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Chemistry Core Concept Cheat Sheet
20: Equilibrium
Key Chemistry Terms
Reversible Reaction: Reaction that can proceed in bothdirections
Equilibrium: When the rate of the forward and reverse ofa reversible process are equal.
Dynamic equilibrium: The number of reactants andproducts do not change, but the reaction continues to occurin both directions.
Equilibrium constant expression: Equation showing theratio of the concentration of products to reactants with the
balanced equation coefficients as powers. Equilibrium constant (K): The value found when
equilibrium concentrations are plugged into the equilibriumconstant expression.
Homogeneous equilibrium: When all species are thesame state of matter.
Heterogeneous equilibrium: When there are at least 2different states of matter present.
Reaction Quotient (Q): When concentrations at any timeare plugged into the equilibrium constant expression. Usedto determine if a system is at equilibrium.
Solubility Product (Ksp): Equilibrium constant for adissolution reaction.
Dissolution reaction: The process of a solid dissolvingand forming ions.
Saturated solution: A solution that is at equilibrium. Solubility: The amount of a solid that will completely
dissolve to form a saturated solution. Le Chateliers Principle: A system at equilibrium will re-
adjust to reach equilibrium again when disturbed. Exothermic reaction: System gives off energy to the
surroundings. Energy can be thought of as a product. Endothermic reaction: System gains energy from the
surroundings. Energy can be thought of as a reactant.
Establishing Equilibrium
Equilibrium is not established instantly. The forwardreaction must produce products, which can then reformreactants. As the forward rate slows and the reverse rateincreases, equilibrium will be established.
Equilibrium Constants
Writing equilibrium constant expressions
Write the concentration of the products over theconcentration of the reactants.
Do not include pure solids or pure liquidsonly gases andsolutions
Use the coefficients of the balanced equations as powersfor each species.
Finding equilibrium constant:
Plug in equilibrium concentrations into the equilibriumconstant expression
Reaction Quotient
Writing Reaction Quotient Expressions:
Same as for K expressionsFinding reaction quotient:
Plug in concentrations at any timeDetermining if a system is at equilibrium:
If Q = K, its at equilibrium If Q > K, the reaction will proceed to the left (remove
exctra products and form more reactants) to reachequilibrium
If Q < K, the reaction will proceed to the right (removeextra reactants and form more products) to reachequlibrium
Using ICE Charts
ICE charts a technique for organizing information in anequilibrium problem
Make a table with the reactants and products across the to Place ICE down the left hand side, for Initial, Change and
Equilibrium. Fill in any given information from the problem. Use the balanced equations stoichiometric ratio to
determine the change row.
Solving problems with ICE Charts:
If an equilibrium concentration is known, you can determinthe change and find the other equilibrium concentrationsto plug in and solve for K
If you dont know any equilibrium concentrations, writeexpressions for them and plug in the expressions into your
K equation to solve for them.o If the K is very tiny (10-5 or smaller), you may
approximate that the change is insignficiantcompared to the orginal value (if the originalvalue is > 0) e.g.: 0.25 M x 0.25 M
Solubility Product
Writing solubility product expressions
Same as writing K expressions. With a dissolution reaction, the reactant is always a puresolid and is therefore never included.
Reaction Quotient can be used in solubility as well:
If Q = K, its at equilibrium (saturated solution) If Q > K, there are too many ions present, some will re-for
solid (precipitate) and leave a saturated solution present. If Q < K, the solution is unsaturatedmore solid could
dissolve
Determining solubility:
Set the initial concentration of the solid to x and theequilibrium concentration to 0. Solve with an ICE chart tfind x and that is the solubility.
Le Chateliers Principle
The system will try to un-do what you did
Change made Reaction will shifttowards
Add reactant Products
Remove reactant Reactants
Add a product Reactants
Remove a product Products
Decrease volume Side with least gasparticles
Increase volume Side with most gasparticles
For endothermic reactions, think of energy (temperature) a reactant and follow guide above. For exothermic reactions, think of energy (temperature) a
a product and follow guide above.
Changes that do not affect equilibrium:
Adding/removing a pure solid or liquid Adding/removing a non-reactive gas Changing the volume of a reaction with equal number of g
particles on each side. Adding a catalyst