Name: shifting equilibrium by changing concentration
Uploaded: 2020-09-24T15:06:50.000Z
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Le Chatelier's Principle: Changing Concentration

A system is in dynamic equilibrium when the rate of the forward reaction equals the rate of the reverse reaction. Though the reaction is still proceeding in both directions, the net concentrations of reactants and products remain constant.
At this point, if a reactant or product&#8217;s concentration is changed, for example by adding additional molecule A or by removing some molecule C, the equilibrium is disturbed. Such changes are stresses on the system.
According to Le Ch&#226;telier&#8217;s principle, when a system at equilibrium is stressed due to a change in concentration, a change in volume or pressure, or a change in temperature, the equilibrium position shifts in a direction to minimize the stress and restore equilibrium.
To understand a system&#8217;s response to changes in concentration, consider two connected water tanks, A and B. Initially, the water level in the two tanks is equal, and the system is in equilibrium. Then, water is added to tank B.
This stress raises the water level in tank B and disturbs the equilibrium. To offset this stress, some of the water flows into tank A to restore the equilibrium.
Conversely, if water is removed from tank B, the lowering of the water level also triggers stress.
Consequently, water from tank A flows into tank B to restore the equilibrium.
For a system at equilibrium, the reaction quotient equals the equilibrium constant. Adding more reactants to the equilibrium mixture decreases Q. According to Le Ch&#226;telier&#8217;s principle, the system minimizes this stress by shifting the equilibrium position towards the products to expend the added reactant and re-establish equilibrium. 
Likewise, removing a product from the equilibrium mixture will lead to a similar response in order to restore equilibrium.
In contrast, adding more products increases Q. Thus, the equilibrium position shifts towards the reactants to expend the added product and restore equilibrium.
A change in concentration shifts the equilibrium position without changing the value of K.

le chatelier's principle: changing concentration

A system at equilibrium is in a state of dynamic balance, with forward and reverse reactions taking place at equal rates. If an equilibrium system is subjected to a change in conditions that affects these reaction rates differently (a stress), then the rates are no longer equal and the system is not at equilibrium. The system will subsequently experience a net reaction in the direction of a greater rate (a shift) that will re-establish the equilibrium. This phenomenon is summarized by Le Ch&#226;telier&#8217;s principle: if an equilibrium system is stressed, the system will experience a shift in response to the stress that re-establishes equilibrium.
Reaction rates are affected primarily by concentrations, as described by the reaction&#8217;s rate law, and temperature, as described by the Arrhenius equation. Consequently, changes in concentration and temperature are the two stresses that can shift an equilibrium.
Effect of a Change in Concentration
If an equilibrium system is subjected to a change in the concentration of a reactant or product species, the rate of either the forward or the reverse reaction will change. As an example, consider the equilibrium reaction:
<img alt="Static equilibrium diagram; &#931;Fx=0, &#931;Fy=0, MA=0 principles illustrated." src="/files/ftp_upload/11391/11391_Equation%201v2.png" style="text-align: center; height: 48px;" data-altupdatedat="1761830439000" data-alt="Eq1" />
When this system is at equilibrium, the forward and reverse reaction rates are equal.
<img alt="Chemical equilibrium constant equation K=([C]^c[D]^d)/([A]^a[B]^b) in chemical reactions" src="/files/ftp_upload/11391/11391_Equation%202.png" style="text-align: center; height: 50px;" data-altupdatedat="1761830439000" data-alt="Eq2" />
If the system is stressed by adding reactant, either N2 or O2, the resulting increase in concentration causes the rate of the forward reaction to increase, exceeding that of the reverse reaction:
<img alt="Static equilibrium diagram; &#931;Fx=0, MA=0 conditions; forces labeled; educational physics concept." src="/files/ftp_upload/11391/11391_Equation%203.png" style="text-align: center; height: 50px;" data-altupdatedat="1761830439000" data-alt="Eq3" />
The system will experience a temporary net reaction in the forward direction to re-establish equilibrium (the equilibrium will shift right). This same shift will result if some product NO is removed from the system, which decreases the rate of the reverse reaction, again resulting in the same imbalance in rates.
The same logic can be used to explain the left shift that results from either removing reactant or adding product to an equilibrium system. Both of these stresses result in an increased rate for the reverse reaction
<img alt="Organic chemistry reaction mechanism diagram showing nucleophilic substitution reaction process." src="/files/ftp_upload/11391/11391_Equation%204.png" style="text-align: center; height: 50px;" data-altupdatedat="1761830439000" data-alt="Eq4" />
and a temporary net reaction in the reverse direction to re-establish equilibrium.
As an alternative to this kinetic interpretation, the effect of changes in concentration on equilibria can be rationalized in terms of reaction quotients. When the system is at equilibrium,
<img alt="Equilibrium of forces; &#931;Fx=0, &#931;Fy=0 diagram with applied and reactionary forces, static analysis." src="/files/ftp_upload/11391/11391_Equation%205.png" style="text-align: center; height: 70px;" data-altupdatedat="1761830439000" data-alt="Eq5" />
If a reactant is added (increasing the denominator of the reaction quotient) or product is removed (decreasing the numerator), then Qc &#60; Kc and the equilibrium will shift right. Note that the three different ways of inducing this stress result in three different changes in the composition of the equilibrium mixture. If N2 is added, the right shift will consume O2 and produce NO as equilibrium is re-established, yielding a mixture with a greater concentration of N2 and NO and a lesser concentration of O2 than was present before. If O2 is added, the new equilibrium mixture will have greater concentrations of O2 and NO and a lesser concentration of N2. Finally, if NO is removed, the new equilibrium mixture will have greater concentrations of N2 and O2 and a lesser concentration of NO. Despite these differences in composition, the value of the equilibrium constant will be the same after the stress as it was before (per the law of mass action). The same logic may be applied for stresses involving removing reactants or adding product, in which case Qc &#62; Kc and the equilibrium will shift left.
This text has been adapted from <a href="https://openstax.org/books/chemistry-2e/pages/13-3-shifting-equilibria-le-chateliers-principle">Openstax, Chemistry 2e, Section 13.3 Shifting Equilibria: LeChatelier&#8217;s Principle</a>.

A system at equilibrium is in a state of dynamic balance, with forward and reverse reactions taking place at equal rates. If an equilibrium system is ...

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KEY TERMS AND CONCEPTS

shifting equilibrium by changing concentration

Chemical equilibrium is a dynamic balance. At equilibrium, the forward and reverse reactions happen at equal rates. If the system is stressed in a way that changes one rate more than the other, it is no longer at equilibrium and will shift until balance is restored. This behavior is described by Le Châtelier’s principle.

Changes in concentration are a major stress on an equilibrium system. Reaction rates depend mainly on concentration and temperature. For that reason, changing the amount of a reactant or product can move the equilibrium in one direction or the other.

Adding a reactant, such as N2 or O2 in the example reaction, increases the forward reaction rate. The forward rate then becomes greater than the reverse rate, so the equilibrium shifts right. Removing the product NO has the same effect because it lowers the reverse reaction rate and creates the same imbalance.

The opposite changes cause a left shift. Removing reactant or adding product increases the reverse reaction rate compared with the forward rate. The system then moves in the reverse direction until equilibrium is reached again.

This shift can also be explained with the reaction quotient, Qc, and the equilibrium constant, Kc. When reactant is added, Qc becomes less than Kc. When product is removed, Qc also becomes less than Kc, so the equilibrium shifts right. If reactant is removed or product is added, then Qc is greater than Kc and the equilibrium shifts left.

Although the final composition changes, the equilibrium constant stays the same after the stress is removed and the system rebalances. If N2 is added, the new mixture has more N2 and NO and less O2. If O2 is added, the mixture has more O2 and NO and less N2. If NO is removed, the mixture ends with more N2 and O2 and less NO. The same idea applies to shifts caused by removing reactant or adding product.

Shifting Equilibrium by Changing Concentration

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the equilibrium constant

Chemical Equilibrium and Kc Rules

Chemical equilibrium and the equilibrium constant Kc describe how reactants and products are balanced in a reaction. In the oxidation of sulfur dioxide, a mixture that starts with reactants only has no product at first. As the reaction moves forward toward equilibrium, reactant concentrations decrease and product concentration increases. When equilibrium is reached, the concentrations of both stay constant. A reaction can also begin with only products present. In that case, the reaction moves...

Chemical equilibrium is a dynamic balance. At equilibrium, the forward and reverse reactions happen at equal rates. If the system is stressed in a way that changes one rate more than the other, it is no longer at equilibrium and will shift until balance is restored. This behavior is described by Le Châtelier’s principle. Changes in concentration are a major stress on an equilibrium system. Reaction rates depend mainly on concentration and temperature. For that reason, changing the amount of a...

homogeneous equilibria for gaseous reactions

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Relating Kc and Kp in Gas Equilibria

Gas-phase equilibrium can be written using either concentration or pressure. The equilibrium constant may be shown as Kc, which uses molar concentration, or as Kp, which uses partial pressure of the reactants and products. The link between Kc and Kp comes from the ideal gas equation and the definition of molarity. Molarity is the number of moles divided by volume. In the ideal gas equation, P stands for partial pressure, V for volume, n for number of moles, R for the gas constant, and T for...

calculating the equilibrium constant

Finding Kc from Equilibrium Data

The equilibrium constant, Kc, can be found from the equilibrium concentrations of reactants and products. When those concentrations are known, the calculation is straightforward. The values are placed into the Kc expression for the reaction. One example uses nitrogen dioxide and dinitrogen tetroxide. When 0.10 mol NO2 is added to a 1.0-L flask at 25 °C, the concentrations change until equilibrium is reached. At equilibrium, [NO2] is 0.016 M and [N2O4] is 0.042 M, and these values are used to...

reaction quotient

Using Q to Track Equilibrium Changes

Reaction quotient Q helps track how far a reversible reaction has moved toward equilibrium. For a balanced reaction such as m A + n B ⇌ x C + y D, Q comes directly from the stoichiometric coefficients in the equation. When concentrations are used, the expression is written as Qc. For gases, a similar form uses partial pressures. These expressions use relative concentration and pressure values, not absolute values. That means reaction quotients are dimensionless and have no units. As a reaction...

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Equilibrium Concentrations from an ICE Table

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dynamic equilibrium

Chemical Equilibrium in Reversible Reactions

Chemical equilibrium in reversible reactions happens when a reaction moves in both directions at the same time. A reversible reaction uses a double arrow to show that products can turn back into reactants. At equilibrium, the concentrations of reactants and products stay constant over time because the forward and reverse reaction rates are equal. The video uses the reversible decomposition of colorless dinitrogen tetroxide to brown nitrogen dioxide as an example. At time zero, there is finite...

le chatelier's principle: changing volume (pressure)

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Gas Equilibrium Under Volume Changes

Gas-phase equilibrium can shift when the volume of the system changes. In these reactions, a change in pressure changes concentration because the partial pressure of an ideal gas is proportional to its molar concentration. That means pressure changes affect equilibrium in the same way as concentration changes. When the volume of a gas mixture changes, the concentrations of reactants and products change together. Since every gas in the equilibrium occupies the same space, the same volume change...

le chatelier's principle: changing temperature

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Le Chatelier's Principle in Heating and Cooling

Le Chatelier's principle helps explain how chemical equilibrium responds to heating and cooling. A system at equilibrium will shift to restore balance after a temperature change, but the equilibrium constant, K, will not stay the same. Instead, the mixture reaches a new equilibrium with a different composition. This temperature effect is different from a concentration change, which shifts equilibrium without changing K. To understand why temperature matters, the reaction can be written with...

the small x assumption

Estimating Equilibrium with Small Changes

A small equilibrium constant can make equilibrium calculations easier. In these reactions, the equilibrium position favors the reactants. When the initial reactant concentration is high and the Kc value is small, the change in concentration may be negligible. In that case, the equilibrium concentration is close to the initial concentration. This idea helps simplify weak acid and weak base problems. Chemists can assume that some equilibrium concentrations are equal to the initial concentrations.

key terms and concepts

Learn how changing concentration shifts chemical equilibrium, including why adding or removing reactants or products moves the reaction left or right.

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Shifting Equilibrium by Changing Concentration

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