AP Chemistry 7.1 Introduction to Equilibrium Study Notes - New Syllabus Effective fall 2024
AP Chemistry 7.1 Introduction to Equilibrium Study Notes- New syllabus
AP Chemistry 7.1 Introduction to Equilibrium Study Notes – AP Chemistry – per latest AP Chemistry Syllabus.
LEARNING OBJECTIVE
Explain the relationship between the occurrence of a reversible chemical or physical process, and the establishment of equilibrium, to experimental observations.
Key Concepts:
- Reactions & Equilibrium
- Reversible Reactions
7.1.A.1 Reversible Processes in Chemistry:
1. Reversibility in Chemical Processes:
| Feature | Reversible Process | Irreversible Process |
|---|---|---|
| Direction | Both forward and reverse reactions occur | Reaction proceeds in one direction |
| Equilibrium | Achieves equilibrium, where forward and reverse rates are equal | Does not reach equilibrium |
| Energy | Energy can be absorbed or released, depending on direction | Typically releases energy (exothermic) |
| Reversibility | Can be reversed by changing conditions | Cannot be easily reversed |
2. Phase Changes and Gas Behavior:
i. Evaporation:
– Liquid → gas below boiling point.
– Occurs on the surface of the liquid.
– Example: Water evaporating from a lake.
ii. Condensation:
– Gas → liquid when cooled.
– Occurs when gas molecules lose energy.
– Example: Dew on grass.
iii. Absorption:
– Gas or liquid is absorbed evenly by another substance (liquid or solid).
– Example: CO₂ absorbed by water in a carbonated drink.
iv. Desorption:
– Gas or liquid released from an absorbing material.
– Example: Volatile gases released by activated charcoal.
These processes specify the way in which materials undergo phase change or interact with each other.
3. Acid-Base and Redox Reactions:
Acid-Base Reactions (Proton Transfer):
– Acids donate protons (H⁺), bases accept them.
– Example: HCl→H++Cl− (HCl loses a proton to water).
Redox Reactions (Electron Transfer):
– Oxidation: Loss of electrons, reduction: Gain of electrons.
– Example: Zn+CuSO4→ZnSO4+Cu (Zn loses electrons, Cu²⁺ gains them).
4. Chemical Equilibrium:
Le Chatelier’s Principle
Le Chatelier’s Principle is that when an equilibrium system is disturbed by a change in conditions (concentration, temperature, or pressure), the system will shift to counteract the disturbance and return to equilibrium.
Factors Affecting Equilibrium:
i. Concentration:
– Increasing the concentration of reactants shifts the equilibrium to produce more products.
– Increasing the concentration of products shifts the equilibrium to produce more reactants.
ii. Temperature:
– Temperature increase shifts equilibrium in the heat-absorbing direction (endothermic direction).
– Temperature decrease shifts equilibrium in the heat-releasing direction (exothermic direction).
iii. Pressure (gas reactions):
– An increase in pressure shifts the equilibrium to the product side having a lower number of moles of gas.
– Decrease in pressure shifts equilibrium in the side of more moles of gas.
iv. Catalysts:
– Catalysts speed up the attainment of equilibrium but do not affect the position of equilibrium.
7.1.A.2 Equilibrium in Chemical Systems:
1. Definition and Characteristics of Equilibrium:
Equilibrium is the state of a reversible reaction where rates of forward and reverse processes are equal and reactant and product concentrations are constant.
Key Features:
– Dynamic process: Reactions continue to occur but no net change in concentration happens.
– No observable changes: No change in concentrations happens.
– Reversible: Forward and reverse reactions happen.
– Balance: Forward and reverse reaction rates are equal.
2. Le Chatelier’s Principle:
A system in equilibrium, when disturbed, will shift to counteract the disturbance.
Shifts in Response to Changes:
– Concentration:
– Add reactants → shift right (greater amount of products).
– Add products → shift left (greater amount of reactants).
– Temperature:
– Raise temp → shift toward endothermic reaction.
– Lower temp → shift toward exothermic reaction.
– Pressure (gases):
– Greater pressure → shift to side with fewer moles of gas.
– Smaller pressure → shift to side with more moles of gas.
3. Equilibrium Constant (K):
The equilibrium constant (K) is a ratio of product concentrations (or partial pressures) to reactant concentrations at equilibrium.
For a reaction:
Significance:
– K > 1: Favors products.
– K < 1: Favors reactants.
– K = 1: Equal reactant and product concentrations.
– Temperature-dependent: K varies with temperature.
K can be used to forecast the equilibrium position and relative yields of products and reactants.
7.1.A.3 Dynamic Nature of Equilibrium:
1. Dynamic Equilibrium:
Dynamic equilibrium is where the forward and reverse reactions of a reversible process are happening at the same rates, so the concentration of the reactants and products are unchanged.
– Key Features:
– Continuous reactions: Forward and reverse reactions are still happening.
– Equal rates: Rate of the forward reaction equals the rate of the reverse reaction.
– Constant concentrations: The concentrations of the reactant and product remain constant with time.
Summary:
In dynamic equilibrium, reactions take place continuously, but no net concentration change is seen because the forward and reverse reaction rates balance each other.
2. No Net Change:
At dynamic equilibrium, the concentration of reactants and products is equal because the rate of the forward reaction is equal to the rate of the reverse reaction.
– Forward reaction: Forms products from reactants.
– Reverse reaction: It reverses products to reactants.
Since these reactions occur at the same rate, the amount of reactants and products remains constant with time, even though both the reactions still go on.
7.1.A.4 Graphs of Concentration, Partial Pressure, or Rate vs. Time in Chemical Equilibrium:
1. Chemical Equilibrium Basics:
Chemical equilibrium is found in a reversible reaction when the forward reaction rate equals the reverse reaction rate, meaning that the reactant and product concentrations remain constant. The reactions still happen at equal rates, however, making equilibrium dynamic.
Key Points:
i. Dynamic: Forward and reverse reactions still proceed at equal rates.
ii. Equilibrium Constant (K): A ratio of product to reactant concentrations at equilibrium, which varies with temperature.
iii. Le Chatelier’s Principle: When a system in equilibrium is disturbed, it will adjust to offset the change and achieve balance.
2. Graphs of Concentration, Pressure, and Rate vs. Time:
i. Concentration vs. Time:
– Reactants decrease, products increase, then both stabilize at equilibrium (horizontal lines).
ii. Pressure vs. Time (for gases):
– Pressure changes initially, then stabilizes at equilibrium.
iii. Rate vs. Time:
– Forward rate decreases, reverse rate increases, and both become equal at equilibrium.
These graphs illustrate that equilibrium occurs when concentrations, pressure, and reaction rates do not change with time.
3. Le Chatelier’s Principle:
i. Concentration Change:
– Addition of reactant or product causes equilibrium to shift towards reversing the change (forward or reverse reaction).
– Graphs (given above) indicate concentration moving, and then leveling off at a new equilibrium.
ii. Pressure Change (gases):
– Adding pressure shifts equilibrium toward fewer moles of gas; removing pressure shifts towards more moles.
– Pressure stabilizes after the shift.
iii. Change in Temperature:
– Rising temperature moves in the endothermic direction; falling shifts in the exothermic direction.
– Graphs (given above) demonstrate changes in rates and stabilization at a new equilibrium.
Equilibrium shifts are illustrated in concentration, pressure, or rate graphs, ultimately stabilizing at new points.