Module 4: Chemical Equilibria

single-headed arrow

Only the products remain after the reaction and these reactions are signified using a ________________.

reversible reactions

products can react to form the original reactant; signified using a double-headed arrow.

Chemical Equilibrium

state when the concentrations of reactants and products remain constant with time because the rates of the forward and reverse reactions are equal

reactant

In the beginning of the reaction, only the ______ is present.

products

As the reaction progresses, some of the reactants are converted to ________ and so the amount starts to decrease

reactant

There comes a time in the reaction when the concentration of the ________ no longer changes.

zero

In the beginning of the reaction, the concentration of product is _____.

increase

As the reaction progresses, the amount of product starts to ________.

Equilibrium

A state when the concentration of reactants and products remain constant with time

time

In chemical equilibrium, the concentration is not equal but does not change through ______.

Dynamic

The reaction may appear to have stopped macroscopically (visible to the unaided eye) since there are no changes in the concentrations of reactants and products. However, on the molecular level, the forward and reverse reactions are constantly occurring.

Dynamic

The concentration does not change because the forward and backward reaction rate is already equal.

Balance

The rate of the forward reaction is equal to the rate of the reverse reaction. Hence, no apparent change is seen.

Approached from any direction

Equilibrium can be established even when (a) only the reactant is initially present, (b) only the product is initially present, (c) both reactant and product are initially present

consumed

Chemical Equilibrium is the state in which the concentrations of reactants and products remain constant with time because the rate at which they are formed equals the rate at which they are ______

constant

The concentrations of the products and reactants remain _____.

Law of Mass Action

Expresses the relationship between the concentrations of reactants and products at equilibrium in any reaction

Keq

equilibrium constant; eq emphasizes that concentrations used are equilibrium concentrations

dependent

Keq is temperature ________.

dimensionless

Keq has no units (_____ quantity)

right

Keq >>> 1, the equilibrium lies far to the ____

left

Keq <<< 1, the equilibrium lies far to the ____

Homogenous Equilibria

reactions where all the species are in the same phase

Heterogeneous Equilibria

reactions where the reactants and products are of different phases

magnitude of Q

allow us to qualitatively predict the shift of the system if it is not yet at equilibrium; very useful for solving quantitative problems involving equilibrium systems

Reaction Quotient

used to determine whether a reaction is at equilibrium

Q > Keq (reaction shift to left)

the ratio of the initial concentrations between the products an reactants is large

Q<Keq (reaction shift to the right)

the ratio of the initial concentrations between the products and reactants is small

Q = Keq

The system is at equilibrium and no shift will occur.

La Chatelier’s Principle

A rule that allows us to qualitatively predict the effects of applying stress (change in concentration, pressure, and temperature) on a system at equilibrium.

The principle states that: If a stress is applied to a system at equilibrium, the system will adjust to minimize the stress

Change in Concentration

If a reactant or product is added to a system at equilibrium, the system will shift away from the added component.

removed

Conversely, if a reactant or product is removed, the system will shift toward the ______ component.

increased

If pressure is _________ (by lowering the volume), the system will shift towards the direction which reduces the total number of gaseous molecules.

decreased

If pressure is _________ (by increasing the volume), the system will shift towards the direction which increases the total number of gaseous molecules.

Change in Temperature

nearly always changes the equilibrium constant value; In general, the sign of triangle H for the reaction will dictate the temperature dependence of Keq.

exothermic reaction

(Negative triangle Ho)

• the equilibrium constant decreases as the temperature increases

endothermic reaction

(positive triangle Ho)

• the equilibrium constant increases as the temperature increases

heat

How do we predict the shift qualitatively using Le Chatelier’s principle?

- treat _____ as one of the reactants or products and follow the trend for the effect of concentration

away

Adding more heat in endothermic. The system relieve the stress by shifting ____ from the added component.

toward

Removing heat in endothermic. The system relieve the stress by shifting ____ from the added component.

away

Adding more heat in exothermic. The system relieve the stress by shifting ____ from the added component.

toward

Removing heat in exothermic. The system relieve the stress by shifting ____ from the added component.

Arrhenius Definition

Acids are solutions that increase the H+ (proton) concentration in solution

Bases are solutions that increase the OH- (hydroxide) concentration in solution

Bronsted-Lowry Definition

donate or receive protons

Acids are substances that donate a proton (H+) to another substance. Bases are substances that receive a proton (H+) from another substance.

Lewis Definition

pair of electrons

Acids are substances that receive a pair of electrons from another substance. Bases are substances that donate a pair of electrons to another substance.

Conjugate Pairs

Species that differ from one another by one proton

Water

a universal solvent, can also act as an acid or base in different situations. It can even form ions, but to a very small extent.

amphirotic

Water is _______.

- Water has the ability to act as an acid and a base in different reactions. Water acts as acid in the presence of a base, conversely water acts as base in the presence of an acid.

pure water

Even in ______, one molecule can donate a proton, while another receivers th proton.

Aqueous solutions

_________therefore contain small concentrations of hydronium and hydroxide ions due to the self-ionization or autoprotolysis of water.

25

At ___ C, Kc is equal to 1.008 x 10-14. For convenience, we shall use the approximation Kw = 1.00 x 1014. Just like a typical Keq value, the Kw value varies with temperature.

Strong Acids

• React with water completely (100% ionized in solution)

• After the reaction, no undissociated solute remains.

Weak Acids

• Those that react incompletely with water (not completely ionized in solution)

• They yield solutions that contain significant amounts of the parent acid (or base) and its conjugate base (or acid)

solvent-dependent

The strength of an acid or base is solvent-dependent. In general, the weakest acids form the strongest base (and vice versa).

position

The strength of an acid depends on the equilibrium ______ of its dissociation reaction.

large

Strong acids tend to have ____ equilibrium constant (Ka) values and weak acids will have small Ka values.

Strong Acids and Bases

high % dissociation; complete ionization with waterv

right

Strong acids have high % dissociation which means that the equilibrium lies far to the _____ (forward reaction is favored).

Weak acids-bases

low % dissociation (reverse reaction is favored); partial ionization

stronger

The ______ the acid, the weaker its conjugate base.

weaker

The stronger the base, the _____ its conjugate acid.

higher

For a particular weak acid, the more dilute the acid becomes, the ____ is % dissociation.

Buffers

Solutions that maintain the pH at a fairly constant level; comprised of a weak acid (or weak base) and its conjugate ase (or conjugate acid)

Electrolyte

substance capable of generating ions in solution and causes an increase in its electrical conductivity

Strong Electrolyte

completely dissociates to form ions in solution

Weak Electrolyte

Substances that are not completely dissociated in solution

Double Headed Arrow

signify incomplete dissociation

Henderson-Hasselbach Equation

pH changes in a buffer system

neutralizing

How buffers resist drastic changes in pH, by _______ the added acids or bases.

Acids and bases

substances that are ubiquitous in the environment and the living organism

Ksp

generated when a slightly soluble salt undergoes dissociation

Slightly Soluble Salts

There are reactions that involve the dissolution or precipitation of a slightly soluble salt (or ionic compound).

increase

As the concentration of K+ and CrO42- ions _____, these ions begin to collide and react with one another and re-form the solid salt.

Common Ion Effect

the presence of a common ion will decrease the solubility of a salt

Complex Ion Formation

the presence of a Lewis base that acts as a ligand will increase the solubility of a salt