# Standard free energy change graph

#### Halcyon32

2+ Year Member

How does this graph show standard free energy change throughout if the standard state is at 25 degrees C? I'm guessing my understanding of standard state is incorrect?

#### popopopop

7+ Year Member
Standard state for these thermo problem uses 25C and 1 atm and equal amounts of reactants/products. The temperature will fluctuate so you predict direction of deltaG in the formula deltaG = H - TS. Is that what you're asking?

OP
H

#### Halcyon32

2+ Year Member
Standard state for these thermo problem uses 25C and 1 atm and equal amounts of reactants/products. The temperature will fluctuate so you predict direction of deltaG in the formula deltaG = H - TS. Is that what you're asking?
Right, so if the temperature fluctuates that means the reaction is not at standard state, say at 250C but the question says that the graph shows standard state, implying all points on the graph are at standard state. That's what I don't understand

#### popopopop

7+ Year Member
Ahhh, it's probably just a phrasing thing in the title. Temperature is just an accepted arbitrary value (25C) in this case, so we can measure the free energy in a "standard" state, as a reference point. So I guess it's standard state in the case of pressure and concentration. From my understanding, I think it's still considered "standard state" even if we fluctuate temperature, BUT we have to keep both temperature and atmosphere constant. It makes sense to me because deltaG is a state function and measures max amount of work in a system. That's why we have accepted deltaG for certain reactions and know if it's spontaneous or no.

Wiki says for standard state, "Strictly speaking, temperature is not part of the definition of a standard state. For example, as discussed below, the standard state of a gas is conventionally chosen to be unit pressure (usually in bar) ideal gas, regardless of the temperature. However, most tables of thermodynamic quantities are compiled at specific temperatures, most commonly 298.15 K (25.00 °C; 77.00 °F) or, somewhat less commonly, 273.15 K (0.00 °C; 32.00 °F)."

Other sources seems to support the above too, so looks like the pressure is what's standard state?

OP
H

#### Halcyon32

2+ Year Member
Ahhh, it's probably just a phrasing thing in the title. Temperature is just an accepted arbitrary value (25C) in this case, so we can measure the free energy in a "standard" state, as a reference point. So I guess it's standard state in the case of pressure and concentration. From my understanding, I think it's still considered "standard state" even if we fluctuate temperature, BUT we have to keep both temperature and atmosphere constant. It makes sense to me because deltaG is a state function and measures max amount of work in a system. That's why we have accepted deltaG for certain reactions and know if it's spontaneous or no.

Wiki says for standard state, "Strictly speaking, temperature is not part of the definition of a standard state. For example, as discussed below, the standard state of a gas is conventionally chosen to be unit pressure (usually in bar) ideal gas, regardless of the temperature. However, most tables of thermodynamic quantities are compiled at specific temperatures, most commonly 298.15 K (25.00 °C; 77.00 °F) or, somewhat less commonly, 273.15 K (0.00 °C; 32.00 °F)."

Other sources seems to support the above too, so looks like the pressure is what's standard state?
Ahhh ok then that makes a ton of sense. Also, would you mind explaining to me the concept of what you meant when you said delta G measures max amount of work in a system? I always had trouble understanding exactly how it worked and what it measured. How does it indicate the max amount of work?