The effect of pressure on an endothermic reaction depends on the change in the number of moles of gas during the reaction. According to Le Chatelier's principle, increasing the pressure will shift the equilibrium to the side with fewer moles of gas.
Here's a breakdown:
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Le Chatelier's Principle: This principle states that if a change of condition (like pressure, temperature, or concentration) is applied to a system in equilibrium, the system will shift in a direction that relieves the stress.
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Endothermic Reactions: These are reactions that absorb heat from their surroundings.
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Effect of Pressure (Assuming Gases are Involved):
- If the reaction produces more moles of gas: Increasing the pressure will shift the equilibrium towards the reactants (the side with fewer moles of gas). This will hinder the forward, endothermic reaction.
- If the reaction produces fewer moles of gas: Increasing the pressure will shift the equilibrium towards the products (the side with fewer moles of gas). This will favor the forward, endothermic reaction.
- If there is no change in the number of moles of gas: Pressure has no effect on the equilibrium.
Example:
Consider the following endothermic reaction:
N2O4(g) ⇌ 2NO2(g) ΔH > 0
In this reaction, one mole of N2O4 gas decomposes into two moles of NO2 gas.
- Increasing the pressure will shift the equilibrium towards the reactants (N2O4) because there are fewer moles of gas on the reactant side. This will reduce the amount of NO2 formed.
- Decreasing the pressure will shift the equilibrium towards the products (NO2) because there are more moles of gas on the product side. This will increase the amount of NO2 formed.
Summary Table:
| Change in Pressure | Shift in Equilibrium | Effect on Endothermic Reaction (Forward) |
|---|---|---|
| Increase | Towards Fewer Gas Moles | Hinders if products have more gas moles, Favors if reactants have more gas moles, No effect if gas moles are equal |
| Decrease | Towards More Gas Moles | Favors if products have more gas moles, Hinders if reactants have more gas moles, No effect if gas moles are equal |
Important Note: The effect of pressure is only significant if gases are involved in the reaction and if there is a change in the number of moles of gas during the reaction. If the reaction involves only liquids or solids, or if the number of moles of gas remains the same, pressure will have a negligible effect.
