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Which factor does NOT increase the rate of a chemical reaction by increasing the number of effective collisions between reactant molecules?
A. Increasing the temperature
B. Increasing the concentration of reactants
C. Decreasing the surface area of solid reactants
D. Adding a catalyst
C. Decreasing the surface area of solid reactants reduces the number of particles available for collisions, thus decreasing the rate of reaction.
What is the primary reason for an increase in the rate of reaction when the temperature is raised?
A. More reactant molecules are present
B. Reactant molecules move slower
C. Reactant molecules have higher kinetic energy
D. Activation energy of the reaction decreases
C. When the temperature is raised, the kinetic energy of the molecules increases, leading to more effective collisions and an increased rate of reaction.
Which statement about catalysts is FALSE?
A. Catalysts lower the activation energy of a reaction
B. Catalysts are consumed in the reaction
C. Catalysts increase the rate of reaction
D. Catalysts do not change the equilibrium position of a reaction
B. Catalysts are not consumed in the reaction; they are regenerated at the end of the reaction
The units of the rate constant for a zero-order reaction are:
A. s^-1
B. M^-1 s^-1
C. M s^-1
D. No units
C. M s^-1 - the rate is directly proportional to the rate constant
Explain the concept of activation energy in the context of the collision theory. (3 marks)
Activation energy is the minimum amount of energy required for reactant molecules to collide in such a way that they form a product. It represents the energy barrier that must be overcome for a reaction to occur. In collision theory, only those collisions that possess energy equal to or greater than the activation energy will lead to a successful reaction.
How does an increase in temperature affect the rate of reaction according to collision theory? (2 marks)
An increase in temperature increases the kinetic energy of the particles, leading to more frequent collisions. Additionally, a greater proportion of these collisions will have energy equal to or greater than the activation energy, thus increasing the rate of reaction.
Describe the effect of a catalyst on the activation energy of a reaction. [2]
A catalyst lowers the activation energy of a reaction. By providing an alternative reaction pathway with a lower energy barrier, a catalyst increases the number of successful collisions, leading to a faster reaction rate without being consumed in the process.
Using collision theory, explain why increasing the concentration of reactants generally increases the rate of a reaction. [3]
Increasing the concentration of reactants results in a higher number of particles in a given volume. This leads to more frequent collisions between reactant particles. According to collision theory, a higher frequency of collisions increases the likelihood of successful collisions, thereby increasing the rate of the reaction.
What is meant by the term "order of reaction"? [2]
The order of reaction indicates the power to which the concentration of a reactant is raised in the rate equation. It provides insight into how changes in the concentration of that reactant affect the overall rate of the reaction.
Define the term "rate constant" in the context of reaction kinetics. [2]
The rate constant, often denoted as k, is a proportionality constant in the rate equation that relates the rate of a reaction to the concentrations of the reactants. It is specific to a particular reaction at a given temperature.
How does the rate constant vary with temperature for a typical chemical reaction? [2]
As the temperature increases, the rate constant typically increases. This is because an increase in temperature leads to more particles having energy greater than the activation energy, resulting in a higher frequency of effective collisions.
c) Using collision theory, explain the effect of increasing the surface area of a solid reactant on the rate of a reaction. [3]
Increasing the surface area of a solid reactant exposes more particles to potential collisions with other reactants. This leads to a higher frequency of effective collisions, and thus an increased rate of reaction, as per collision theory.
a) Describe the role of a catalyst in terms of its effect on the energy profile of a reaction. [3]
) A catalyst provides an alternative reaction pathway with a lower activation energy. While the initial and final energy levels remain unchanged, the energy barrier that needs to be overcome is reduced in the presence of a catalyst.
b) What is meant by the standard enthalpy change of formation? [2]
The standard enthalpy change of formation is the energy change when one mole of a compound is formed from its elements in their standard states under standard conditions (usually 298 K and 1 atm).
How can Hess's Law be used to determine the enthalpy change of a reaction? [3]
Hess's Law states that the total enthalpy change for a reaction is independent of the pathway by which the reaction occurs. It remains constant provided the initial and final conditions are the same. Therefore, if a reaction can be expressed as the sum of two or more other reactions, the enthalpy change for the overall reaction is the sum of the enthalpy changes for those reactions.
Which factor does NOT affect the activation energy of a reaction?
A. Temperature
B. Catalyst
C. Concentration
D. Surface area
C. Activation energy is a fixed value for a given reaction and is not affected by the concentration of reactants
n the context of collision theory, which statement is TRUE?
A. All collisions lead to a reaction
B. Only collisions with energy greater than the activation energy lead to a reaction
C. Collisions with energy less than the activation energy lead to a reaction
D. The orientation of colliding particles does not matter
B. For a reaction to occur, the colliding particles must have energy greater than or equal to the activation energy and must collide with the correct orientation
How does increasing the surface area of a solid reactant affect the rate of reaction?
A. It decreases the rate of reaction
B. It does not affect the rate of reaction
C. It increases the rate of reaction
D. It makes the reaction reach equilibrium faster
C. Because as surface area increases it provides more sites for collisions, leading to an increased rate of reaction
For a reaction to be second order with respect to a particular reactant, the rate of reaction will:
A. Remain unchanged if the concentration of the reactant is doubled
B. Double if the concentration of the reactant is doubled
C. Quadruple if the concentration of the reactant is doubled
D. Halve if the concentration of the reactant is doubled
C. because for a second-order reaction with respect to a particular reactant, the rate is proportional to the square of its concentration
Explain the significance of the activation energy in determining the rate of a reaction. [3]
The activation energy is the minimum energy required for reactant molecules to undergo a successful collision leading to the formation of products. A higher activation energy means fewer molecules possess the necessary energy to react, leading to a slower reaction rate. Conversely, a lower activation energy means more molecules can react, resulting in a faster rate.
How does the concentration of reactants influence the rate of a reaction? [2]
An increase in the concentration of reactants leads to a higher frequency of collisions between reactant molecules. This increases the likelihood of successful collisions, thereby increasing the rate of the reaction.
What factors can affect the activation energy of a reaction? [3]
Factors affecting the activation energy include the nature of the reactants (e.g., bond strengths, molecular size), the presence of a catalyst, and the specific reaction pathway or mechanism.
Describe the effect of a catalyst on the rate of a reaction using collision theory. [3]
A catalyst increases the rate of a reaction by providing an alternative reaction pathway with a lower activation energy. This means that a greater proportion of collisions will have sufficient energy to overcome the energy barrier, leading to more successful collisions and a faster reaction rate.
How can the order of a reaction be experimentally determined? [2]
The order of a reaction can be experimentally determined by observing how the rate of the reaction changes as the concentration of one of the reactants is varied, while keeping other conditions constant. The resulting data can be plotted and analysed to deduce the order with respect to that reactant
c) Using collision theory, explain why an increase in temperature generally leads to an increase in the rate of a reaction. [3]
An increase in temperature provides molecules with more kinetic energy. This means a greater proportion of molecules will have energy exceeding the activation energy, leading to a higher frequency of successful collisions and, consequently, an increased reaction rate.
Describe the relationship between the rate constant, temperature, and the activation energy of a reaction. [4]
The relationship between the rate constant (k), temperature (T), and activation energy (Ea) is given by the Arrhenius equation: k = Ae^(-Ea/RT), where A is the pre-exponential factor and R is the universal gas constant. As the temperature increases, the rate constant typically increases, indicating a faster reaction rate. Conversely, a higher activation energy at a given temperature will result in a smaller rate constant and a slower reaction rate.
What is meant by the "order" of a reaction? [2]
The order of a reaction with respect to a particular reactant indicates the power to which its concentration must be raised to predict the rate of the reaction. The overall order of the reaction is the sum of the orders with respect to each reactant.
Describe how the concentration of a reactant affects the rate of a zero-order reaction. [3]
For a zero-order reaction, the rate of the reaction is independent of the concentration of the reactant. This means that changing the concentration of the reactant will have no effect on the reaction rate.
Explain the role of a catalyst in terms of its effect on the activation energy and the rate of a reaction. [3]
A catalyst provides an alternative reaction pathway with a lower activation energy compared to the uncatalysed reaction. This results in a higher frequency of effective collisions and an increased rate of reaction, even though the catalyst itself is not consumed in the reaction.
Using collision theory, describe the effect of increasing the concentration of reactants on the rate of a reaction. [4]
Increasing the concentration of reactants leads to a higher frequency of collisions between reactant molecules. This increases the likelihood of successful collisions, thereby increasing the rate of the reaction, as per collision theory.
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