Gases in reactions
Many of the processes seen so far in this topic involve gases. When dealing with gas volumes, you have already seen that it is possible to convert between the volume of a gas and the number of moles present using the relationship shown below.
moles (n) =
However, this relationship will only be valid if the gas volume is measured at standard temperature and pressure (0°C and 100.0 kPa). In industry, reactions often take place at high temperatures and/or pressures, so to be able to have a relationship that can be used in other conditions would be very useful.
The relationship that is used is called the ideal gas equation and is shown here.
PV = nRT
P = pressure measured in kPa (1.000 atm 
= 101.3 kPa)
V = volume in litres
n = number of moles
R = 8.315 J K-1 mol-1 (the universal gas constant)
T = temperature measured in Kelvin (see below)
This expression can be rearranged to give the following relationships.
V =
or n = 
For information regarding rearranging equations and units of pressure, refer to the number skills section of the Toolkit.
The Kelvin temperature scale
When using the ideal gas equation, we need to use the Kelvin temperature scale and not temperature values given in Celsius (°C). The Celsius scale is not an absolute scale because 0 °C does not really mean zero temperature, rather it is the freezing point of water which is the reference point for this scale. Because of this, temperatures measured on the Celsius scale can have negative values, but temperature values measured on the Kelvin scale are always positive.
Temperature and energy
You should remember, from previous studies, that temperature is a measure of the average kinetic energy of the particles in a substance. As the temperature of the substance decreases, the kinetic energy of the particles also decreases. As the total kinetic energy approaches zero, the temperature of the substance also approaches a lower limit. The temperature at this limit is termed absolute zero, or zero kelvin (0 K). This is equivalent to -273.1 °C. In theory this is the point at which the kinetic energy of the particles will be zero.
Fortunately, the intervals used in the two scales are the same, where an increase in one kelvin is equal to an increase in one degree celsius, so it is easy to convert from one to the other using the relationship shown below.
temperature in K = temperature in °C + 273.1
Convert the following temperatures. Type in the answers to one decimal place and then click the check answer button.
Boiling temperature of pure water
1 |
2 |
3 |
Boiling point of oxygen
4 |
