The Gaseous State: Ideal & Real Gases & pV = nRT (Cambridge (CIE) A Level Chemistry): Flashcards
Exam code: 9701
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- FrontGas Pressure
Define gas pressure.
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Cards in this collection (18)
Define gas pressure.
Gas pressure is the force exerted per unit area on the walls of a container, caused by gas molecules constantly colliding with the walls.
What happens to the pressure of a gas when its volume is decreased at constant temperature?
The pressure increases. Reducing the volume squashes molecules closer together, increasing the frequency of collisions with the container walls.
True or False?
At constant temperature, gas pressure is inversely proportional to volume.
True.
As volume decreases, pressure increases proportionally. A graph of volume against 1/pressure gives a straight line.
Increasing the temperature of a gas at constant volume causes molecules to gain more .......... energy, so they move .......... and collide with container walls more frequently, increasing .......... .
Increasing the temperature of a gas at constant volume causes molecules to gain more kinetic energy, so they move faster and collide with container walls more frequently, increasing pressure.
What is the relationship between temperature and pressure at constant volume, and what graph shape confirms this?
Temperature is directly proportional to pressure. A graph of temperature (x-axis) against pressure (y-axis) gives a straight line through the origin.
True or False?
Decreasing the volume of a gas at constant temperature decreases the frequency of collisions with the container walls.
False.
Decreasing the volume brings molecules closer together, increasing collision frequency with the walls and therefore increasing pressure.
Define collision frequency.
Collision frequency is the number of collisions that gas particles make with the container walls per unit time. It increases when volume is decreased or temperature is increased, both of which raise gas pressure.
A graph of the .......... of a gas plotted against .......... gives a straight line, confirming the inverse relationship between these two quantities at constant temperature.
A graph of the volume of a gas plotted against 1/pressure gives a straight line, confirming the inverse relationship between these two quantities at constant temperature.
Why does increasing the temperature of a gas increase its pressure at constant volume?
Higher temperature gives gas molecules more kinetic energy. They move faster and collide with the container walls more frequently and with greater force, raising the pressure.
Define ideal gas.
An ideal gas is a theoretical gas that obeys the kinetic theory assumptions: molecules have negligible volume, no intermolecular forces exist, collisions are perfectly elastic and temperature is proportional to average kinetic energy.
State the ideal gas equation and define every symbol and its unit.
PV = nRT, where P = pressure (Pa), V = volume (m3), n = moles of gas (mol), R = 8.31 J K-1 mol-1, T = temperature (K).
True or False?
At very high pressures and low temperatures, real gases deviate from ideal gas behaviour.
True.
At high pressures and low temperatures, molecules are closer together, so intermolecular forces become significant and the molecules' own volume can no longer be ignored.
To convert Celsius to Kelvin, add .......... to the Celsius temperature. To convert cm3 to m3, divide by .......... .
To convert Celsius to Kelvin, add 273 to the Celsius temperature. To convert cm3 to m3, divide by 1 000 000.
Define elastic collision.
An elastic collision is a collision in which no kinetic energy is lost. It is one of the key assumptions of the kinetic theory of gases: gas molecules lose no energy when they collide with each other or with the container walls.
Why does the volume of an ideal gas increase when it is heated at constant pressure?
Heating gives molecules more kinetic energy, causing more frequent collisions. To maintain constant pressure, the molecules must spread further apart, so the volume increases. Volume is directly proportional to temperature at constant pressure.
True or False?
For an ideal gas, volume is inversely proportional to pressure at constant temperature.
True.
This follows directly from PV = nRT: at constant n and T, doubling P halves V.
A flask (1000 cm3, 300 kPa, 23 °C) contains 6.39 g of gas. Using PV = nRT, the moles of gas = .......... and the molar mass = .......... g mol-1.
A flask (1000 cm3, 300 kPa, 23 °C) contains 6.39 g of gas. Using PV = nRT, the moles of gas = 0.12 and the molar mass = 53.25 g mol-1.
Give two reasons why real gases deviate from ideal gas behaviour at high pressure.
At high pressure: (1) intermolecular forces (e.g. London forces) become significant, reducing the actual pressure below the ideal value. (2) The volume of the gas molecules themselves can no longer be ignored, so the free volume is less than predicted.
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