Equilibrium

Equilibrium

refers to systems that have opposing processes happening at the same time and rate

• almost all reactions are reversible at certain conditions

• represented by ⇌

⇌ represents that the equation is both

going in the forwards direction to products and back towards reactants; considered dynamic

at equilibrium the rate of the forward reaction is equal to

the rate of the reverse reaction

some reactions “favor” products or reactants more than the other resulting in

one side of the reaction having a higher [] than the other

forward favored reaction

25 25 → 50

backwards favored reaction

50 50 → 25

Le Chetalier

when a system experiences stress the equilibrium will shift to relieve the stress

Factors that cause a shift

• Pressure change (only for gases)

• Concentration change (volume changes for gas)

• Temperature change

Pressure (P)

• equilibrium systems involving gases the total mol of gas on the reactant side and product side must be unequal

• If P increases the [] of all the species in the reaction

  • in order to reduce the stress the reaction will shift torward the side with fewer moles

• changing P or adding an inert gas (such as a noble gas) will not affect the equilibrium constant

• will go to side with least amount of moles when increasing pressure

Concentration

A + B ⇌ C + D

• increase [] of one species in a system stresses equilibrium and will result in a shift away from the increase []

• If [] of A or B is increased it will cause ⇌ to shift toward right

• If [] of C or D is increased it will cause ⇌ to shift toward left

• A decrease in [] will cause the equilibrium to shift toward to decrease species, if [A] decreases the ⇌ will shift left

• Changing the [] will not affect the ⇌ constant

Temperature

• endo: heat + A + B ⇌ C (increasing heat = shift right) (decrease = left)

• exo: A + B ⇌ C + heat (decreasing heat =shift right) (increasing = left)

• increasing heat go away/ decreasing heat go towards

• change in temp will affect the equilibrium constant

  • only way to change ⇌ constant

Equilibrium constant [] of reactants and products

nA + mB ⇌ xC + yD

• where n, m ,x, and y are the coefficients used to balance the chemical equation

• assume initial [] of C and D are both zero since none to start

• initial [] of A and B are at their max at start of reaction

• as the system reacts and reaches equilibrium the concentration will undergo no more changes

Equilibrium constant (K)

shows us the mathematical relationship between products and reactants

• determined with just the balanced equation of the system at equilibrium

K= [C]^x[D]^y / [A]^n [B]^m

• if [] known of products and reactants in reaction you can use equilibrium expression to calculate K

about equilibrium expression/constant

• equilibrium is always temp dependent

• only aqueous and gaseous substances are used it the equilibrium expression

  • Solids/liquids never in equilibrium expressions

• Keq is how you show aq []

• Ksp is how you show the solubility product constant for ions

  • only for dissolving a solid

• the lower the K value the higher the equilibrium [] of the reactants

Keq < 1 favor

reactants

Keq > 1 favor

products

Molarity=

mol solute/L solution

[Brackets] refers to

concentration of your substances

Ice Table

• used to organize equilibrium calc info

• acronym that stands for initial concentration, change in concentration, and equilibrium concentration

• Reaction, Initial, Change, Equilibrium

solubility equilibrium

• many ionic compounds dissolve only a little in H2O

  • we use concept of solubility equilibrium to quantify the degree of solubility

• equilibrium = rate of dissolving is equal to rate of precipitation

• write the solubility product constant Ksp expression for this “reaction”, remembering that solids not included

Solubility Product constant

the smaller the Ksp, the less soluble the compound is

The Common Ion Effect

• a common ion is an ion that is found in both compounds in solution

• The lowering of the solubility of an ionic compound by adding a common ion is called the common ion effect

• If common ion, will lower the Ksp

AgCl(s) ⇌ Ag + Cl in Cl solution (equilibrium shifted left)

how can K be changed

temperature change (only thing that changes the value of K)

do catalysts affect the equilibrium?

no

if an endothermic reaction at equilibrium were placed in an ice bath, what would be the effect (A + heat ⇌ B)

more reactants (heat decreasing, so shift left)

What is true of the rates of the forward and reverse reactions at equilibrium?

they are equal, a catalyst affects both equally, and temperature changes both

Things are more soluble when

Ksp is closer to 1

Dynamic equilibrium

equation going both forwards and reverse

How does catalyst affect equilibrium?

lowers the activation energy for the reactions

LeChatelier’s Principle

when a system experiences stress the equilibrium will shift to relieve the stress

• increase side=favor opposite

• decrease side=favor same side

• pressure only affects gas (P increases = favors less mol)

Equilibrium constant

products divided by reactants to the coefficient power

What is the meaning of the magnitude of Keq?

• Keq<1 favors reactants

• Keq>1 favors products

How is Keq writen if any species is a pure liquid or solid?

not shown in it

how can Keq be changed

temperature change

Equilibrium position

the reactants or products will be favored

Solubility product constant

start with solid and put in H2O so reactant is (s) and products are (aq)

What is the meaning of the magnitude of Ksp?

the larger the value (x10^x) or greater the more soluble it is (more free floating ions)

• the smaller the value, the more insoluble it is (less free floating ions)