Chem 30: Equilibrium, Acids, and Bases

Dynamic equilibrium

- Occurs when there is a balance between two opposing processes at the same rate

- Related to collision reaction theory which states that particles are in constant motion and constantly collide, rearrange and reorient themselves

- Results in equal competition between collisions of reactants to form products and products to form reactants

Solution/solubility equilibrium

the physical state in which the opposing process of dissolution and crystallization of a solute occur at equal rates

Phase equilibrium

rate of cooling is equal to the rate of cooling

Chemical reaction equilibrium

a dynamic equilibrium between reactants and products of a chemical reaction in a closed system

Percent yield and formula

actual yield/theoretical yield x 100

When percent yield is <50%

Reactants favoured

When percent yield is >50%

Products favoured

Equilibrium law

When a chemical system is at equilibrium, the value for this equilibrium is constant

Equilibrium Constant

Numerical value that is mathematically equal to the concentration of all product species, divided by the concentration of all reactant species, raised to the power of the co-efficient of that species

Kc

Equilibrium constant (Kc) formula

[products]/[reactants]

Steps for predicting the Predominant Acid-Base Reaction

1) List all entities as they appear in solution, including water

2) Label all possible aqueous acids and bases

3) Label the strongest acid and strongest base using table of pages 8 and 9 in data booklet (Acids higher up = stronger acids, Bases lower down = stronger bases)

4) Write an equation showing the transfer of one proton from the strongest acid to the strongest base, with the products being the conjugate base and acid of the reactants

5) Predict the position of the equilibrium, based on the fact that the side that is OPPOSITE the strongest acid is favoured

Kw

Ionization constant for water

1.0 x 10^-14

[H3O+][OH-]

Acid ionization constant

Ka

indicates the extent to which an acid will react with water. It's a ratio of the dissociated form of the acid to the undissociated form

Base ionization constant

Kb

Value of a week based is determined using an equilibrium law expression for a Bronsted-Lowry reaction between the weak base and water first and a substitution of concentration values second

Indicates the extent to which a base will react with water

Kc greater that 1

products favoured at equilibrium

Kc equal to 1

Neither is favoured, equilibrium yield is equal to 50%

Kc less than 1

reactants favoured at equilibrium

Le Chatelier's Principle

when a chemical system at equilibrium is distributed by a change in property of the system, the system always appears to react to oppose the change, until a new equilibrium is reached

Increase one or both reactants in a chemical system...

shifts to the RIGHT, produces more product

Increase the amount of product in a chemical system...

shifts to the LEFT, produces more reactant

Decrease the amount of reactant in a chemical system...

shifts to the LEFT, produces more reactant

Decrease the amount of product in a chemical system...

Shifts RIGHT, produces more product

Changing concentration effect on equilibrium constant

- has no effect on the value of equilibrium constant

Increased concentration of a reactant

Collisions between reactants are much more frequent, significantly increasing the rate of the forward reaction, shift to opposite side

Increased concentration of products

Rate of reverse reaction increases, and rate of forward reaction decreases as reactant molecules are used up

Decreased concentration shifts...

To same side

Equilibrium constant is dependent on

temperature

Heating endothermic shifts

right

Cooling endothermic shifts

left

Heating exothermic shifts

left (system tries to reduce the excess heat being let out, so shifts to reverse reaction)

Cooling exothermic shifts

right (system wants to produce more heat to offset the heat being lost due to cooling)

Changing volume effect on equilibrium constant

No effect

Increase in volume (decrease pressure)

shift toward the side with the larger number of moles of gaseous entities

Decrease in volume (increase pressure)

shift toward the side with the smaller number of moles of gaseous entities

Steps for creating ICE tables

1. Determine reaction occurring at equilibrium

2. Set up ICE table

3. Fill in all info given by the problem

4. Calculate the change for one chemical species

5. Using stoich, determine the change in concentration for each chemical species

6. Calculate equilibrium concentration

pH =

-log [H3O+]

pOH =

-log[OH-]

[H3O+] =

10^-pH

[OH-] =

10^-pOH

conjugate acid-base pairs

two substances that are related by the loss or gain of a single hydrogen ion (proton)

Endpoint

Observable colour change

Equivalence point

When the amount of acid and base are precisely chemically equal

Buffers

- Resist changes in pH if the quantity of excess acid or base added is less than the quantities of the conjugate pair entities present in the buffer

- Combination of any weak acid with its conjugate base, in the same solution

Indicators

A Bronstead-Lowry conjugate weak acid-base pair formed when an indicator dye dissolves in water

How can you identify what your sample is or your titrant based on what pH your titration curve starts and ends at

- A very low initial pH (around 1-2) indicates a strong acid sample.

- A slightly higher initial pH (around 2-3) indicates a weak acid sample.

- A very high initial pH (around 13-14) indicates a strong base sample.

- A slightly lower initial pH (around 10-11) indicates a weak base sample.

- pH Increases during the titration: A base is being added to an acid, so the titrant is a base. The final pH will be high (basic).

- pH Decreases during the titration: An acid is being added to a base, so the titrant is an acid. The final pH will be low (acidic).

Amphiprotic species

A species that may either gain or lose a proton, and can act as either an acid or base

Check to see if it can form both a conjugate acid and base

The stronger an acid

The less is attracts its own proton

The stronger a base

The more it attracts another proton