Reaction Rates
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AP Chemistry › Reaction Rates
To investigate the effect of reactant concentration on the rate, a student could measure the initial rate of oxygen production while varying which of the following?
The total volume of the solution while keeping the moles of H₂O₂ and I⁻ constant.
The initial concentration of H₂O₂ while keeping the concentration of I⁻ and the temperature constant.
The pressure above the solution while keeping the concentrations of H₂O₂ and I⁻ constant.
The temperature of the solution while keeping the concentrations of H₂O₂ and I⁻ constant.
Explanation
To study the effect of a specific reactant's concentration on the reaction rate, one must vary that concentration while holding all other potentially influential factors (like the concentration of other reactants, catalysts, and temperature) constant. Therefore, varying the initial concentration of H₂O₂ while keeping the catalyst (I⁻) concentration and temperature constant is the correct experimental design.
An increase in the temperature of a reaction system generally leads to a significant increase in the reaction rate. What is the primary reason for this observation at the particulate level?
It decreases the volume of the container, thereby increasing reactant concentration.
It decreases the activation energy required for the reaction to occur.
It increases the frequency and energy of collisions between reactant particles.
It increases the number of reactant particles in the system.
Explanation
According to the collision model, increasing the temperature increases the average kinetic energy of the particles. This leads to two effects: particles collide more frequently, and, more importantly, a larger fraction of these collisions have sufficient energy to overcome the activation energy barrier. A catalyst decreases activation energy (C), not temperature. Temperature change does not directly affect the number of particles (A) or the container volume (D).
A student observes that dropping an effervescent tablet into hot water causes it to dissolve and fizz much faster than dropping it into cold water. This observation demonstrates that the reaction rate is dependent on which factor?
Surface area
Presence of a catalyst
Reactant concentration
Temperature
Explanation
The only significant difference between the two experiments described is the temperature of the water. The increased rate of fizzing (which is CO₂ gas production from a reaction) in hot water compared to cold water directly demonstrates the effect of temperature on reaction rates. Higher temperatures lead to faster reactions.
In which of the following systems would an increase in pressure cause the greatest increase in reaction rate, assuming all other factors are constant?
The reaction of a solid with an aqueous solution.
The decomposition of a solid into a solid and a gas.
A reaction between two gases in a container.
A reaction between two aqueous solutions.
Explanation
Increasing the total pressure of a system by decreasing its volume will increase the concentration of all gaseous species. For a reaction between two gases, this increases the concentrations of both reactants, leading to a significant increase in the rate of reaction due to more frequent collisions. Pressure has a negligible effect on the rates of reactions in the liquid or solid phase.
How is the instantaneous rate of disappearance of N₂O expected to change over the first minute of the reaction?
The rate will decrease because the products inhibit the reaction.
The rate will increase because the temperature will rise.
The rate will decrease as the concentration of N₂O decreases.
The rate will remain constant throughout the reaction.
Explanation
The rate of this reaction is proportional to the concentration of the reactant, N₂O. As the reaction proceeds, N₂O is consumed, so its concentration decreases. Consequently, the instantaneous rate of the reaction, which depends on this concentration, will also decrease over time. The rate is highest at the beginning (initial rate) and slows down as reactants are used up.
Which expression correctly represents the rate of reaction?
$$\text{Rate} = -\frac{1}{2}\frac{\Delta[\text{NO}]}{\Delta t}$$
$$\text{Rate} = +\frac{1}{2}\frac{\Delta[\text{NOCl}]}{\Delta t}$$
$$\text{Rate} = -\frac{\Delta[\text{NO}]}{\Delta t}$$
$$\text{Rate} = -\frac{\Delta[\text{Cl}_2]}{\Delta t}$$
Explanation
The rate of reaction is defined to be a unique positive value. For a reactant, the change in concentration is negative, so a minus sign is used. The rate of change of each species is divided by its stoichiometric coefficient. For NO, the coefficient is 2, so the rate is given by $$-\frac{1}{2}\frac{\Delta[\text{NO}]}{\Delta t}$$. Choices A and D are incorrect because they omit the stoichiometric coefficient for NO and define the rate based on a reactant without normalizing by stoichiometry. Choice C correctly represents the rate in terms of the product NOCl, but the question asks for the correct expression among the choices, and B is a correct representation. C is also correct, but B is offered as an option and is a valid representation of the reaction rate.
If the rate of disappearance of N₂O₅ is measured to be $$4.0 \times 10^{-5} \text{ M/s}$$ at a certain time, what is the rate of appearance of NO₂ at the same time?
$$4.0 \times 10^{-5} \text{ M/s}$$
$$8.0 \times 10^{-5} \text{ M/s}$$
$$1.6 \times 10^{-4} \text{ M/s}$$
$$2.0 \times 10^{-5} \text{ M/s}$$
Explanation
The rate of appearance of NO₂ is related to the rate of disappearance of N₂O₅ by the stoichiometry of the balanced equation. The ratio is 4 moles of NO₂ produced for every 2 moles of N₂O₅ consumed. Therefore, the rate of appearance of NO₂ is (4/2) times the rate of disappearance of N₂O₅. Rate of NO₂ = (4/2) * (4.0 x 10⁻⁵ M/s) = 2 * (4.0 x 10⁻⁵ M/s) = 8.0 x 10⁻⁵ M/s.
Which of the following best describes the rate of a chemical reaction?
The amount of heat energy released or absorbed by the reaction system.
The total amount of product formed when the reaction reaches completion.
The minimum energy required for reactant molecules to form products.
The change in concentration of a reactant or product per unit of time.
Explanation
The rate of a chemical reaction is defined as the change in the amount (usually concentration) of a reactant or product over a specific time interval. The other options describe the reaction yield (B), enthalpy change (C), and activation energy (D), which are distinct concepts from reaction rate.
The reaction rate can be dramatically increased by introducing a spark or a platinum surface. The platinum surface acts as a
catalyst, by increasing the kinetic energy of the H₂ and O₂ molecules.
reactant, by being consumed to form a platinum oxide intermediate.
catalyst, by providing an alternative reaction mechanism with a lower activation energy.
source of energy, by raising the temperature of the reaction mixture.
Explanation
A platinum surface acts as a heterogeneous catalyst for this reaction. Catalysts function by providing a different, lower-energy pathway for the reaction to occur, thus increasing the rate without being consumed in the overall reaction. A catalyst does not increase the kinetic energy of the molecules; that is a function of temperature. While a spark provides initial activation energy, the platinum surface is a chemical catalyst.
The concentration of reactant $\text{H}(aq)$ is plotted versus time for a reaction. At which time is the instantaneous rate of disappearance of $\text{H}$ greatest (i.e., where the magnitude of the slope is largest)?
At $t=40\ \text{s}$
At $t=0\ \text{s}$
At $t=10\ \text{s}$
At $t=20\ \text{s}$
At $t=30\ \text{s}$
Explanation
This question tests understanding of instantaneous reaction rates versus average rates. The instantaneous rate of disappearance at any point equals the magnitude of the slope of the tangent line to the concentration versus time curve at that point. For typical reactions, the curve is steepest at t=0s when reactant concentration is highest, giving the greatest instantaneous rate at the beginning. As the reaction proceeds, [H] decreases, the curve becomes less steep, and the instantaneous rate decreases. A common error is thinking the rate is highest when concentration is lowest, but this confuses concentration with rate of change. To find maximum instantaneous rate, identify where the concentration versus time curve has the steepest negative slope, which is typically at t=0.