Chp 16 Chem 2

Select common laboratory methods for determining the rate of a reaction.

-Change in pressure over time -Change in electrical conductivity -Spectrometric methods

In order to determine reaction orders experimentally, a series of experiments are run in which the initial concentration of one reactant is ___________ while the initial concentration of the other is kept __________ . The effect of the initial concentration on the initial rate is then evaluated in each case.

changed, constant

The steps for studying kinetics are summarized in the illustration. Match each lettered step with the appropriate action required.

A Determine the slope of tangent at t0 for each plot. B Compare the initial rates when the concentration of one reactant changes and the other is held constant (and vice versa). C Substitute the initial rates, orders, and concentrations into the rate equation and solve for k.

Match each method for determining reaction rate with the correct description.

Change in pressure over time: Monitors reactions involving gaseous reactants or products Change in electrical conductivity: Monitors reactions involving ionic reactants or products Spectrometric methods: Monitor reactions where one component absorbs or emits a specific wavelength of light

Which of the following statements correctly describe chemical kinetics? Select all that apply.

Chemical kinetics measures how fast reactants are converted into products. Chemical kinetics is the study of reaction rates. Chemical kinetics describes chemical reactions using collision theory.

Which of the following options correctly interpret the rate law rate = k [HNO2]4/[NO]2? Select all that apply

If [NO] doubles the rate decreases by a factor of 4. The reaction order is -2 with respect to NO.

Which of the following are possible forms of a reaction order? Select all that apply.

Negative integers Positive integers Fractional numbers Zero

In the illustration, the purple and blue spheres represent reactants and the green and yellow spheres represent products. From the information conveyed in the illustration, match the reaction with the appropriate reaction rate. Instructions

Reaction with purple and green spheres - Greater reaction rate Reaction with blue and yellow spheres- Lower reaction rate

Which of the following statements correctly describe average rate and initial rate? Select all that apply.

The average rate is an average of instantaneous rates for a given period of time. The initial rate is an instantaneous rate at t = 0.

Which of the following factors are affected by an increase in reactant concentration? Select all that apply.

The number of reactant collisions The number of reactant particles in a given volume The rate of reaction

What can be deduced about the rate for the plot of concentration versus time for the reaction of C → D?

The rate is decreasing over the course of the reaction. Rate is decreasing because the slope of the curve approaches zero over time. Rate = slope.

Which of the following statements correctly describe a rate law for a reaction? Select all that apply. (Assume that the reaction is not reversible to any appreciable extent.)

The rate law relates the rate of reaction to the concentrations of the reactants. The rate law includes the rate constant for the reaction.

Which of the following options correctly describe the reaction CH3CHO (g) → CH4 (g) + CO (g) where the rate = k[CH3CHO]3/2? Select all that apply.

The reaction order is 3/2 with respect to CH3CHO. The overall order is 3/2.

Which of the following options correctly describe the effect of increasing temperature on a reacting system? Select all that apply.

The reaction rate increases. Particles collide more frequently. Particles collide more energetically.

Why does a hot steel nail placed in oxygen burn slowly (illustration on left) but the same mass of a piece of steel wool burns quickly (illustration on right)?

The steel wool has a greater surface area for reaction.

If only reactants are initially added to a reaction flask, which species decrease in concentration during the course of the reaction N2 (g) + 3H2 (g) → 2NH3 (g)? Select all that apply.

[N2][H2]

The reaction _________ typically has units of mol⋅L-1⋅s-1.

rate

Match each rate law with the statement that correctly describes it. Instructions

rate = k[Br-][BrO3-][H+]2 = The overall reaction order is four. rate = k[O3]2[O2]-1 =The overall reaction order is one. rate = k[NO]2[O2] =The rate is first order with respect to O2. rate = k[NH3]0 =The rate is not dependent on the reactant concentration.

A plot of reactant concentration vs. time gives a downward sloping curve such as the one shown, which indicates that the _____.

reaction rate decreases over time Reason: The rate is decreasing because the slope of the curve approaches zero over time. Rate = slope.

Write a balanced chemical equation for the reaction that has the following rate equation: Rate = - 1/2 Δ[N2O5]/Δt = 1/4 Δ[NO2]/Δt = Δ[O2]/Δt

2N2O5 (g) → 4NO2 (g) + O2 (g)

Determine the overall reaction order for the reaction 4A (g) + 3B (g) → 2C (g) from the experimental data provided. Multiple choice question.

3

Determine the rate constant for the reaction 4A (g) + 3B (g) → 2C (g) given the accompanying experimental data. The rate law for this reaction is given by rate = k[A]2[B].

5.00 × 103 L2/mol2⋅min

Which of the following statements best describes the effect of reactant concentration on reaction rate?

As reactant concentration increases, the number of reactant collisions increases, which in turn increases the reaction rate.

Why does the rate of a reaction generally decrease over time?

As reactant is used up, there are fewer reactant particles to collide and therefore fewer effective collisions.

Match the order of the reaction with the information provided about the effect of changing [A] for the reaction A → products. Instructions

If the rate doubles when [A] doubles, the reaction is: first order with respect to A. If the rate quadruples when [A] doubles, the reaction is: second order with respect to A. If the rate does not change when [A] doubles, the reaction is: zero order with respect to A.

Which of the following statements correctly describe how a rate law is determined? Select all that apply.

If there is more than one reactant present the reaction orders will usually be determined individually. The rate law is determined by measuring the initial reaction rate for different reactant concentrations. The value of k can be calculated from a single experiment once the reaction orders are known.

True or False: In rate law expressions, reaction orders may be negative or fractions.

True

Express the rate of reaction in terms of the change in concentration of each of the reactants and products in the reaction A (g) + 2B (g) → C (g).

- Δ[A] / Δt = - 1/2 Δ[B] / Δt = Δ[C] / Δt

The reaction of acetone with bromine under acidic catalyzed conditions can be represented by the balanced equation CH3COCH3 + Br2 → CH3COCH2Br + H+ + Br-. The rate of disappearance of Br2 was measured for several different concentrations of the various reactants at a certain temperature. The data are summarized in the table below. Given that the rate law is given by rate = k[CH3COCH3][H+], the rate constant at this temperature is equal to ________.

0.0038 M-1⋅s-1

Place the steps necessary to calculate the rate constant for the reaction A (g) + B (g) + C (g) → D (g) from experimental data. Start with the first step at the top of the list. Instructions Choice 1 of 4. Solve for the rate constant in the rate law by using the data from any of the experiments. toggle button Choice 2 of 4. Compare reaction rates when the concentration of one reactant changes while the other remains constant. toggle button Choice 3 of 4. Determine the reaction orders for each reactant individually. Choice 4 of 4. Write a rate law, using the experimentally determined reaction orders.

1. Compare reaction rates when the concentration of one reactant changes while the other remains constant 2. Determine the reaction orders for each reactant individually 3. Write a rate law, using the experimentally determined reaction orders 4. Solve for the rate constant in the rate law by using the data from any of the experiments

All other factors being equal, which of the following mixtures will have the fastest reaction rate?

An aqueous reactant mixed with another aqueous reactant with a stirring device Reason: The surface area of the reactants is maximized for this situation. The reactants are mixed and in the same phase.

Match the type of reaction rate with the correct point on a graph of [reactant] versus time. Instructions

Average rate: The slope of a line joining two points on a curve Instantaneous rate: The slope of a line tangent to the curve at a particular point Initial rate: The slope of a line tangent to the curve at t = 0

In the plot of [A] versus time for the reaction of A → products, match the plot with the order of reaction. Assume k is the same for all cases. Instructions

Black line - zero order green line- first order red line- second line

The molecular scenes show three experiments at a given temperature involving reactants X (black) and Y (green), with their initial rates (in mol/L⋅s). If the rate law is rate = k[X]2, which options correctly identify the initial rate for Expt 2 and the expected appearance of the scene for Expt 3?

Expt 3 would depict 6 black spheres. Initial rate for Expt 2 = 0.25 x 10-5

True or false: Based on the balanced equation N2 (g) + 3H2 (g) → 2NH3 (g), the rate law is given by rate = k[N2][H2]3.

False Reason: Correct. The rate law can only be determined from experiment. It cannot be deduced from a balanced chemical equation unless the equation represents an elementary step in the mechanism.

Which of the following changes would cause an increase in the reaction rate? Select all that apply.

Increasing the concentration of the reactants Increasing the surface area of the reactants

Correctly interpret the plot of rate vs. reactant concentration, [A], in the attached figure, where a, b, and c refer to different reaction orders.

The rate is directly proportional to [A] for a first-order reaction. The rate increases exponentially with [A] for a second-order reaction. The rate does not change no matter what the value of [A] is for a zero-order reaction.

Which of the following options correctly interpret the rate law rate = k[H2][NO]2? Select all that apply.

The reaction is second order with respect to NO. If [NO] is doubled while [H2] remains the same, the reaction will occur 4 times faster.

Match the plot of rate versus [A] for the reaction of A → products with the appropriate order of reaction. Instructions

a = Zero-order reaction b = First-order reaction c = Second-order reaction

When determining reaction orders by experiment, the most common procedure is to ______.

change the concentration of one reactant while keeping the other concentrations constant

Each reaction has its own characteristic reaction rate, which is determined by the nature of the reacting species. However, reaction rate can be influenced by increasing the frequency and energy of the ________ between molecules.

collisions

During the course of a reaction, reactant concentrations _____ and product concentrations _____.

decrease; increase

The rate of a reaction _____ over time because as the reactant is used up, there are _____ reactant particles to collide.

decreases The rate of any reaction decreases over time as the reactant is used up and fewer reactant particles are present to collide.

Raising the temperature of a reaction increases the reaction rate by increasing the _______ and _________ of the collisions.

frequency and energy

The __________ rate of a reaction is typically used to find other kinetic parameters, such as reaction orders.

initial

What type of reaction rate is typically used to find other kinetic parameters?

initial rate

For the reaction aA + bB → cC + dD, the rate law is given by rate = k[A]m[B]n. Match the appropriate definition with each term of the rate law.

k= Rate constant m, n = Reaction orders [A], [B] = Reactant concentrations

The units for reaction rate are given by _____.

mol/L⋅s

Reactant concentration decreases _____ quickly in a reaction with a higher rate than in a reaction with a lower rate.

more If a reaction is faster (i.e., occurs at a greater rate), reactant concentration will decrease more quickly.

The rate of a reaction is expressed as Δ[A]/Δt. If A is a reactant a _____ sign must be added in front of this expression in order for the rate to have a _____ value. This is not necessary if A is a product.

negative; positive Since [A] decreases over time, a negative sign must be placed in front of this expression so that the rate will have a positive value. Reaction rates are always expressed as positive values.

Chemical kinetics studies the ___________ of chemical reactions, i.e., how fast reactants are converted into products.

rate

Which of the following options correctly express rate for the reaction H2 (g) + O2 (g) → H2O2 (l)? Select all that apply.

Δ[H2O2] / Δt - Δ[H2] / Δt