Name: equilibrium concentrations from an ice table
Uploaded: 2020-09-24T15:06:50.000Z
Duration: 2 min 5 s
Channel: JoVE

K12 Standards

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North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.5.2

Calculating Equilibrium Concentrations

Onbekende evenwichtsconcentraties kunnen worden bepaald uit de beginconcentratie van de reactanten en de evenwichtsconstante, K, voor de reactie. Beschouw de reactie waarbij 0, 30 molair stikstofgas en 0, 40 molair zuurstofgas reageren om stikstofoxidegas te produceren en waarbij K 0, 10 is. Om de evenwichtsconcentraties te berekenen, worden de bekende waarden in een ICE-tabel weergegeven.Voor de concentratieverandering wordt de toename of afname voor elk respectievelijk product of reactant aangegeven met x maal de stoichiometrische coëfficiënt. De som of het verschil wordt gebruikt om de evenwichtsconcentraties te vinden, die vervolgens worden gesubstitueerd in de evenwichtsuitdrukking. Om x op te lossen, wordt de uitdrukking uitgebreid en worden alle termen op één kant gezet om deze om te zetten in de vorm:ax kwadraat plus bx plus c.Deze vergelijking kan worden opgelost met behulp van de kwadratische formule. Oplossen resulteert in twee waarden voor x:0, 047 en min 0, 065. Omdat de negatieve concentratie van een stof onmogelijk is, kan de waarde worden verworpen.Als je 0, 047 gebruikt voor x, zijn de evenwichtsconcentraties van stikstof, zuurstof en stikstofmonoxide respectievelijk 0, 25, 0, 35 en 0, 094 molair. De perfecte kwadratische voorwaarde is een situatie waarin een snelkoppeling kan worden gebruikt om de kwadratische formule te vermijden. De beginconcentraties van stikstof en zuurstof in de bovenstaande reactie waren bijvoorbeeld elk 0, 30 molair, de vergelijking wordt een perfect vierkant.In dergelijke gevallen kan de vergelijking worden vereenvoudigd door de vierkantswortel van beide zijden te nemen om x op te lossen.

evenwichtsconcentraties berekenen

Het kunnen berekenen van evenwichtsconcentraties is essentieel voor veel gebieden van wetenschap en technologie, bijvoorbeeld bij het formuleren en doseren van farmaceutische producten. Nadat een medicijn is ingenomen of geïnjecteerd, is het doorgaans betrokken bij verschillende chemische evenwichten die de uiteindelijke concentratie ervan in het lichaamssysteem van belang beïnvloeden. Kennis van de kwantitatieve aspecten van deze evenwichten is vereist om een doseringshoeveelheid te berekenen die het gewenste therapeutische effect zal bewerkstelligen. Een meer uitdagend type evenwichtsberekening kan er een zijn waarbij evenwichtsconcentraties worden afgeleid van initiële concentraties en een evenwichtsconstante. Voor deze berekeningen is doorgaans een aanpak in vier stappen nuttig: <ol> <li>Identificeer de richting waarin de reactie zal verlopen om een evenwicht te bereiken.</li> <li>Ontwikkel een ICE-tabel.</li> <li>Bereken de concentratieveranderingen en vervolgens de evenwichtsconcentraties.</li> <li>Bevestig de berekende evenwichtsconcentraties.</li> </ol> Berekening van evenwichtsconcentraties Onder bepaalde omstandigheden bedraagt de evenwichtsconstante Kc voor de ontleding van PCl5(g) in PCl3(g) en Cl2(g) 0,0211. De bovenstaande procedure kan worden gebruikt om de evenwichtsconcentraties van PCl5, PCl3 en Cl2 te bepalen in een mengsel dat aanvankelijk alleen PCl5 bevatte bij een concentratie van 1,00 M. Stap 1. Bepaal de richting waarin de reactie verloopt. De uitgebalanceerde vergelijking voor de ontleding van PCl5 is <img alt="Eq1" src="/files/ftp_upload/11390/11390_Equation 1.png" style="text-align: center; height: 50px;"> Omdat aanvankelijk alleen de reactant aanwezig is, is Qc = 0, en zal de reactie naar rechts verlopen. Stap 2. Ontwikkel een ICE-tabel. <table style="margin-left: auto; margin-right: auto;"> <tbody> <tr> <td style="color: black; border: 1px solid black; text-align: center;"></td> <td style="color: black; border: 1px solid black; text-align: center;">     PCl5 (g)     </td> <td style="color: black; border: 1px solid black; text-align: center;">  PCl3 (g)   </td> <td style="color: black; border: 1px solid black; text-align: center;">  Cl2 (g)  </td> </tr> <tr> <td style="color: black; border: 1px solid black; text-align: center;">Initial Concentration (M)</td> <td style="color: black; border: 1px solid black; text-align: center;">1.00</td> <td style="color: black; border: 1px solid black; text-align: center;">0</td> <td style="color: black; border: 1px solid black; text-align: center;">0</td> </tr> <tr> <td style="color: black; border: 1px solid black; text-align: center;">Change (M)</td> <td style="color: black; border: 1px solid black; text-align: center;">−x</td> <td style="color: black; border: 1px solid black; text-align: center;">+x</td> <td style="color: black; border: 1px solid black; text-align: center;">+x</td> </tr> <tr> <td style="color: black; border: 1px solid black; text-align: center;">Equilibrium Concentration (M)</td> <td style="color: black; border: 1px solid black; text-align: center;">1.00 − x</td> <td style="color: black; border: 1px solid black; text-align: center;">x</td> <td style="color: black; border: 1px solid black; text-align: center;">x</td> </tr> </tbody> </table> Stap 3. Los de verandering en de evenwichtsconcentraties op. Het vervangen van de evenwichtsconcentraties in de evenwichtsconstantevergelijking geeft <img alt="Eq2" src="/files/ftp_upload/11390/11390_Equation 2.png" style="text-align: center; height: 170px;"> Een vergelijking van de vorm ax2 + bx + c = 0 kan worden herschikt om x op te lossen: <img alt="Eq3" src="/files/ftp_upload/11390/11390_Equation 3.png" style="text-align: center; height: 90px;"> In dit geval a = 1, b = 0,0211 en c = −0,0211. Het vervangen van de juiste waarden voor a, b, en c levert het volgende op: <img alt="Eq4" src="/files/ftp_upload/11390/11390_Equation 4.png" style="text-align: center; height: 90px;"> De twee wortels van de kwadraten zijn daarom <img alt="Eq5" src="/files/ftp_upload/11390/11390_Equation 5.png" style="text-align: center; height: 50px;"> Voor dit scenario is alleen de positieve wortel fysiek betekenisvol (concentraties zijn nul of positief), en dus x = 0,135 M. De evenwichtsconcentraties zijn <img alt="Eq6" src="/files/ftp_upload/11390/11390_Equation 6.png" style="text-align: center; height: 130px;"> Stap 4. Bevestig de berekende evenwichtsconcentraties. Vervanging in de uitdrukking voor Kc (om de berekening te controleren) geeft <img alt="Eq7" src="/files/ftp_upload/11390/11390_Equation 7.png" style="text-align: center; height: 90px;"> De evenwichtsconstante berekend op basis van de evenwichtsconcentraties is gelijk aan de waarde van Kc gegeven in de opgave (afgerond op het juiste aantal significante cijfers).

Evenwichtsconcentraties berekenen

Being able to calculate equilibrium concentrations is essential to many areas of science and technology&#8212;for example, in the formulation and dosing ...

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Chemical Equilibrium

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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 effect.

A common approach is needed when equilibrium concentrations must be derived from starting values and a known Kc. The first step is to decide which way the reaction will move to reach equilibrium. Next, an ICE table is set up. ICE stands for initial, change, and equilibrium. After that, the concentration changes are solved and used to find the equilibrium concentrations. The final step is to check the answer.

The decomposition of PCl5(g) into PCl3(g) and Cl2(g) shows how this works. In the example, Kc is 0.0211, and the mixture starts with only PCl5 at 1.00 M. Because only the reactant is present at the start, Qc equals 0, so the reaction moves to the right. The ICE table gives an initial concentration of 1.00 M for PCl5 and 0 for the products, with changes of -x, +x, and +x.

Substituting the equilibrium values into the Kc expression leads to a quadratic equation. Solving it gives two roots, but only the positive root is physically meaningful because concentrations cannot be negative. In this case, x = 0.135 M. The equilibrium concentrations are then found for PCl5, PCl3, and Cl2. A final substitution into the Kc expression confirms that the calculated value matches the given equilibrium constant when rounded correctly.

Equilibrium Concentrations from an ICE Table

equilibrium-concentrations-from-an-ice-table

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

le-chatelier-s-principle-changing-concentration

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

homogeneous-equilibria-for-gaseous-reactions

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...

calculating equilibrium concentrations

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...

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 to find equilibrium concentrations with an ICE table, use Kc, and check answers with a PCl5 decomposition example for high school chemistry.

Equilibrium Concentrations from an ICE Table | JoVE K12

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3.2.9-12.F

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Equilibrium Concentrations from an ICE Table

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