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Questions and Answers
What is required for reactants to form products in a chemical reaction?
What is required for reactants to form products in a chemical reaction?
Kinetic energy of molecules does not influence the likelihood of a reaction occurring.
Kinetic energy of molecules does not influence the likelihood of a reaction occurring.
False
Who proposed the concept of activation energy?
Who proposed the concept of activation energy?
Svante Arrhenius
In order for reactants to successfully form products, they must collide with the correct ______.
In order for reactants to successfully form products, they must collide with the correct ______.
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Match the following concepts with their descriptions:
Match the following concepts with their descriptions:
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What is a catalyst?
What is a catalyst?
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The overall rate law can be derived from experimental data.
The overall rate law can be derived from experimental data.
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What must be done if the proposed mechanism does not match the experimental rate law?
What must be done if the proposed mechanism does not match the experimental rate law?
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A catalyst affects the rate law by altering the value of the ______.
A catalyst affects the rate law by altering the value of the ______.
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Match the following steps of evaluating a proposed mechanism with their descriptions:
Match the following steps of evaluating a proposed mechanism with their descriptions:
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What does large activation energy imply about the reaction rate?
What does large activation energy imply about the reaction rate?
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Molecules with kinetic energy greater than the activation energy can react.
Molecules with kinetic energy greater than the activation energy can react.
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What is the term used to describe the energy needed to reach the transition state?
What is the term used to describe the energy needed to reach the transition state?
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In the context of activation energy, a larger !a results in a bigger energetic ______ to overcome.
In the context of activation energy, a larger !a results in a bigger energetic ______ to overcome.
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Match the following terms with their definitions:
Match the following terms with their definitions:
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In the kinetic-molecular theory, what do molecules carry into a reaction?
In the kinetic-molecular theory, what do molecules carry into a reaction?
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The transition state is the most stable state during a chemical reaction.
The transition state is the most stable state during a chemical reaction.
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What happens to the reaction rate when the activation energy is high?
What happens to the reaction rate when the activation energy is high?
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What does the half-life represent in a chemical reaction?
What does the half-life represent in a chemical reaction?
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Temperature has no effect on the speed of a chemical reaction.
Temperature has no effect on the speed of a chemical reaction.
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What is the relationship between collision frequency and temperature?
What is the relationship between collision frequency and temperature?
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The rate of a reaction is influenced by its ______ order.
The rate of a reaction is influenced by its ______ order.
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Match the terms related to reaction rates with their descriptions:
Match the terms related to reaction rates with their descriptions:
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Which factor does NOT influence the rate of a reaction according to the collision model?
Which factor does NOT influence the rate of a reaction according to the collision model?
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Ant speed doubles with an increase of 10°C.
Ant speed doubles with an increase of 10°C.
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What are the necessary conditions for two molecules to react according to the collision model?
What are the necessary conditions for two molecules to react according to the collision model?
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What does the symbol $k$ represent in the Arrhenius Equation?
What does the symbol $k$ represent in the Arrhenius Equation?
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The exponential term in the Arrhenius equation increases with temperature.
The exponential term in the Arrhenius equation increases with temperature.
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What effect does temperature have on reaction rates according to the Arrhenius Equation?
What effect does temperature have on reaction rates according to the Arrhenius Equation?
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The fraction of collisions that occur with a proper orientation in the Arrhenius Equation is represented by ______.
The fraction of collisions that occur with a proper orientation in the Arrhenius Equation is represented by ______.
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Which of the following factors is NOT included in the Arrhenius Equation?
Which of the following factors is NOT included in the Arrhenius Equation?
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Match the following variables with their meanings in the Arrhenius equation:
Match the following variables with their meanings in the Arrhenius equation:
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The slope of the natural logarithm of the rate constant $ln(k)$ versus $1/T$ is equal to $-Ea/R$.
The slope of the natural logarithm of the rate constant $ln(k)$ versus $1/T$ is equal to $-Ea/R$.
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In the Arrhenius equation, as temperature increases, the rate constant $k$ tends to ______.
In the Arrhenius equation, as temperature increases, the rate constant $k$ tends to ______.
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What does a positive change in activation energy (ΔE) indicate for the reverse reaction?
What does a positive change in activation energy (ΔE) indicate for the reverse reaction?
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A bimolecular reaction involves one molecule participating in the reaction.
A bimolecular reaction involves one molecule participating in the reaction.
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What is an elementary reaction?
What is an elementary reaction?
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In the reaction 2 O3(g) → 3 O2(g), O(g) appears as a(n) _______ in the steps.
In the reaction 2 O3(g) → 3 O2(g), O(g) appears as a(n) _______ in the steps.
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Match the types of reactions with their corresponding molecularity:
Match the types of reactions with their corresponding molecularity:
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Which of the following describes how a reaction proceeds?
Which of the following describes how a reaction proceeds?
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Elementary reactions must add together to give the overall reaction.
Elementary reactions must add together to give the overall reaction.
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What is the rate law for the reaction A + B → C + D?
What is the rate law for the reaction A + B → C + D?
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For ΔE < 0, the reverse reaction is _______ than the forward reaction.
For ΔE < 0, the reverse reaction is _______ than the forward reaction.
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In the example of the reaction sequence 2 O3(g) → 3 O2(g), which step consumes the intermediate O(g)?
In the example of the reaction sequence 2 O3(g) → 3 O2(g), which step consumes the intermediate O(g)?
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Study Notes
Lecture 11 Announcements
-
Today's Topics: Chemical Kinetics II (Brown Ch. 14)
- Temperature and Rate (14.5)
- Reaction Mechanisms (14.6)
- Catalysis (14.7)
- Problem Set 10: Due before Exercise #11 tomorrow; upload on Moodle link
- Problem Set 11: Posted on Moodle; due before Exercise #12 next week
- Study Center: Wednesdays 18:00–20:00 in ETA F 5
- Office Hours: Prof. Norris and Brisby, Thursdays 17:00–18:00 in LEE P 210
- Final Exam: Monday, February 3, 2025, at 8:30–10:30
Lecture 12 Next Week
-
Topics: Brown Ch. 15 Chemical Equilibrium
- 15.1 The Concept of Equilibrium
- 15.2 The Equilibrium Constant
- 15.3 Understanding and Working with Equilibrium Constants
- 15.4 Calculating Equilibrium Constants
- 15.5 Le Châtelier's Principle
- 19.7 Free Energy and the Equilibrium Constant
Red Thread
- Topics progression: Properties → Kinetics → Equilibrium → Acid-Base → Catalysis → Christmas! → Batteries
Review
- Chemical Kinetics: Studies the speed of reactions/processes, quantified by reaction rate
- Reaction Rate: Depends on reactant concentrations (Rate = k[A]m[B]n[C]p...); determined by experiment
- Reaction Order: Determined by experiment; can be zero-, first-, second-order, etc.
- Half-life (t1/2): Time when half the initial reactant is gone
Important Open Questions
- Rate Law: What do terms m, n, p mean in Rate = k[A]m[B]n[C]p…?
- Reaction Speed: Why can one spontaneous reaction be fast and another slow?
- Reaction Activation: Why are reactions often heated?
- Temperature Influence: What role does temperature play in kinetics?
Speed of Ants versus Temperature
- Ants as Model: Ants are "cold-blooded", body temperature changes with the environment
- Experiment: A classic experiment revealed doubling of ant speed with a 10°C rise in temperature
- Relevance: This relates to analogous concepts in chemistry
Why Heat Chemical Reactions?
- Collision Model: Molecules must collide to react; reaction rate is proportional to collision frequency
- Collision Frequency: Depends on both concentration and temperature.
- Kinetic Energy and Orientation: Molecules must collide with proper kinetic energy and orientation for reaction
Why Does Orientation Matter?
- Correct Collision: Correct molecular orientation is crucial for reaction; a "hit from the wrong side" will not form product
- Kinetic Energy: Proper kinetic energy must also be present at the collision point
Why Does Kinetic Energy Matter?
- Activation Energy (Ea): Minimum energy needed for molecules to react
- Reactants Stability: Reactants are generally stable molecules
- Products Stability: Products are generally stable molecules
- Transition State: Molecules in the unstable transition state during reaction
Activation Energy, Ea
- Chemical Bonds: In transition state, bonds are half broken and half formed; relatively unstable
- Energy Barrier: Energy required to achieve transition state is the activation energy; the "hill" to overcome
- Transition State/Complex: At peak reactants are in transition state/activated complex.
Implications of Activation Energy, Ea
- Energy Effect: High activation energy results in slow reaction because only a small fraction of molecules have enough kinetic energy to reach transition state; in simpler terms, a high hill means less success
- Temperature effect: At higher temperatures, the number of molecules with kinetic energy exceeding activation energy will increase
- Kinetics: Increasing temperature will generally speed up a reaction by increasing the fraction of molecules with kinetic energy sufficient to make it over the energy barrier
Temperature Dependence of Rate Law
- Rate Constant (k): Rate constant depends on temperature, thus often called a rate coefficient
- Arrhenius Equation: k(T) = A exp(-Ea/RT) = A exp[-Ea/RT].
- Frequency Factor: A is a temperature-independent constant that includes collision frequency.
- Exponential term: exp[-Ea/RT] is the Boltzmann factor, shows temperature dependence
Arrhenius Equation
- Natural Log: Taking natural log of both sides gives ln[k] = ln(A) - Ea/RT
- Linear Relationship: This gives a linear relationship useful for determining Ea graphically from plotting ln(k) vs. 1/T
Ants at +10°C?
- Rate constants at different temperatures: k1(T1) = Aexp(-Ea/RT1) ; k2(T2) = Aexp(-Ea/RT2); k1/k2 = exp(-Ea/R(1/T1 - 1/T2))
- Rule of thumb: Reaction rate doubles for a 10°C increase in temperature - a rough idea of temperature effect
Role of ΔE
- Reaction rate: ΔE (change in enthalpy/energy) does not directly affect reaction rate itself
- Reverse reaction: ΔE does affect reverse reaction rate.
Reaction Mechanisms
- Elementary Reactions: Simple reactions that are fundamental steps in a chemical reaction; reactions that occur in single steps.
- Overall Reaction: Summation of elementary reactions.
- Molecularity: Number of molecules participating in an elementary reaction (unimolecular, bimolecular, termolecular).
Rate Laws for Elementary Reactions
- Rate Law vs. Molecularity: m and n in rate law are not always equal to stoichiometric coefficients a and b - depends on the molecularity of the elementary reaction.
Elementary Reactions and Their Rate Laws
- Rate Laws for different molecularities: Summary table for elementary reactions of different molecularities and their associated rate laws - unimolecular, bimolecular, termolecular.
Notes
- Complexity of Reactions: Not all reactions are elementary, as many are multi-step processes
- Rate Law Determination: Rate laws for complex reactions cannot always be determined by inspection; requires analysis of elementary reactions and steps
- Multistep Reactions: Multistep reactions (including intermediates) are common
Process to Determine Reaction Mechanism
- Procedure: Measuring rates, proposing a mechanism, and checking consistency with measured laws to determine reaction mechanism
- Identifying intermediates: Rate-determining step (slowest step) is the key to determine the overall reaction rate.
Rate Law of Proposed Mechanism
- Rate expressions: Combining rate expressions for each step can yield an expression for the overall rate
- Rate Limiting Step: One step (typically slowest step) is rate-limiting; sets overall reaction rate.
Rate Law of Proposed - 2
- Comparison: Compare proposed rate law with experimentally measured rate law
- Inconsistency: If not consistent, propose an alternate mechanism and repeat the process
Catalysts
- Effect on Reaction Rate: Substances that speed up reactions without being consumed themselves
- Mechanism: Catalysts may alter reaction path, leading to a lower activation energy. They may also help reactants come together in the right way or orientation
- Effect on Rate Law: Catalysts affect reaction rate by changing the rate constant (k) often by affecting activation energy
Catalysts: Graphically
- Reaction Coordinate: Catalysis can influence reaction paths and transition states; catalysed pathways with multiple steps show multiple transition state peaks.
Catalysts: Types
- Homogeneous: Reactants, products, and catalysts are in the same phase (like acid catalysis).
- Heterogeneous: Reactants, products, and catalysts are in different phases (like solid catalysts for gas phase reactions)
Natural Catalysts: Enzymes
- Biological Role: Enzymes lower activation energies and are essential for biological processes.
- Example: Nitrogen fixing enzyme nitrogenase plays a role in fertilizers
The Crisis
- Historical Concern: William Crookes predicted widespread starvation in the 1930s due to fertilizer shortages; the prediction came true.
- Background on Agriculture: Plants require nitrogen-based nutrients, and traditional methods used crop rotation, manure composting, etc., to maintain soil fertility.
Fertilizers
- Historical Sources: Early sources of nitrogen based fertilizers (bird guano, Chilean salts).
- Need of solutions: Crookes predicted shortage of natural sources; needing alternative sources
Fritz Haber and Carl Bosch
- Development: Developed a process to produce synthetic ammonia, a crucial fertilizer; fixed nitrogen efficiently
- Scaled up production: Significant engineering challenges overcome in scaling up the Haber-Bosch process
- Industrial Significance: Crucial for producing ammonia based fertilizers; this process became a major factor in feeding the growing population
Synthetic Fertilizers
- Ammonium-based fertilizers: Different types of synthetic fertilizers, like ammonium sulfate and nitrate, their composition, and nitrogen content
- Nitric Acid: Produced from ammonia using the Ostwald process; also used to produce explosives like TNT.
Oppau Plant
- Disaster: Explosion at BASF plant in Oppau, Germany, highlights some hazards associated with ammonia based fertilizer production
- Cause: Poor mixing of nitrogen fertilizers combined with moisture; caused explosion
But Why Is High Pressure Needed for NH3?
- Kinetics of Haber process: The reaction forming ammonia is exothermic at room temperature but very slow due to high activation energy
- Pressure Effect: High pressure improves reaction rates by effectively increasing concentration and thus collisions, needed to overcome the kinetics barrier.
What We Learned
- Arrhenius Equation: Summarizes the relationship between rate constant, temperature, and activation energy; important for understanding temperature impacts
- Activation Energy Difference: Different activation energies can affect speed of reaction.
- Reaction mechanisms: Defined molecularity clarifies rate laws for elementary reactions
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Description
This quiz covers key concepts from Chapter 14 of Brown's chemistry textbook, focusing on chemical kinetics. Topics include how temperature affects reaction rates, reaction mechanisms, and the role of catalysis. Perfect for students preparing for exams or seeking to reinforce their understanding of kinetic principles.
