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.

Explanation:

Usually:

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.