What is the circle on top of variables in chem?

Like dS^o, dH^o, and dG^o. Do they represent standard state? Or standard condition? What's the difference between these two anyway?

1 Answer
Dec 20, 2017

Circle refers to standard conditions, Plimsoll symbol refers to standard state.



The circle superscript refers to standard conditions.

The Plimsoll symbol (circle with a line through) refers to standard state . ⦵

You will find that they are used interchangeably, which is a bit confusing. But as long as you can recognise the difference between standard state and standard conditions, and when to use each, you should be okay.

Standard conditions , represented by the initialism STP (stnading for standard temperature pressure) refers to a temperature of #273.15 K# (#0^@C#) and a pressure of #1 atm # (#10^5 Pa#). STP is normally used in calculations using the Ideal Gas Law to calculate gas volume, pressure etc.

Standard State supplies several different conditions, including:

  • The standard state temperature is #25°C# (#298 K#). Sometimes #0^@C# is used for standard state, but if another temperature is not supplied assume #298 K#.
  • All gases are at #1 atm# pressure.
  • All liquids and gases are pure.
  • All solutions are at #1M# concentration.
  • The energy of formation of an element in its normal state is defined as zero.

Standard state is used in thermodynamic calculations for entropy, enthalpy, Gibbs, equilibrium constant etc.