CHEM 1212 Exam 3

What does Le Charlier’s principle state?

• For any change you make to a system at equilibrium, the system will attempt to compensate and return to equilibrium

How does changing concentrations affect equilibrium?

If you add reactant to the mixture or remove product

• Q decreases

• The reaction will shift to the right

If you add product or remove reactant from the system

• Q increases

• The reaction will shift to the left

How does changing volume or pressure affect equilibrium?

• Remember, PV=nRT

Decreasing volume

• This will increase pressure in the system

• Reaction will try to decrease pressure by shifting towards side with fewer gas molecules

Increasing volume

• This will decrease pressure of the system

• Reaction will compensate by shifting towards the side with more gas molecules

What happens if you add an inert gas to a system at constant volume? Constant pressure?

Constant Volume

• Adding any inert gas not involved in the reaction will not shift the equilibrium of the reaction at all

Constant pressure

• Adding an inert gas a constant pressure will increase the volume, meaning less moles per volume

• This will shift the reaction towards the side with more moles of gas

How does temperature change affect equilibrium?

• Changing temp doesn’t’t affect Q, but it changes K

For endothermic reactions

• Increasing temp increases K

• Decreasing temp decreases K

For Exothermic

• Increasing temp decreases K

• Decreasing temp increases K

What is the Van’t Hoff Equation for changing temperature of a system?

How does adding a catalyst affect equilibrium?

• A catalyst has no effect on the equilibrium condition

• A catalyst will only get a system the the equilibrium condition faster

What are the rules for altering reactions and how they affect K?

• Reversing a reaction means taking the inverse of K

• When the coefficients of a reactions are multiplied by a number, K is raised to the power of that number

• When two equations are added, the K for the new equation is the multiplied product of the old equations

Explain Gibbs free energy and how its related to thermo

What is the equation for finding Gibbs free energy of a reaction? What does this reaction tell us?

If G < 0

• The reaction is spontaneous

• Q < K, so the reaction will go towards products

If G > 0

• The reaction is non spontaneous

• Q > K, so the reaction will go towards reactants

If G = 0

• The reaction is at equilibrium

What is the Gibbs free energy of a system at equilibrium?

What does K tell us about spontaneity?

When K > 1

• The mixture is mostly products

• ln(K) > 0, so G < 0

• This means the forward reaction is spontaneous

When K < 1

• The equilibrium mixture is mostly reactants

• The reverse reaction is spontaneous

When K = 1

• The reaction is at equilibrium

What are the three definitions of acids and bases?

Arrhenius

• Simplest

What is the Arrhenius definition of acids and bases?

Acids

• Produce H+ ions in aqueous solution

Bases

• Produce OH- ions in aqueous solution

• This definition is very restrictive and doesn’t encompass most acids and bases

What is the Bronsted-Lowry definition of acids and bases?

Acids

• Donate protons to another substance (H+ ion)

Bases

• Accepts a proton (H+) from another substance

• Acids and bases form conjugate acid-base pairs

• Acids become conjugate bases and bases become conjugate acids

What is the Lewis definition of acids and bases?

Acids

• Acids accept an electron pair

• They do this by either kicking off an H+ ion, having an incomplete octet, expanding octet, or higher order bonds

Bases

• Bases donate an electron pair

• They must have a lone pair to donate

• These acids and bases must occur in pairs

What is amphoterism?

Amphoteric species

• Can act as both an acid or a base

• Water is a very good example

• H2O can become OH- or H3O+

Explain auto ionization of water

• Because water is amphoteric, it can also react with itself without any other substance present

• Kw is the auto ionization equilibrium constant, AKA ion product constant

• At 25C Kw = 1.0 × 10-14

How do you convert from ion concentration to pH scale? And vice versa?

What are the sigfig rules for converting between logs and exponents?

• For logs, the # of SF inside the log becomes the # of decimals in the result

• For exponents, the # of decimals in the exponent becomes the # of SF in the result

How does the pH scale with concentration of H3O+?

• A change in concentration by a factor of 10 causes the pH to change by 1

• pH value can be much higher than 14 or much lower than 1

How do you convert from OH- concentration to pOH scale?

How is pH different than pOH, how do you convert between the two

• pOH has the same neutral value as pH, but the scale is flipped, meaning pOH > 7 is acidic

How are strong acids different than weak acids?

Strong acids

• Completely ionize in aqueous solution

• Conjugate base is weak

Weak acids

• Only partially ionize in aqueous solution

• Conjugate base will be much stronger

List the strong acids you need to know

Explain monoprotic vs polyphonic acids

Monoprotic acids

• They only have one proton (H+ ion) to donate

Polyprotic acids

• They have two or more protons to donate

• Conjugates of these acids can be amphoteric

• BEING POLYPROTIC DOES NOT MEAN THE ACID IS STRONGER

List the strong bases you need to know

What is the acid equilibrium constant? How is it calculated?

• This constant describes how much acid is dissociated, indicating strength of the acid

• Polyphonic acids will have multiple equilibrium constants

Explain the base equilibrium constant

Same as acid constant, but for bases

What is pKa and pKb?

• Just like with pH and pOH, the -log of Ka and Kb is easier the understand

• pKa = -log(Ka)

• pKb = -log(Kb)

• Lower pKa and pKb indicates stronger acids and bases

What is the relationship between Ka and Kb, pKa and pKb?

• Ka x Kb = Kw at that temperature

Strong acids completely dissociate in water, so they donate too of their protons into the solution

• This means that the concentration of the strong acid and hydronium will be the same

How do you determine hydronium concentration for weak acid solutions?

• Use an equilibrium equation

What do you need to be careful of with strong base solutions when calculated OH- concentration?

• All hydroxide ions will dissociate, BUT

• Monobasic strong bases produce 1 hydroxide

• Dibasic strong bases produce 2 hydroxides

• This means that for dibasic the OH- M will be double the base M

How do you calculate OH- M for a weak base solution?

How do you calcuate the percent of weak acid or base that has dissociated in a solution?

• Also remember that the percent dissociation of a weak acid increases as the acid concentration decreases

What do you do if a solution has multiple acids of different strengths?

• We know that strong acids completely dissociate, so the strong acid dissociation will essentially overpower the weak acid dissociation

• Strong acid will contribute basically all of the hydronium

IF THERE ARE TWO WEAK ACIDS

• The stronger of the two will contribute the most

What is hydrolysis?

• Hydrolysis happens when a salt dissociates and dissolves in water, reacting to form H3O+ and OH-

• Depending on the salt, the solution after hydrolysis can be acidic, neutral, or basic

What are neutral ions of salts?

• Neutral ions are ions that do not react with water and form a neutral solution

Neutral cations from strong bases

• Li+, Na+, K+, Ca2+, Sr2+, Ba2+

• All from groups 1 and 2

Neutral anions from strong monoprotic acids

• Cl-, Br-, I-, NO3-, CLO4-, CLO3-

• These strong acids/bases completely dissociate, so they cannot react with water to re-form acids/bases

Explain basic ions in solution

• When non0neutral ions react with water, they typically behave like bases and produce OH-, making the solution basic

Explain acidic ions in solution

• When non-neutral cations react with water, they typically behave like acids and produce H3O+

How do you determine what the pH of a solution will be if both ions going into it are non-neutral?

How can we predict the direction of an acid-base reaction for weak acids and bases?

• All acid-base reactions favor a stronger acid or base forming their weaker conjugates

• Use pKa or pKb to find equilibrium constant and direction