GEN CHEM 2 | WW3 Reviewer

Irreversible Reaction

• goes essentially to completion

• reactants form products and do not significantly reform reactants

Reversible Reaction

• products can convert back into reactants under the same conditions

• written with a double arrow

Forward Reactions

reactants become products

Reverse Reactions

products become reactants

Reaction Rates and Equilibrium

• There is a rate of the forward reaction and rate of the reverse reaction.

• At the start, the forward rate is high and reverse rate is low.

• As the reactants decrease, the forward rate decreases.

• As the products increase, the reverse rate increases.

Chemical Equilibrium

• rate forward = rate reverse

• concentrations stop changing (they become constant)

• reaction does not stop

Dynamic Equilibrium

• molecules continue reacting in both directions

• the forward and reverse processes occur simultaneously

• as they occur at the same rate, so there is no net change in concentration

Factors Affecting Equilibrium

• concentration

• pressure/volume

• temperature

Concentrations Over Time

The reactants decrease exponentially over time while the products increase exponentially over time until they reach equilibrium and no net change occurs and the concentrations are constant

Rates Over Time

The forward reaction rate and reverse reaction rate become equal over time.

Equilibrium Constant

ratio of the equilibrium concentration of the products and reactants multiplied to their coefficients

Equilibrium Constant Formula (K_C)

K_C = [products]/[reactants] = ([C]^c[D]^d)/([A]^a[B]^b)

What does a large value of K entail?

The reaction is mostly products at equilibrium.

What does a small value of K entail?

The reaction is mostly reactants at equilibrium.

Alternative Equilibrium Constant Formulas for Gases (K_P)

K_P = [P_B]^b/[P_A]^a (where P is for the partial pressures of the substances of equilibrium

Calculating K_C from K_P

• K_P = K_C(0.0821T)^∆n

• K_C = K_P/((0.0821T)^∆n)

T = temperature in Kelvin and ∆n is (moles of gas products - moles of gas reactants)

Rules in Writing K

1. Do not include solid and liquid substances as their concentrations remain constant throughout the reaction.

2. Do not include solvents (e.g. water in liquid form).

3. The equilibrium constant of the reverse of a reaction is the reciprocal.

4. It depends on the balanced equation.

What does it mean if K_C is greater than 1?

The forward reaction is favored, meaning the concentration of the products will increase and the concentration of the reactants will decrease. In this context, these are numbers greater than 10.

What does it mean if K_C is less than 1?

The reverse reaction is favored, meaning the concentration of the products will decrease and the concentration of the reactants will increase. In this context, these are numbers less than 0.1.

Reactant Quotient

• ratio of the initial concentration of the products over the reactants, regardless if equilibrium is reached

• follows the same formula as the equilibrium constant

What does it mean if the reaction quotient is less than than the equilibrium constant?

The forward reaction must proceed to produce more products.

What does it mean if the reaction quotient is less than than the equilibrium constant?

The reverse reaction must proceed to produce more reactants.

What does it mean if the reaction quotient is equal to the equilibrium constant?

The reaction is in equilibrium.

How to calculate equilibrium concentrations from initial concentrations?

1. Write the equilibrium constant expression.

2. Use the ICE table and write the initial concentrations in the initial row.

3. Express the change in concentrations of the substances using x. Multiply x according to the stoichiometric rations. Use a negative x for reactants and a positive x for products.

4. Set the equilibrium concentrations as initial - x. Substitute these values in the equilibrium equation.

5. Get the value of x.

6. Substitute x into the equilibrium concentration expressions (initial - x) to obtain their values.

Le Chatelier's Principle

If a stress is applied to a system at equilibrium, the equilibrium shifts to relieve stress or counteract the change.

What happens when a substance is added to one side of a system?

The equilibrium shifts to use the added substance to produce the substances on the other side.

What happens when a substance is removed?

The equilibrium shifts to produce the removed substance back.

What happens when the concentration of the reactants increases?

The equilibrium must shift to the right, creating more products.

What happens when the concentration of the reactants decreases?

The equilibrium must shift to the left, creating more reactants.

What happens when the concentration of the products increases?

The equilibrium must shift to the left, creating more reactants.

What happens when the concentration of the products decreases?

The equilibrium must shift to the right, creating more products.

What happens when the pressure of the system increases?

The equilibrium must shift to the side with fewer gas moles.

What happens when the pressure of the system decreases?

The equilibrium must shift to the side with more gas moles.

What happens when the volume of the system increases?

The equilibrium must shift to the side with more gas moles.

What happens when the volume of the system decreases?

The equilibrium must shift to the side with fewer gas moles.

Heat’s Effect on Equilibrium Reactions

• In endothermic reactions, heat is treated as a reactant and tends to be added on the reactant side.

• In exothermic reactions, heat is treated as a product and tends to be added on the product side.

What happens if the temperature increases in an endothermic reaction?

The equilibrium shifts to the right, producing more products.

What happens if the temperature decreases in an endothermic reaction?

The equilibrium shifts to the left, producing more reactants.

What happens if the temperature increases in an exothermic reaction?

The equilibrium shifts to the left, producing more reactants.

What happens if the temperature decreases in an exothermic reaction?

The equilibrium shifts to the right, producing more products.

Catalysts’ Effect on Equilibrium

• The activation energy is lowered for both reverse and forward reactions.

• The system will reach equilibrium sooner.

• The equilibrium constant and the position of equilibrium does not change.

Titration

• volumetric analysis

• common laboratory method to determine the concentration of an unknown solution

Titrant

• acid or base of known concentration that is added to the substance being analyzed

• placed in burette

Analyte

• substance that is being analyzed or unknown

• placed in flask

Burette

where titrant is placed

Burette clamp

holds burette above the flask

Stopcock

controls whether the titrant can flow into the flask

Flask

where analyte is placed

Initial Reading

the initial volume of the titrant in the burette

Final reading

• the final volume of the titrant in the burette once the end point is reached

• usually occurs when the indicator causes a color change

• initial is subtracted from the final volume to obtain the final

Acid-Base Titration

• determines the unknown acidic or basic substance through acid-base reaction

• involves strong/weak acids or bases

• determines the concentration of an acid or base, and whether it is strong or weak

Titration Curve

• graphical representation of how the pH of a solution changes when a base or acid is added

• follows a neutralization reaction

Equivalence Point

• which just an adequate reagent/titrant is added to react completely with a substance

• added titrant and analyte are chemically equal

• theoretical point

Endpoint

• point wherein solution changes color

• practical point

• detected using indicator

• may or may not coincide exactly with the equivalence point

Acid-base Indicators

• substance which change color or develop turbidity at a certain pH

• locate equivalence points and measure pH

• includes phenolphthalein, litmus, methyl orange, etc.

• must change over the vertical section of the curve where there is a large change in pH for the addition of a very small volume of alkali/base

Phenolnaphthalein

color changes from clear to pink at around pH = 9

Litmus

color changes from red to blue at around pH = 7

Methyl Orange

color changes from red to yellow at around pH = 4

Types of Titration Curves

• Strong Acid vs Strong Base

• Strong Acid vs Weak Base

• Weak Acid vs Strong Base

• Weak Acid vs Weak Base

Strong Acid vs Strong Base

• starts at a very low pH

• pH rises very slowly at first then shows a very steep jump

• equivalence point is around 7

• after eq. pt., pH is high

Strong Acid vs Weak Base

• starts at a very low pH

• pH rises but the curve is less steep

• equivalence point is < 7

• after eq. pt., pH is in the acidic and neutral range

Weak Acid vs Strong Base

• starts at a moderately low pH

• early in the titration, there is a buffer region where pH changes gradually

• equivalence point is above 7

• after the eq. pt, the pH becomes strongly basic

Weak Acid vs Weak Base

• starts at a moderate pH

• curve is smoother and less steep overall

• equivalence point clearly not at 7

• choosing an indicator is hard due to the less steep curve