On the previous page, you learned that if stress is put on the system, such as reactants or products being added or removed, then the system will shift the equilibrium to relieve the added stress. This is called Le Chatelier’s Principle. Study the tabs to learn how Le Chatelier’s Principle predicts how changes in concentration and temperature will affect the equilibrium of a reaction.
A reaction can be stressed by adding more of one of the substances (i.e., increasing its concentration). The equilibrium will shift away from the addition so that it can use up the excess chemical and get back to the equilibrium ratio.
Conversely, a reaction can also be stressed by removing some of a substance (i.e., decreasing its concentration). In this case, the equilibrium will shift towards the removal so that it can make up the deficit of the missing chemical and get back to the equilibrium ratio.
Click each stress in this chart to see how adding or removing different substances would affect the equilibrium for this reaction:
N2 (g) + 3 H2 (g) ⇌ 2 NH3 (g)
| Stress | Equilibrium Shift | Reasoning |
|---|---|---|
|
→ (towards products) |
More reactants were added, so the reaction will favor the forward direction, using up the excess reactants and making more products until it can get back to the equilibrium ratio. |
|
|
→ (towards products) |
More reactants were added, so the reaction will favor the forward direction, using up the excess reactants and making more products until it can get back to the equilibrium ratio. |
|
|
← (towards reactants) |
More products were added, so the reaction will favor the
reverse direction, using up
the excess products and making more reactants until it can
get back to the equilibrium ratio.
|
|
|
← (towards reactants) |
Reactants were removed, so the reaction will favor the
reverse direction, making
more reactants to make up for the deficit until it can get
back to the equilibrium ratio.
|
|
|
← (towards reactants) |
Reactants were removed, so the reaction will favor the reverse direction, making more reactants to make up for the deficit until it can get back to the equilibrium ratio. |
|
|
→ (towards products) |
Products were removed, so the reaction will favor the forward direction, making more products to make up for the deficit until it can get back to the equilibrium ratio. |
Recall that reactions can be described as endothermic or exothermic. An endothermic reaction is one that absorbs heat energy, while an exothermic reaction is one that releases heat energy. A reversible reaction is endothermic in one direction and exothermic in the opposite direction.
Raising the temperature requires adding heat, so the reaction will shift in the endothermic direction so that it can use up the excess heat and get back to the equilibrium ratio. Conversely, lowering the temperature requires removing heat, so the reaction will shift in the exothermic direction to produce more heat to make up for the deficit and get back to the equilibrium ratio.
Because an exothermic reaction produces heat, it can be helpful to think of the heat energy as a product of that reaction. Conversely, because an endothermic reaction absorbs heat, think of the heat energy as a reactant necessary for that reaction to proceed.
Click each stress in this chart to see how increasing or decreasing the temperature affects the equilibrium for this exothermic reaction:
N2 (g) + 3 H2 (g) ⇌ 2 NH3 (g) + 22 kcal
| Stress | Equilibrium Shift | Reasoning |
|---|---|---|
|
← (towards reactants) |
Heat is a “product” so when more heat energy is added, the reaction will favor the reverse (endothermic) direction, using up the excess heat until it can get back to the equilibrium ratio. |
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|
→ (towards products) |
Heat is a “product” so when heat energy is removed, the reaction will favor the forward (exothermic) direction, producing more heat until it can get back to the equilibrium ratio. |
Click each stress in this chart to see how increasing or decreasing the temperature affects the equilibrium for this endothermic reaction:
12.6 kcal + H2 (g) + I2 (g) ⇌ 2HI (g)
| Stress | Equilibrium Shift | Reasoning |
|---|---|---|
|
→ (towards products) |
Heat is a “reactant” so when more heat energy is added, the reaction will favor the forward (endothermic) direction, using up the excess heat until it can get back to the equilibrium ratio. |
|
|
← (towards reactants) |
Heat is a “reactant” so when heat energy is removed, the reaction will favor the reverse (exothermic) direction, producing more heat until it can get back to the equilibrium ratio. |
Question
How does a reaction at equilibrium respond to changes in concentration or temperature?
Changing concentrations makes the reaction shift to replace or use up what was added or removed. Changing temperature makes the reaction favor either the heat-absorbing (endothermic) or heat-releasing (exothermic) side.
Question
What do changes in concentration and temperature have in common in how they affect a reaction at equilibrium?
Both cause the reaction to shift in a way that counteracts the change. For concentration, the reaction shifts to use up what was added or replace what was removed. For temperature, the reaction shifts to absorb added heat or release heat if the temperature decreases. In both cases, the system tries to restore balance according to Le Chatelier’s Principle.
