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Equilibrium
Equilibrium is typically in AP Chemistry Course Content in Unit 7. This topics covers foundational ideas for conceptual ideas on this topic. Below will be a short summary covering topics 7.1 Introduction to Equilibrium to 7.11 Introduction to Solubility Equilibria.
This page will serve as a basis to this difficult unit, or quick review for others.
7.1: Introduction to Equilibrium
most chemical rxns are reversible, indicated with a double arrow
at equilibrium, the RATE of the forward rxn is equal to the RATE of the backward rxn
all phase changes are reversible salts/gases/acid-base/redox rxn
if water is left in an open flask, it will start to evaporate
being OPEN means vapor can escape, therefore there is no reverse rxn
if water is left in a closed flask, it will start to evaporate
however, since water vapor CANNOT escape until h2o (l) ←--> h2o (g), the reverse rxn is possible
we cannot always see if equilibrium has been reached, occurring when the RATES of both sides are equal not concentrations (they just need to remain constant)
equilibrium is DYNAMIC meaning the forward/reverse rxns DO NOT stop when equilibrium is reached, they just continue at the same rate
7.2: Direction of Reversible Reactions
vapor pressure builds up inside a closed container, due to an increase in gas particles colliding, to change into vapor from liquid particles, one must overcome a minimum amount of kinetic energy breaking the IMFs
at higher temperatures, more molecules will have this energy, thus vapor pressure will increase (substances with strong IMFs, have lower vapor pressure)
volume & surface area will not affect the final vapor pressure, dependent only on temperature
forward reaction favored a net conversion of reactants into products
reverse reaction favored a net conversion of products into reactants
almost always a reaction is favored
if the forward rxn is exothermic, then the forward will be favored, an exothermic rxn is energetically favored
catalysts have no effect, both reactions benefit the same amount
7.3: Reaction Quotient and Equilibrium Constant & 7.4 Calculating K and Q
K (equilibrium constant): has no units
Kc = [PRODUCTS]^x / [REACTANTS]^y
stoichiometric coefficients represented as x and y
Kc is expressed in molarity [mol/L], Kp is expressed in pressure (atm)
SOLIDS AND LIQUIDS ARE NOT INCLUDED, concentrations do not change during an rxn, and they get a value of “1”
reaction quotient (Q) is the same, but values do not have to be at equilibrium
Q will dictate which direction the rxn will need to proceed to reestablish equilibrium
K>Q Think about, if there are too few products, so a net conversion FORWARD will need to happen products/REACTANTS
K<Q think about it, too much product, so a net conversion REVERSE will need to happen PRODUCTS/reactants
7.5: Magnitude of the Equilibrium Constant & 7.6: Properties of the Equilibrium Constant
if K is large (>1) products are favored, if very large (>1x10^3) rxn goes to completion
if K is small (<1) reactants are favored, if very small (<1x10^-3) rxn barely goes at all
3 Properties (similar to Hess’s Law)
Reversing the rxn
Multiplying the rxn
Combining the rxn
When a reaction is reversed, K becomes the reciprocal of the original (eg. 10 → 1/10)
When the stoichiometry is multiplied by a number, K is raised to that POWER (eg. 2X → (X)^2
When reactions are combined, the K overall is the PRODUCT, implying you gotta multiply all the K values together
ofc you can cancel out ions and stuff that appear on both sides of the equation
7.7: Calculating Equilibrium Concentrations
RICE/ICE TABLES (rxn/initial/change/equilibrium values)
Another gives Initial values and K value solving for final concentrations
First Calculate the Q value and compare it to K to decide which way the reaction is going
Then plug everything into the Numeric Solver (REMEMBER SQUARE ROOT POWERS + COEFFICIENT WITH X)
Then plug X back into the Concentrations and solve for final concentrations
1 gives initial and final making it easy to calculate X and plugin for final concentrations/K values at equilibrium
7.8: Representations of Equilibrium
we can model rxns to show the relative number of particles before & after the equilibrium
use mole ratios, you can also assume 1 particle is 1 mole unless specified
DETERMINE whether Q is larger or smaller than K by counting the particles, then decide which direction the reaction will need to proceed to establish equilibrium
remember PRODUCTS/REACTANTS
7.9: Introduction to Le Châtelier’s Principle & 7.10: Reaction Quotient and Le Châtelier’s Principle
if a system at equilibrium is stressed, the reaction will proceed a certain way to counteract the disturbance
stress=changes in concentration/pressure/temperature
concentration, the system will shift away from the added component (or toward the removed component) Q can be to check
OH- can also be added (reacting with the (aq) reactants, thus removing reactants so the reverse reaction will occur to reform the lost (aq) reactant
Adding more reactants will shift the equilibrium forward to consume reactants and form products
diluting/concentrating the solution
diluting will always cause a shift toward the more aqueous species
Fe+3 (aq) + SCN- (aq) → FeSCN+2 (aq)
(eg.) [FeSCN] / [Fe] [SCN] = Q>K so shift towards reactants (cause more aq reactants)
(eg.) [FeSCN] [NaOH] = [npc] = Q<K so shift towards products (cause more aq products)
concentrating by allowing the solution to evaporate
concentrations of more (aq) species will decrease
concentrations of fewer (aq) species will increase
pressure
MORE PRESSURE → SHIFT LESS MOLES OF GAS
LESS PRESSURE → SHIFT MORE MOLES OF GAS
addition of an inert gas increases the pressure but has NO effect on the equilibrium
changing the volume of the system affects the pressure (↑V) (↓P)
THE ONLY THING THAT CAN CHANGE THE VALUE OF K IS TEMPERATURE
endothermic = heat is a reactant (ΔH is positive)
exothermic = heat is a product (ΔH is negative)
at equilibrium Q=K, when stress is applied means longer equal
7.11: Introduction to Solubility Equilibria
Ksp tells us how soluble something is “sp” stands for solubility product
Since solids are not included in K expression, Ksp NEVER have denominators
Ksp = [reactants]
Unsaturated Solution = more solute dissolves (Q<K)
Saturated Solution = no more solute dissolves (Q=K)
Supersaturated Solution = becomes unstable crystals form (Q>K)
Ksp > 1 = Soluble, the closer Ksp is to 1, the more soluble the solid is
Ksp will only allow us to compare the solubility of solids that fall apart into the SAME amount of ions
The bigger the Ksp the more soluble
if they have a different amount of ions, solve for S
Q<Ksp → not at equilibrium, no more solute can dissolve, no precipitate formed
Q>Ksp → “past equilibrium” not all solute will dissolve, precipitate formed
YOU CAN USE RICE TABLES BUT NO DENOMINATOR!!!
S CAN BE SOLVED SUBSTITUTED FOR X
DISSOLVING=SOLUBILITY
THIS MEANS IF SOMETHING IS MORE SOLUBLE IT DISSOLVED MORE MEANING MORE PRODUCT MEANS IT DISSOLVED A LOT MEANING PRECIPITATE FORMED MEANING A LOT OF PRODUCT MEANING HIGHER KSP
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