Chemical Equilibria & Le Chatelier's Principle
In a closed system, a reversible reaction reaches dynamic equilibrium when the rates of the forward and reverse reactions become equal. This guide covers how to apply Le Chatelier's principle to changes in temperature, pressure, and concentration, and how to calculate the equilibrium constants Kc and Kp.
- Kc
- The equilibrium constant calculated using the equilibrium concentrations of reactants and products.
- Kp
- The equilibrium constant calculated using the partial pressures of gaseous reactants and products.
- Mole Fraction
- The number of moles of a specific gas divided by the total number of moles of gas in a mixture.
Chemical Equilibria & Le Chatelier's Principle: Reaction Profile (ΔH & E_a) + Le Chatelier Yield Simulator
Toggle a catalyst on the enthalpy profile diagram or adjust temperature and pressure to see how dynamic equilibrium shifts.
Modelled on N₂(g) + 3H₂(g) ⇌ 2NH₃(g) (ΔH = −92 kJ/mol) (4 moles gas → 2 moles gas):
Higher pressure shifts equilibrium to the side with fewer gas moles (right, 4 → 2). Lower temperature favours the exothermic forward reaction.
Key Concepts & Exam Definitions (5)
- Kc
- The equilibrium constant calculated using the equilibrium concentrations of reactants and products.
- Kp
- The equilibrium constant calculated using the partial pressures of gaseous reactants and products.
- Mole Fraction
- The number of moles of a specific gas divided by the total number of moles of gas in a mixture.
- Partial Pressure
- The pressure exerted by an individual gas in a mixture, calculated by multiplying its mole fraction by the total pressure.
- Haber Process
- An industrial process for producing ammonia from nitrogen and hydrogen, using principles of equilibrium to maximize yield.
Quick-Fire Flashcards (5)
Click any card to check the answer, then mark it as Known to track your progress.
If an exothermic reaction is heated, which way does the equilibrium shift?
How does an increase in pressure affect an equilibrium involving gases?
What is the only factor that changes the value of an equilibrium constant (Kc or Kp)?
Write the expression for Kc for the reaction: aA + bB ⇌ cC + dD.
In the Haber process, why is a high pressure (e.g., 200 atm) used?
Exam-Style Questions & Mark Schemes (3)
Write or think through your answer first, then click Check Mark Scheme to see the exact points examiners award marks for.
- Q1
The reaction A(g) + 3B(g) ⇌ 2C(g) is exothermic. Explain the effect of increasing temperature and pressure on the equilibrium yield of C.
Click “Check Mark Scheme” to compare your answer, Increasing temperature: The forward reaction is exothermic. By Le Chatelier's principle, the system shifts to the left in the endothermic direction to oppose the increase in temperature, decreasing the yield of C. Increasing pressure: There are 4 moles of gas on the left and 2 moles on the right. The system shifts to the right (side with fewer moles) to reduce pressure, increasing the yield of C.
- Q2
For an endothermic reaction, explain why the value of Kc increases as temperature increases.
Click “Check Mark Scheme” to compare your answer, Increasing the temperature favors the forward endothermic reaction to oppose the change. This means at equilibrium, the concentration of products increases and the concentration of reactants decreases. Since Kc = [Products]/[Reactants], a larger numerator and smaller denominator result in a larger value for Kc.
- Q3
Why are compromise conditions used in industrial processes like the Contact Process?
Click “Check Mark Scheme” to compare your answer, While a low temperature might give a higher equilibrium yield for an exothermic reaction, the rate of reaction would be too slow to be economically viable. High pressures give high yields but are expensive to maintain safely. Compromise conditions provide an acceptable yield at a reasonable rate and cost.
Common Exam Questions (3)
What is dynamic equilibrium?
Dynamic equilibrium is reached in a closed system when the forward and reverse reactions continue at the same rate, so the concentrations of reactants and products remain constant.
What is Le Chatelier's principle?
Le Chatelier's principle states that if a system at dynamic equilibrium is subjected to a change in temperature, pressure, or concentration, the position of equilibrium shifts in the direction that opposes the change.
Does a catalyst affect the position of equilibrium?
No. A catalyst increases the rate of both the forward and reverse reactions equally by providing an alternative pathway with a lower activation energy, so equilibrium is reached faster without changing the equilibrium position or the value of Kc.
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