Q value with species in different states

In summary, the conversation discusses how to set up the Q equation for the half reaction 2H2O(l) → O2(g) + 4H+(aq) + 4e-. It is mentioned that Q can be calculated using the partial pressure of oxygen in bars and concentration in M, and that there is no issue with mixing states. It is also mentioned that Q is different from K, the equilibrium constant, and is useful in explaining situations where Q does not equal K. The use of bars as a unit for pressure is also discussed and a source from 1982 is referenced. The conversation ends with a request for a source on the topic.
  • #1
MathewsMD
433
7
Given the half reaction:

2H2O(l) → O2(g) + 4H+(aq) + 4e-

How would you set up the Q equation for this reactions?

Would it be Q = [H+]4 only or do we assume the partial pressure of oxygen gas is 1 atm? Is there a general way to write the Q value for species in different states?
 
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  • #2
Typically we will just express the partial pressure of the oxygen in bars and concentration in M. No problem with mixing states, as technically these are just approximations of the activity.
 
  • #3
Borek said:
Typically we will just express the partial pressure of the oxygen in bars and concentration in M. No problem with mixing states, as technically these are just approximations of the activity.

Oh really? My class didn't delve too deeply into fugacity, but I was told when assigning partial pressure values in the Q equation, to ensure units were always in atm. I wasn't told exactly why, but why do you say to use bars? Do you mind referring me to a derivation or reasoning perhaps?

Also, in my class, the professor completed an example where he solved for Q but omitted the gaseous species involved since it was a half-reaction in a voltaic cell. It was not at standard conditions, though. Is it reasonable to assume the partial pressure was 1atm, though?
 
  • #4
What is Q? I only see people talking about Q recently in this forum? Is there a new book which decided to use Q instead of K?
 
  • #5
Q is the reaction quotient, K is the equilibrium constant. I found it is convenient to treat them separately, as they are not always identical.
 
  • #6
MathewsMD said:
Oh really? My class didn't delve too deeply into fugacity, but I was told when assigning partial pressure values in the Q equation, to ensure units were always in atm. I wasn't told exactly why, but why do you say to use bars?

This is just a convention. Pressure should be in bar, since 1982. atm was an earlier standard.

Sorry, there are some problems with IUPAC servers and I can't trace it right now to any acceptable source, have it just on disk in a private discussion with people I trust.
 
  • #7
Borek said:
Q is the reaction quotient, K is the equilibrium constant. I found it is convenient to treat them separately, as they are not always identical.
Ok, reaction quotient was new to me as a concept. I still don't see what it is good for. What I want to say is that the Nernst equation is derived for an electrochemical equilibrium situation, so Q=K.
 
  • #8
When you mix some slowly reacting reagents Q doesn't equal K - it gets there, but slowly. Without introducing Q it is difficult to explain how come we have a solution in which... K doesn't equal K? But you can easily say Q doesn't equal K and the situation is clear.
 
  • #9
Borek said:
This is just a convention. Pressure should be in bar, since 1982. atm was an earlier standard.

Sorry, there are some problems with IUPAC servers and I can't trace it right now to any acceptable source, have it just on disk in a private discussion with people I trust.

Okay, thanks for the help. If you ever do find the source you could link me to, that would be great!
 

Related to Q value with species in different states

1. What is Q value with species in different states?

The Q value with species in different states refers to the amount of energy released or absorbed during a nuclear reaction when the reactants and products are in different nuclear states. It is a measure of the stability of a nuclear reaction.

2. How is Q value with species in different states calculated?

The Q value with species in different states can be calculated using the following equation: Q = (Mass of reactants - Mass of products) x c^2, where c is the speed of light. This equation takes into account both the mass and energy of the particles involved in the reaction.

3. How does Q value with species in different states affect nuclear reactions?

The Q value with species in different states plays a crucial role in determining whether a nuclear reaction is exothermic (releasing energy) or endothermic (absorbing energy). A higher Q value indicates a more stable reaction, while a lower Q value indicates a less stable reaction.

4. Can Q value with species in different states be negative?

Yes, the Q value with species in different states can be negative. This means that the reactants have a higher mass than the products, indicating an endothermic reaction where energy is absorbed instead of released.

5. What factors can affect the Q value with species in different states?

The Q value with species in different states can be affected by changes in mass of the particles involved in the reaction, as well as changes in the nuclear states of the reactants and products. Additionally, the presence of other particles or external factors, such as temperature, can also impact the Q value.

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