How Do You Calculate Kc and Kp for a Reversible Gas Reaction?

In summary, the conversation discusses finding the equilibrium concentrations and equilibrium constant for the reaction PCl5 --> PCl3 + Cl2 at 500K and in a 5.000L container with an initial concentration of 1.000 mol PCl5. The equilibrium mixture contains 78.50% PCl5 and the remaining percentage is made up of PCl3 and Cl2. The given information is used to find the equilibrium constant, Kc, and the equilibrium concentrations of the reactants and products. The attempt at a solution involves using the molarity of the reactant (.2) and multiplying it by .785 to get the percent at equilibrium, but there is a mistake in not accounting for the remaining percentage in the
  • #1

Homework Statement

When 1.000 mol PCl5 is intorduced into a 5.000L container @500K, 78.50% of PCl5 dissociates to give equilibrium mixture PCl5, PCl3, Cl2 : PCl5--->PCl3+Cl2 (this eq. is reversible)

(a) calculate Kc and Kp
(b) if the initial concentrations are [PCl5]=.500M, [PCl3]=.150M, and [Cl2=.600], in which direction does the rx proceed to reach equilibrium? what are the concentrations when the mixture reaches equilibrium?

Homework Equations


The Attempt at a Solution

I attempted to find Kc of the initial problem, by getting the molarity (.2) of the reactant. I then multiplied by .785 to get the percent to put in the numerator, but am not getting the right answer in the book. I know in part b i must use the chart to convert to quadratic equation, but am having trouble with the first part (surprisingly, as its usually the second part which gives me trouble). I have tried this either which way and still can not get the right answer. Any input would be great. Thanks!
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  • #2
Not seeing all the numbers it is hard to say - you can be missing something, it can be just a math error. Please show your complete work, step by step.
  • #3
Yeah turns out I had the right idea, but didn't do it right. I went and saw my professor and he walked me through it. I had assumed that the .2M stayed the same for the PCl5, however, when the equation went to equilibrium, I had forgotten to use the 21.5% that was left, as the PCl5.

.2M *.785 gives .157, then .2-.157 gives .043, which is what I should have used for the PCl5 @ equilibrium, rather than the .2. Thanks for the input. Glad to know that people actually respond and are willing to help out.

Related to How Do You Calculate Kc and Kp for a Reversible Gas Reaction?

What is chemical equilibrium with gases?

Chemical equilibrium with gases is a state where the concentration of reactants and products in a closed system remain constant over time. This means that the rate of the forward reaction is equal to the rate of the reverse reaction, resulting in no overall change in the concentrations of substances involved.

How is equilibrium constant (K) calculated for a gas phase reaction?

The equilibrium constant (K) for a gas phase reaction is calculated by taking the ratio of the products' partial pressures to the reactants' partial pressures, where each pressure is raised to the power of its coefficient in the balanced chemical equation. This can also be written in terms of concentration, using the ideal gas law.

Why do changes in pressure affect equilibrium with gases?

According to Le Chatelier's principle, changes in pressure can affect equilibrium with gases because gases are compressible and changes in pressure can alter their volume and therefore their concentration. An increase in pressure will shift the equilibrium towards the side with fewer moles of gas, while a decrease in pressure will shift it towards the side with more moles of gas.

What is the effect of temperature on equilibrium with gases?

Changes in temperature can affect equilibrium with gases because it can alter the direction and extent of the reaction. An exothermic reaction will favor the reverse reaction at higher temperatures, while an endothermic reaction will favor the forward reaction. Additionally, an increase in temperature will increase the rate of reaction, which can also impact equilibrium.

How can catalysts impact equilibrium with gases?

Catalysts do not affect the position of equilibrium in a gas phase reaction, but they can speed up the rate of both the forward and reverse reactions equally. This means that the system will reach equilibrium faster, but the equilibrium concentrations of the reactants and products will remain the same.

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