Unit 2: Equilibrium, Acids & Bases, Buffers Flashcards
What do the different relationships between Q (reaction quotient) and K (equilibrium constant) mean?
(<, >, and =)
- If Q > K, the reaction proceeds leftward toward reactants because product concentration is higher than at equilibrium.
- If Q < K, the reaction proceeds rightward toward products because product concentrations are lower than at equilibrium.
- If Q = K, equilibrium has been reached and the reaction remains stable.
What is chemical equilibrium?
- A state that occurs when the forward and reverse rates of a reaction are equal and concentrations of reactants and products are no longer changing with time
Quadratic Formula
Homogeneous Equilibrium
- all species are in the same phase
What is Kp?
- the equilibrium constant with respect to pressure of gases
- it is only expressed in terms of gases, SOLIDS AND LIQUIDS DO NOT COUNT towards Kp
- it equals the partial pressures of products over the partial pressures of reactants, at equilibrium and raised to their stoichiometric coefficients
What is the equation for the relationship between Kp and Kc?
- Kp is equal to Kc times the product of the ideal gas constant and temperature (Kelvin) raised to the power of the difference in moles between products and reactants
the picture shows the inverse:
Heterogeneous Equilibrium
- a reversible reaction in which the chemical species of reactants and prodcuts are in different phases
- pure solids and liquids DO NOT COUNT in the equilibrium expression (their values remain constant at a given temperature)
Le Châtelier’s Principle
If external stress is applied to a system at equilibrium, the system adjusts such that the stress is offset and equilibrium is re-established.
Factors that affect equilibrium
- temperature
- pressure
- volume
- concentration
How does temperature affect equilibrium?
- Depending on the ΔH of the reaction, heat is either a reactant or a product
- ΔH > 0, reaction is endothermic, heat is reactant, raised temperature drives reaction productsward
- ΔH < 0, reaction is exothermic, heat is a product, raised temperature drives reaction reactantsward
How does pressure affect equilibrium?
- An increase in pressure drives a reaction towards whichever side has fewer moles
How do catalysts affect equilibrium?
They DO NOT. They only affect the rate at which it is reached.
What are standard state conditions?
1 atm pressure and 1 M concentration
How is ΔG related to the equilibrium constant?
- By a proportionality constant R x T, with R being the constant 8.314 J/(mol K) and T being temperature (Kelvin)
ΔG (standard state) = - RT x ln K
or for any conditions
ΔG (any conditions) = RT x ln Q + ΔG (standard state)
What’s a Bronsted-Lowry Acid/Base?
- acid is capable of yielding an H+ ion and is therefore a “proton donor”
- base yields OH- ion and is a “proton acceptor”
What are conjugate acids/bases?
A conjugate acid or base is produced from the gain or loss of H+ ions, respectively.
What are the 7 strong acids?
And what makes them “strong”?
- HBr (Hydrobromic Acid)
- HI (Hydroiodic Acid)
- HCl (Hydrochloric Acid)
- HClO3 (Chloric Acid)
- HClO2 (Chlorous Acid)
- H2SO4 (Sulfuric Acid)
- HNO3 (Nitric Acid)
- They completely dissociate in solution.
What are the strong bases? And what makes them strong?
- Group I and II Metal Hydroxides ( ex: NaOH, Ca(OH)2 )
- Group I and II Metal Oxides ( ex: MgO, CaO )
- They dissociate completely in solution to produce H+ accepting compounds.
What is a weak acid?
- Any acid not within the 7 strong acids which dissociates only partially in solution
What are the weak bases?
Amines - contain N with a lone electron pair
Carbonates - CO32- + H20 <—> HCO3- + OH-
What determines the strength of an acid bond?
- smaller atoms bond more tightly to their H+s and therefore produce weaker acids (because they give off their H+ ions less readily)
- within a group, elements closer to the top of the table (smaller atoms) produce weaker acids, whereas elements further down the table (bigger atoms) produce stronger acids
Determining strength of oxy acids with different central atoms, but same # of oxygens.
- the more electronegative central atom inductively draws electron density from H-O bond, weakening it and therefore strengthening the acid
- ex: HClO4 is stronger than HBrO4
Determining strength of oxy acids with same central atoms but different #s of oxygens.
- more oxygens inductively weaken the O-H bond, strengthening the acid
ex: HClO3 is stronger than HClO2
Lewis Acids and Bases
- Acid accepts a pair of electrons
- Base donates a pair of electrons
What is Kw?
- it is the ion product of water at a given temperature
Kw = [H+][OH-] = 1.0 x 10-14
What is pH?
what is pOH?
- log [H+]
- log [OH-]
What should one do to find the pH of extremely dilute acids?
- Add back in the H+ ions from water
ex: pH of 1.0x10-8 M HCl is equal to: - log (1.0x10-8 + 1.0x10-7)
Polyprotic Acids
- some acids have more than one acidic proton and more than one Ka value
- most H+ concentration comes from the 1st dissocation so usually Ka1 is used
What are the acidic metals?
Small cations (above group 4) with large charges (> +1)
ex: Al, B
What is the relationship between Ka and Kb?
Ka x Kb = Kw (1.0 x 10-14)
What is the composition of a salt that forms a neutral solution?
An anion of a strong acid and a cation of a strong base
What is the composition of a salt that forms an acidic solution?
- An anion of a strong acid and a cation of a weak base
What is the composition of a salt that forms a basic solution?
Anion of a weak acid and cation of a strong base
How can you determine the acidity or basicness of a salt formed from a weak base and weak acid?
Compare the Ka of the cation with the Kb of the anion.
Whichever is larger determines pH.
Amphoteric Salts
- can act as acids or bases, compare Ka and Kb values of reacting species
- the higher K value determines acidity/basicity
Henderson-Hasselbach Equation
- used for buffer systems only
Equivalence Point of a Titration Curve
(and how to calculate its pH)
- the point at which the moles of acid and moles of conjugate base are equal
- usually found where pH is changing rapidly
calculation:
- Write out ICF table, acid and base react completely, leaving only conjugate and water
- Write out ICE table for conjugate and water reaction (resulting in acid and OH-
- Use Kb and ICE to find [OH-]
- Use [OH-] to find pH
Calculating pH before an equivalence point on a titration curve.
- use the Henderson-Hasselbalch Equation
Calculating pH at 1/2 of the equivalence point of a titration curve.
pH = pKa
Calculating pH beyond the equivalence point of a titration curve (acid titrated with base).
- Base is now in excess of acid, so use the unreacted base to determine the pH
