Topic 3 - Thermodynamics3.2 – Thermal Properties of Matter
The Particle Model● Suppose that all objects are made of particles.● Solid objects have lower temperatures thantheir liquid phase.● Therefore the particles have lower average kineticenergy in the solid phase than in the liquid phase.● We assume that in a solid the particles canvibrate around fixed points, in liquids they canmove slowly and in gases they can movequickly.
The Particle Model● An object in its solid phase has a fixed shape.● When it melts it moves to fill the shape of itscontainer.● In the gaseous phase it will completely fill itscontainer.● We assume that in solids, the particles aretightly bound in fixed shapes (crystalline), thatin liquids they are bound close together but inno fixed shape, and in gases they are virtuallyunbound.
Changing Phases● When thermal energy flowsinto an object 2 things canhappen.● Either the kinetic energy ofthe particles can increase(Temperature goes up)● Or the potential energy ofthe particles can increase(bonds break andtemperature remainsconstant)● This can be shown on aphase change graph.Temperature/KTime /s
Evaporation versus Boiling● Evaporation is the process by which particles inthe liquid phase change into the gaseous phaseat temperatures below the boiling point of theliquid.● This is because the individual particles have arange of kinetic energies even though theaverage kinetic energy gives rise to atemperature less than the boiling point.● If a particular particle close to the surface hasenough energy to escape the bulk of the liquidthen it will evaporate and form a vapour.
Evaporation versus Boiling● Now that the highly energetic particle hasescaped, the average kinetic energy of theremaining liquid is reduced● The temperature has decreased.● This is the basis of evaporative cooling.● A volatile liquid evaporates rapidly.● A liquids volatility is controlled by a factor calledits equilibrium vapour pressure.
Boiling● A liquid boils when its vapour pressure fallsbelow that of atmospheric pressure.● This means that the temperature at which waterboils is dependent on air pressure.● It boils at lower temperatures at the top of amountain where the pressure is less than at sealevel.
Thermal Capacity● An object that is heated is able to store thermalenergy as Internal energy.● The amount of energy required to raise anobjects temperature by 1K is known as itsthermal capacity.● An object with a large thermal capacity will takelonger to cool down than an object with a lowthermal capacity.● This is because the rate of cooling depends on thetemperature difference but the total thermal energyavailable depends on the thermal capacity.
Thermal Capacity● The thermal capacity of an object is given by:Where● ΔQ is the thermal energy transferred into the objectin J● ΔT if the temperature rise of the object in KThermal Capacity= Q T
Specific Heat Capacity● In order to compare the thermal capacity ofdifferent objects made of different materials thethermal capacity per unit mass is calculated.● This is called the specific heat capacity● The specific heat capacity of a material is theenergy required to raise 1kg of sample by 1K.● The symbol for specific heat capacity is c.● Often this is quoted in data books as per g not kg!!!c= Qm T
Questions● A 750g sample of unknown metal is heated untilits temperature is raised to 70oC from 15oC. Indoing this 20kJ of energy is transferred into thesample. What is the specific heat capacity ofthe metal?● How much energy is required to bring a 90gsample of water from 10oC to boiling point?● A 600kg block of Pyrex loses 8.7x106J ofenergy as it cools from 95oC. What is itstemperature after cooling? c=8.4x102Jkg-1K-1
Mixing materials● Often two materials of different temperaturesare added together.● The hotter material loses energy and the colderone gains this. They then both end up at thesame equilibrium temperature.● This can be used to find the specific heatcapacity of different materials and is known asthe method of mixtures.
Questions● A 2kg block of copper at 150oC is placed into a3L bucket of water at 15oC. What is the finaltemperature of the water?● A 4kg block of aluminium at -100oC is placedinto a 2L vat of mercury at 250oC. What is thefinal temperature.
Latent Heat● The energy required to break the bonds in a 1kg sample ofsubstance is known as the specific latent heat.● The specific latent heat of transformation is the energyrequired to completely change a the sate of a 1kg sampleof material.● The latent heat of vaporisation LVchanges liquids to gases● The latent heat of fusion LFchanges solids to liquids● The latent heat of sublimation LSchanges solids to gasesL= Qm
Questions● How much energy is required to completelymelt a 35kg block of water ice?● How much energy is required to raise a 2kgblock of water ice from -20oC to 45OC.● A 500g block of ice at -10oC is added to a 1.5Lbeaker of warm water at 70oC. What is the finaltemperature of the mixture?
Questions● How much energy is required to completely boil5L of water?● A pressure kettle is used to heat 0.5L of waterfrom 15oC to 110oC in 5 minutes. What is theminimum input power needed to do this?● Hot rocks at 450OC are placed into a kettle.Each rock has a mass of 0.5kg. If the kettlehas 5.5L of water at 20oC in it then how manyrocks are needed to boil the water.● crock= 980 Jkg-1K-1
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