Chemistry Core Concept Cheat Sheet
20: Equilibrium
Key Chemistry Terms
Using ICE Charts
• Reversible Reaction: Reaction that can proceed in both
directions
• Equilibrium: When the rate of the forward and reverse of
a reversible process are equal.
• Dynamic equilibrium: The number of reactants and
products do not change, but the reaction continues to occur
in both directions.
• Equilibrium constant expression: Equation showing the
ratio 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 equilibrium
constant expression.
• Homogeneous equilibrium: When all species are the
same state of matter.
• Heterogeneous equilibrium: When there are at least 2
different states of matter present.
• Reaction Quotient (Q): When concentrations at any time
are plugged into the equilibrium constant expression. Used
to determine if a system is at equilibrium.
• Solubility Product (Ksp): Equilibrium constant for a
dissolution reaction.
• Dissolution reaction: The process of a solid dissolving
and 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 Chatelier’s Principle: A system at equilibrium will readjust 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.
ICE charts a technique for organizing information in an
equilibrium problem
• Make a table with the reactants and products across the top
• Place “ICE” down the left hand side, for Initial, Change and
Equilibrium.
• Fill in any given information from the problem.
• Use the balanced equation’s stoichiometric ratio to
determine the “change” row.
Establishing Equilibrium
Equilibrium is not established instantly. The forward
reaction must produce products, which can then reform
reactants. As the forward rate slows and the reverse rate
increases, equilibrium will be established.
Equilibrium Constants
Writing equilibrium constant expressions
• Write the concentration of the products over the
concentration of the reactants.
• Do not include pure solids or pure liquids—only gases and
solutions
• Use the coefficients of the balanced equations as powers
for each species.
Finding equilibrium constant:
• Plug in equilibrium concentrations into the equilibrium
constant expression
Reaction Quotient
Writing Reaction Quotient Expressions:
• Same as for “K” expressions
Finding reaction quotient:
• Plug in concentrations at any time
Determining if a system is at equilibrium:
• If Q = K, it’s at equilibrium
• If Q > K, the reaction will proceed to the left (remove
exctra products and form more reactants) to reach
equilibrium
• If Q < K, the reaction will proceed to the right (remove
extra reactants and form more products) to reach
equlibrium
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Solving problems with ICE Charts:
• If an equilibrium concentration is known, you can determine
the “change” and find the other equilibrium concentrations
to plug in and solve for K
• If you don’t know any equilibrium concentrations, write
expressions 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 insignficiant
compared to the orginal value (if the original
value 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 pure
solid and is therefore never included.
Reaction Quotient can be used in solubility as well:
• If Q = K, it’s at equilibrium (saturated solution)
• If Q > K, there are too many ions present, some will re-form
solid (precipitate) and leave a saturated solution present.
• If Q < K, the solution is unsaturated—more solid could
dissolve
Determining solubility:
• Set the initial concentration of the solid to “x” and the
equilibrium concentration to “0”. Solve with an ICE chart to
find “x” and that is the solubility.
Le Chatelier’s Principle
The system will try to un-do what you did
Change made
Add reactant
Remove reactant
Add a product
Remove a product
Decrease volume
Increase volume
Reaction will shift
towards
Products
Reactants
Reactants
Products
Side with least gas
particles
Side with most gas
particles
• For endothermic reactions, think of energy (temperature) as
a reactant and follow guide above.
• For exothermic reactions, think of energy (temperature) as
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 gas
particles on each side.
• Adding a catalyst
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