham discovered that the rate of effusion is inversely proportional to the square root of the gas’s molar mass(M)
- This is known as Graham’s law of Diffusion

- v
_{a }and v_{b}are the molecular speeds where v stands for velocity - v
_{a}/ v_{b}= the square root of M_{b}/ M_{a} - Particles of lower molar masses travel much faster than heavier particles
- Gay-Lussac’s law of combining volumes states that the volume of gases involved in chemical changes can be represented by a ratio of small whole numbers
- In a mixture of gases, each individual gas will exert a certain amount of pressure
- The pressure of each had in a mixture of gases is called the partial pressure
- The total pressure of the mixture can be found by adding the partial pressures. This is known as Dalton’s laws of partial pressure
- P
_{total}= P_{A }+ P_{B }+ P_{C}

- P
- The ratio of gas volumes is the same as the ratio of moles in a balanced equation

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