Topic 3 - Thermodynamics

3.1 Thermal Concepts

Energy in Substances

All objects have internal energy (U)

This is because the internal particles are vibrating randomly in the substance and there are electrical forces between them.Internal energy is the sum of the kinetic and potential energies of the internal particles of a substance.

The potential energy is due mainly to the electric fields between the particles, both bonding and intermolecular

Energy in a Substance

The energy in an object is internal energy

The transfer of energy between objects by non-mechanical methods is thermal energy.

The temperature of a body is a measure of the average internal kinetic energy of the particles in the substance.

Energy Flows

Internal Energy flows as thermal energy from areas of high temperature to areas of low temperature.

The rate of flow is directly proportional to the temperature difference between the two objects.

This means that the internal energy of the colder object increases at the same rate as the internal energy of the hot object decreases.

Energy Flows

Hot object

Cold object

Warm objects

Energy Flows

Thermal Energy can transferred by 3 methods

It can be conducted by molecules vibrating into the ones around it

It can be convected, where energy moves due to the movement of particles

It can be radiated where the energy is given off as electromagnetic waves due to the temperature of the object.

Energy Flows - Conduction

Conduction works best in solids than in fluids.

Conduction is due to the particles vibrating into each other causing the energy to be transferred from one to the next

Conduction works
better in metals
than non-metals
due to the addition
of free electrons.

Energy Flows - Convection

Convection works as a method of thermal transfer in fluids.

As one area of fluid warms up, the particles move further apart from each other.

This means there the local
density decreases.

Colder, higher density fluid
flows in from the
surroundings pushing the
warm fluid upwards and
taking the energy with it.

Energy Flows - Radiation

All hot objects emit electromagnetic radiation of various wavelengths.

This is due to the vibration of the charges in the object causing radiation to be emitted.

The hotter the object the higher the peak intensity, and the wider the spectrum emitted

The rate of emission also
depends on the surface
nature.Dull black bodies are better
emitters and absorbers than
shiny silver surfaces

Temperature Scales

The thermal energy absorbed or emitted by a body cannot be directly measured.

The temperature, the average kinetic energy per molecule, can be measured using a thermometer.The common scales are Celsius, Fahrenheit and Kelvin.

Temperature Scales

A temperature scale is usually defined by either two fixed points, or 1 fixed point and a fixed interval.Celsius is defined by the fixed points of the melting and boiling point of pure water at standard pressures. (0 and 100 oC)

Fahrenheit is defined by the coldest temperature that could be achieved in a lab at the time (0F) and the blood temperature of a race horse (100F)

The kelvin scale is a measure of the absolute temperature of an object and is defined as 0K is the coldest temperature possible with an interval equal to 1oC.

Kelvin Temperatures

As temperature is proportional to the kinetic energy of the molecules, it makes no sense to talk of negative temperature.Negative Kinetic energy doesn't exist!)

The Kelvin scale avoids this problem and must always be used in thermodynamics questions.

The Celsius and Kelvin temperatures are linked by:

Absolute Zero

At a temperature of 0K, the average kinetic energy of the particles is zero. They are not moving on average.

This temperature is called absolute zero and is currently thought to be -273.15oC.However, this cannot be actually attained in the laboratory.

Particles in Substances

When we think about substances we often think about their macroscopic properties.Mass, dimension, average temperature.

However, we sometimes need to think about the properties of individual particles.

These are the microscopic properties.Internal energy, molecular mass, shape.

Particles in Substances

A sample of a pure elemental substance will contain a fixed number of atoms.

The amount of substance is known as the mole (mol)

The number of atoms in 1 mole of carbon-12 is the same as the number of atoms in 1 mole of uranium-238.Clearly, these will not have the same total mass.

Molar Mass

The mass of 1 mole of substance is given by:

By definition, 12g of carbon-12 is defined to contain 1 mol of atoms.

Therefore 238g of uranium-238 has 1 mol of atoms.The molar mass of the elements can be found from the periodic table (atomic mass)

Avagadro's Number

Avagadro's found that equal volumes of different gases at standard temperature and pressure contained equal numbers of atoms.STP = 273K & 101.3kPa

The number of particles in one mole of any gas is the Avagadro number.

It is defined as NA=6.022x1023 mol-1

The actual number of entities in a sample is thus:

N is the number of particles, n is the number of moles.

Molecular Mass

It is often important to work with molecules and not elements.e.g. SiO2 , O2, CO2

To find the mass of 1 mole of this molecule, simply sum the atomic masses of each component atom.e.g. SiO228 + 2x16 = 60g mol-1

Questions

Express the following temperature in K22oC, 45oC, 37oC, 578oC, -96oC

Express the following temperatures in oC.6600K, 75K, 4K, 373K, 687K

Calculate the number of moles present in:82g of O2 , 76g of H2, 97g of H2O, 350g of Fe2O3

Calculate the number of atoms present in:24g of SiO2, 98g of Al2O3, 45g of NH4NO3