GChem VI

Question 1

Define: Reaction rate

Answer 1

How fast the concentration of a reactant decreases over a period of time, or how that of a product increases over time.

(insert: equation)

Question 2

Define: Chemical Kinetics

Answer 2

The study of the rate of chemical reactions and the pathway in which these reactions occur. Therefore, chemical kinetics is path dependent.

Question 3

What are the five factors that affect the rate of a reaction?

Answer 3

1. nature of the reactants
2. concentration of reactants
3. temperature
4. catalyst
5. surface area

Question 4

What is the rate of reaction of the following reaction?

H<font><span>2O2(aq)+3I-(aq)+2H+--&gt;I3- +2H20(l)</span></font>

Answer 4

Rate= -∆[H2O2]/∆t= -1/3 ∆ [I-]/∆t=-1/2 ∆[H+]/∆t=∆[I3-]/∆t=(1/2 ∆[H20])/∆t

(note)To write a rate law, remember:

aA +bB –> cC

Rate= -1/a [A]/t = -1/b [B]/t = 1/c[C]/t

Question 5

How can the rate law be determined for a multi-step reaction, if the following table of rates vs. reaction concentrations is given:

(table format)

[A] (M) [B] (M) [C] (M) rate (M/s)
1 1 1 1
2 1 1 4
1 2 1 2

Answer 5

Rate=<font><span>k[A]^x[B]^y[C]^z</span></font>

• x and y can only be found experimentally from the data provided
• As [A] doubles, the rate quadruples, therefore 2^x=4, x=2

Question 6

How does temperature affect the rate of a reaction?

Answer 6

The rate constant, k, is solely dependent on temperature. Therefore, as temperature increases, k increases, and the overall rate of reaction increases.

Question 7

Identify the rate determining step in the following reaction and write the overall reaction rate law:

NO2(g) + CO(g) →NO(g) +CO2(g)
——————————————————–
NO2(g) + NO2(g)→ NO3(g) + NO(g):slow
NO3(g) + CO(g)→NO2(g) +CO2(g): fast

Answer 7

The rate determining step is always the slow step in the reaction mechanism. Therefore, the overall rate law will be derived from the slow step to show that:

Rate=k[NO2]2

Question 8

Label the following designated parts on the energy profile.

Label:

Transition state, activation energy, and heat of enthalpy on the energy profile below.

Answer 8

Question 9

Define: Catalyst

Answer 9

• A substance that increases the rate of a chemical reaction but is not consumed by the reaction.
• works by lowering the activation energy of the rate determining step.
• alter kinetics but not thermodynamics
• help a system achieve equilibrium faster, but does not alter the position of the equilbirum
• increases rate constant, k, but does not alter Keq

Question 10

What is the difference between kinetic control and thermodynamic control of a reaction?

Answer 10

• kinetics: describes how fast a reaction will take place
• thermodynamics: reveals whether a reaction will occur or not based on whether it is spontaneous or nonspontaneous

Question 11

What is the equation:

An equation that shows the relationship between the equilibrium constant,Keq, and Gibbs free energy, ΔG°

Answer 11

ΔG° = -RT ln (Keq)

• R= constant
• T= temperature(K)

Question 12

What is the equation:

How can the Keq be derived from a reaction?

aA(s) + bB(g) → cC(g)+2dD(l)

Answer 12

Keq= [C]^c/[B]^{<font><span>b</span></font>}

• Keq= products/reactants
• pure liquids and solids are excluded from the equilibrium expression
• the coefficients in the chemical equation become the exponents in the equilibrium expression

Question 13

Define: Biocatalysis

Answer 13

The use of natural catalysts, such as protein enzymes, to perform chemical transformations on organic compounds

Question 14

Describe the process of: Enzyme-substrate binding

Answer 14

The substrate, which is the reactant molecule, binds with the enzyme’s active site and an enzyme-substrate complex is formed. The substrate is transformed into one or more products, which are then released from the active site.

Question 15

What is the equation?

Two equations that show the relationship between the rate constant, k, and temperature,T

Answer 15

Arrhenius equation:
k = Ae-Ea/RT

Eyring equation:
**k=(k_b T)/h e^((-∆G)/RT) **

• Ae= activation energy
• R= constant
• T=temperature(Kelvins)
• b=constant
• h=constant

Question 16

What can be determined from the equilibrium constant, K, of a reaction?

Answer 16

• if K<< 1: reverse reaction is favored, so reactants are favored
• if K≈1: neither direction is favored
• if K>>1: forward reaction is favored, so products are favored

Question 17

According to Le Chatelier’s Principle, a change in what three factors can disturb a system at equilibrium?

Answer 17

1. concentration of the reactants or products
2. volume and pressure
3. temperature

Question 18

What happens if NO₂ is added to the following reaction?

N₂O_4(g)→2NO_2(g)

Answer 18

The reaction will proceed to the left in order to consume some of the added NO_2.

(Note: Add=Away: equilibrium shifts away from added material, Take away= toward: equilibrium shifts toward removed material)

Question 19

According to Le Chatelier’s Principle, what happens if the volume of a reaction mixture is decreased?

Answer 19

If the volume is decreased, then pressure is increased. Therefore, the reaction will proceed toward the direction with fewer moles of gas particles to minimize the pressure increase.

Question 20

Analyze the data:

What reaction order is depicted by the graph below?

Answer 20

A first order reaction is depicted because as concentration of the reactant increases, the rate of reaction proportionally increases as well

Question 21

Analyze the data:

Based on the energy profile, determine whether the reaction is exothermic or endothermic.

Answer 21

Because the products have a lower standard free energy than the reactants,energy must have been released during the reaction. Therefore, the reaction is exothermic.