Thanks, this helped a lot. It's very easy to understand.

<h2>The effect of pressure on equilibrium</h2>
If the pressure of a gaseous reaction <a data-lesson="mixtures" data-subject="chemistry" data-tutorial="classification-of-matter" href="https://app.ulearngo.com/chemistry/classification-of-matter/mixtures">mixture</a> is changed the equilibrium will shift to minimise that change.
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If the pressure is increased the equilibrium will shift to favour a decrease in pressure.
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If the pressure is decreased the equilibrium will shift to favour an increase in pressure.
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</ul>
When the volume of a system is decreased (and the temperature is constant), the pressure will increase. There are more collisions with the walls of the container. If there are fewer gas molecules there will be fewer collisions, and therefore lower pressure. The equilibrium will shift in a direction that reduces the number of gas molecules so that the pressure is also reduced. So, to predict in which direction the equilibrium will shift to <a data-lesson="chemistry-104-summary-1" data-subject="chemistry" data-tutorial="physical-and-chemical-change" href="https://app.ulearngo.com/chemistry/physical-and-chemical-change/chemistry-104-summary-1">change</a> pressure you need to \(\color{darkgreen}{\text{look at the number of gas molecules in the balanced}}\) \(\color{darkgreen}{\text{reactions}}\).
<div class="ns-school-defn2">
<h4>Tip:</h4>
Recall that:
\[\text{p} \propto \dfrac{\text{T}}{\text{V}}\]
That is, if the temperature remains constant, and the volume is increased, the pressure will decrease.
<img alt="76077fbc630b44164944dec819bd878c.png" src="https://data.ulearngo.com/assets/c24ab917-a349-4684-ba03-16bad98d7029"/>
The figure below shows a decrease in pressure by an increase in the volume, and an increase in pressure by a decrease in the volume.
</div>
For example, the equation for the reaction between <a data-lesson="occurrence-preparation-and-properties-of-nitrogen" data-subject="chemistry" data-tutorial="metals-metalloids-nonmetals" href="https://app.ulearngo.com/chemistry/metals-metalloids-nonmetals/occurrence-preparation-and-properties-of-nitrogen">nitrogen</a> and <a data-lesson="hydrogen-reactions" data-subject="chemistry" data-tutorial="metals-metalloids-nonmetals" href="https://app.ulearngo.com/chemistry/metals-metalloids-nonmetals/hydrogen-reactions">hydrogen</a> is shown below:
\(\color{blue}{\text{N}_{2}\text{(g)}} + \color{blue}{\text{3H}_{2}\text{(g)}} \leftrightharpoons \color{red}{\text{2NH}_{3}\text{(g)}}\)
The ratio in the balanced equation is \(1:3:2\). That is, for every \(\text{1}\) \(\text{molecule}\) of \(\color{blue}{\text{N}_{2}\text{ gas}}\) there are \(\text{3}\) \(\text{molecules}\) of \(\color{blue}{\text{H}_{2}\text{ gas}}\) and \(\text{2}\) \(\text{molecules}\) of \(\color{red}{\text{NH}_{3}\text{ gas}}\) (from the balanced equation). Therefore the ratio is \(\color{blue}{\textbf{4 molecules of reactant gas}}\) to \(\color{red}{\textbf{2 molecules of product gas}}\).
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An increase in pressure will:
<ul style="list-style-type: square;">
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Favour the reaction that decreases the number of gas molecules.
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There are \(\color{red}{\textbf{fewer molecules of product gas}}\) than reactant gas, so the forward reaction is favoured.
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The equilibrium will shift to the right and the yield of \(\color{red}{\textbf{NH}_{3}}\) will increase.
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</ul>
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A decrease in pressure will:
<ul style="list-style-type: square;">
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Favour the reaction that increases the number of gas molecules.
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There are \(\color{blue}{\textbf{more molecules of reactant gas}}\), so the reverse reaction is favoured.
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The equilibrium will shift to the left and the yield of \(\color{red}{\textbf{NH}_{3}}\) will decrease.
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</ul>
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</ul>
Consider illustration in the figure below.
<div class="wp-caption alignnone" style="width: 754px"><img alt="44cf1731fd60e4b9e8825d77dffc53fa.png" height="813" src="https://data.ulearngo.com/assets/766a450f-ef8d-4e61-a274-33006c4a334d" width="754"/>(a) A decrease in the pressure of this reaction favours the reverse reaction (more gas molecules), the equilibrium shifts to the left. (b) An increase in the pressure of this reaction favours the forward reaction (fewer gas molecules), the equilibrium shifts to the right.</div>
The figure above shows how <a data-lesson="factors-affecting-the-equilibrium-constant" data-subject="chemistry" data-tutorial="chemical-equilibrium" href="https://app.ulearngo.com/chemistry/chemical-equilibrium/factors-affecting-the-equilibrium-constant">changing</a> the pressure of a system results in a shift in the equilibrium to counter that change. In the original system there are 12 molecules in total: \({\color{skyblue}{6{\text {H}}_{2}}} + {\color{blue}{2{\text{N}}_{2}}} \leftrightharpoons {\color{red}{4{\text{NH}}_{3}}}\)
If you decrease the pressure (shown by an increase in volume), the equilibrium will shift to increase the number of gas molecules. That shift is to the left and the number of \(\text{H}_{2}\) and \(\text{N}_{2}\) molecules will increase while the number of \(\text{NH}_{3}\) molecules will decrease:
<img alt="526494ffbf8fcd1bbb64d55647484a70.png" src="https://data.ulearngo.com/assets/cfd30be8-bf1a-4a79-bbed-3d307e89a4bc"/>
If you increase the pressure (shown by a decrease in volume), the equilibrium will shift to decrease the number of gas molecules. That shift is to the right and the number of \(\text{H}_{2}\) and \(\text{N}_{2}\) molecules will decrease while the number of \(\text{NH}_{3}\) molecules will increase:
<img alt="0cac7dda5e8db7deeca7f1ac70cc7af0.png" src="https://data.ulearngo.com/assets/00a351f0-47bb-46ae-938c-90d16abbf6bc"/>
Note that the total number of nitrogen and hydrogen <a data-lesson="chemical-formulas" data-subject="chemistry" data-tutorial="atoms-molecules-ions" href="https://app.ulearngo.com/chemistry/atoms-molecules-ions/chemical-formulas">atoms</a> remains the same in all three situations. Equations (a) and (b) are not balanced equations.
Another example is the reaction between <a data-lesson="sulfur" data-subject="chemistry" data-tutorial="metals-metalloids-nonmetals" href="https://app.ulearngo.com/chemistry/metals-metalloids-nonmetals/sulfur">sulfur</a> dioxide and oxygen:
\(\color{blue}{\text{2SO}_{2}\text{(g)}} + \color{blue}{\text{O}_{2}\text{(g)}} \leftrightharpoons \color{red}{\text{2SO}_{3}\text{(g)}}\)
In this reaction, \(\color{red}{\textbf{two molecules of product gas}}\) are formed for every \(\color{blue}{\textbf{three molecules of}}\) \(\color{blue}{\textbf{reactant gas}}\).
<ul>
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An increase in pressure will:
<ul style="list-style-type: square;">
<li>
Favour the reaction that decreases the number of gas molecules.
</li>
<li>
There are \(\color{red}{\textbf{fewer molecules of product gas}}\) than reactant gas, so the forward reaction is favoured.
</li>
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The equilibrium will shift to the right and the yield of \(\color{red}{\textbf{SO}_{3}}\) will increase.
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</ul>
</li>
<li>
A decrease in pressure will:
<ul style="list-style-type: square;">
<li>
Favour the reaction that increases the number of gas molecules.
</li>
<li>
There are \(\color{blue}{\textbf{more molecules of reactant gas}}\) than product gas, so the reverse reaction is favoured.
</li>
<li>
The equilibrium will shift to the left and the yield of \(\color{red}{\textbf{SO}_{3}}\) will decrease.
</li>
</ul>
</li>
</ul>
<section></section>

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Effect of Pressure on Equilibrium

Chemistry generally explores the substances of which matter is composed, the investigation of their properties and reactions, and the use of such reactions to form new substances. 

Chemistry

Discover the principles of Chemical Equilibrium including reversible reactions, dynamic equilibrium, and the Equilibrium Constant, learn how to calculate and interpret RICE tables, and explore the effects of concentration, temperature, pressure, and catalysts on equilibrium reactions.


Effect of Pressure on Equilibrium

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