Name: shifting equilibrium by changing concentration
Uploaded: 2020-09-24T15:06:50.000Z
Duration: 2 min 27 s
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Le Chatelier's Principle: Changing Concentration

Een systeem is in dynamisch evenwicht wanneer de snelheid van de voorwaartse reactie gelijk is aan de snelheid van de omgekeerde reactie. Hoewel de reactie nog steeds in beide richtingen verloopt, blijven de nettoconcentraties van reactanten en producten constant. Als op dit punt de concentratie van een reactant of product wordt veranderd, bijvoorbeeld door extra molecuul A toe te voegen of door een molecuul C te verwijderen, wordt het evenwicht verstoord.Dergelijke veranderingen zijn spanningen op het systeem. Volgens het principe van Le Châtelier verschuift de evenwichtspositie in een richting om de spanning te minimaliseren en het evenwicht te herstellen wanneer een systeem in evenwicht wordt gestrest als gevolg van een verandering in concentratie, een verandering in volume of druk, of een verandering in temperatuur. Denk, om de reactie van een systeem op concentratieveranderingen te begrijpen, aan twee verbonden watertanks, A en B.Aanvankelijk is het waterpeil in de twee tanks gelijk en is het systeem in evenwicht.Vervolgens wordt water toegevoegd aan tank B.Deze spanning verhoogt het waterpeil in tank B en verstoort het evenwicht. Om deze spanning te compenseren, stroomt een deel van het water in tank A om het evenwicht te herstellen. Omgekeerd, als water uit tank B wordt verwijderd, veroorzaakt het verlagen van het waterpeil ook stress.Bijgevolg stroomt water uit tank A in tank B om het evenwicht te herstellen. Voor een systeem in evenwicht is het reactiequotiënt gelijk aan de evenwichtsconstante. Het toevoegen van meer reactanten aan het evenwichtsmengsel verlaagt Q.Volgens het principe van Le Châtelier minimaliseert het systeem deze spanning door de evenwichtspositie naar de producten te verschuiven om de toegevoegde reactant te verbruiken en het evenwicht te herstellen.Evenzo zal het verwijderen van een product uit het evenwichtsmengsel leiden tot een soortgelijke reactie om het evenwicht te herstellen. Daarentegen verhoogt het toevoegen van meer producten de Q.Aldus verschuift de evenwichtspositie naar de reactanten om het toegevoegde product te verbruiken en het evenwicht te herstellen. Een verandering in concentratie verschuift de evenwichtspositie zonder de waarde van K te veranderen.

principe van le chatelier: concentratie veranderen

Een systeem in evenwicht bevindt zich in een staat van dynamisch evenwicht, waarbij voorwaartse en achterwaartse reacties met gelijke snelheden plaatsvinden. Als een evenwichtssysteem wordt onderworpen aan een verandering in de omstandigheden die deze reactiesnelheden op een andere manier beïnvloeden (een stress), dan zijn de snelheden niet langer gelijk en is het systeem niet in evenwicht. Het systeem zal vervolgens een netto reactie ervaren in de richting van een grotere snelheid (een verschuiving) die het evenwicht zal herstellen. Dit fenomeen wordt samengevat door het principe van Le Châtelier: als een evenwichtssysteem onder druk staat, zal het systeem een verschuiving ervaren als reactie op de spanning die het evenwicht herstelt. Reactiesnelheden worden voornamelijk beïnvloed door concentraties, zoals beschreven door de snelheidswet van de reactie, en temperatuur, zoals beschreven door de Arrhenius-vergelijking. Bijgevolg zijn veranderingen in concentratie en temperatuur de twee spanningen die een evenwicht kunnen verschuiven. Effect van een verandering in concentratie Als een evenwichtssysteem wordt onderworpen aan een verandering in de concentratie van een reactant of productsoort, zal de snelheid van de voorwaartse of de achterwaartse reactie veranderen. Beschouw als voorbeeld de evenwichtsreactie: <img alt="Eq1" src="/files/ftp_upload/11391/11391_Equation 1v2.png" style="text-align: center; height: 48px;"> Wanneer dit systeem in evenwicht is, zijn de voorwaartse en achterwaartse reactiesnelheden gelijk. <img alt="Eq2" src="/files/ftp_upload/11391/11391_Equation 2.png" style="text-align: center; height: 50px;"> Als het systeem onder druk wordt gezet door het toevoegen van reactant, N2 of O2, zorgt de resulterende toename van de concentratie ervoor dat de snelheid van de voorwaartse reactie toeneemt en die van de omgekeerde reactie overtreft: <img alt="Eq3" src="/files/ftp_upload/11391/11391_Equation 3.png" style="text-align: center; height: 50px;"> Het systeem zal een tijdelijke nettoreactie in voorwaartse richting ervaren om het evenwicht te herstellen (het evenwicht zal naar rechts verschuiven). Deze zelfde verschuiving zal het gevolg zijn als een deel van het product NO uit het systeem wordt verwijderd, waardoor de snelheid van de omgekeerde reactie afneemt, wat opnieuw resulteert in dezelfde onbalans in de snelheid. Dezelfde logica kan worden gebruikt om de verschuiving naar links te verklaren die het gevolg is van het verwijderen van reactant of het toevoegen van product aan een evenwichtssysteem. Beide spanningen resulteren in een verhoogde snelheid van de omgekeerde reactie <img alt="Eq4" src="/files/ftp_upload/11391/11391_Equation 4.png" style="text-align: center; height: 50px;"> en een tijdelijke nettoreactie in de omgekeerde richting om het evenwicht te herstellen. Als alternatief voor deze kinetische interpretatie kan het effect van concentratieveranderingen op evenwichten worden gerationaliseerd in termen van reactiequotiënten. Wanneer het systeem in evenwicht is, <img alt="Eq5" src="/files/ftp_upload/11391/11391_Equation 5.png" style="text-align: center; height: 70px;"> Als een reactant wordt toegevoegd (waardoor de noemer van het reactiequotiënt toeneemt) of een product wordt verwijderd (waardoor de teller wordt verlaagd), dan zal Qc < Kc en zal het evenwicht naar rechts verschuiven. Merk op dat de drie verschillende manieren om deze spanning te induceren resulteren in drie verschillende veranderingen in de samenstelling van het evenwichtsmengsel. Als N2 wordt toegevoegd, zal de rechterverschuiving O2 verbruiken en NO produceren naarmate het evenwicht wordt hersteld, waardoor een mengsel ontstaat met een grotere concentratie N2 en NO en een lagere concentratie O2 dan voorheen aanwezig was. Als O2 wordt toegevoegd, zal het nieuwe evenwichtsmengsel hogere concentraties O2 en NO en een lagere concentratie N2 hebben. Als tenslotte NO wordt verwijderd, zal het nieuwe evenwichtsmengsel hogere concentraties N2 en O2 en een lagere concentratie NO hebben. Ondanks deze verschillen in samenstelling zal de waarde van de evenwichtsconstante na de spanning hetzelfde zijn als daarvoor (volgens de wet van massa-actie). Dezelfde logica kan worden toegepast voor spanningen die gepaard gaan met het verwijderen van reactanten of het toevoegen van product, in welk geval Qc > Kc en het evenwicht naar links zal verschuiven.

Principe van Le Chatelier: concentratie veranderen

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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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...

le chatelier's principle: 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...

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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: 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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