Question

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?

Answer 1

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.