Q versus K

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Gabriella Bates 2L
Posts: 113
Joined: Thu Jul 11, 2019 12:15 am

Q versus K

Postby Gabriella Bates 2L » Thu Jan 09, 2020 1:07 pm

Can someone explain the different between K and Q? How can Q be used to tell us the direction the reaction will proceed? Thanks in advance!

Phuong Tran 1G
Posts: 33
Joined: Sat Sep 14, 2019 12:16 am

Re: Q versus K

Postby Phuong Tran 1G » Thu Jan 09, 2020 1:27 pm

Q is the reaction quotient and has the same formula as K. Q would be equal to the concentrations of the product over the concentrations of the reactants. However, unlike K, Q can be used even when the equation isn’t in equilibrium. You then use Q and compare it to K to see how far the reaction is from equilibrium.

If Q>K, there is more product than at equilibrium (or not enough reactant) and so the reaction would proceed in the reverse direction

If Q=K, the equation is already at equilibrium

If Q<K, there is less product than at equilibrium and so the reaction would proceed forward.

Andrew Liang 1I
Posts: 105
Joined: Fri Aug 30, 2019 12:18 am

Re: Q versus K

Postby Andrew Liang 1I » Thu Jan 09, 2020 2:02 pm

We calculate Q and K the same way (product/reactant). However we use Q when we are unsure whether the chemical reaction is at equilibrium yet. If Q is greater than K then it means that there is still more product that need to be reacted; therefore, it is a reverse reaction going from product to reactants. On the other hand, if Q is smaller than K then it means that there is still more reactants that need to be reacted; therefore it is a forward reaction going from reactants to product. Last scenario, if Q is equal to K then it means that the reaction has reached equilibrium.

Caroline Beecher 2H
Posts: 51
Joined: Wed Nov 14, 2018 12:21 am

Re: Q versus K

Postby Caroline Beecher 2H » Thu Jan 09, 2020 2:45 pm

An example of using Q in comparison to K we did as an example in class with the reaction: N2O4(g) == 2NO2(g). Partial pressures of the two gases were given, and the goal was to see if the system was at equilibrium at this specific point in the reaction. The Qp ended up being less than the Kp, meaning at that specific time the reaction was not in equilibrium and would therefore proceed toward the products (to the right).

Cooper Baddley 1F
Posts: 100
Joined: Wed Sep 18, 2019 12:19 am

Re: Q versus K

Postby Cooper Baddley 1F » Thu Jan 09, 2020 2:46 pm

K can only be taken when the reaction is at equilibrium while Q can be taken at any point and through comparing them we can see where the reaction is overall. Other than that they use the same formula and rules.

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