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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