Equilibrium constant changes???

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NeverBackDown

Full Member
10+ Year Member
So while doing rate law equations, I've noticed that certain questions ask how the Keq constant is affected by changing the Temp(or pressure) of a (i'm assuming isolated) system.

Generally, I get these questions wrong because in my head Keq doesn't change because it is a constant for a given equation at STP or Standard conditions....and the rate constant used in any reaction not at these conditions is the reaction quotient Q not K.

What am i not getting? Should I assume once the conditions change the reaction quotient Q is still called K on the actual test?

thanks

fyi the particular kaplan Qbank question i'm talking about said Keq rises with increasing temp.

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Just stumbled upon the answer to my question...

First off changing pressure cannot be used to predict how the Keq will change.

The Keq doesn't have to be at 273K, you can change the temp... what you have to know for the MCAT is the general trend as you increase temp, the Keq, Ksp, Kp, will increase.

Reaction quotient is used as the reaction moves away from the eq position for the given Keq value. For example if you are given a Keq value which is valid for a specific reaction at 49K and the temp increases to 273K, the reaction constant at 273K would actually be the reaction quotient, not the Keq because the Keq is only valid at the initial temp of 49K. Confusing...lol

Just stumbled upon the answer to my question...

First off changing pressure cannot be used to predict how the Keq will change.

The Keq doesn't have to be at 273K, you can change the temp... what you have to know for the MCAT is the general trend as you increase temp, the Keq, Ksp, Kp, will increase.

Reaction quotient is used as the reaction moves away from the eq position for the given Keq value. For example if you are given a Keq value which is valid for a specific reaction at 49K and the temp increases to 273K, the reaction constant at 273K would actually be the reaction quotient, not the Keq because the Keq is only valid at the initial temp of 49K. Confusing...lol

I don't think so. Remember Le Chatelier's principle? Increasing temperature pushes a reaction in endothermic way and decreasing temperature pushes a reaction in exothermic way. Depending on which way the reaction is pushed, Keq can increase or decrease.

Yea, to clarify. Increasing temp increases the Keq and decreasing temp lowers Keq.

Changing temp and adding/releasing heat are two different things. Endothermic reactions are the adding of heat. Exothermic reactions are the release of heat.

If a reaction release heat, ie exothermic, and you heat the reaction up, you DO NOT INCREASE Keq in the forward written way. Go back and read Le Chatliers principle.