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4. States of MatterI The gaseous state:(i) Ideal gas behaviour and deviations from it(II) pV = nRT and its use in determining a value for MrII The liquid stateThe kinetic concept of the liquid state and simple kinetic-molecular descriptions of changes of stateIII The solid stateLattice structures

- Candidates should be able to:
- state the basic assumptions of the kinetic theory as applied to an ideal gas
- (b) explain qualitatively in terms of intermolecular forces and molecular size:
- (i) the conditions necessary for a gas to approach ideal behaviour
- (ii) the limitations of ideality at very high pressures and very low temperatures
- (c) state and use the general gas equation pV = nRT in calculations, including
- the determination of Mr
- (d) *describe, using a kinetic-molecular model, the liquid state; melting;
- vaporisation and vapour pressure
- (e) *describe, in simple terms, the lattice structure of a crystalline solid which is:
- (i) ionic, as in sodium chloride, magnesium oxide
- (ii) simple molecular, as in iodine
- (iii) giant molecular, as in graphite; diamond; silicon(IV) oxide
- (iv) hydrogen-bonded, as in ice
- (v) metallic, as in copper
- [the concept of the ‘unit cell’ is not required]

(f) explain the strength, high melting point and electrical insulating properties of

- ceramics in terms of their giant molecular structure
- (g) relate the uses of ceramics, based on magnesium oxide, aluminium oxide
- and silicon(IV) oxide, to their properties (suitable examples include furnace
- linings; electrical insulators; glass; crockery)
- (h) describe and interpret the uses of the metals aluminium, including its alloys,
- and copper, including brass, in terms of their physical properties
- understand that materials are a finite resource and the importance of
- recycling processes
- (j) outline the importance of hydrogen bonding to the physical properties
- of substances, including ice and water
- (k) suggest from quoted physical data the type of structure and bonding
- present in a substance

Avogadro’s law:

Equal volumes of any gas measured at the same temperature and pressure contain the same numbers of particles (atoms and molecules

In order for volumes of gases to be comparable, they must be measured under the same conditions of temperature and pressure. Alternatively the volumes at the required temperature can be worked out using the Ideal Gas Equation.

PV = constant (for a fixed mass of gas)

T

If we take 1 mole of gas the constant is given the symbol R and is called the gas constant, and for n moles of gas we have

PV = nRT

R is a constant, 8.314 KJ-1mol-1

P, pressure must be in Pascals, Pa; V, volume must be in m3 (1m3 = 106 cm3 = 103 dm3), T, temperature must be in Kelvin, K

Kinetic theoryis an attempt to explain the observed properties of gases

- The particles are moving randomly

- We can neglect the volume of the particles themselves in comparison
- to the total volume of the gas

- The particles do not attract one another

- The average kinetic energy of the particles is proportional to the

temperature of the gas

- No energy is lost in collisions between particles

- Bombardment of the walls of the container explains pressure and
- increasing temperature makes them hit walls harder, so pressure
- increases

Deviations from Ideal Gas Behaviour

When gases are put under high pressure or cooled down the gas molecules get closer together (or move slower at lower temperatures) and they become attracted to each other using intermolecular forces and start to form a liquid. So there are no gases at 0 K!

What volume is needed to store 0.050 moles of helium gas at 202.6kPa and 400K?

What pressure will be exerted by 20.16g hydrogen gas in a 7.5L cylinder at 20oC?

A 50L cylinder is filled with argon gas to a pressure of 10130.0kPa at 30oC. How many moles of argon gas are in the cylinder?

To what temperature does a 250mL cylinder containing 0.40g helium gas need to be cooled in order for the pressure to be 253.25kPa?

What volume is needed to store 0.050 moles of helium gas

at 202.6kPa and 400K?

PV = nRT

P = 202.6 kPa

n = 0.050 mol

T = 400K

V = ? L

R = 8.314 J K-1 mol-1

202.6V=0.050x8.314x400

202.6 V = 166.28

V = 166.28 ÷ 202.6

V = 0.821 L (821mL)

What pressure will be exerted by 20.16g hydrogen gas

in a 7.5L cylinder at 20oC?

PV = nRT

P = ? kPaV = 7.5Ln = mass ÷ MM mass=20.16g MM(H2)=2x1.008=2.016g/mol

n=20.16 ÷ 2.016=10molT=20o=20+273=293KR = 8.314 J K-1 mol-1

Px7.5=10x8.314x293Px7.5 = 24360.02P = 24360.02 ÷ 7.5 = 3248kPa

A 50L cylinder is filled with argon gas to a pressure of 10130.0kPa

at 30oC. How many moles of argon gas are in the cylinder?

PV = nRT

P = 10130.0kPaV = 50Ln = ? molR = 8.314 J K-1 mol-1T=30oC=30+273=303K

10130.0x50=nx8.314x303506500=nx2519.142n=506500 ÷ 2519.142=201.1mol

To what temperature does a 250mL cylinder containing 0.40g helium

gas need to be cooled in order for the pressure to be 253.25kPa? PV = nRT

P = 253.25kPaV=250mL=250 ÷ 1000=0.250Ln=mass ÷ MM mass=0.40g MM(He)=4.003g/moln=0.40 ÷ 4.003=0.10molR = 8.314 J K mol-1T = ? K

253.25x0.250=0.10x8.314xT63.3125 = 0.8314xTT=63.3125 ÷ 0.8314=76.15K

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