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Effect of Pressure

Discusses how pressure affects equilibrium direction in systems involving gaseous components.

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Effect of Pressure

Ammonia storage tanks are under high pressure to prevent the ammonia from decomposing

Credit: Bill Whittaker (User:Billwhittaker/Wikipedia)
Source: http://commons.wikimedia.org/wiki/File:Meth_ammonia_tank_Otley_iowa.JPG
License: CC BY-NC 3.0

What is the dual purpose here?

The ammonia storage tank in the picture above does two things, One it stores ammonia at high pressure to minimize the reverse reaction that would lead to less ammonia and more nitrogen and hydrogen. Secondly, it sends an important message. Ammonia is used to make methamphetamine, a dangerous drug of abuse. Locks and other safety mechanisms built into the tanks help stop the theft of ammonia to be used in this illicit activity.

Effect of Pressure

Changing the pressure of an equilibrium system in which gases are involved is also a stress to the system. A change in the pressure on a liquid or a solid has a negligible effect. We will return again to the equilibrium for the Haber-Bosch process. Imagine the gases are contained in a closed system in which the volume of the system is controlled by an adjustable piston as shown in Figure below.

Compressing a mixture of nitrogen and hydrogen creates more ammonia

Credit: CK-12 Foundation - Zachary Wilson
License: CC BY-NC 3.0

Effect of pressure on ammonia formation.[Figure2]

On the far left, the reaction system contains primarily N2 and H2, with only one molecule of NH3 present. As the piston is pushed inwards, the pressure of the system increases according to Boyle’s Law. This is a stress to the equilibrium. In the middle image, the same number of molecules is now confined in a smaller space and so the pressure has increased. According to LeChâtelier’s principle, the system responds in order to relieve the stress. In the image on the right, the forward reaction has been favored, in which one molecule of N2 combines with three molecules of H2 to form two molecules of NH3. The overall result is a decrease in the number of gas molecules in the entire system. This in turn decreases the pressure and provides a relief to the original stress of a pressure increase. An increase in pressure on an equilibrium system favors the reaction which produces fewer total moles of gas. In this case, it is the forward reaction that is favored.

A decrease in pressure on the above system could be achieved by pulling the piston outward, increasing the container volume. The equilibrium would respond by favoring the reverse reaction in which NH3 decomposes to N2 and H2. This is because the overall number of gas molecules would increases and so would the pressure. A decrease in pressure on an equilibrium system favors the reaction which produces more total moles of gas. This is summarized in Table below.

Stress Response
pressure increase reaction produces fewer gas molecules
pressure decrease reaction produces more gas molecules

Like changes in concentration, the \begin{align*}K_{eq}\end{align*} value for a given reaction is unchanged by a change in pressure.

It is important to remember when analyzing the effect of a pressure change on equilibrium that only gases are affected. If a certain reaction involves liquids or solids, they should be ignored. Calcium carbonate decomposes according to the equilibrium reaction:

\begin{align*}\text{CaCO}_3(s) \rightleftarrows \text{CaO}(s)+\text{O}_2(g)\end{align*}

Oxygen is the only gas in the system. An increase in the pressure of the system slows the rate of decomposition of CaCO3 because the reverse reaction is favored. When a system contains equal moles of gas on both sides of the equation, pressure has no effect on the equilibrium position, as in the formation of HCl from H2 and Cl2.

\begin{align*}\text{H}_2(g)+\text{Cl}_2(g) \rightleftarrows 2\text{HCl}(g)\end{align*}


  • The influence of pressure on an equilibrium system is described.


  1. Why does the pressure increase in the middle picture?
  2. How is the stress due to increased pressure relieved?
  3. Why does an increase in pressure slow the rate of CaCO3 decomposition?

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Image Attributions

  1. [1]^ Credit: Bill Whittaker (User:Billwhittaker/Wikipedia); Source: http://commons.wikimedia.org/wiki/File:Meth_ammonia_tank_Otley_iowa.JPG; License: CC BY-NC 3.0
  2. [2]^ Credit: CK-12 Foundation - Zachary Wilson; License: CC BY-NC 3.0

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