Chapter 14 Chemical Kinetics
If 0.260 mol of a nonvolatile nonelectrolyte are dissolved in 3.00 mol of water, what is the vapor pressure PH2O of the resulting solution? The vapor pressure of pure water is 23.8 torr at 25 ∘C .
(23.8-P)/23.8=0.26/(0.26+3). P= 21.9torr
Biomolecular reaction
An elementary reaction involving the collision of two reactant species
Unimolecular reaction
An elementary reaction that involves the decomposition or rearrangement of a single particle.
If the rate = [P]2[G][O] then the overall reaction order is what?
Fourth order overall
Which kind of cation has greater ion-dipole interactions and frequent ion-pairings?
The cations with a small size and large. (Also have most frequent ion-pairings)
For the reaction 2AsH3 (g) → As2 (g) + 3H2 (g), the reaction rate can be given by which of the following?
+∆[As2]/∆t (+3∆[H2]/∆t, +1/2∆[As2]/∆t ,+1/2∆[AsH3]/∆t are not correct)
second order reaction
-rate is proportional to either the concentrations of 2 reactants or to the square of the concentrations of a single reactant *Rate = k[A]1[B]1 or rate = k [A]2 or rate = k [B]2 1/([A]) vs time line slope = k A graph of 1/[A]A graph of 1/[A] versus time yields a straight line for a second-order reaction. versus time yields a straight line for a second-order reaction.
What will affect reaction rates?
1. Physical state of the reactants; 2. Reactant concentrations; 3. Reaction temperature; 4. Presence of catalyst;
Evaporation and volatility
1. When a volatile solvent is kept in a sealed container, a vapor forms above the solvent as a result of the physical process known as evaporation. 2. When the two phases are at equilibrium, the force exerted by the solvent molecules in the gas phase is called the vapor pressure. 3. Therefore, the vapor pressure is a measurable indicator of the volatility of a solvent. 4. Higher vapor pressures, which increase with temperature and solvent purity, correspond to more molecules in the gas phase.
Initial rate data were collected for the reaction 2H2(g) + 2NO(g) → 2H2O(g) + N2(g). What is the overall order for the reaction based on the experimental data?
3
Heterogeneous catalyst
A catalyst that is in a different phase from that of the reactant substances.
collision model
A model based on the idea that molecules must collide to react; used to account for the observed characteristics of reaction rates
Ion pairing
A phenomenon occurring in solution when oppositely charged ions aggregate and behave as a single particle (associated with van't Hoff factor and can be neglected in non-electrolyte)
Elementary reaction
A reaction in which reactants are converted to products in a single step
Activated complex (transitional form)
A transitional structure that results from an effective collision and that persists while old bonds are breaking and new bonds are forming
Termolecular reaction
An elementary reaction that involves three reactant molecules (not trimolecular)
An elementary reaction includes only reactant "A". When the concentration of "A" increases by 50%, the rate increases by a factor of 2.25. What is the molecularity of the elementary reaction?
Bimolecular
A reaction mechanism __________ be proven to be correct using experimental data________.
Cannot, because different mechanism can lead to the same rate law. (Please make sure the determining step)
It is determined experimentally that the reaction E + F → X + Y is second order with respect to E and first order with respect to F. Which of the following statements is correct?
Doubling the concentration of E will increase the reaction rate by a factor of four.
What typically happens to the concentration of an unsaturated solution when it evaporates at constant temperature and the volume decreases?
Increases, then remains constant.
Boiling point and vapor pressure of solution
Increasing the solute concentration in a solution lowers the freezing point, increases the boiling point, lowers the vapor pressure (of the solvent), and increases the osmotic pressure. The colligative properties of vapor pressure and boiling point elevation are directly related. Boiling occurs when the vapor pressure of the solvent reaches ambient pressure (typically 1 atm ). Heating a liquid raises its vapor pressure, but if the vapor pressure of the solvent is lowered (due to the addition of solute), then more heat is required for the vapor pressure to be raised to ambient pressure, which is why the boiling point increases.
Imagine a solution of two liquids that are so similar that their interactions together are the same as in the individual liquids. Will this solution deviate positively from, deviate negatively from, or ideally follow Raoult's law?
It will be an ideal solution.
Large ion-dipole interaction
Large charge & small radius
Order the ion-dipole interactions for lithium, sodium, and magnesium cations in water from weakest to strongest.
Na+ < Li+ < Mg2+
A solution is composed of 1.90 mol cyclohexane (P∘cy=97.6 torr) and 2.00 mol acetone (P∘ac=229.5 torr). What is the total vapor pressure Ptotal above this solution?
Ptotal=PA+PB+PC=XAP∘A+XBP∘B+XCP∘C Significant Figures Feedback: Your answer 165.26torr was either rounded differently or used a different number of significant figures than required for this part.
Reaction intermediates
Reaction intermediates are formed in one step and then consumed in a later step of the reaction mechanism.
Reactants A and B combine to give the product AB. Under what conditions would the rate constant k have units of M-1 s-1?
Reaction is first order in A and first order in B.
Consider the following hypothetical molecular collisions and predict which of the following will form potential products, given the values for the energy of activation, Ea, and enthalpy of the reaction, ΔH, in combination with the molecular orientation.
Reactions in which Ea>ΔH are either endothermic, when ΔH is positive, or exothermic, when ΔH is negative. Reactants must also be properly orientated for a reaction to occur. When molecules are properly oriented and there is sufficient energy to break bonds products may be form
Which type of NaCl(aq) solution is experiencing the fastest rate of crystallization? All of the solutions are at the same temperature.
Saturated
An emulsion is a suspension of two liquids that are usually immiscible. Emulsifying agents are substances that keep parts of an emulsion mixed together and __________ is an example.
Soap
chemical kinetics
The area of chemistry that is concerned with reaction rates and reaction mechanisms
Activation energy
The minimum amount of energy required to start a chemical reaction
Molecularity
The number of molecules that participate as reactants in an elementary reaction
Match the words in the left column to the appropriate blanks in the sentences on the right.
The orientation factor does not dependThe orientation factor depends on temperature. It is combined with frequency of collision into the frequency factor, A , which increases with temperature. on temperature. It is combined with frequency of collision into the frequency factor, A, which is essentially constantThe orientation factor depends on temperature. It is combined with frequency of collision into the frequency factor, A , which increases with temperature. with temperature.
What factors determine whether a collision between two molecules will lead to a chemical reaction?
The orientation of the molecules The energy of the collision
Consider a reaction taking place at 20oC with an energy of activation = 25 kJ/mol. If the temperature is doubled to 40oC, how much does the rate increase?
The rate increases by 1.95 times.
Rate determining or rate-limiting step.
The slowest step in the mechanism is called the rate determining or rate-limiting step.
Reaction mechanism
The step-by-step sequence of reactions by which the overall chemical change occurs
Imagine a solution of two liquids in which the molecules interact less favorably than they do in the individual liquids. Will this solution deviate positively from, deviate negatively from, or ideally follow Raoult's law?
This solution will have a vapor pressure that is slightly higher than the value predicted by Raoult's law. This is called a positive deviation.
Testing the hypothesis would call for various vapor pressure measurements. What materials and equipment would you need to measure the vapor pressure of a pure solvent?
This type of apparatus could be used to measure the vapor pressure of your solvent. As the vapor pressure changes in the container, the relative fluid levels change, and the difference in height corresponds to the difference in pressures between the vapor pressure (from the closed end) and atmospheric pressure (from the open end). When the pressure in the container is the same as atmospheric pressure, the difference in height is equal to 0. There are no calculations involving mass that would allow you to determine the pressure. Therefore, any equipment related to obtaining the mass will not be useful in determining the pressure.
True or false: Heterogeneous catalysts have different phases from reactants.
True
True or false: The rate limiting step in a reaction is the slowest step in the reaction sequence.
True
Which factor is most sensitive to changes in temperature?
fraction of molecules with energy greater than the activation energy
