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Intro to Chemistry
Table of Contents
Unit 12 Overview: Kinetics
12.1 Chemical Reaction Rates
12.2 Factors Affecting Reaction Rates
12.3 Rate Laws
12.4 Integrated Rate Laws
12.5 Collision Theory
12.6 Reaction Mechanisms
12.7 Catalysis

💏intro to chemistry review

12.2 Factors Affecting Reaction Rates

Citation:

Chemical reactions are the heart of chemistry. Their rates are influenced by temperature, concentration, and catalysts. These factors affect how often molecules collide and how much energy they have when they do.

Physical properties like state and surface area also impact reaction speeds. The chemical nature of reactants, including bond strength and polarity, plays a crucial role. Understanding these factors helps predict and control reaction rates in various applications.

Factors Influencing Reaction Rates

Factors affecting reaction rates

  • Temperature
    • Higher temperatures increase average kinetic energy of molecules
      • Molecules move faster and collide more frequently (gas particles)
      • Collisions are more energetic, increasing likelihood of successful collisions (activation energy)
    • Arrhenius equation: $k = Ae^{-E_a/RT}$
      • $k$ is rate constant, $A$ is frequency factor, $E_a$ is activation energy, $R$ is gas constant, and $T$ is absolute temperature (Kelvin)
      • As temperature increases, rate constant $k$ increases exponentially (doubling rate for every 10℃ rise)
  • Concentration
    • Higher concentrations of reactants lead to faster reaction rates
      • More molecules per unit volume increases probability of collisions (mol/L)
      • Collision theory states rate is proportional to frequency of effective collisions
        • Collision frequency affects the overall reaction rate
    • Rate law: $\text{Rate} = k[A]^m[B]^n$
      • $[A]$ and $[B]$ are concentrations of reactants, $m$ and $n$ are reaction orders, and $k$ is rate constant
      • Doubling concentration of reactant with order of 1 will double reaction rate (first-order reaction)
  • Catalysts
    • Catalysts lower activation energy $E_a$ of reaction without being consumed
      • Provide alternative reaction pathway with lower energy barrier (transition state)
      • Increase rate constant $k$ by allowing more successful collisions at given temperature
    • Catalysts can be homogeneous (same phase as reactants) or heterogeneous (different phase)
      • Enzymes are biological catalysts highly specific to their substrates (active sites)

Impact of physical properties on reactions

  • Physical states
    • Reactions involving solids are generally slower than those involving liquids or gases
      • Solid reactants have limited contact area and require diffusion for molecules to interact (lattice structure)
    • Reactions between gases are usually faster than those between liquids
      • Gas molecules move more freely and have higher collision frequency (mean free path)
    • Reactions in solution (liquid phase) are often faster than those in solid state
      • Solvents can help bring reactants together and stabilize transition states (solvent cage effect)
  • Surface area
    • Increasing surface area of solid reactants can increase reaction rates
      • Breaking solids into smaller pieces or using powders exposes more surface for reactions to occur (grinding, crushing)
      • Heterogeneous catalysts often have high surface areas to maximize their effectiveness (porous materials)
    • Porous materials, such as zeolites or activated carbon, have high surface areas due to their internal structure
      • These materials can act as effective catalysts or adsorbents (gas storage, water purification)

Chemical nature and reaction speeds

  • Bond strength
    • Reactants with weaker bonds generally react faster than those with stronger bonds
      • Less energy required to break weaker bonds during reaction process (single vs double bonds)
      • For example, single bonds are easier to break than double or triple bonds (C-C vs C=C)
  • Polarity and solubility
    • Polar reactants often react faster in polar solvents, while nonpolar reactants react faster in nonpolar solvents
      • "Like dissolves like" principle: similar polarities lead to better solubility and higher reaction rates (water vs oil)
    • Solubility affects ability of reactants to interact and collide effectively
      • Reactants with higher solubility in given solvent will have greater chance of successful collisions (salt in water)
  • Steric hindrance
    • Bulky substituents or functional groups can slow down reaction rates by blocking access to reactive sites
      • Steric hindrance can prevent reactants from getting close enough to collide and react (branched vs straight chain)
    • Reactants with less steric hindrance generally react faster than those with more steric hindrance
      • For example, primary alcohols react faster than tertiary alcohols in substitution reactions (SN2 mechanism)

Reaction Mechanisms and Energy Profiles

  • Reaction mechanism describes the step-by-step process of a chemical reaction
    • Includes information about intermediates and transition states
    • Can involve multiple elementary steps (reaction mechanism)
  • Potential energy diagram illustrates energy changes during a reaction
    • Shows reactants, products, and transition states (reaction coordinate)
    • Activation energy and overall energy change can be visualized
  • Thermodynamics deals with energy changes and spontaneity of reactions
    • Determines whether a reaction is favorable overall
  • Kinetics focuses on the rate of chemical reactions and factors affecting it
    • Studies how quickly reactions occur and the pathways they follow

Key Terms to Review (39)

Activation energy (Ea): Activation energy (Ea) is the minimum amount of energy required for a chemical reaction to occur. It determines the rate at which reactants transform into products.
Arrhenius equation: The Arrhenius equation describes the temperature dependence of reaction rates. It shows how the rate constant $k$ increases exponentially with an increase in temperature.
Chemical thermodynamics: Chemical thermodynamics studies the interrelation of heat and work with chemical reactions or physical changes. It applies principles of thermodynamics to predict the direction and extent of chemical processes.
Collision theory: Collision theory explains how and why chemical reactions occur by describing the conditions under which reactant particles must collide. Effective collisions require proper orientation and sufficient energy to overcome activation energy.
Integrated rate laws: Integrated rate laws describe the concentration of reactants as a function of time. They are derived from differential rate laws and are used to determine reaction order and rate constants.
Mean free path: Mean free path is the average distance a gas molecule travels between collisions with other molecules. It is influenced by factors such as temperature, pressure, and particle size.
Overall reaction order: Overall reaction order is the sum of the exponents of the concentration terms in a rate law equation. It indicates how the rate of reaction depends on the concentration of reactants.
Rate constant: The rate constant, often denoted as $k$, is a proportionality factor in the rate equation that relates the reaction rate to the concentration of reactants. Its value is specific to a particular reaction and changes with temperature.
Reaction mechanism: A reaction mechanism describes the step-by-step sequence of elementary reactions by which overall chemical change occurs. It provides detailed information on the intermediates, transition states, and energy changes throughout the process.
Transition state: The transition state is a high-energy, unstable configuration of atoms during a chemical reaction that represents the point of maximum energy. It is the state through which reactants must pass to be converted into products.
Thermodynamics: Thermodynamics is the branch of physics that deals with the relationships between heat, work, temperature, and energy. It describes the fundamental physical laws governing the transformation of energy and the flow of heat, which are essential to understanding the behavior of chemical systems and processes.
Reaction Mechanism: A reaction mechanism is the step-by-step sequence of elementary reactions that describes how reactants are transformed into products during a chemical reaction. It provides a detailed understanding of the pathways and intermediates involved in the overall chemical process.
Maxwell-Boltzmann Distribution: The Maxwell-Boltzmann distribution describes the distribution of molecular speeds or kinetic energies in an ideal gas at a given temperature. It is a fundamental concept in the kinetic-molecular theory of gases and is crucial for understanding factors affecting reaction rates and collision theory.
Mean Free Path: The mean free path is the average distance a particle, such as a gas molecule, travels between successive collisions with other particles. It is a fundamental concept in the kinetic theory of gases and plays a crucial role in understanding the behavior of gases, including effusion, diffusion, and reaction rates.
Rate Constant: The rate constant is a measure of the speed or rate at which a chemical reaction occurs. It is a fundamental parameter that describes the intrinsic reactivity of the reactants and the reaction mechanism, and it is an essential component in understanding and predicting the kinetics of chemical processes.
Enzyme: An enzyme is a biological catalyst that accelerates the rate of a chemical reaction without being consumed or altered itself. Enzymes are essential for life, as they facilitate the numerous biochemical reactions that sustain living organisms.
Kinetics: Kinetics is the study of the rates of chemical reactions and the factors that influence those rates. It is a fundamental aspect of chemistry that examines how quickly reactants are converted into products, providing insights into the mechanisms and pathways of chemical transformations.
Reaction Rate: Reaction rate is the measure of the speed at which a chemical reaction occurs, quantifying the change in the concentration of reactants or products over time. It is a fundamental concept in understanding the dynamics of chemical processes and how they can be influenced and controlled.
Rate Law: The rate law is an equation that describes the relationship between the rate of a chemical reaction and the concentrations of the reactants. It is a fundamental concept in chemical kinetics that helps quantify and predict the speed of a reaction under specific conditions.
Reaction Order: Reaction order is a measure of how the rate of a chemical reaction changes with the concentrations of the reactants. It describes the relationship between the rate of a reaction and the concentrations of the reactants involved, providing insights into the mechanism of the reaction.
Elementary Reaction: An elementary reaction is the simplest type of chemical reaction, involving the transformation of reactants into products in a single step without any intermediate steps. It is the fundamental unit of a complex chemical process and forms the basis for understanding reaction kinetics and mechanisms.
Catalysis: Catalysis is the process by which a substance, called a catalyst, increases the rate of a chemical reaction without being consumed or altered itself. Catalysts work by providing an alternative pathway for the reaction, lowering the activation energy required and allowing the reaction to proceed more quickly.
Transition State: The transition state is a critical point in the course of a chemical reaction where the reactants have not yet fully transformed into the products, but rather exist in a high-energy, unstable intermediate configuration. This transient state is a key concept in understanding the factors that affect reaction rates and the mechanisms by which reactions occur.
Rate-Determining Step: The rate-determining step is the slowest elementary step in a reaction mechanism that controls the overall rate of the chemical reaction. It is the step that limits the speed at which the reaction can proceed and is the crucial factor in determining the reaction rate.
Active Site: The active site is the specific region on an enzyme or catalyst where the substrate molecule binds and the chemical reaction takes place. It is the key functional part of these biomolecules that facilitates catalysis and allows them to carry out their essential roles in biological processes.
Activation Energy: Activation energy is the minimum amount of energy required to initiate a chemical reaction. It represents the energy barrier that must be overcome for the reaction to occur, acting as a catalyst for the transformation of reactants into products.
Reaction Coordinate: The reaction coordinate is a graphical representation of the progress of a chemical reaction, depicting the changes in energy as the reactants are converted into products. It is a fundamental concept in understanding the factors that influence reaction rates and the mechanisms by which reactions occur.
Collision Theory: Collision theory is a model that explains how chemical reactions occur by describing the necessary conditions for reactant molecules to collide and form products. It is a fundamental concept in understanding the factors that affect the rates of chemical reactions.
Heterogeneous Catalyst: A heterogeneous catalyst is a catalyst that exists in a different physical phase than the reactants in a chemical reaction. It is typically a solid material that facilitates reactions between gaseous or liquid reactants, without becoming permanently incorporated into the final products.
Potential Energy Diagram: A potential energy diagram is a graphical representation that depicts the changes in potential energy as a reaction progresses. It provides a visual aid to understand the energy profile of a chemical reaction, including the reactants, products, and any intermediate steps or transition states.
Zeolite: Zeolites are a group of naturally occurring or synthetic aluminosilicate minerals with a porous, crystalline structure. They are known for their ability to act as molecular sieves, selectively adsorbing and desorbing molecules based on their size and shape, which makes them particularly useful in various industrial and chemical applications.
Arrhenius Equation: The Arrhenius equation is a mathematical formula that describes the relationship between the rate of a chemical reaction and the temperature at which the reaction occurs. It is a fundamental concept in the field of chemical kinetics and is widely used to understand and predict the behavior of chemical reactions.
Frequency Factor: The frequency factor, also known as the pre-exponential factor or the collision frequency, is a measure of the frequency of collisions between reactant molecules in a chemical reaction. It is a crucial parameter in the Arrhenius equation, which describes the relationship between the rate constant of a reaction and the temperature at which the reaction occurs.
Homogeneous Catalyst: A homogeneous catalyst is a catalyst that is in the same phase as the reactants in a chemical reaction. It is dissolved or dispersed within the reaction mixture, allowing for intimate contact with the reactants and facilitating the reaction process.
Solvent Cage Effect: The solvent cage effect refers to the phenomenon where reactant molecules are temporarily trapped or confined within a 'cage' formed by the surrounding solvent molecules. This encapsulation can influence the reaction kinetics and dynamics.
Substrate: A substrate is the substance upon which an enzyme acts, or the reactant that is converted into product(s) by a chemical reaction. It is the starting material in a biochemical or chemical process.
Collision Frequency: Collision frequency refers to the number of collisions between reactant molecules per unit of time. It is a critical factor in determining the rate of a chemical reaction, as more frequent collisions between reactants increase the probability of successful reactions occurring.
Activated Carbon: Activated carbon is a highly porous, carbon-rich material that has been treated to increase its surface area and adsorption capacity. It is widely used in various applications, including water purification, air filtration, and chemical processing, due to its exceptional ability to remove impurities and contaminants from different media.
Steric Hindrance: Steric hindrance, also known as steric effects, refers to the obstruction of chemical reactions or molecular interactions due to the bulky size or spatial arrangement of atoms or molecules. This phenomenon can significantly impact the rate and outcome of chemical processes.