Exam 3 Review

Reaction mechanisms

100

what is the molecularity of 2NO(g) + O₂(g) --> 2NO₂(g)

trimolecular

200

Draw a diagram for the following reaction mechanism and label the axis, intermediates, transition states, activation energies, and ΔH.

Step 1 is fast and exothermic

Step 2 is slow and endothermic. 

The overall reaction is endothermic

2nd Ea is larger than the first (bigger hump)

intermediate should be lower than reactants and products should be higher than reactants

300

Does this mechanism have a catalyst? Does it have an intermediate? How do you know?

H₂O₂ + I^- ⇋ H₂O + IO^- (fast)

H₂O₂ + IO^- --> H₂O + O₂ + I^- (slow)

IO- is the intermediate and I^- is the catalyst.

Intermediates and made then used up (in the inner part of reaction)

Catalysts are used and then made again (because they need to go catalyze other reactions)

400

H₂O₂(aq) + I^-(aq) ⇋ H₂O(aq) + IO^-(aq) (fast)

H₂O₂(aq) + IO^-(aq) --> H₂O(l) + O₂(aq) + I^-(aq) (slow)

overall: 2H₂O₂(aq) --> 2H₂O(l) + O₂(aq)

What is the overall rate law?

rate = k[H₂O₂]²[I^-]/[H₂O]

k=k₂k₁/k_-1

Thermodynamics

100

Calculate the Gibbs free energy change (G) for the following chemical reaction: 

glutamate + NH3 —--> glutamine + H2O

The reaction occurs at 37°C, the change in heat (H) = 2.8 kJ/mol, and the change in entropy (S) = 5.6 J/K*mol.

1.1x10³ J/mol = 1100 kJ/mol

200

What assumption do we have to make to answer this question?

At what temperature (in ^oC) does a reaction go from being nonspontaneous to spontaneous if it has 

ΔH = 284 kJ/mol and ΔS = 986 J/(K*mol)

ΔG=0

300

VERBALLY explain how you would solve these problems:

This reaction occurs at 298K , Find ΔG^o and determine if it is thermodynamically favorable:

                CH₄(g)   + 2O₂(g) ⇋  CO₂(g)   + 2H₂O(g)

Enthalpy:  -75 kJ        0 kJ         393.5 kJ     -242 kJ

Entropy:    186 J/K     208 J/K    214 J/K       184 J/K

1) Use these equations and plug in the given enthalpy and entropy of formation values:

ΔH^∘ = ∑(n⋅ΔH_f^∘_(products)) − ∑(m⋅ΔH_f^∘_(reactants))

ΔS^∘ = ∑(n⋅ΔS_f^∘_(products)) − ∑(m⋅ΔS_f^∘_(reactants)) 

2) Then plug what you know into ΔG^o=ΔH^o−TΔS^o

3) If G is negative, then it is thermodynamically favorable.

400

One statement contradicts the other 3. Which is it?

ΔG^o < 0 

lnK > 0

ΔS^o > ΔH^o/T

K < 1  

K < 1 contradicts the other 3

Equilibrium

100

At some point along the reaction below, the partial pressure of H₂ is 5.25 atm and the partial pressure of hydrogen bromide is 1.75 atm

H₂(g) + Br₂(l) ⇌ 2HBr(g)             ΔG° = 5.7 kJ/mol at 25 °C

What is the equilibrium constant at 25 °C?

K = 0.10

200

Fill in an ICE table for the following problem (do not need to solve for E column):

The concentration-based equilibrium constant for the reaction below is 1.1 × 10−3 at 1300 °C.

Br₂(g) ⇌ 2Br(g)

If the initial concentrations are [Br₂] = 0.065 M and [Br] = 0.015 M, calculate the concentrations of these two species at equilibrium.

Respectively,

I : 0.065M, 0.015M

C: +x, -2x

E: 0.065+x, 0.015-2x

300

If the K value for S + O₂ ⇌ SO₂ is 25 and the K value for A + B ⇌ C is 14, what is the K value for SO₂ + 0.5A + 0.5B ⇌ S + O₂ + 0.5C

0.15

400

solve for x:

K_c = 4.3x10^-3 = (0.034-2x)²/(0.098+x)

x = 0.006406

Le Chatelier

100

What will happen to the concentration of NO₂ if the temperature is increased?

N₂O₄(g) ⇌ 2NO₂(g)        ΔH>0

NO₂ will increase

200

If the volume of the container decreases, what will happen to the concentration of SO₃?

2SO₂(s)+O₂(g)⇌2SO₃(g)

SO₃ decreases

300

 

CaCO₃(s)⇌CaO(s)+CO₂(g)ΔH>0

If additional CaCO₃ (s) is added to the system at equilibrium, what happens to the concentration of CO₂

no change

400

What effect does removing NaCl have on the equilibrium position of the following reaction:

AgCl(s) ⇌ Ag⁺(aq) + Cl^-(aq)

Equilibrium shifts to the right

Acid/Base (Exam 4) and previous exams review

100

Label the acid, base, conjugate acid, and conjugate base:

HCl(aq) + NH₃(aq) --> Cl^-(aq) + NH₄⁺

acid: HCl (strong)         conj base: Cl^-

base: NH₃ (weak)            conj acid: NH4⁺

200

Write the chemical equation for the dissociation of solid silver chromate.

Ag₂CrO₄(s) ⇌ 2Ag⁺(aq) + CrO₄^2-(aq)

- correct states

- equilibrium arrows

- correct charges/balanced correctly

- water doesn't need to be shown (implied by aq state)

300

Which of the following statements is false:

(i) The rate of a reaction depends on the energy of collisions between reactants.
(ii) The rate of a reaction depends on the orientation of colliding molecules.
(iii) The rate of a reaction depends on the energy difference between the reactants and products

(iii) The rate of a reaction depends on the energy difference between the reactants and products

it depends on the activation energy: the energy difference between reactants and transition state(s)

400

Make a given list with very specifically labeled values and explain how to solve this problem (don't do any math):

If 61.2 g of I₂ are dissolved in 567 g of hexane (C 6 H14 ), what is the boiling point of the solution? The boiling point of pure hexane is 68.7 °C, and the molal boiling point elevation constant for hexane is 2.90 °C/m

mass I₂ (solute) = 61.2g

mass C₆H₁₄ (solvent) = 567g

T_b^o = 68.7^oC

K_b = 2.90 ^oC/m

Find: T_b

ΔT_b = imK_b

ΔT_b =T_b^o + T_b 

use masses to solve for m then isolate T_b and solve

(if you're practicing outside of PAL, the answer is T_b = 69.9^oC)