A-level Chemistry/OCR (Salters)/Standard conditions

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Definition[edit]

In chemistry, the standard conditions are defined as:

  • a specified temperature
  • 1 atm pressure
  • 1 mol dm−3 concentration

To indicate that a quantity was measured under standard conditions, a superscript plimsoll line symbol, o, is added to the quantity's symbol.

Why do we use standard conditions?[edit]

Measurements depend on conditions[edit]

If you measure a physical quantity (like enthalpy change), you will get different values under different conditions. For example, the enthalpy change of a particular reaction will be different at different temperatures, different pressures or different concentrations of reactants.

Using standard conditions avoids infinitely long data tables[edit]

You could imagine having tables of values for every possible combination of physical quantity, temperature, pressure, concentration and so on. In practice, this is unfeasable and unnecessary. A more elegant solution is to define a set of conditions (called standard conditions) and always measure physical quantities under these conditions. You can calculate the value of the physical quantity under non-standard conditions using appropriate mathematical techniques.

Standard conditions can often be used unmodified[edit]

Often, you won't even need to adjust the value of a quantity measured under standard conditions to what it would be if it were measured under specific non-standard set of conditions. For example, if you are looking at trends in the periodic table by comparing, say, ionisation enthalpy, the pattern should be equally clear under standard conditions or not. In fact, having the values for different elements all measured under the same conditions saves a great deal of effort — imagine having to adjust hundreds of pieces of data to correct for the fact that they were each recorded under different conditions.