Effect Of Volume Changes On Gas-phase Equilibria
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Jun 5, 2019
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Le Chatlier's Principle states that when a system at equilibrium undergoes a change in temperature,
volume, or amount of moles present in a reactant or product, the system will respond in order to reach
equilibrium. Think of a system at equilibrium as a balanced scale (equal weights on both sides) and when
one side gains more weight, the scale will have to adjust the other side in order to reach equilibrium.
Introduction
In terms of volume changes within a system at equilibrium, the following applies:


When there is a decrease in volume, the equilibrium will shift to favor the direction that produces
fewer moles of gas.
When there is an increase in volume, the equilibrium will shift to favor the direction that
produces more moles of gas.
It's important to remember that these rules only apply to equations in which gases are involved. If only
solids and aqueous solutions are present, volume changes will have no effect on the equilibrium.
Conceptual Questions
1. What happens when you increase the volume in a system at equilibrium that has equal moles of
reactants and products and you increase the volume, in which direction will a net change occur in
order to restore equilibrium? Decrease?
2. What happens when you increase the volume in an equilibrium system in which there are more
moles of reactants than products?
3. What happens when you decrease the volume in an equilibrium system in which there are more
moles of products than reactants?
4. What happens when you increase the volume in an equilibrium system in which there are more
moles of products than reactants?
5. What happens when you decrease the volume in an equilibrium system in which there are more
moles of reactants than products?
Practice Problems
How could one increase the amount of reactants produced in the following equation:
N2(g)+3H2(g)⇌2NH3(g)
a) increase volume b) decrease volume
Explain what would happen in each of the following equations if there is a DECREASE in volume:
2SO2(g)+O2(g)⇌2SO3(g)
H2(g)+I2(g)⇌2HI(g)
CaCO3(s)⇌CaO(s)+CO2(g)
AgCl(s)+⇌Ag+1(aq)+Cl−(aq)
Summary
Le Chatlier's Principle applies to changes in the following:
1. pressure changes
2. concentration changes
3. volume changes
When any system at equilibrium is disturbed, it will attempt to reach a new equilibrium.
Answers
1. Because there is an equal number of moles on both sides of the reaction, an increase in volume
will have no effect on the equilibrium and thus there is no shift in the direction. Similarly, when
you decrease the volume there is no effect on the equilibrium.
2. Because there are more moles of reactants, an increase in volume will shift the equilibrium to the
left in order to favor the reactants.
3. When there is a decrease in volume, the equilibrium will shift towards the side of the reaction
with fewer moles. In this case, there are fewer moles of reactants and so the equilibrium will
favor the reactants and shift to the left.
4. An increase in volume always favors the direction that produces more moles of gas and because
in this case there are more moles of products, the reaction will shift to the right and produce more
moles products.
5. Because a decrease in volume always favors the direction that produces fewer moles, this system
will shift to right and produce more moles of products.
6. Answers
1. In order to increase the amount of reactants formed, increase the volume so that it will shift to the
right (towards the side with more moles of gas).
2. Effects of a Decrease in Volume

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
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The products are favored; shift to right.
Same number of moles on each side of equation; no effect.
The reactants are favored; shift to left.
Because no gases are present there is no change.