Chemical Jeopardy 1

Intermolecular Forces

100

Describe the intermolecular forces that must be overcome to convert each of the following from a liquid to a gas:

1. Br₂
2. CH₃COOH
3. H₂S

1. London dispersion forces
2. hydrogen bonding, dipole-dipole, London dispersion
3. dipole-dipole, London dispersion

200

Which member of the following pairs has the strongest intermolecular forces?

1. H₂O or H₂S
2. CO₂ or CO
3. CH₄ or SiH₄

1. H₂O
2. CO₂
3. SiH₄

300

Which member of the following pairs has the lowest vapor pressure and why:

1. CCl₄ or CBr₄
2. CH₃OCH₃ or CH₃CH₂OH

1. CBr₄
because of higher MW, therefore stronger IMF (London).
2. CH₃CH₂OH
because of Hydrogen Bonding (they both have MW = 46), therefore stronger IMF.

400

Rank the following substances: CH₃OH, HCl, Ne, NaF in order of increasing of boiling points, and explain the reasoning.

Ne < HCl < CH₃OH < NaF

Explanation: Ne (MW=20; L) < HCl (MW=36.5; L + D-D) < CH₃OH (MW=32; L + D-D + H-B)< NaF (MW=42; ionic)

500

Butane (left) and 2-methylpropane (right) have the same molecular formula but butane has a higher boiling point. Explain.

Both butane and 2-methylpropane experience London dispersion forces, however, the larger contact surface area (rod shape, smaller radius) between butane molecules facilitates stronger forces and produces a higher boiling point.

State Changes

100

Name the phase transition in each of the following situations and indicate whether the transition is exothermic or endothermic:

1. bromine vapor to bromine liquid
2. crystals of iodine disappear in the fume hood
3. rubbing alcohol from an open container disappears

1. condensation (exothermic)
2. sublimation (endothermic)
3. evaporation (endothermic)

200

Explain why the heat of fusion of any substance is generally lower than its heat of vaporization.

Melting does not require separation of molecules, so the energy requirement is smaller than for vaporization, where the molecules must be completely separated.

300

Define the critical point. What is a supercritical fluid? Give an example.

The critical point is the temperature and pressure beyond which the gas and liquid phases are indistinguishable. Supercritical fluid is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist. Example: CO₂, which is used to extract caffeine from coffee beans.

400

Explain how each of the following affects the vapor pressure of a liquid:

1. volume of the liquid
2. surface area
3. intermolecular attractive forces
4. temperature

1. no effect
2. no effect
3. vapor pressure decreases with increasing IMFs
4. increases with increasing temperature

500

At 1 atm, how much energy in kJ is required to freeze 72.0 g of water, H₂O(l), at 0.0^oC to ice, H₂O(s), at 0.0^oC? The enthalpy of fusion is +6.00 kJ/mol.

- 24.0 kJ

Colligative Properties

100

What are the four properties of a solution that depend on the total concentration but not the type of particle(s) present as a solute.

freezing point depression, boiling point elevation, osmotic pressure, vapor pressure lowering

200

Indicate the type of solute-solvent interaction that should be most important in each of the following solutions:

1. CCl₄ in benzene (C₆H₆)
2. CH₃CH₂OH in water
3. KBr in water
4. HCl in acetonitrile (CH₃CN)

1. dispersion
2. hydrogen bonding
3. ion-dipole
4. dipole-dipole

300

The enthalpy of solution of KBr in water is about +198 kJ/mol. Nevertheless the solubility of KBr in water is relatively high. Why does the solution process occur even though it is endothermic?

KBr is quite soluble in water because of the increase in disorder of the system (KBr lattice ---> K+ Br- hydrated ions) associated with dissolving process. Since Delta S >0 (or Delta G < 0) the process is spontaneous.

400

List the following aqueous solutions in order of increasing boiling point: 0.05 m LiBr ; 0.05 m Zn(NO₃)₂; 0.05 m urea (CO(NH₂)₂); 0.05 m H₂SO₄

0.05 m urea <0.05 m LiBr < 0.05 m H₂SO₄ < 0.05 m Zn(NO₃)₂

500

1. Does the molarity of a solution change with the change of temperature? Explain
2. Does the molality of a solution change with the change of temperature? Explain

1. Yes, molarity changes with temperature. Molarity is defined as moles of solute per unit volume of solution. A change in temperature changes the volume and thus molarity
2. No, molality does not change with temperature. Molality is defined as moles of solute per unit of kg of solvent. Temperature affects neither mass nor moles.

Reaction Rates

100

Define the term " reaction rate". What is the instantaneous rate?

Reaction rate is the change in the amount of products or reactants in a given amount of time. The instantaneous rate is the slope of the line drawn tangent to the concentration vs time curve at a specific time.

200

What are the four factors that can affect the rate of a chemical reaction? Which of the above four factors affect the rate constant?

concentration of reactants, surface area of reactants, temperature, and presence of a catalyst. Only temperature.

300

How do the half-lives of first-order and second-order reactions differ?

For first order reactions, the half-life depends solely on the reaction rate constant, k. For second order reactions, the half-life depends on both the initial concentration, C₀, and the rate constant, k.

400

For the following reaction: A + B ---> C it is seen that when [A] doubles the initial rate quadruples; and when both [A] and [B] double, the initial rate increases by a factor of 8. What are the orders with respect to A and B? What is the overall order of the reaction?

2nd order in [A]; 1st order in [B], overall 3rd order.

500

For the reaction CV + OH ---> CVOH the absorbances of the solution mixture were collected over time using a Vernier spectrometer. By graphing the data it was found that the line of [CV] as a function of time has an R²= 0.889; the line of Ln[CV] as function of time has an R² = 0.987; and the line of 1/[CV] as a function of time has an R² = 0.955. What is the order with respect to [CV]? It was found that when [OH] doubled, the rate constant, found by the slope of the best-fit line, doubled. What is the order with respect to [OH]? What is the overall order of the reaction?

1st order in [CV]; 1st order in [OH]; 2nd order overall.

Reaction Mechanism

100

What is a reaction mechanism? What is meant by the term elementary reaction?

A reaction mechanism is a series of elementary reactions that describe how an overall reaction occurs and explain the experimentally determined rate law. An elementary reaction is a process that occurs in a single event.

200

What factors determine whether a collision between two molecules will lead to a chemical reaction? What is the activation energy?

The energy of the collision and the orientation of the molecules when they collide. The activation energy is the minimum kinetic energy required for the collision to result in a reaction.

300

Define an intermediate. What is a catalyst? What part of the energy profile of a reaction is affected by a catalyst?

An intermediate is produced in one elementary step and is consumed in a later elementary step and therefore does not appear in the overall equation for the reaction. A catalyst is a substance that increases the rate of the reaction without undergoing any chemical change itself. So the catalyst appears in the first and last elementary steps of the reaction. A catalyst increases the rate of reaction by decreasing the activation energy (E_a).

400

Write the Arrhenius Equation? Define all the terms.

k = A e ^-(E_a/RT)

k - rate constant

A - frequency factor

E_a - activation energy

T- temperature in K

R- gas constant

500

The reaction 2 NO (g) + Cl₂ (g) ----> 2 NOCl (g) was performed under conditions of constant [Cl₂]. After processing the data, it was found that the graph of 1/[NO] as a function of time was linear with an R² = 0.999.

a) Is the following mechanism consistent with the data?

NO(g) + Cl₂ (g) <--->NOCl₂ (g) (fast)

NOCl₂(g) + NO (g) ----> 2 NOCl (g) (slow)

b) Does the linear plot guarantee that the rate law is second order in [NO]?

1. The two-step mechanism is consistent with the data, assuming that the second step is rate determining.
2. The linear plot guarantees that the overall law will include [NO]².