Chemical equilibria flashcards

Chemical equilibria describes the state reached in a reversible reaction when the rate of the forward reaction equals the rate of the reverse reaction. At this point, the concentrations of reactants and products remain constant, though both reactions continue. Students must understand the equilibrium constant Kc, how to write expressions for it, and how its value changes only with temperature. Le Chatelier's principle explains how a system responds to changes in concentration, pressure, and temperature by shifting the position of equilibrium. Students should distinguish between a shift in equilibrium position and a change in Kc, and understand the role of catalysts. Applying these ideas to industrial processes, such as the Haber process, is also essential.

1. Card 1

   What is meant by a dynamic equilibrium?

   A state where the forward and reverse reactions occur at equal rates, so concentrations remain constant. It only exists in a closed system.

2. Card 2

   What does the equilibrium constant Kc represent?

   Kc is the ratio of product concentrations to reactant concentrations, each raised to the power of their stoichiometric coefficients, at equilibrium.

3. Card 3

   Write the Kc expression for: aA + bB ⇌ cC + dD

   Kc = [C]^c [D]^d / [A]^a [B]^b. Square brackets denote molar concentration in mol/dm3.

4. Card 4

   How does temperature affect Kc?

   Temperature is the only factor that changes the value of Kc. Increasing temperature raises Kc for endothermic reactions and lowers it for exothermic reactions.

5. Card 5

   What effect does a catalyst have on equilibrium?

   A catalyst speeds up both forward and reverse reactions equally, so it does not change the position of equilibrium or the value of Kc.

6. Card 6

   State Le Chatelier's principle.

   If a system at equilibrium is subjected to a change, it will shift its position to oppose that change and restore a new equilibrium.

7. Card 7

   What happens to equilibrium position if concentration of a reactant is increased?

   The equilibrium shifts in the forward direction to use up the added reactant, producing more products until equilibrium is re-established.

8. Card 8

   How does increasing pressure affect a gaseous equilibrium?

   The equilibrium shifts towards the side with fewer moles of gas, reducing pressure. If moles are equal on both sides, there is no shift.

9. Card 9

   If Kc is very large, what does this tell you about the equilibrium?

   A large Kc means the equilibrium lies to the right, so products are favoured and the reaction goes almost to completion.

10. Card 10

    Why is a compromise temperature used in the Haber process?

    High temperature speeds up the reaction but shifts equilibrium left (exothermic reaction), so a compromise temperature around 450 degrees C balances yield and rate.

11. Card 11

    Does changing pressure change the value of Kc?

    No. Changing pressure shifts the position of equilibrium but does not alter Kc, which only changes with temperature.

12. Card 12

    What are the units of Kc and how are they determined?

    Units of Kc depend on the overall change in moles in the expression. Cancel mol/dm3 terms; if moles balance, Kc is dimensionless.

• Many students incorrectly believe that a catalyst shifts the position of equilibrium or changes Kc, when in fact it only helps the system reach equilibrium more quickly.
• Students often confuse a change in the position of equilibrium with a change in Kc, not realising that only a change in temperature alters the value of Kc.
• A common error is assuming that equal concentrations of reactants and products exist at equilibrium, when in reality the position of equilibrium depends on the value of Kc and can lie far to one side.

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