Chem Exam 3
What are the three rules for manipulating equilibrium constant expressions?
1. Multiply a reaction by a coefficient, x, then raise the equilibrium constant expression to the power of x 2. Reverse reaction, then take reciprocal of equilibrium constant expression 3. Add reactions; then take the product of equilibrium constant expressions
N₂ (g) + 3 H₂(g) ⇌ 2NH₃ (g) Kc= [NH3]² / [N₂] [H₂]³ = 780 What is Kp for this reaction at 298 K?
1.304 [atm⁻²]
Identify the conjugate acid, and the conjugate base: - CH₃COOH (aq) + H₂O (aq) ⇌ H₃O⁺ (aq) + CH₃COO⁻ (aq)
H₃O⁺ is the conjugate acid CH₃COO⁻ is the conjugate base CH₃COOH, CH₃COO⁻ are a conjugate acid-base pair H₂O, H₃O⁺ are a conjugate acid-base pair
Identify the conjugate acid, and the conjugate base: - HCl (aq) + H₂O (aq) ⇌ H₃O⁺ (aq) + Cl⁻ (aq)
H₃O⁺ is the conjugate acid Cl⁻ is the conjugate base HCl, Cl⁻ are a conjugate acid-base pair H₂O, H₃O⁺ are a conjugate acid-base pair
Type 2 Problems, When can we use a helpful approximation?
If Kc or Kp is >10² or <10⁻² -Or if the algebraic expression of the equilibrium constant leads to a polynomial beyond a quadratic.
When is the approximation of a weak base valid?
If [B]init/Kb is greater than 100, then you can use the equation of: Square root Kb∙[B]init
When is the approximation of a weak acid valid?
If [HA]init/Ka is greater than 100, then you can use the equation of: Square root Ka∙[HA]init
What is the leveling effect?
In aqueous solution, the strongest acid is H₃O⁺ and the strongest base is OH⁻.
What does the reaction quotient (Qc or Qp) indicate?
Indicates the relative amounts of product and reactant when the reaction is NOT at equilibrium.
What does H₂O being amphiprotic mean?
It both donates and accepts protons.
What is the general pattern to identify an organic acid?
Look for a "H" to be bonded to the COO -For example CH₃COOH
Identify the conjugate acid and the conjugate base: - NH₃(aq) + H₂O (aq) ⇌ NH₄⁺ (aq) + OH⁻ (aq)
NH₄⁺ is the conjugate acid OH⁻ is the conjugate base NH₄⁺, NH₃ are a conjugate acid-base pair H₂O, OH⁻ are a conjugate acid-base pair
What happens when you put strong acids into solution?
"Every molecule" donates its proton
What happens when you put a weak acid into solution?
"Only some of the molecules" donate their proton
What are the 2 key principles of using helpful approximation in type 2 problems?
-If x is less than 5% of A, then we approximate A + x ~ A OR A - x ~ A -When x (by itself) is directly involved in multiplication or division, leave it there.
The reaction: 2 COF₂ (g) ⇌ CO₂ (g) + CF₄ (g) has Kc = 2.00 at 1000°C. Determine the value of Kc for these reactions: 1) CO₂ (g) + CF₄ (g) ⇌ 2 COF₂ (g) 2) COF₂ (g) ⇌ 1/2 CO₂ (g) + 1/2 CF₄ (g) 3) 2 CO₂ (g) + 2 CF₂ (g) ⇌ 4 COF₂ (g)
1) .5 2) 1.41 3) .25
What are the two factors to explain the molecular structure and acid-base strength? Look at pg.3 of ch.15 handout 1
1. Acidic O-H bond is polar, electron withdrawal cause by electronegative O atoms make O-H bond more polar; easier to donate proton=STRONGER ACID 2. The negative charge on the oxoanion conjugate base is stabilized by the delocalization of multiple resonance structures: more resonance structures, more delocalization for the same amount of charge
What is the generic monoprotic acid dissociation?
HA (aq) + H₂O (aq) ⇌ H₃O⁺ (aq) + A⁻ (aq) -A is a variable
For our purpose, all organic acids can be considered?
They can be considered weak acids.
Do acids accept or donate protons?
Acids donate protons
What effects does an aromatic ring have have the strength of a base?
An aromatic ring draws electrons away from N, causing a weaker base.
What are the 6 strong acids?
HCl HBR HI HNO₃ HClO₄→Monoprotic H₂SO₄→Diprotic
What is the generic weak base dissociation?
B (aq) + H₂O (aq) ⇌ BH⁺ (aq) + OH⁻ (aq) -B is a variable
Do bases accept or donate protons?
Bases accept protons
What is the relationship between an acid and a conjugate base?
The weaker an acid the stronger the conjugate base will be.
Identify the conjugate acid and the conjugate base: - NaOH (s) ↔ Na⁺ (aq) + OH⁻ (aq) H₂O
Na⁺ is the conjugate "acid" OH- is the conjugate "base"
For strong bases, what is the Kc value?
The Kc value for strong bases is usually greater than one due to the formation of products.
For weak acids, what is the Kc value?
The Kc value for weak acids is generally less than one due to small amount of products formed.
For weak bases, what is the Kc value?
The Kc value for weak bases is usually less than one due to a small amount of products forming.
What are the three primary quantities that can be altered in the Le Chatelier's principle?
The amounts of products or reactants -For equilibria involving gases, the total pressure for the mixture -The temperature
The more oxygen atoms an acid has means what?
The more oxygen atoms an acid has the stronger the acid will be.
What is the equation to solve for ∆n?
The number of moles of gaseous product - number of moles of gaseous reactant
What way does a reaction move if Q<K?
The reaction as written proceeds in a forward direction.
What way does a reaction move if Q>K?
The reaction as written proceeds in the reverse direction.
What happens when some of a reactant or product is added to a reaction mixture?
The reaction shifts to consume some of the substance that was added.
What happens when some of a reactant or product is removed from a reaction?
The reaction shifts to produce more of the substance that was removed.
What happens when the total pressure of a reaction mixture is decreased?
The reaction will attain a new equilibrium by moving in the direction that increases the number of gas phase molecules.
What happens when the total pressure of a reaction mixture is increased?
The reaction will attain a new equilibrium by moving in the direction that reduces the number of gas phase molecules.
What is the relationship between a base and a conjugate acid?
The stronger a base the weaker the conjugate acid will be.
For strong acids, what is the Kc value?
Typically very high because it forms all products.
What are [H₃O⁺] and [OH⁻] in a solution that has a pH of 3.50?
[H₃O⁺] → 3.16x10⁻⁴ mol/L [OH⁻] → 3.16x10⁻¹¹ mol/L