Gases and Solutions Flashcards
1
Q
Kinetic Theory of Gases
A
- Gases are composed of particles in rapid, continuous, random motion.
- Attraction and repulsion between particles in gases is negligible.
- Particles in a gas are widely spaced – the total volume of all the particles is negligible compared to the volume the gas occupies.
- Particles in a gas have kinetic energy, given by EK = ½ mv2.
- The average EK of all particles in a gas is proportional to its temperature and is the same for all gases at the same temperature.
- Particle collisions are elastic – as particles collide, they do not lose speed or energy.
2
Q
Behaviour of Gases
A
- Take the shape of their container: constant random motion with negligible forces of attraction, results in gas particles spreading out to occupy the entire volume of the container.
- Have a very low density: large empty spaces between gas particles, total volume of gas particles is negligible.
- Exert a pressure: constant random motion results in collisions with container walls – pressure increases with the frequency & force of these collisions.
- Can be compressed: large empty spaces between gas particles, when compressed, the amount of space between the gas particles decreases (particle size doesn’t change)
- Readily diffuse through other gases: because gas particles have rapid, random motion and a lot of empty space between them, so they quickly spread and mix.
3
Q
Temperature
A
- A measure of average kinetic energy of particles within a substance.
- Heavier gases have lower speeds than lighter gases.
- As T (Temperature) increases, EK (Kinetic Energy)increases.
- At any temperature, gas particles will have a range of speeds.
4
Q
Temperature Units
A
- Absolute zero: the theoretical lowest temperature possible (0 K = -273.15 ºC)
- Kelvin vs ºC
- 0 K = -273.15°C
- 10 K = -263.15°C
- To convert from ºC to K do the following:
- T(K) = TºC + 273.15
- To convert from K to ºC do the following:
- T(ºC) = T(K) – 273.15
5
Q
Pressure
A
- Determined by the frequency and force of collisions of the gas particles and the container walls.
- SI unit pascal (Pa)
- Standard Pressure = 100 kPa
6
Q
Gas Laws
A
- The relationships between T, P & V of gases is described by different laws
- Boyle’s Law (P, V)
- Charles’ Law (T, V)
- Combined Gas law (P, V & T)
7
Q
Boyle’s Law
A
- “At constant temperature, the volume of a given mass of gas is inversely proportional to the pressure”.
- Increasing the volume of a container results in a decrease in the frequency of collisions.
- More space = less chance of a particle colliding with the container walls.
- Inversely proportional relationship:
- Double the volume, halve the pressure!
- Halve the volume, double the pressure!
8
Q
Charles’ law
A
- “At constant pressure, the volume of a fixed quantity of gas is proportional to its absolute temperature”
- Increasing temperature of the gas increases the average EK and the average velocity of the particles.
- The gas particles collide with the container walls more frequently and with a greater force.
- This results in an increase in pressure.
- For the pressure inside a container to remain constant, the volume of the gas will increase.
9
Q
Ideal Gas vs Real Gas
A
Real Gas:
* particles in a real gas do occupy volume
* forces of attraction do exist between particles of a real gas
Ideal Gas:
* particle in an ideal gas have negligible volume
* forces of attraction are negligible between particles of an ideal gas
These differences only become important in extreme conditions – at very low temperatures:
1. Theoretically an ideal gas at 0 K would occupy zero volume, but a real gas will never have zero volume because its particles occupy space.
2. If a real gas is compressed or cooled, it will condense to form a liquid due to the intermolecular forces that exist between the particles. However, an ideal gas will never condense because these attractive forces are negligible.
10
Q
Volume of Gas
A
volume of a gas depends on:
- pressure
- temperature
- amount of gas particles present (moles)
11
Q
Molar Volume of gases (STP)
A
- Avogadro’s hypothesis: equal volumes of gases at the same temperature and pressure contain equal numbers of particles
- the volume of 1 mol of all gases, at the same temperature & pressure is identical
- At STP (standard temperature and pressure)
- Temperature: 273.15 K or 0 ºC
- Pressure: 100.0 kPa
- n=V/22.71
- n = number of moles
- V = volume in litres (L)
12
Q
Ideal Gas Law
A
- We need at least one of the conditions of P, T or V and quantity of gas remain constant.
- In reality, all conditions vary
- Boyle’s law, Charles’ law and Avogadro’s hypothesis combine to give the Ideal Gas law which includes all conditions as variables.
- PV=nRT
- P = pressure in kPa
- V = volume in L
- n = number of moles of gas
- R = universal gas constant 8.314 J K-1 mol-1
- T = temperature in K
13
Q
Solutions
A
- solute: the substance that ‘is dissolved’
- solvent: the substance that ‘does the dissolving’
- solution: a homogenous mixture (uniform composition) made up of two or more substances
- solubility: the ability for a given substance, the solute, to dissolve in a solvent.
- qualitative: soluble, insoluble, slightly soluble
- quantitative: mass that can dissolve in 100g of water
- soluble: able to dissolve
- insoluble: unable to dissolve
- solubility is better thought of as a range from infinitely soluble (ethanol in water) to poorly soluble (silver chloride in water)
14
Q
Water - the Universal Solvent
A
- Water is often referred to as the universal solvent, (despite the fact it DOES NOT dissolve many compounds).
- It is such a good solvent due to its polarity.
- A POLAR molecule has a slight +ve charge at one end and a slight –ve charge at the other end (or DIPOLE).
- This charge is not as big as the charge on an ion, but none-the-less creates an electrostatic attraction and repulsions with neighbouring molecules.
15
Q
Solutes
A
- The solute in a solution is the part in small amounts which dissolves in the solvent.
- Substances that readily dissolve in water are called HYDROPHILLIC (water loving).
- Substances that do not dissolve in water are called HYDROPHOBIC (water hating).
- Solutes can be either:
- Ionic (metal and non-metal)
- Polar (molecules containing –OH, C=O, or –NH groups)
- Non-Polar – but these don’t dissolve in water (long C chains)
- Substances may dissolve in water will either be polar (like water) or charged (like Ionic substances)
