What is normal temperature and pressure

Normal condition in physical chemistry

Every reaction is carried out under certain conditions (pressure, temperature, brightness, etc.), whereby in chemistry the parameters pressure and temperature are of great importance. By varying these two parameters appropriately, the equilibrium of a reaction can be shifted accordingly (=> LeChatelier's principle).

In general and inorganic chemistry we always speak of “room temperature” or “normal condition” without going into further details. This is due to the fact that in these sub-disciplines a chemical reaction is viewed more qualitatively, i.e. which product is created and why. Only (mainly) in the sub-area “Physical Chemistry” are the reaction conditions recorded quantitatively.

The normal condition (in a chemical reaction)

Before we deal with the “normal condition” in more detail, this term should be separated from the often confused term “standard condition”. The two terms “normal condition” and “standard condition” do not mean the same thing. Normal conditions (certain pressure and temperature) are the same worldwide, while standard conditions differ from country to country.

The normal condition (also called standard condition) is defined worldwide and a certain temperature and pressure are assigned to this condition (and defined by corresponding German industrial standards (DIN)):

  • Temperature: T = 273.15 K (= 0 ° C)
  • Pressure: p = 101,325 Pa = 1,013.25 hPa = 1,013.25 mbar = 1.01325 bar

The so-called “standard conditions” also exist. The most frequently used standard condition in Germany is:

  • Temperature: T = 298.15 K (= 25 ° C)
  • Pressure: p = 101,300 Pa = 1,013 hPa = 1,013 mbar = 1,013 bar

(Under these conditions one often speaks of the standard temperature and the standard pressure). Standard conditions are not globally valid and everywhere the same conditions, this can be seen, for example, from the fact that in chemistry a (slightly) changed standard condition is often used:

  • Temperature: T = 273.15 K (= 0 ° C)
  • Pressure: p = 100,000 Pa = 1,000 hPa = 1,000 mbar = 1,000 bar

Why do “standard conditions” exist at all?

For example, if we look in a lexicon for the standard enthalpy of formation of certain values, then these have mostly been measured under standard conditions (25 ° C). If you were to use normal conditions (0 ° C), you would have to convert your own measurement result over and over again. In addition, the “setting” of standard conditions (25 ° C) in a laboratory is easier than normal conditions (0 ° C). The main advantage of the standard conditions (for students) is that you do not have to carry out any (complicated) conversions.