# Le Chatelier's Principle

## Le Chatelier's Principle states that changes to an equilibrium system will result in opposing shifts to achieve a new equilibrium.

#### Key Points

• Le Chatelier's Principle can be used to predict the behavior of a system due to changes in pressure, temperature, or concentration.

• Le Chatelier's Principle implies that the addition of heat to a reaction will favor the endothermic direction of a reaction as this reduces the amount of heat produced in the system.

• Increasing the concentration will drive the reaction to the right, while increasing the concentration of products will drive the reaction to the left.

#### Terms

• the state of a reaction in which the rates of the forward and reverse reactions are the same

• Any theory that relates collisions among particles to reaction rate; reaction rate depends on such factors as concentration, surface area, temperature, stirring, and the presence of either a catalyst or an inhibitor.

#### Figures

1. ##### Henry Le Chatelier

A photograph of Henry Le Chatelier

2. ##### NO2 to N2O4

The value of K changes with temperature. In the endothermic reaction N2O4(g) ⇌ 2NO2(g), the equilibrium position can be shifted by changing the temperature. When heat is added and the temperature increases, the reaction shifts to the right and the flask turns reddish brown due to an increase in NO2. When heat is removed and the temperature decreases, the reaction shifts to the left and flask turns colorless due to an increase in N2O4. This demonstrates Le Chatelier’s Principle because the equilibrium shifts in the direction that consumes energy.

3. ##### Le Chatelier's Principle

This lesson shows how Le Chatelier's Principle predicts changes in a equilibrium, as well as demonstrates a easy and convienent method for making predictions for the effect of temperature, concentration, and pressure.

Le Chatelier's Principle Figure 3 is an observation about chemical equilibria of reactions. It states that changes in the temperature, pressure (or volume), or concentration of a system will result in predictable and opposing changes in the system in order to achieve a new equilibrium state. Le Chatelier's Principle can be used in practice to understand reaction conditions that will favor increased product formation. This idea was discovered and formulated independently by Henri Louis Le Chatelier (pictured below) and Karl Ferdinand Braun.

Figure 1

## Changes in Concentration

According to Le Chatelier's Principle, the addition of product or the removal of reactants to a system will shift the equilibrium towards the reactants, while the addition of more reactants or the removal of product will provide the opposite shift towards product, or the right-hand side of the reaction equation. This can be illustrated by the equilibrium of carbon monoxide and hydrogen gas, reacting to form methanol.

$CO + 2 H_2 ⇌ CH_3OH$

Suppose we were to increase the concentration of CO in the system. Using Le Chatelier's principle, we can predict that the amount of methanol will increase, thereby decreasing the total change in CO. If we are to add a species to the overall reaction, the reaction will favor the side opposing the addition of the species. Likewise, the subtraction of a species would cause the reaction to fill the "gap" and favor the side where the species was reduced.

This observation is supported by the collision theory. As the concentration of CO is increased, the frequency of successful collisions of that reactant would increase also, allowing for an increase in forward reaction, and generation of the product. Even if a desired product is not thermodynamically favored, the end-product can be obtained if it is continuously removed from the solution.

## Changes in Pressure

A change in pressure (or volume) will result in an attempt to restore equilibrium by creating more or less moles of gas.  For example, if the pressure in a system increases (or the volume decreases), the equilibrium will shift to favor the side of the reaction that involves fewer moles of gas. Similarly, if the volume of a system increases (or the pressure decreases), the production of additional moles of gas will be favored.

Considering the reaction of nitrogen gas with hydrogen gas to form ammonia:

$N_2 + 3 H_2 ⇌ 2 NH_3$       ΔH = -92kJ mol-1

Note the number of moles of gas on the left-hand side and the number of moles of gas on the right-hand side. When the volume of the system is changed, the partial pressures of the gases change. If we were to decrease pressure by increasing volume, the equilibrium of the above reaction will shift to the left, because the reactant side has greater number of moles than the product side. The system tries to counteract the decrease in partial pressure of gas molecules by shifting to the side that exerts greater pressure.

Similarly, if we were to increase pressure by decreasing volume, the equilibrium shifts to the right, counteracting the pressure increase by shifting to the side with fewer moles of gas that exert less pressure. If the volume is increased because there are more moles of gas on the reactant side, this change is more significant in the denominator of the equilibrium constant expression, causing a shift in equilibrium.

## Changes in Temperature

Finally, increases in temperature will favor the reaction direction that consumes heat, while decreases in reaction temperature will favor the direction that produces heat.  In other words, the addition of heat to an exothermic reaction will shift the equilibrium towards the reactants, while the same change will shift an endothermic reaction towards product formation. This can be viewed in the endothermic reaction of

$N_2O_4(g) ⇌ 2NO_2(g)$

Production of NO2 consumes heat. When heat is added and the termperature increases, it will shift the equilibrium to the right as more NO2 is produced. The color of the gas changes, as shown in Figure 2.

#### Key Term Glossary

chemical equilibrium
the state of a reversible reaction in which the rates of the forward and reverse reactions are the same
##### Appears in these related concepts:
Chemical equilibrium
In a chemical reaction, chemical equilibrium is the state in which both reactants and products are present at concentrations which have no further tendency to change with time.
##### Appears in these related concepts:
collision theory
Any theory that relates collisions among particles to reaction rate; reaction rate depends on such factors as concentration, surface area, temperature, stirring, and the presence of either a catalyst or an inhibitor.
##### Appears in these related concepts:
Collision Theory
Collision theory qualitatively explains how chemical reactions occur and why reaction rates differ for different reactions.
##### Appears in these related concepts:
concentration
the proportion of a substance in a mixture
##### Appears in these related concepts:
constant
Consistently recurring over time; persistent
##### Appears in these related concepts:
endothermic
of a chemical reaction that absorbs heat energy from its surroundings
##### Appears in these related concepts:
endothermic reaction
a process in which the system absorbs energy from its surroundings in the form of heat
##### Appears in these related concepts:
equilibrium
the state of a reaction in which the rates of the forward and reverse reactions are the same
##### Appears in these related concepts:
equilibrium constant
referring to a numerical value derived from the ratio of concentrations of products to reactants of a reversible reaction
##### Appears in these related concepts:
exothermic
of a chemical reaction that releases energy in the form of heat
##### Appears in these related concepts:
exothermic reaction
a transformation that releases energy in the form of light or heat
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favored
to proceed in a particular direction
##### Appears in these related concepts:
frequency
The number of occurrences of a repeating event per unit of time.
##### Appears in these related concepts:
gas
Matter in a state intermediate between liquid and plasma that can be contained only if it is fully surrounded by a solid (or held together by gravitational pull); it can condense into a liquid, or can (rarely) become a solid directly.
##### Appears in these related concepts:
heat
Heat is defined as the energy transferred from one system to another by thermal interaction.
##### Appears in these related concepts:
mole
In the International System of Units, the base unit of the amount of substance; the amount of substance of a system that contains as many elementary entities as there are atoms in 0.012 kg of carbon-12. Symbol: mol. The number of atoms in a mole is known as Avogadro’s number.
##### Appears in these related concepts:
molecule
the smallest particle of a specific element or compound that retains the chemical properties of that element or compound; two or more atoms held together by chemical bonds
##### Appears in these related concepts:
nitrogen
a chemical element (symbol N) with an atomic number of 7 and atomic weight of 14.0067
##### Appears in these related concepts:
partial pressure
the pressure that one component of a mixture of gases would contribute to the total pressure
##### Appears in these related concepts:
Pressure
the amount of force that is applied over a given area divided by the size of this area
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product
a chemical substance formed as a result of a chemical reaction
##### Appears in these related concepts:
reactant
Any of the participants present at the start of a chemical reaction. Also a molecule before it undergoes a chemical change.
##### Appears in these related concepts:
solution
A homogeneous mixture, which may be liquid, gas or solid, formed by dissolving one or more substances.
##### Appears in these related concepts:
Solution
A homogeneous mixture, which may be liquid, gas or solid, formed by dissolving one or more substances.
##### Appears in these related concepts:
state
The physical property of matter as solid, liquid, gas or plasma
##### Appears in these related concepts:
system
the part of the universe being studied, arbitrarily defined to any size desired
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temperature
A measure of cold or heat, often measurable with a thermometer.
##### Appears in these related concepts:
volume
A unit of three-dimensional measure of space that comprises a length, a width, and a height. It is measured in units of cubic centimeters in metric, or cubic inches or cubic feet in English measurement.