Chemical Kinetics: Rate Expressions

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Questions and Answers

What happens to the backward reaction in an irreversible reaction?

It is considered in the rate expression.

It influences the equilibrium constant.

It proceeds at the same rate as the forward reaction.

It is ignored entirely. (correct)

What does the term 'K' represent in the equilibrium expressions?

The equilibrium constant. (correct)

The concentration of reactants.

The rate constant for the forward reaction.

The change in enthalpy.

In the law of mass action, a correct rate expression is obtained when applied to:

Elementary reaction steps that are rate controlling. (correct)

Overall reactions only.

Irreversible reactions exclusively.

All types of chemical reactions.

According to the Arrhenius Equation, how does temperature affect the rate constant?

Higher temperatures increase the rate constant. Signup and view all the answers

What does the Van’t Hoff equation relate to in terms of chemical reactions?

The change in enthalpy per mole of reaction. Signup and view all the answers

In the context of rate constants, what does the symbol 'A' represent?

Frequency factor. Signup and view all the answers

What information does the change in enthalpy ($ abla H$) provide in chemical reactions?

The heat change associated with a reaction at constant pressure. Signup and view all the answers

What is the relationship between the forward rate constant ($K_f$) and the backward rate constant ($K_b$)?

They relate through the equilibrium constant ($K$). Signup and view all the answers

What characterizes a homogeneous reaction?

It occurs entirely within one phase. Signup and view all the answers

In a heterogeneous reaction, where does the reaction primarily occur?

At the interface of two phases. Signup and view all the answers

Which of the following statements is true regarding the rate of heterogeneous reactions?

It is proportional to the interfacial area. Signup and view all the answers

Which process is NOT involved in a heterogeneous reaction?

Bulk phase reactions. Signup and view all the answers

What is the primary factor that affects the rate of a heterogeneous reaction involving solids?

The surface area of the solid. Signup and view all the answers

What is the nature of the rate of reaction in a homogeneous reaction?

It can be defined as a negative quantity. Signup and view all the answers

Which statement describes the reaction mechanisms in homogeneous vs. heterogeneous reactions?

Homogeneous reactions have simpler mechanisms. Signup and view all the answers

What may represent a heterogeneous reaction?

Solid-solid reduction of ZnO with carbon. Signup and view all the answers

Which reaction is likely to proceed faster due to the involvement of a gas?

MO(s) + CO(g) = M(s) + CO2(g) Signup and view all the answers

What is the role of a catalyst in a chemical reaction?

To alter the rate of a reaction without being consumed. Signup and view all the answers

How does the concentration of hydrochloric acid change with time in the reaction CaCO3 + 2HCl?

It decreases with time. Signup and view all the answers

Which of the following can be considered a negative catalyst?

Glycerol Signup and view all the answers

What defines the order of a reaction?

The sum of the powers of the concentrations of reactants. Signup and view all the answers

Which of the following statements is true regarding Reaction (3)?

It is limited by surface area and low diffusivity. Signup and view all the answers

In the context of chemical reactions, what effect does increasing temperature have on Reaction (1)?

It increases the concentration of CO2 over time. Signup and view all the answers

Which catalyst is commonly used in the reforming of natural gas?

Al2O3 Signup and view all the answers

What does the integration constant (C) equal when $t = 0$ and $x = 0$?

$\frac{(a)^{1-n}}{n-1}$ Signup and view all the answers

At what point is $x$ equal to $rac{a}{2}$?

At the half-life $t = t_{1/2}$ Signup and view all the answers

In the differential method for a first-order reaction, what is the relationship between the rate of change of concentration and the concentration itself?

Rate is equal to the product of rate constant and concentration. Signup and view all the answers

How can the order of a reaction (n) be determined?

By taking two values of concentration and measuring the rate of reaction at these concentrations. Signup and view all the answers

What is the expression for the rate of reaction in a general nth order reaction involving one substance?

$-\frac{d[A]}{dt} = K[A]^n$ Signup and view all the answers

Which equation describes the relationship between the two different initial concentrations for the same nth order reaction?

$-\frac{d[A_1]}{dt} = K[A_1]^n$ and $-\frac{d[A_2]}{dt} = K[A_2]^n$ Signup and view all the answers

What does the value of K represent in the context of reaction order?

The rate constant for the reaction. Signup and view all the answers

In the context of nth order reactions, what condition must be satisfied to simplify the expression for K?

The reaction must reach completion. Signup and view all the answers

Which formula(s) relate to the derivation of the rate of reaction for nth order?

$-\frac{d[A]}{dt} = K[A]$ and $-\frac{d[A]}{dt} = K[A]^n$ Signup and view all the answers

What is the significance of $ log K[A_1] - log K[A_2] $ in determining reaction order?

It helps in determining the exponent n. Signup and view all the answers

What does the activation energy (E) represent in a chemical reaction?

The energy barrier that reactants must overcome to form products Signup and view all the answers

In the Arrhenius equation K = A exp(−E/RT), what does the variable K represent?

The rate constant of the reaction Signup and view all the answers

What does ln K = ln A − (E/RT) illustrate when plotted as ln K vs (1/T)?

The relationship between the rate constant and temperature Signup and view all the answers

Which of the following factors does NOT affect the rate of heterogeneous reactions?

Concentration of reactants in the bulk solution Signup and view all the answers

In the context of kinetic studies, what is the rate controlling step?

The slowest step that determines the overall reaction rate Signup and view all the answers

If the activation energy of a reaction increases, what is the likely effect on the reaction rate with temperature?

The reaction rate will still increase, but at a lower rate compared to lower activation energy Signup and view all the answers

What does the frequency factor (A) in the Arrhenius equation represent?

The probability of effective collisions between reactants Signup and view all the answers

Which statement accurately describes the Arrhenius equation?

It provides a mathematical relationship between temperature and reaction rate. Signup and view all the answers

Study Notes

Rate Expressions

Reversible Reactions: The rate expression for a reversible reaction is given by:
- Kf / Kb = (CxC_Y) / (C_MC_N) = K
  - Where K is the equilibrium constant, Kf is the forward rate constant, Kb is the backward rate constant, Cx is the concentration of X, C_Y is the concentration of Y, C_M is the concentration of M, and C_N is the concentration of N.
Irreversible Reactions: For irreversible reactions, the backward reaction is ignored. The rate expression simplifies to:
- (n₀/V) = KfC_MC_N
  - Where n₀ is the initial number of moles, V is the volume, and Kf is the forward rate constant.

Rate Controlling Step

The rate constant is determined by the slowest step in a multi-step reaction.
Elementary Reactions: The law of mass action can be applied to elementary steps, which represent the individual steps in a reaction mechanism.
Rate Controlling Step: The slowest step in the mechanism dictates the overall rate of the reaction.

Arrhenius Equation

Describes the relationship between temperature and the rate constant of a reaction.
Arrhenius Equation: K = Ae^(-E/RT)
- Where K is the rate constant, A is the frequency factor, E is the activation energy, R is the ideal gas constant, and T is the absolute temperature.
Activation Energy (E): The minimum amount of energy that reactant molecules must possess to overcome the energy barrier and form products
Frequency Factor (A): Represents the frequency of collisions between reactant molecules.

Reaction Order

Represents the power to which the concentration of each reactant is raised in the rate law.
Overall Order: Sum of the individual orders of each reactant.
First Order Reaction: Rate is directly proportional to the concentration of one reactant.
Second Order Reaction: Rate is proportional to the square of the concentration of one reactant or the product of the concentrations of two reactants.

Types of Reactions

Homogeneous Reactions: Occur entirely within a single phase, where reactants and products are in the same phase.
Heterogeneous Reactions: Involve reactants and products in different phases and occur at the interface between phases.

Rate of Reaction

Represents the change in concentration of a reactant or product over time.
Rate Law: Mathematical expression that relates the rate of reaction to the concentrations of reactants.

Factors Affecting Reaction Rate

Surface Area: Increased surface area in heterogeneous reactions leads to a faster rate of reaction due to more opportunities for collisions at the interface.
Catalyst: Alters the rate of a reaction without being consumed itself.
- Positive Catalyst: Increases the rate of reaction.
- Negative Catalyst (Inhibitor): Decreases the rate of reaction.
Concentration: Increasing the concentration of reactants generally leads to a faster reaction rate due to more frequent collisions.
Temperature: Increased temperature typically leads to a faster reaction rate due to increased kinetic energy of molecules, leading to more successful collisions.

Heterogeneous Reactions

Involve reactants and products in different phases.
Key Steps:
- Transport of Reactants: Movement of reactants to the reaction site.
- Chemical Reaction: Conversion of reactants to products at the reaction site.
- Transport of Products: Movement of products away from the reaction site.
Rate Controlling Step: The slowest step in the process that determines the overall rate of reaction.

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Description

Test your understanding of rate expressions in chemical reactions, including reversible and irreversible reactions. Learn how to calculate equilibrium constants and identify rate-controlling steps in multi-step reactions. This quiz will challenge your knowledge of the fundamental principles of chemical kinetics.