Standard State Conditions and Reaction Quotient

[cwpatter]

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[Blueprint MCAT Tutor]

We're told that the reaction quotient for any reaction under standard state conditions is one. Meaning there must be an equal amount of products and reactants.

Where did you read this? I find this really confusing and want to get some context.

Obviously it's not going to be at equilibrium unless you just happen to have a reaction whose Keq is exactly equal to 1.

I mean I think the answer is that your first question is right - we just throw everything in a beaker, set the conditions to "standard state" and then let 'er rip.

 

[inasensegone]

In standard state conditions we're told that everything (products and reactants) has a molarity of 1.

We're told that the reaction quotient for any reaction under standard state conditions is one. Meaning there must be an equal amount of products and reactants.

But at what point in the reaction are we taking this reaction quotient? When it's at equilibrium? Before it's even started?

Do we just throw a 1 molar concentration of products and reactants in a pot at 298K and 1atm and say STANDARD STATE?

Or do we get that 1 molar concentration of both AFTER the reaction reaches equilibrium?

Where did you read this? I find this really confusing and want to get some context.

Obviously it's not going to be at equilibrium unless you just happen to have a reaction whose Keq is exactly equal to 1.

I mean I think the answer is that your first question is right - we just throw everything in a beaker, set the conditions to "standard state" and then let 'er rip.

What the OP appears to be talking about is the Standard Free Energy Change... and yes what Bryan said at the end of his post is basically what is going on here:

The conditions are: 1 molar concentration of all solute species, including reactants and products, and 1 atmosphere of pressure, and 25 degrees C.

You do this so the reaction quotient is equal to 1 and so basically, you get the above equation from the following equation (or perhaps vice versa but the 2 are related):

When the reaction quotient is equal to 1, by the rules of logs, you get the 1st equation from the 2nd.

So what the 1st equation basically tells you is if the reaction is spontaneous or not. After you dump 1 molar of reactants and products into your environment, will they proceed in the forward or reverse direction? Think of it as a reference value for a given reaction. Is it spontaneous or not? If the Keq is less than 1, the natural log is a negative number and you get a positive standard free energy change - the reaction is non-spontaneous. If the Keq is greater than 1, the natural log is a positive number and you get a negative standard free energy change - the reaction is spontaneous. This makes sense because the Keq will be less than 1 when reactants exceed products at equilibrium, and the Keq will be greater than 1 when products exceed reactants at equilibrium.

 

[inasensegone]

Awesome explanation, the concept is crystal clear now - thanks!

You're welcome! Glad it helped.