- An experiment is performed with [X]=0.40M and [Y]=0.40M. A second experiment is performed with [X]=0.40M and [Y]=0.80M. How will the initial rate of the second experiment compare to the first?
Answer: The initial rate will be the same.
The rate law, Rate = k[X]2, shows that the reaction is second order in X and zero order in Y. Since the rate does not depend on the concentration of Y, doubling [Y] while keeping [X] constant will have no effect on the initial rate. Therefore, the rate will be the same in both experiments. - The rate law for a chemical reaction provides a mathematical relationship between the rate of reaction and which of the following?
Answer: The concentrations of the reactants.
The rate law expresses how the rate of a reaction depends on the concentration of the reactants. While temperature and catalysts affect the rate constant (k), the rate law itself is an expression in terms of reactant concentrations.
- The units of a rate constant for a certain reaction are given as Lmol−1min−1. What is the overall order of this reaction?
Answer: Second order
The units of the rate constant depend on the overall reaction order. For a second-order reaction, Rate = k[concentration]2. The units would be k=Rate/[concentration]2=(mol L−1min−1)/(mol L−1)2=Lmol−1min−1. This matches the given units. - Which statement best explains why reaction orders are determined from experimental data rather than from the stoichiometry of the balanced chemical equation?
Answer: Reaction orders reflect the molecularity of the elementary steps in the reaction mechanism, which is not revealed by the overall stoichiometry.
The correct answer is A. The rate law, including the reaction orders, is dictated by the slowest step (the rate-determining step) of the reaction mechanism. The overall balanced equation shows only the net change and does not provide information about the individual steps, so orders must be found experimentally.
- Based on these data, the reaction is what order with respect to C?
Answer: Zero order
The correct answer is A. Comparing the two trials, the initial concentration of C was doubled (from 0.50 M to 1.0 M), but the initial rate of the reaction remained unchanged. This indicates that the rate is independent of the concentration of C, which defines a zero-order reaction.
- What is the correct rate law for this reaction?
Answer: Rate = k[NO]2[Cl2]
The correct answer is C. Comparing experiments 1 and 2, [NO] is constant while [Cl₂] doubles, and the rate doubles (3.6/1.8=2). Thus, the reaction is first-order in [Cl₂]. Comparing experiments 2 and 3, [Cl₂] is constant while [NO] doubles, and the rate quadruples (1.44×10−4/3.6×10−5=4). Thus, the reaction is second-order in [NO]. The rate law is Rate = k[NO]2[Cl2]. - A reaction is found to be zero-order with respect to one of its reactants, Z. What does this imply about the role of Z in the reaction kinetics?
Answer: The rate of the reaction is independent of the concentration of Z.
The correct answer is A. For a reaction to be zero-order with respect to a reactant Z, the rate law includes the term [Z]0, which equals 1. This means that changing the concentration of Z has no effect on the rate of the reaction. However, Z is still a reactant and is consumed (B is incorrect). A catalyst's concentration does affect the rate (C is incorrect). D is nonsensical. - If the rate of reaction is 3.6×10−5mol L−1s−1 when [A]=0.20M and [B]=0.10M, what is the value of the rate constant, k, including units?
Answer: 1.8×10−3Lmol−1s−1
First, rearrange the rate law to solve for k: k=Rate/([A][B]). Substitute the given values: k=(3.6×10−5mol L−1s−1)/((0.20mol L−1)(0.10mol L−1))=(3.6×10−5)/(0.020)=1.8×10−3. The overall reaction order is 1+1=2, so the units for k are Lmol−1s−1. - The rate law for a reaction is determined to be Rate = k[A]2. Which of the following experimental procedures would be most useful for determining the value of the rate constant, k?
Answer: Measuring the initial rate of reaction at a known initial concentration of A and a constant temperature.
The correct answer is C. Once the rate law is known (Rate = k[A]2), the rate constant k can be calculated by rearranging the equation to k = Rate / [A]2. This requires measuring the initial rate for a known initial concentration of A at a constant temperature. Choice A would be used to find the activation energy. Choices B and D relate to equilibrium and thermochemistry, not kinetics. - If the initial rate of decomposition is measured to be R at a certain initial concentration of N2O5, what will the initial rate be if the concentration of N2O5 is tripled?
Answer: 3R
The reaction is first order with respect to N2O5, as indicated by the exponent of 1 on [N2O5] in the rate law. In a first-order reaction, the rate is directly proportional to the concentration of the reactant. Therefore, if the concentration of N2O5 is tripled, the rate will also triple, becoming 3R. - For a given reaction, how does an increase in temperature typically affect the reaction rate and the value of the rate constant, k?
Answer: The reaction rate increases, and the rate constant k also increases.
The correct answer is B. An increase in temperature increases the kinetic energy of molecules, leading to more frequent and energetic collisions, which increases the reaction rate. The rate constant, k, is temperature-dependent as described by the Arrhenius equation; it increases exponentially with temperature. - A reaction has the rate law: Rate = k[P]2[Q]. If the concentration of P is halved and the concentration of Q is kept constant, the rate of the reaction will
Answer: decrease by a factor of 4.
The correct answer is A. The rate is proportional to [P]2. If the concentration of P is changed by a factor of 1/2, the rate will change by a factor of (1/2)2 = 1/4. Therefore, the rate decreases by a factor of 4. - Which of the following statements correctly describes the reaction based on this rate law?
Answer: The reaction is second order with respect to NO2 and zero order with respect to CO.
The rate law explicitly shows the dependence of the rate on reactant concentrations. The exponent for [NO2] is 2, so the reaction is second order with respect to NO2. Because CO is a reactant but does not appear in the rate law, its exponent is 0, meaning the reaction is zero order with respect to CO. - For a reaction that is determined to be zero order with respect to a particular reactant, what is the effect on the overall reaction rate if the concentration of only that reactant is doubled?
Answer: The reaction rate remains the same.
If a reaction is zero order with respect to a reactant, the rate of the reaction is independent of the concentration of that reactant. The concentration term is raised to the power of zero ([reactant]0=1), so changing its concentration has no effect on the rate. - For a reaction with the general rate law, Rate = k[A]m[B]n, what do the exponents m and n represent?
Answer: The reaction orders with respect to reactants A and B, which must be determined experimentally.
The correct answer is A. The exponents m and n in a rate law are the orders of the reaction with respect to each reactant. These orders indicate how the rate is affected by the concentration of that reactant and must be determined through experimental data, not from the stoichiometry of the overall reaction. - Based on this data, what is the reaction order with respect to reactant B?
Answer: Zero order
Comparing trial 1 and trial 2, the concentration of A is held constant at 0.10M while the concentration of B is doubled from 0.10M to 0.20M. The initial rate remains unchanged at 2.0×10−4M/s. Since changing the concentration of B has no effect on the rate, the reaction is zero order with respect to B. - A reaction involving reactants X and Y has an overall reaction order of three. If concentrations are measured in mol L⁻¹ and time in seconds, what are the units of the rate constant, k?
Answer: L² mol⁻² s⁻¹
The correct answer is A. For a third-order reaction, the units of k can be found from k = Rate / [conc]³. This gives (mol L⁻¹ s⁻¹) / (mol L⁻¹)³ = (mol L⁻¹ s⁻¹) / (mol³ L⁻³) = L² mol⁻² s⁻¹. Choice B is for a second-order reaction, C is for a first-order reaction, and D represents the units of reaction rate.
- If the initial concentration of A is 0.20M, what is the initial rate of the reaction?
Answer: 8.0×10−4mol L−1s−1
The initial rate can be calculated by substituting the given values into the rate law equation: Rate = k[A]. Rate = (4.0×10−3s−1)(0.20mol L−1)=8.0×10−4mol L−1s−1. - Which of the following is the rate law for this reaction?
Answer: Rate = k[B]2
The correct answer is B. Comparing experiments 1 and 2, [B] is constant while [A] doubles, and the rate remains unchanged. Therefore, the reaction is zero-order with respect to A. Comparing experiments 1 and 3, [A] is constant while [B] doubles, and the rate increases by a factor of 4 (2.0×10−2/5.0×10−3=4). Therefore, the reaction is second-order with respect to B. The rate law is Rate = k[A]0[B]2, which simplifies to Rate = k[B]2. - To determine the rate law for a chemical reaction, which of the following properties is typically measured experimentally as a function of time?
Answer: The concentration of a reactant or a product.
The rate of a reaction is defined as the change in concentration of a species per unit time. Therefore, experimental methods for determining a rate law involve monitoring a property that is proportional to concentration, such as absorbance, pressure, or conductivity, over time.
