To show how the steam engine works we must look at the study of gases (for this example we will assume that the steam is behaving as an ideal gas). If we consider the simple equation:

H₂O_(l)+energy → H₂O_(g), ΔHº_vap = + 44kJ^-1mol^-1

This is the vapourisation reaction which accounts for the movement of the piston in the cylinder.

Now look at the relationship between temperature, pressure and volume for an ideal gas.

Because the volume of the cylinder is variable (ie. the piston moves up and down) the pressure of the steam inside the piston will remain constant. Therefore if we take the pressure at 1atm the following relationship will be true.

-This tells us that the volume of the cylinder is proportional to the number of moles of steam and the temperature of the steam in the cylinder. If we increase the temperature of the steam then the volume of the cylinder will increase...

...But by how much?

If we undertake an experiment to discover the expansion of 5 moles of steam (given by the vaporisation of water) in the cylinder due to a temperature increase of 50° and if we keep the pressure constant at 1atm, then we can disregard the pressure value. Firstly we should calculate the initial volume of the cylinder at its lowest possible temperature as a gas at 1atm pressure (100°) temperature.

@ 373K,

V = (5mol×0.08206 dm³ atm K^-1 mol^-1×373K) / 1atm = ~153 dm³

Now if we do the same calculation for when the temperature of the gas has risen 50° we will be able to see the change in volume of the cylinder.

@ 423K,

V = (5mol×0.08206 dm³ atm K^-1 mol^-1×423K) / 1atm = ~173.5 dm³

Increase in the gas volume = 173.5 dm³-153 dm³ = ~20.5 dm³

So we now know that if we heat 5 moles of steam 50° above its boiling point it will increase in volume by approximatley 20.5 dm³.

But what if we heat it further???

No I'm not going to bore you senseless with MORE mind numbing calculations, instead I'll brighten up your day by disscussing...

Charles' Law

Whilst studying the properties of gases, in 1787, French physicist Jacques Charles discovered that the volume of a gas at constant pressure increases linearly with the temperature of the gas. ie. The plot of of volume of gas (V) against the gas' temperature (°C), at constant pressure, gives us a striaght line. This experiment was conducted using several gases, all of which showed that the volume that a gas occupies at constant pressure is directly proportional to the temperature of the gas.

This means that if we have a gas (at fixed pressure) at temperature, T₁, that occupies a volume, V₁, and we heat it to temperature, T₂, it will occupy a new volume, V₂. The relationship between the gas at temperature and volume 1 and the gas at temperature and gas 2 is:

Ok. Now we know that the volume of the steam engine cylinder increases at a rate that is directly proportional to the temperature of the gas, we also know the relationship for this.

But what about when the steam is cooled in the condenser?...

Well, it does exactly the opposite! The volume of the gas decreases at the same rate as before. (Directly proportional to the decreasing temperature).

This occurs over and over and the upward and downwards movement of the piston rotates the flywheel and VOILA! we have the energy we need to move.

On to todays transport I think!