AP Chemistry 7.10 Reaction Quotient and Le Chateilier’s Principle Study Notes - New Syllabus Effective fall 2024
AP Chemistry 7.10 Reaction Quotient and Le Chateilier’s Principle Study Notes- New syllabus
AP Chemistry 7.10 Reaction Quotient and Le Chateilier’s Principle Study Notes – AP Chemistry – per latest AP Chemistry Syllabus.
LEARNING OBJECTIVE
Explain the relationships between Q, K, and the direction in which a reversible reaction will proceed to reach equilibrium.
Key Concepts:
- Calculating the Equilibrium Constant
- Magnitude of the Equilibrium Constant
- Manipulating the Equilibrium Constant
7.10.A.1 Disturbance and Response in Equilibrium Systems:
1.Equilibrium in Chemical Systems:
Equilibrium is the condition in a reversible reaction of chemicals when the rate of the forward and backward processes are equal so that there is constant reactant as well as product concentration.
Key Characteristics:
i. Dynamic: There is proceeding of reactions, but there is no net concentration change.
ii. Constant Concentrations: The reactants as well as products’ concentration remains constant.
iii. Forward = Reverse Rate: Production rate of products is equal to rate of formation of reactants.
iv. Closed System: Equilibrium can be attained only in a closed system.
v. Equilibrium Constant (K): The ratio of amount of products to amount of reactants remains constant at a given temperature.
vi. Affected by Changes: The equilibrium position changes with changes in temperature, pressure, or concentration (Le Chatelier’s Principle).
2. Reaction Quotient (Q):
| Property | Reaction Quotient (Q) | Equilibrium Constant (K) |
|---|---|---|
| Definition | Ratio of product to reactant concentrations at any point | Ratio of product to reactant concentrations at equilibrium |
| Calculation Time | Can be calculated at any time during the reaction | Calculated only when the system is at equilibrium |
| Formula | Same as Q, but only at equilibrium | |
| Use | Predicts the direction of the reaction | Indicates the position of equilibrium at a given temperature |
| Equilibrium Condition | Not necessarily at equilibrium | Always at equilibrium |
| Comparison with K | Compares with K to predict shift: Q < K (forward), Q > K (reverse) | Constant value for a reaction at a specific temperature |
3. Le Chatelier’s Principle:
A system that is in equilibriated will adjust to counteract alterations.
– Adding reactants: Shifts towards products →
– Adding products: Shifts towards reactants ←
– Raising temperature: Shifts towards endothermic reaction (absorbs heat) →
– Lowering temperature: Shifts towards exothermic reaction (releases heat) ←
– Applying pressure: Shifts towards fewer gas molecules →
– Relaxing pressure: Shifts towards more gas molecules ←
4. Re-establishing Equilibrium:
When the system is disturbed, the reaction quotient (Q) changes, but the system will adjust so that Q equals the equilibrium constant (K) again.
i. If Q < K:
The system shifts toward the products to force Q higher, using up reactants and creating additional products until Q equals K.
ii. If Q > K:
The system shifts towards the reactants to reduce Q, using up products and producing more reactants until Q is equal to K.
7.10.A.2 Effect of Stresses on Q and K in Equilibrium:
1. Effect of Concentration Changes on Q:
| Change in Concentration | Effect on Q | Shift in Equilibrium |
|---|---|---|
| Increase in reactants | Q decreases | Shifts towards products |
| Increase in products | Q increases | Shifts towards reactants |
| Decrease in reactants | Q increases | Shifts towards reactants |
| Decrease in products | Q decreases | Shifts towards products |
2. Effect of Temperature Changes on K:
| Temperature Change | Endothermic Reaction (Heat Absorbed) | Exothermic Reaction (Heat Released) |
|---|---|---|
| Increase in Temperature | K increases (shifts towards products) | K decreases (shifts towards reactants) |
| Decrease in Temperature | K decreases (shifts towards reactants) | K increases (shifts towards products) |
Temperature affects K by shifting the equilibrium in favor of either the forward or reverse reaction, depending on whether heat is absorbed or released.
3. Le Chatelier’s Principle:
| Disturbance | System Response |
|---|---|
| Increase in Reactants | Shifts towards products |
| Increase in Products | Shifts towards reactants |
| Decrease in Reactants | Shifts towards reactants |
| Decrease in Products | Shifts towards products |
| Increase in Temperature | Endothermic: towards productsExothermic: towards reactants |
| Decrease in Temperature | Endothermic: towards reactantsExothermic: towards products |
OLD Content
Reaction Quotient and Le Chateilier’s Principle
- A disturbance to a system at equilibrium causes Q to differ from K
- The reaction will “shift” to bring Q back into agreement with K
Effects of Changes to a System
1. Pressure (only affects gasses); there are three ways to change the pressure of a system with gas
- Changing pressure may alter the equilibrium position, but it does not alter the equilibrium constant (K)
a. Effect of changing Pressure:
- Increase pressure, the equilibrium will shift to the side with less moles of gas
- Pressure decreases, equilibrium will shift to side to side with more moles of gas
b. Addition of an inert (unreactive) gas:
- Increases the total pressure but has no effect on the equilibrium of the system or the concentrations or partial pressures of the reactants or products
c. Effect of changing Volume:
- Increase volume: shift to side with more moles of gas
- Decrease volume: shift to side with less moles of gas
d. Changing the volume of the system affects pressure (Boyle’s Law) → See letter a…
- V increases → P decreases; V decreases → P increases
- When volume of the container decreases, the system responds by reducing its own volume by decreasing the total number of gaseous molecules
- V increases → P decreases; V decreases → P increases
2. Temperature: K will change depending on temp (treat energy as either a reactant or product)
- Ex: Endothermic
- Treat heat as reactant
- Effect on K: Adding heat will shift in forward direction so K > 1
- Forward direction → high temp & LP; Reverse direction → low temp & HP
- Ex: Exothermic:
- Treat heat as a product
- Effect on K: Adding heat will shift in reverse reaction so K < 1
- Forward direction → low temp & HP; Reverse direction → high temp and LP
3. Concentration: the system will shift away from the added component (or toward the removed component)
- If question adds something in reaction, added component will likely react with smthn in the reaction
- If add something that forms a precipitate (often OH-) → are taking reactant out of reaction
- What if dilute the solution? (ex: add water vapor)
- Dilute → all molarities (products and reactants) will decrease
- Diluting will always cause a shift toward more aqueous species
- If there are more reactants (denominator) than products → Q > K
- If there are more products (numerator) than reactants → Q < K
- What if increase the concentration of the solution?
- Concentrate → all molarities (products and reactants) will increase
- Concentrating will always cause a shift toward less aqueous species!
- If there are more reactants (denominator) than products → Q < K
- If there are more products (numerator) than reactants → Q > K
- Changing the amount of liquid/solid or adding a catalyst will have NO SHIFT