Definition of reaction quotient Q, and how it is used to predict the direction of reaction
What is Q?
The reaction quotient
is a measure of the relative amounts of products and reactants present in a reaction at a given time.
For reversible reaction
, where , , , and are the stoichiometric coefficients for the balanced reaction, we can calculate using the following equation:
This expression might look awfully familiar, because
is a concept that is closely related to the equilibrium constant . Unlike , which is based on equilibrium concentrations, can be calculated whether we are at equilibrium or not.
The magnitude of
tells us what we have in our reaction vessel. What does that mean exactly? Let's start by thinking about the extremes. For a reaction that has only starting materials, the product concentrations are . Since our numerator is zero, then . For a reaction that has only products, we have in the denominator of our equation, so is infinitely large. Most of the time, we will have some mixture of reactants and products, but you can still remember that very small values of tell you that you have mostly reactants and very large values of result from having mostly products in the reaction vessel.
and for a given reaction tells us which direction the reaction needs to go to reach equilibrium. You can think of this as another way to use Le Châtelier’s principle.
Using Q to predict the direction of reaction
From Le Châtelier’s principle, we know that when a stress is applied that moves a reaction away from equilibrium, the reaction will try to adjust to get back to equilbrium. By comparing
and , we can see how our reaction is adjusting—is it trying to make more product, or is it consuming product to make more reactant? Alternatively, are we at equilibrium already?
There are three possible scenarios to consider:
Let’s think back to our expression for
above. We have our product concentrations, or partial pressures, in the numerator and our reactant concentrations, or partial pressures, in the denominator. In the case where , this suggests that we have more product present than we would have at equilibrium. Therefore, the reaction will try to use up some of the excess product and favor the reverse reaction to reach equilibrium.
In this case, the ratio of products to reactants is less than that for the system at equilibrium. In other words, the concentration of the reactants is higher than it would be at equilibrium; you can also think of it as the product concentration being too low. In order to reach equilibrium, the reaction will favor the forward reaction and try to use up some of the excess reactant to make more product.
Hooray! The reaction is already at equilibrium! Our concentrations won't change since the rates of the forward and backward reactions are equal.
We know that
can have possible values starting from zero (all reactants) to infinitely large (all products). We also know that our reaction will adjust the concentrations to reach equilibrium if it isn't at equilibrium already. Another way we can think about these ideas is to draw out a number line for all possible values of :
To simplify things a bit, the line can be roughly divided into three regions. For very small values of
, ~ or less, the reaction has mostly reactants. For intermediate values of , between ~ and , we have significant amounts of both products and reactants in our reaction vessel. Finally, when is large, greater than ~ , we have mostly products.
If we plot both
and on our number line, the direction we move to get from to tells us about how the reaction is trying to adjust. If we are moving to the right, we are shifting the concentrations to make more products and favoring the forward reaction. If we are moving to the left toward zero, we are moving in the direction of making more reactants and favoring the reverse reaction.
Given the following concentrations, what is
, which side of the reaction is favored at that value of ?
We can calculate
by writing out the equation using the balanced reaction and then using the given concentrations.
If we compare
to , we can see that . This tells us that we have excess product compared to equililbrium and therefore the reverse reaction will be favored.
If we draw out the number line with our values of
and , we get something like this:
We can see that
falls near the region where we have mostly products, which is to the right of . Since the reaction will adjust to move closer to , we can draw an arrow for the direction of that shift. This arrow starts at and points toward , and it also points to the mostly reactants region. This tells us that our reaction will be favoring the reverse reaction in order to make more reactants and consume excess products.
As you can see, both methods give the same answer, so you can decide which one works best for you!
We can compare the reaction quotient
to the equilibrium constant to predict what a reaction will do to reach equilibrium. In addition, you might see pop up in other chemistry topics and equations because we are often interested in what happens to various thermodynamic quantities when we are not at equilibrium. Stay tuned for more!
Want to join the conversation?
- I'm confused with the difference between K and Q. I'm sorry if this is a stupid question but I just can't see the difference. How can you have a K value of 1 and then get a Q value of anything else than 1?(29 votes)
- K is the equilibrium constant. Therefore K is revealing the amount of products to reactants that there should be when the reaction is at equilibrium. Q is used to determine whether or not the reaction is at an equilibrium. At any given point, the reaction may or may not be at equilibrium. By calculating Q (products/reactants), you can compare it to the K value (products/reactants AT EQUILIBRIUM) to see if the reaction is at equilibrium or not. If Q=K, the reaction is at equilibrium.(73 votes)
- Can i get help on how to do the table method when finding the equilibrium constant(10 votes)
Try this website if you need help with ICE tables and calculating the equilibrium constant K. I don't think Khan Academy has any videos on ICE tables but I am not sure; if they do not, then I will try to get some ICE table videos in. It is a fundamental topic of chemistry and, in my opinion, just as important as stoichiometry.(14 votes)
- when setting up an ICE chart where and how do you decide which will be -x and which will be x?(6 votes)
- if the reaction will shift to the right, then the reactants are -x and the products are +x.
If the reaction will shift left, do the opposite.(13 votes)
- in the example shown, I'm a little confused as to how the 15M from the products was calculated.(3 votes)
- How can we identify products and reactants? For example, in the reactions: 2HI <=> H2 plus I2 and H2 plus I2 <=> 2HI, the values of Q differ. In the section "Visualizing Q," the initial values of Q depend on whether initially the reaction is all products, or all reactants. Can't we just assume them to be always all reactants, as definition-wise, reactants react to give products?(6 votes)
- As you say, it's a matter of definition.
The arrow points away from the reactants and towards the products.(1 vote)
- in the above example how do we calculate the value of K or Q ? As in how is it
1.0M or 15M?(4 votes)
- Would adding excess reactant effect the value of the equilibrium constant or the reaction quotient? or neither? or both?(4 votes)
- The equilibrium constant of a chemical reaction is the value of the reaction quotient when the reaction has reached equilibrium. An equilibrium constant value is independent of the analytical concentrations of the reactant and product species in a mixture, but depends on temperature and on ionic strength.(1 vote)
- How is the Reaction Constant (Q) affected by change in temperature, volume and pressure ?(4 votes)
- the reaction quotient is affected by factors just the same way it affects the rate of reaction(0 votes)
- This is a little off-topic, but how do you know when you use the 5% rule? Cause I'm not sure when I can actually use it.(1 vote)
- You use the 5% rule when using an ice table. When you plug in your x's and stuff like that in your K equation, you might notice a concentration with (2.0-x) or whatever value instead of 2.0. If the K value given is extremely small (something time ten to the negative exponent), you can elimintate the minus x in that concentration, because that change is so small it does not matter. After finding x, you multiply 0.05 to the 2.0 from 2.0-x and compare that value with what you found for x. If x is smaller than 0.05(2.0), then you're good to go!(5 votes)
- why shouldn't K or Q contain pure liquids or pure solids? and isn't hydrofluoric acid a pure liquid coz i remember Sal using it in the video of Heterogenous equilibrium so why did he use it?(1 vote)
- The equilibrium constant is a ratio of the concentration of the products to the concentration of the reactants. Keyword- concentration. While gas changes concentration after the reaction, solids and liquids do not (the way they are consumed only affects amount of molecules in the substance). Concentration of the molecule in the substance is always constant. For hydrofluoric acid, it is an aqueous solution, not a liquid, therefore it is dissolved in water (concentration can change - moles per unit volume of water).(5 votes)