Questions and Answers
If a catalyst is added to the N2(g) + 3H2(g) ⇌ 2NH3(g) system at equilibrium, what will happen?
The equilibrium will not shift, as the catalyst does not affect the equilibrium position.
If the total pressure of the N2(g) + 3H2(g) ⇌ 2NH3(g) system is increased by adding an inert gas, such as helium, what will happen to the equilibrium?
The equilibrium will not shift, as the inert gas does not participate in the reaction.
If the temperature of the N2(g) + 3H2(g) ⇌ 2NH3(g) system is increased, what will happen to the equilibrium constant, Kc?
Kc will increase, as the forward reaction is endothermic.
If the temperature of the N2(g) + 3H2(g) ⇌ 2NH3(g) system is decreased, what will happen to the equilibrium?
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According to Le Châtelier's principle, if the pressure of the N2(g) + 3H2(g) ⇌ 2NH3(g) system is increased, what will happen?
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What happens to an equilibrium reaction when an external stress is applied, according to Le Châtelier's principle?
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If the initial concentrations of the N2(g) + 3H2(g) ⇌ 2NH3(g) system are not at equilibrium, and a catalyst is added, what will happen?
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Which factor does not affect the equilibrium constant Kc in a chemical reaction?
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In the system involving cis-stilbene and trans-stilbene, if more cis-stilbene is added at equilibrium, what will happen to the concentrations of both compounds?
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What does Kc = 24.0 at 200 °C represent in the context of a chemical equilibrium reaction?
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If the concentration of cis-stilbene is initially 0.5 mol/L instead of 1.0 mol/L in the system, how would this change affect the equilibrium concentrations of both cis- and trans-stilbene?
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How does a change in volume affect a system at equilibrium, based on Le Châtelier's principle?
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What is the effect of adding iron(III) nitrate [Fe(NO3)3] to the original solution containing the FeSCN^{2+} equilibrium?
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How does the addition of oxalic acid (H2C2O4) affect the original red solution containing the FeSCN^{2+} equilibrium?
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According to Le Châtelier's principle, how does an increase in pressure affect the equilibrium of a gaseous reaction?
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Which of the following relationships correctly describes the ideal gas law?
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According to Le Châtelier's principle, how do changes in pressure affect the concentrations of species in condensed phases (liquids and solids)?
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In the expression $P = \frac{nRT}{V}$, what does the variable C represent?
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Study Notes
Equilibrium System
- Increasing pressure in a system decreases volume, resulting in an increase in concentration (n/V) of both reactants and products.
- This increase in pressure causes Qc > Kc, leading to a shift in the reaction to the left until Qc = Kc.
- An increase in pressure (decrease in volume) favors the net reaction that decreases the total number of moles of gases.
- A decrease in pressure (increase in volume) favors the net reaction that increases the total number of moles of gases.
Le Châtelier's Principle: Changes in Temperature
- A change in temperature can alter the equilibrium constant.
- If the forward reaction is endothermic (absorbs heat, ΔH° > 0), an increase in temperature will favor the endothermic direction, resulting in a decrease in [N2O4] and an increase in [NO2].
- If the forward reaction is exothermic (releases heat, ΔH° < 0), a decrease in temperature will favor the exothermic direction, resulting in a decrease in [NO2] and an increase in [N2O4].
- The equilibrium constant Kc increases when the system is heated and decreases when the system is cooled.
Le Châtelier's Principle: The Effect of a Catalyst
- A catalyst enhances the rate of a reaction by lowering the reaction's activation energy.
- The presence of a catalyst does not alter the equilibrium constant or shift the position of an equilibrium system.
- Adding a catalyst to a reaction mixture that is not at equilibrium will cause the mixture to reach equilibrium sooner.
Factors That Affect Chemical Equilibrium
- Concentration
- Pressure
- Volume
- Temperature
- Catalyst (does not change K)
Le Châtelier's Principle: Changes in Concentration
- If an external stress is applied to a system at equilibrium, the system adjusts in such a way that the stress is partially offset as the system reaches a new equilibrium position.
- Examples of concentration changes:
- Adding sodium thiocyanate (NaSCN) to a solution of iron(III) thiocyanate [Fe(SCN)3] increases the concentration of SCN-, causing the equilibrium to shift from right to left.
- Adding iron(III) nitrate [Fe(NO3)3] to the original solution increases the concentration of Fe3+, causing the equilibrium to shift from right to left.
- Adding oxalic acid (H2C2O4) to the original solution decreases the concentration of Fe3+, causing the equilibrium to shift from left to right.
Le Châtelier's Principle: Changes in Volume and Pressure
- Changes in pressure generally do not affect the concentrations of species in condensed phases.
- Gases are compressible and are affected by changes in pressure.
- The ideal gas law (PV = nRT) can be used to describe the behavior of gases in response to changes in pressure.
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