Can you make a solution under standard conditions?

In summary, the standard state in thermodynamics is a hypothetical reference state that is often used in calculations and experiments. It is typically determined by taking the limit of a certain parameter, such as activity or concentration. In order to measure properties of solutions under standard conditions, one must conduct experiments at various concentrations and extrapolate to the standard state. This can be done using specialized calorimeters that measure the heat of solution.
  • #1
Confusus
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This is coming from junior-level physical chemistry.

Lots of thermodynamic properties of reactions involving solutions are tabulated under standard conditions; I'm particularly thinking of standard half-cell voltages, but any property will do.

How do you make, say, a sodium chloride solution under those conditions?

My best understanding is that you can't. The definition of the standard state of both solute and solvent specifies (1) the pure solvent, which isn't in the sample, and (2) the solute with some hypothetical properties in the infinitely-dilute limit (as determined by the Henry's law constant).

So if my understanding here is correct, what does a standard state property involving solutions even mean? How would you make the solution(s) and measure that property?
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To be a little more concrete, think about it in terms of the equation for chemical potential: [tex] \mu_i = \mu_i^° + RT \ln a_i[/tex]
to get standard conditions the solute must have activity = 1. The formula for activity breaks down to [tex] a_i = \gamma_i c_i/c^° [/tex]
Its common to say "the standard concentration c° = 1 M" but you see in this equation that the nonideality plays a role in the activity coefficient, making the activity NOT unity, and the chemical potential not the standard value.

So what's going on?
 
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  • #2
The standard state is a reference state, and is typically considered to be a hypothetical one that is arrived upon after taking the limit of a certain parameter. For example, the IUPAC definition of standard states includes that, for gases, the standard state of a gas is determined as if it is an ideal gas, which as we know is an approximation to the behavior of real gases.

So, practically speaking, you want to measure the properties of a number of solutions where one varies one of the parameters in order to extrapolate/calculate the standard state. As the aspiring comedians will note, the goal is to keep you from dividing by zero in any calculation one may ever have to do.
 
  • #3
Thanks Mike H, that was a good reminder of something I knew/should have known. I get confused thinking about it each time it comes up.

Can you be a little more concrete? Let's say I wanted to measure the standard enthalpy change of dissolving sucrose in water. The standard states of pure water and pure sucrose are straight-forward. But the solution, what is its "standard state"? If I have this right, it is an imaginary sample where you have pure water in one beaker and an imaginary solution that obeys Henry's law (as if the solute were infinitely dilute) which defines its activity coefficient but at Molarity of the reciprocal of its activity coefficient, so that the solute has activity=1.

I still have a hard time wrapping my head around what series of experiments would lead to measuring the standard enthalpy of solution of sucrose, even if just in the limit of several related experiments as concentration changes.

Can anyone make that last point more clear?

Thanks again, Mike H
 
  • #4
I'm not sure just how "concrete" a hypothetical state can be, given that one arrives at it by calculation, so I'm not sure if I can help out there without confusing the matter even more. As I said, it's an idealized notion which is intended to serve as a baseline of sorts when doing thermodynamic calculations. For example, the standard state quantities one sees in a standard general chemistry or physical chemistry textbook are most likely the ones at 298.15 K and 101.325 kPa (or thereabouts) - someone doing work in geochemistry is probably going to need the standard state quantities for salts at much higher temperatures & pressures.

But I can tell you how people measure these standard state thermodynamic quantities. To take your example of measuring enthalpies of solution...

There are these fairly cool calorimeters that let you place a known amount of compound in a bulb, submerse the bulb in solvent, and then trigger the bulb's breakage, and then the compound dissolves, and one measures the heat of solution from the compound dissolving in your solvent. One needs to correct for the volume change of the bulb breaking, any heat evolved from the bulb breakage, the change in vapor pressure, and so on. But once you've measured the heat of solution at your chosen concentrations, one can then calculate the enthalpy of solution.
 
  • #5


I can confirm that it is not possible to make a solution under standard conditions. The definition of standard conditions for a solution involves the hypothetical properties of the solute in the infinitely-dilute limit, which is not achievable in a real solution. Therefore, any solution made under real conditions will deviate from the standard state and the properties measured will not be truly under standard conditions.

In terms of making a sodium chloride solution under standard conditions, it is not possible to do so because the standard state of the solute would require an infinitely-dilute solution of sodium chloride, which is not physically attainable.

The concept of standard state properties for solutions is a theoretical construct that allows for comparison and prediction of thermodynamic properties. It is not a real state that can be achieved in a laboratory setting.

In order to measure standard state properties involving solutions, scientists typically use extrapolation techniques to estimate the properties at the standard state from experimental data obtained at non-standard conditions. This allows for the comparison of different solutions under a standardized set of conditions.

Overall, the concept of standard conditions for solutions is a useful tool in physical chemistry, but it is important to understand that it is not a real state that can be achieved in a laboratory setting.
 

1. What are standard conditions?

Standard conditions refer to a set of specific environmental and experimental conditions that are commonly used as a baseline for scientific experiments. These conditions include a temperature of 25 degrees Celsius, a pressure of 1 atmosphere, and a concentration of 1 mole per liter.

2. Why is it important to make a solution under standard conditions?

Making a solution under standard conditions allows for a more accurate and reliable comparison between different experiments. It ensures that the results obtained are not affected by variations in environmental conditions and can be replicated by other scientists.

3. How do you prepare a solution under standard conditions?

To prepare a solution under standard conditions, the solute (substance being dissolved) and solvent (liquid medium) are measured and mixed at a specific temperature and pressure. The solution is then diluted to a concentration of 1 mole per liter.

4. Can you make a solution under standard conditions for any type of experiment?

No, not all experiments require solutions to be made under standard conditions. Some experiments may have specific conditions that need to be maintained in order to obtain accurate results. It is important to understand the requirements of each experiment before preparing a solution under standard conditions.

5. Are there any exceptions to the standard conditions?

Yes, there are some situations where standard conditions may not be applicable. For example, experiments involving extreme temperatures or pressures may require different standard conditions. It is important to consider the specific needs of each experiment when determining the appropriate conditions for making a solution.

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