12 Equilibrium — Applying Le Chatelier’s Principle to Determine Impact of Changing Pressure
Question
Consider the following equilibria:
2NH3 (g) ⇌ N2 (g) + 3H2 (g); ΔH = 92kJ
- How will an increase in temperature affect the following equilibria?
- How will a decrease in the volume of the reaction vessel affect the equilibria?
Show/Hide Answer
- ΔT increases = System shifts right
- ΔV decreases = System shifts left
Refer to Section 5.5: Shifting Equilibria – Le Chatelier’s Principle (1).
Strategy Map
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| Strategy Map Steps |
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1. Identify if the reaction is endothermic or exothermic.
Show/Hide HintLook at the sign of the provided enthalpy ΔH value:
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2. Identify which direction the reaction would shift to minimize this disturbance.
Show/Hide HintIs heat a reactant or a product for the reaction? If the amount of reactant or product increases, the equilibria will shift in the direction that reduces the reactant or product. |
3. Identify which side of the reaction (the products or reactants) has higher moles.
Show/Hide HintGases expand to fill the entire volume of their container. The concentration of a gas (mol/L) is defined by the container volume. |
4. Identify which direction the reaction would shift to minimize this disturbance.
Show/Hide HintEquilibrium can be defined through concentration (Kc) or pressure (Kp). How does the disturbance to volume relate to pressure? Shifting to the side that has fewer total moles of gas will decrease the pressure in the container; meanwhile, shifting to the side with a higher total of gas moles will increase the pressure. |
Solution
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a. ΔT increases = System shifts right
This is an endothermic reaction with a positive ΔH value. The reaction minimizes the disturbance of the increased temperature by favouring the forward reaction to absorb heat from the surroundings to form more products.
b. ΔV decreases = System shifts left
Pressure increases as volume decreases. The reaction minimizes the disturbance of the shrinking container by favouring the reverse reaction, forming more reactants, which have less total moles of gas.
Guided Solution
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The guided solution below will give you the reasoning for each step to get your answer, with reminders and hints.
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| Guided Solution Ideas |
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This question is a theory problem where you will use your knowledge of Le Chatelier’s Principle to predict how a system will shift to counteract different disturbances to re-achieve equilibrium.
Show/Hide ResourceRefer to Section 5.5: Shifting Equilibria – Le Chatelier’s Principle (1). |
| 1. When the temperature is increased.
Recall how to identify if a reaction is endothermic or exothermic. In this question you are given the enthalpy for the reaction. In this case the enthalpy is a positive value. Show/Hide Don’t Forget!A positive enthalpy value means heat energy must be absorbed by the system from the surroundings for this reaction to occur. This reaction is endothermic. Knowing the reaction type will help because we now know which side of the reaction the energy (in the form of heat) is. Show/Hide Think About This!Consider ‘heat’ to be a reactant for an endothermic reaction. Since the temperature is increasing (adding more heat), we can predict how the system will shift to counteract the disturbance. Show/Hide Watch Out!The disturbance is increasing the amount of a reactant (i.e., heat). Therefore, the reaction will shift right to favour the forward reaction, absorbing (i.e., decreasing) heat and forming more products. |
| 2. When the contained volume is decreased.
We can predict the shift of the reaction due to a change in volume by analyzing which side of the reaction has the greatest number of moles of gas. Show/Hide Don’t Forget!To identify which side has a higher number of moles, look at the physical states of each species as well as the stoichiometric coefficients of all gas species on both the reactant and product sides. Show/Hide Watch Out!In this case the stoichiometry is two on the reactant side and four on the product side. Consider how a change in volume impacts concentration and pressure of gases. Show/Hide Think About This!As volume decreases, concentration (mol/L) increases. As volume increases, concentration decreases. Because the K expression uses the stoichiometric coefficients as exponents, if the total number of gas moles on the reactant and product sides of the reaction is different, the concentrations will be disproportionately impacted by a change in volume. Show/Hide Don’t Forget!Recall that if the volume increases, the reaction will shift to the side with the higher moles. If the volume decreases, the reaction will shift to the side with lower moles. |
| Complete Solution |
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| a. ΔT increases = Shifts right
Heatenergy + 2NH3 (g) ⇌ N2 (g) + 3H2 (g) The reaction is endothermic, meaning heat is a reactant. Equilibrium is achieved when the concentration of reactants and products no longer changes over time. An increase in temperature is an increase in the amount of a reactant. The reaction minimizes the disturbance of the increased temperature (added heat) by proceeding in the forward direction, absorbing heat from the surroundings. |
| b. ΔV decreases = Shifts left
2NH3 (g) ⇌ N2 (g) + 3H2 (g) Total moles of gas on the reactant side vs. product side (2 vs. 4); the reactants have fewer total moles of gas species. The reaction minimizes the disturbance of the shrinking container (which increases the pressure) by proceeding in the reverse direction. Shifting towards the side of the reaction with the least amount of moles of gas (reactants) will reduce the pressure inside the container, counteracting the disturbance to return to equilibrium. |
Check Your Work
Think about the K expression. How does the disturbance impact the distribution of products and reactants.
Show/Hide Think About This!
- Consider heat as a reactant for endothermic reactions and a product for exothermic reactions.
- Consider how the change in volume of a container impacts the total pressure.
Show/Hide Answer
Systems at equilibrium can be disturbed by changes to temperature, concentration, and, in some cases, volume and pressure. If the number of moles of gas is different on the reactant and product sides of the reaction equation, volume and pressure changes will disproportionately impact the distribution of products and reactants, disturbing the equilibrium.
Le Chatelier’s Principle describes the system’s response to these disturbances:
- The system will respond in a way that counteracts the disturbance.
Increasing the temperature will always shift an endothermic reaction right towards the formation of products and an exothermic reaction left towards the formation of reactants.
Decreasing the volume of a container increases the concentration of gas species and the internal pressure. The reaction will always shift to the side of the reaction equation with the fewest total moles of gases (reducing the internal pressure). Increasing the volume of a container decreases the concentration of gas species and the internal pressure. The reaction will always shift to the side of the reaction equation with the greatest total moles of gases (increasing the internal pressure).
PASS Attribution
- LibreTexts PASS Chemistry Book CHEM 1510/1520 (2).
- Question 5.E.24a from LibreTexts TRU: Fundamentals and Principles of Chemistry (CHEM 1510 and CHEM 1520) (3) is used under a CC BY 4.0 license.
- Question 5.E.24a is question 13.E.3.13: Q13.3.8 in LibreTexts Chemistry 1e (4), which is under a CC BY 4.0 license.
- Question 13.E.3.13: Q13.3.8 is question 39a from OpenStax Chemistry 2e (5), which is under a CC BY 4.0 license. Access for free at https://openstax.org/books/chemistry-2e/pages/1-introduction
References
1. OpenStax. 5.5: Shifting Equilibria – Le Chatelier’s Principle. In TRU: Fundamentals and Principles of Chemistry (CHEM 1510 and CHEM 1520). LibreTexts, 2022. https://chem.libretexts.org/Courses/Thompson_Rivers_University/TRU%3A_Fundamentals_and_Principles_of_Chemistry_(CHEM_1510_and_CHEM_1520)/05%3A_Chemical_Equilibrium/5.05%3A_Shifting_Equilibria_-_Le_Chateliers_Principle.
2. Blackstock, L.; Brewer, S.; Jensen, A. In PASS Chemistry Book CHEM 1510/1520; LibreTexts, 2023. https://chem.libretexts.org/Courses/Thompson_Rivers_University/PASS_Chemistry_Book_CHEM_1510%2F%2F1520.
3. OpenStax. 5.E: Fundamental Equilibrium Concepts (Exercises). In TRU: Fundamentals and Principles of Chemistry (CHEM 1510 and CHEM 1520). LibreTexts, 2022. https://chem.libretexts.org/Courses/Thompson_Rivers_University/TRU%3A_Fundamentals_and_Principles_of_Chemistry_(CHEM_1510_and_CHEM_1520)/05%3A_Chemical_Equilibrium/5.E%3A_Fundamental_Equilibrium_Concepts_(Exercises).
4. OpenStax. 13.E: Fundamental Equilibrium Concepts (Exercises). In Chemistry 1e (OpenSTAX). LibreTexts, 2022. https://chem.libretexts.org/Bookshelves/General_Chemistry/Chemistry_1e_(OpenSTAX)/13%3A_Fundamental_Equilibrium_Concepts/13.E%3A_Fundamental_Equilibrium_Concepts_(Exercises).
5. Flowers, P.; Robinson, W. R.; Langley, R.; Theopold, K. Ch. 13 Exercises. OpenStax, 2015. https://openstax.org/books/chemistry/pages/13-exercises.
