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    Unit 7: Equilibrium: Introduction To Equilibrium and Reversible Reactions

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    Rylee Lippenholz
    This document provides an overview of key concepts in the AP Chemistry unit on chemical equilibrium: - Chemical equilibrium occurs when the rates of the forward and reverse reactions are equal, resulting in constant concentrations of reactants and products over time. - The equilibrium constant, K, is a measure of the favorability of a reaction at equilibrium. A high K value indicates the reaction proceeds essentially to completion in the forward direction, while a low K value means the reaction only occurs to a small extent. - Le Châtelier's principle states that if a stress is applied to a system at equilibrium, the equilibrium position will shift in a way that reduces the effect of the stress. Common stresses include changes in concentration, pressure

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    Unit 7: Equilibrium: Introduction To Equilibrium and Reversible Reactions

    Uploaded by

    Rylee Lippenholz
    125 views5 pages
    This document provides an overview of key concepts in the AP Chemistry unit on chemical equilibrium: - Chemical equilibrium occurs when the rates of the forward and reverse reactions are equal, resulting in constant concentrations of reactants and products over time. - The equilibrium constant, K, is a measure of the favorability of a reaction at equilibrium. A high K value indicates the reaction proceeds essentially to completion in the forward direction, while a low K value means the reaction only occurs to a small extent. - Le Châtelier's principle states that if a stress is applied to a system at equilibrium, the equilibrium position will shift in a way that reduces the effect of the stress. Common stresses include changes in concentration, pressure

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    AP Chem Review

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    Unit 7_ Equilibrium

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    This document provides an overview of key concepts in the AP Chemistry unit on chemical equilibrium: - Chemical equilibrium occurs when the rates of the forward and reverse reactions are equal, resulting in constant concentrations of reactants and products over time. - The equilibrium constant, K, is a measure of the favorability of a reaction at equilibrium. A high K value indicates the reaction proceeds essentially to completion in the forward direction, while a low K value means the reaction only occurs to a small extent. - Le Châtelier's principle states that if a stress is applied to a system at equilibrium, the equilibrium position will shift in a way that reduces the effect of the stress. Common stresses include changes in concentration, pressure

    Copyright:

    © All Rights Reserved
    Download as DOCX, PDF, TXT or read online from Scribd
    Download as docx, pdf, or txt
    125 views5 pages

    Unit 7: Equilibrium: Introduction To Equilibrium and Reversible Reactions

    Uploaded by

    Rylee Lippenholz
    This document provides an overview of key concepts in the AP Chemistry unit on chemical equilibrium: - Chemical equilibrium occurs when the rates of the forward and reverse reactions are equal, resulting in constant concentrations of reactants and products over time. - The equilibrium constant, K, is a measure of the favorability of a reaction at equilibrium. A high K value indicates the reaction proceeds essentially to completion in the forward direction, while a low K value means the reaction only occurs to a small extent. - Le Châtelier's principle states that if a stress is applied to a system at equilibrium, the equilibrium position will shift in a way that reduces the effect of the stress. Common stresses include changes in concentration, pressure

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    © All Rights Reserved
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    AP Chemistry Cram Review 2020

    Unit 7: Equilibrium

    Unit 7: Equilibrium
    Introduction to Equilibrium and Reversible Reactions
    Chemical equilibrium:
    ● State where concentrations of all reactants and products remain constant with time.
    ● Equilibrium is a dynamic situation; forward and reverse reactions running in tandem.
    ● When a reaction is at equilibrium, the rates of the forward and reverse reactions are equal.

    Equilibrium Constant
    Law of mass action:

    [C ] j [D] m
    Given the reaction jA + kB ↔lC + mD: K=
    [ A ]i [ B]k
    ● [ ] : concentrations of chemical species at equilibrium
    ● K: equilibrium constant
    Ignore pure solids and liquids when writing K, they have no concentration!
    K is always constant at a given temperature.
    Equilibrium concentrations, aka the equilibrium position, may vary.
    Kc and Kp

    [C ] j [ D]m
    Kc : Concentration equilibrium constant. K c =
    [ A ]i [B ]k
    P C j P Dm ❑
    Kp: Partial pressure equilibrium constant. K p = i k
    PA PB
    Both Kc and Kp work the same way because pressure is a reflection of concentration, i.e. how closely
    packed particles are.
    Converting between Kc and Kp will not be assessed.

    Properties of the Equilibrium Constant


    Manipulating K:
    ● Reverse reaction (flip products with reactants) → Reciprocal K
    ● Multiply the balanced equation by factor n → raise K to the n
    ● Adding two reactions → Multiply K1 and K2

    Interpreting K:
    AP Chemistry Cram Review 2020
    Unit 7: Equilibrium

    The value of K reflects the tendency for a reaction to occur.


    ● High value of K (K>>1): Reaction is very favorable, proceeds essentially to completion
    ● Low value of K (K<1): Reaction is not favorable, occurs only to small extent
    ● Very low value of K (K<<1): Reaction is not favorable, does not occur to any significant extent.
    The position of K does not reflect the time needed to reach equilibrium! In other words, kinetics and
    equilibrium are unrelated. Favorable does not mean fast.

    Reaction Quotient and Equilibrium Constant


    Reaction Quotient (Q): Written like K but using initial (instantaneous) concentrations

    [C ] j [D] m
    Q= where [ ] : Initial concentrations of species
    [ A ]i [B ]k
    Comparing Q and K: When Q and K are not equal, there is an imbalance in the system and the reaction
    will shift until equilibrium is established (Q=K). Comparing Q and K determines which direction a
    system will shift to reach equilibrium.
    ● Q=K: System is at equilibrium, no shift will occur.
    ● Q>K: Reaction shifts to the left
    ● Q<K: Reaction shifts to the right
    This can be justified mathematically by thinking about fractions.

    Calculating Equilibrium Concentrations


    RICE Tables: Use to calculate equilibrium pressures/concentrations.
    ● Small K approximation: When calculating RICE tables with a small equilibrium constant
    (K≤1x10-5 or so), the initial concentration of the “reactants” (whatever substance is decreasing)
    can be estimated to be equal to the final concentration. That is, [C]initial −x ≈ [C ]initial. You will
    NEVER have to use quadratic formula to solve a RICE table, use this approximation instead!

    Le Chatelier’s Principle
    If a change is imposed on a system at equilibrium, the position of the equilibrium shifts in a direction that
    tends to reduce that change.
    Remember, K is always constant at a given temperature.
    Concentration change:
    If the concentration of a species is changed, then the system shifts to compensate for that change. This is
    caused by kinetics: Increased concentration of a reactant increases the rate of reaction!
    ● [Reactants] increase or [Products] decrease → Shift to right
    ● [Reactants] decrease or [Products] increase → Shift to left
    AP Chemistry Cram Review 2020
    Unit 7: Equilibrium

    ● If multiple changes are happening, use Q vs. K to predict equilibrium shift instead
    Pressure change:
    ● If adding/removing a gaseous reactant/product: Works like concentration change
    ● If adding an inert gas at constant volume: No change (no concentration or partial pressure of
    species is changed)
    ● If increasing/decreasing volume at constant temperature: Look at the coefficients in the
    balanced equation. The system responds to volume changes by changing its own volume to adapt
    to the volume of the container. This is done by changing the total number of gaseous molecules in
    the system. K does not change!
    ○ Increase volume → shifts to side with more moles of gas
    ○ Decrease volume → shifts to side with less moles of gas
    ● Adding an inert gas at constant pressure: This serves as a volume change! See above.
    Temperature change:
    Changes both the equilibrium position and the value of the equilibrium constant! Otherwise, K is always
    constant at a given temperature.
    Determine if the reaction is exothermic or endothermic and place “heat” as a reactant or product
    accordingly. Then treat it like a concentration change.
    ● Exothermic: Heat is product
    ● Endothermic: Heat is reactant
    Determining change in K from a temperature change:
    ● Reaction shift to right: K increases
    ● Reaction shift to left: K decreases

    Solubility Equilibria
    Solubility product constant ( K sp ): Equilibrium constant for a solid dissolving in a liquid. a.k.a.
    solubility product
    A(s)→ B(aq)+C (aq)
    K sp =[B ][C]
    Ksp works just like K. Constant at constant temperature, but solubility is an equilibrium position and can
    change. Generally, increasing temperature will increase solubility, but not if the dissolution process is
    exothermic.
    Common ion effect: The solubility of a solid is lowered if the solution already contains ions common to
    the solid (or if the concentrations of those ions is increased artificially). This is due to an equilibrium shift
    to the left (Le Chatelier’s)
    Ion product (Q): Defined like Ksp but using initial concentrations.
    Q=[B ]initial [C]initial
    AP Chemistry Cram Review 2020
    Unit 7: Equilibrium

    Predicting precipitation with Q vs. Ksp:


    ● Q > Ksp: Precipitation occurs (solid is formed, reaction shifts to left) and will continue to occur
    until Q=Ksp (saturated solution)
    ● Q < Ksp: No precipitation occurs

    Solubility and pH
    If the anion X- is a strong base (weak acid), then the salt MX will be more soluble in acidic conditions.
    Mg(OH)2 (s) ↔ Mg2+ (aq) + 2OH- (aq)
    ● Basic conditions: Adding OH- (by making the solution more basic) reduces solubility via
    common ion effect.
    ● Acidic conditions: Adding H+ (by making the solution more acidic) increases solubility because
    H+ reacts away OH-, decreasing [OH-], causing equilibrium shift to the right.
    This same logic works when adding something that will precipitate out species in equilibrium!

    Complex ion equilibria:


    Complex ion: A charged species consisting of a metal ion surrounded by ligands (other molecules).
    Often form in solution (aqueous).
    Ag+ (aq) + NH3 (aq) → Ag(NH3)+ (aq)
    K for a complex ion formation is also called a formation constant or a stability constant.
    Don’t get scared! The calculations are no different than for regular equilibrium.

    Predicting equilibrium shifts when adding species:


    Adding something weird to your equilibrium solution (not just a reactant or product), and need to know
    how it shifts?
    1. Write out your equilibrium reaction!
    2. List all the species that are in your mixture at equilibrium (ions, etc). Draw a beaker if you have
    to!
    3. Look at the properties of what you’re adding. What species will it contribute to the equilibrium?
    How will the species you’re adding interact with what’s already in your mixture? Look for:
    a. Changes in the pH of the solution
    b. Precipitates forming (know/recognize your solubility rules!!)
    c. Reactions that produce a gas that leaves/bubbles out of the system
    d. Any other reactions that proceed essentially to completion
    4. Determine the relative change in concentration of equilibrium species and predict equilibrium
    shift.
    AP Chemistry Cram Review 2020
    Unit 7: Equilibrium

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