Name: le chatelier's principle in heating and cooling
Uploaded: 2020-09-24T15:06:51.000Z
Duration: 2 min 19 s
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Le Chatelier's Principle: Changing Temperature

Temperature affects the rate of a chemical reaction; therefore, a change in temperature for a reaction at equilibrium acts as a stress on the system. Le Ch&#226;telier&#39;s principle predicts how the system will respond to minimize such disturbances.
A change in the temperature changes the value of the equilibrium constant, unlike a change in concentration or volume, which shifts the equilibrium without changing the value of K.
Consider the decomposition of gaseous phosphorus pentachloride into phosphorus trichloride and chlorine gas. For this endothermic reaction, the heat absorbed can be thought of as a reactant.
An increase in temperature adds heat to the system, similar to adding more of a reactant. Thus, the equilibrium position shifts towards the products and produces more phosphorus trichloride and chlorine to consume the extra heat because the value of the equilibrium constant, K, has increased.
On the other hand, a decrease in temperature removes heat from the system, similar to removing a reactant. The equilibrium position shifts towards the reactants and produces more phosphorus pentachloride to release heat because the value of K has decreased.
For an exothermic reaction, such as the gaseous reaction between sulfur dioxide and oxygen to produce sulfur trioxide, the heat released can be thought of as a product.
An increase in temperature is similar to adding more of a product. This causes the equilibrium position to shift toward the reactants, producing more sulfur dioxide and oxygen to absorb some of the added heat because the value of K has decreased.
Conversely, decreasing the temperature of this exothermic reaction removes heat, like removing a product. The equilibrium position shifts towards the products and produces more sulfur trioxide to release heat as K has increased.
Thus, an increase in temperature favors the products in an endothermic reaction, whereas a decrease in temperature favors the products in an exothermic reaction.

le chatelier's principle: changing temperature

Consistent with the law of mass action, an equilibrium stressed by a change in concentration will shift to re-establish equilibrium without any change in the value of the equilibrium constant, K. When an equilibrium shifts in response to a temperature change, however, it is re-established with a different relative composition that exhibits a different value for the equilibrium constant.
To understand this phenomenon, consider the elementary reaction:
 <img alt="Static equilibrium diagram; sum of forces &#931;Fx=0, &#931;Fy=0 shown; illustrating balance concepts" src="/files/ftp_upload/11393/11393_Equation_1.png" style="height: 50px;" data-altupdatedat="1754904545000" data-alt="Bacterial conjugation diagram; plasmid transfer via pilus; gene transfer method in genetics study." />
Since this is an elementary reaction, the rates laws for the forward and reverse may be derived directly from the balanced equation&#8217;s stoichiometry:
 <img alt="Chemical reaction mechanism diagram; synthesis of aspirin via esterification from salicylic acid and acetic anhydride." src="/files/ftp_upload/11393/11393_Equation_2.png" style="height: 65px;" data-altupdatedat="1754904545000" data-alt="Optical wave interference pattern; Young's double-slit experiment diagram; light wave analysis." />
When the system is at equilibrium,
 <img alt="Polymerase chain reaction diagram, illustrating DNA amplification, PCR primers and cycles." src="/files/ftp_upload/11393/11393_Equation_3.png" style="height: 50px;" data-altupdatedat="1754904545000" data-alt="Chemical reaction diagram showing oxidation of cyclohexane to adipic acid using nitric acid." />
Substituting the rate laws into this equality and rearranging gives
 <img alt="Lewis structure diagram of benzene, showing hexagonal ring with alternating double bonds." src="/files/ftp_upload/11393/11393_Equation_4.png" style="height: 100px;" data-altupdatedat="1754904545000" data-alt="Thermodynamics process diagram, state changes, heat transfer, work interactions, energy conservation." />
The equilibrium constant can be expressed as a mathematical function of the rate constants for the forward and reverse reactions. Since the rate constants vary with temperature as described by the Arrhenius equation, it stands to reason that the equilibrium constant will likewise vary with temperature (assuming the rate constants are affected to different extents by the temperature change). For more complex reactions involving multistep reaction mechanisms, a similar but more complex mathematical relation exists between the equilibrium constant and the rate constants of the steps in the mechanism. Regardless of how complex the reaction may be, the temperature-dependence of its equilibrium constant persists.
Predicting the shift an equilibrium will experience in response to a change in temperature is most conveniently accomplished by considering the enthalpy change of the reaction. For example, the formation of ammonia by the Haber&#8217;s process is an exothermic (heat-producing) process:
 <img alt="Hydraulic lift diagram, mechanical advantage, force distribution, lever principle, educational use." src="/files/ftp_upload/11393/11393_Equation_5.png" style="height: 50px;" data-altupdatedat="1754904545000" data-alt="Newton's second law diagram: &#931;F=ma with vector forces, mass m, acceleration a. Physics illustration." />
For purposes of applying Le Ch&#226;telier&#8217;s principle, heat, q, may be viewed as a product:
 <img alt="Sn1 reaction mechanism diagram with carbocation intermediate, nucleophilic attack, kinetic steps." src="/files/ftp_upload/11393/11393_Equation_6.png" style="height: 50px;" data-altupdatedat="1754904545000" data-alt="static equilibrium diagram, &#931;F=0, showing forces on beam, torque balance, educational use, vector analysis" />
Raising the temperature of the system is akin to increasing the amount of a product, and so the equilibrium will shift to the left. Lowering the system temperature will likewise cause the equilibrium to shift right. For endothermic processes, heat is viewed as a reactant of the reaction and so the opposite temperature dependence is observed.
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: Le </a><a href="https://openstax.org/books/chemistry-2e/pages/13-3-shifting-equilibria-le-chateliers-principle">Ch</a>&#226;<a href="https://openstax.org/books/chemistry-2e/pages/13-3-shifting-equilibria-le-chateliers-principle">t</a><a href="https://openstax.org/books/chemistry-2e/pages/13-3-shifting-equilibria-le-chateliers-principle">elier&#8217;s Principle</a>.

Consistent with the law of mass action, an equilibrium stressed by a change in concentration will shift to re-establish equilibrium without any change in ...

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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 forward and reverse rate laws based on the balanced equation. At equilibrium, the forward and reverse reaction rates are equal. When those rate laws are combined and rearranged, the equilibrium constant can be expressed as a function of the forward and reverse rate constants.

Because rate constants change with temperature, as described by the Arrhenius equation, the equilibrium constant can also change with temperature. This idea applies to both elementary reactions and more complex multistep mechanisms. In multistep reactions, the math is more complicated, but the temperature dependence of the equilibrium constant still remains.

A practical way to predict the direction of the shift is to use the reaction’s enthalpy change. In the Haber process, ammonia formation is exothermic, which means it produces heat. For Le Chatelier's principle, heat can be treated like a product. Raising the temperature adds heat, so the equilibrium shifts left. Lowering the temperature removes heat, so the equilibrium shifts right. For endothermic processes, heat acts like a reactant, so the temperature effect is opposite.

Le Chatelier's Principle in Heating and Cooling

le-chateliers-principle-heating-and-cooling

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...

le chatelier's principle: changing concentration

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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...

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...

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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

Equilibrium concentrations can be found from initial concentrations and an equilibrium constant. This skill matters in many areas of science and technology, including pharmaceutical product formulation and dosing. After a drug is ingested or injected, it may take part in several chemical equilibria that change its final concentration in the body system of interest. Scientists use these quantitative equilibrium relationships to help calculate a dosage that will produce the desired therapeutic...

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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 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 heating and cooling change chemical equilibrium, why K changes with temperature, and how exothermic and endothermic reactions shift.

Le Chatelier's Principle in Heating and Cooling | JoVE K12

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Le Chatelier's Principle in Heating and Cooling

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