4B5 Chemical Kinetics Flashcards
Explain models of chemical kinetics based on collision theory, including rates, activation energy, and catalysts.
Define:
rate of a reaction
The change in concentration of reactants or products divided by time.
The rate of reaction quantifies how fast the concentration of reactants or products changes over time.
Equation: Rate = change in concentration / time; expressed in Molarity per second (M/s) or per minute (M/min), depending on the reaction speed.
List the four factors that affect the rate of reaction.
- Temperature
- Concentration
- Surface area
- Catalysts
These factors all influence how often reactant molecules collide and how much energy they have for successful collisions.
How does concentration affect the rate of reaction?
Increasing concentration increases the rate of reaction.
Higher concentration means more reactant molecules are present, which leads to more collisions and thus a faster reaction.
What happens to the rate of reaction as temperature increases?
It increases.
Higher temperature increases the kinetic energy of molecules, making them move faster and collide more frequently.
How does surface area affect the rate of reaction?
More surface area increases the rate of reaction.
More surface area exposes more molecules to collisions, speeding up the reaction.
Example: Crushing a solid into powder increases surface area.
Fill in the blank:
In a chemical reaction, rate is proportional to the _________.
number of collisions
The rate of reaction depends on how often molecules collide and how much energy is available for the collision to be effective.
Define:
collision theory
Chemical reactions occur when reactant molecules collide with sufficient energy.
More frequent and energetic collisions increase the rate of the reaction.
What is the rate law in chemistry?
A quantitative expression linking the concentration of reactants to the rate of a chemical reaction.
The rate law defines how the concentration of reactants affects the speed of a chemical reaction.
How can the rate law equation be expressed?
rate = k[A]m[B]n [C]o
Where [A], [B], and [C] are the concentrations and m, n, and o are the partial reaction orders of reactants A, B and C.
What is the reaction order?
The sum of the partial reaction orders for all reactants in the rate law equation.
The reaction order provides a way to understand how reactant concentrations affect the reaction rate.
Fill in the blank:
The reaction order in a first-order reaction is equal to _____.
1
In first-order reactions, the sum of the exponents (m+n) is 1.
In a first-order reaction, how is the rate related to the concentration of reactant A?
The rate is directly proportional to the concentration of A.
If the concentration of A doubles, the rate also doubles in a first-order reaction.
How is the rate constant (k) determined?
By experimental data, which involves:
- Using different concentrations.
- Measuring reaction rates.
After determining the reaction order, k can be calculated from the experimental data by substituting experimental values into the rate equation.
The rate constant k is specific to each chemical reaction under particular conditions.
What does a higher value of the rate constant (k) indicate?
A faster reaction speed.
The rate constant (k) varies with conditions like temperature.
What happens when the concentration of reactant A triples in a first-order reaction?
The rate of the reaction triples.
In first-order reactions, the rate is directly proportional to the concentration of the reactant.
What is the unit of the rate constant (k) in a first-order reaction?
s⁻¹
In a first-order reaction, the rate constant has units of inverse seconds.
What is the unit of the rate constant (k) in a second-order reaction?
L mol⁻¹ s⁻¹
In a second-order reaction, how does the rate change when the concentration of A doubles?
The rate quadruples.
In second-order reactions, the rate is proportional to the square of the reactant concentration.
What is the overall reaction order in the equation rate = k[A]²[B]⁰?
2
The reaction order is the sum of the exponents in the rate law equation (m+n).
True or False:
The reaction order can sometimes be a fraction or negative number.
True
Reaction orders are typically whole numbers but can occasionally be fractions or negative.
What happens to the rate of reaction in a zero-order reaction with respect to reactant B?
The rate is independent of the concentration of B.
If the exponent for B is zero, the concentration of B does not affect the rate of the reaction.
List the steps involved in determining the rate law for a reaction.
- Conduct experiments to measure reaction rates.
- Calculate reaction orders for each reactant.
- Solve for the rate constant (k).
The rate law is determined through experimentation and calculation based on observed data.
What equation is used to calculate the rate constant k?
The Arrhenius equation.
Arrhenius equation: k=Ae^(-Ea/RT).
It expresses the relationship between the rate constant (k), temperature (T), and activation energy (Ea).
Which component in the Arrhenius equation represents the frequency factor?
A
The frequency factor is the number of times reactants approach the activation energy per unit time.
It depends on factors like molecular orientation and collision frequency.
Which factor increases when temperature rises?
The exponential factor.
e^(-Ea/RT) is the exponential factor.
How does the activation energy affect the exponential factor?
The higher the activation energy, the smaller the exponential factor.
A higher activation energy means that fewer reactants can overcome the barrier, reducing the exponential factor.
What is the value of the exponential factor if the activation energy is 0?
1
If the activation energy is 0, the exponential factor is 1, meaning all reactants can overcome the barrier.
What does the Arrhenius equation indicate about the relationship between rate constant and temperature?
The rate constant increases as temperature increases.
According to the Arrhenius equation, k increases as the temperature T increases, leading to a faster reaction rate.
Define:
activation energy
The minimum amount of energy required for a chemical reaction to occur.
A higher activation energy means a slower reaction rate at a given temperature since more energy is required to overcome the barrier.
List two factors that contribute to the activation energy of a reaction.
- Molecular orientation
- Bond breaking
Reactant molecules must collide with proper orientation, and bonds must break before new bonds can form.
What is the effect of thermal energy on the activation energy?
It increases the kinetic energy of molecules, helping them overcome the activation energy.
This energy is usually provided by heat sources like Bunsen burners or hot plates in laboratory settings.
Define:
transition state
A high-energy state in which reactants rearrange to form products.
It is a temporary unstable state in which bonds are breaking and forming, and the system has reached its maximum potential energy.
What is the effect of a catalyst on the activation energy of a reaction?
It lowers the activation energy by providing an alternative reaction mechanism.
Catalysts speed up reactions without being consumed by reducing the energy barrier needed for the reaction to proceed.
List three examples of catalysts.
- Zinc oxide (ZnO)
- Chromium oxide (Cr₂O₃)
- Nickel (Ni)
These catalysts are used in commercial reactions like methanol synthesis, steam reforming, and the water-gas shift reaction.
Define:
activation complex
The unstable intermediate formed during the transition state when bonds are breaking and forming.
It is the state where the reactants are in the process of converting into products but have not yet completed the reaction.
True or False:
Catalysts change the products of a reaction.
False
Catalysts do not alter the products of a reaction; they only affect how quickly the reaction reaches the products.
What type of catalysts are found in living organisms?
Enzymes
These are biological catalysts, typically proteins or RNA.
They have a unique active site that specifically interacts with a particular substrate, allowing for more efficient catalysis.
How do enzymes work as catalysts in biological systems?
They lower activation energy by binding to substrates and aligning them in the correct orientation for the reaction to occur.
Enzymes differ from catalysts since they are highly specific to their substrates and use the induced fit mechanism to bind and catalyze reactions.
What is the purpose of a catalyst in a chemical manufacturing laboratory?
- To increase the efficiency of reactions.
- To reduce the time required to complete reactions.
Catalysts are used to speed up reactions in industrial settings where time and resource efficiency are critical.
What is a reaction mechanism?
The step-by-step sequence of elementary steps by which reactants become products.
A reaction mechanism describes how molecules interact and transform during a chemical reaction.
Which step in a reaction mechanism is considered the rate-determining step?
The slowest elementary step in the reaction mechanism.
This step has the highest energy barrier and controls the overall reaction rate.
Define:
intermediate
A molecule produced in one elementary step and consumed in a subsequent step.
Intermediates are crucial to understanding reaction mechanisms but do not appear in the overall reaction equation.
Fill in the blank:
In a reaction mechanism, a reaction with multiple steps has a _______ transition state for each step.
unique
Each elementary step in a multi-step reaction has its own transition state, with a corresponding activation energy.
Define:
homogeneous reactions
Reactions that involve reactants and products in the same physical state.
This makes the rate laws for homogeneous reactions simpler compared to heterogeneous reactions.
List the steps to find the rate law for a reaction.
- Write out the elementary steps, including intermediates.
- Identify the rate-determining step.
- Find its rate law.
- If necessary, substitute intermediates with initial reactants.
These steps help determine the rate law by focusing on the slowest step and any intermediates involved.
What happens when there are intermediates in the rate law equation?
Intermediates must be substituted with the concentrations of initial reactants.
This substitution is done using the rate laws of prior elementary steps.
True or False:
The rate law for the rate-determining step is always the rate law for the overall reaction.
True
Since the rate-determining step controls the overall reaction rate, its rate law is used for the entire reaction.
Define:
molecularity
It is the number of molecules that must collide to undergo a reaction in an elementary step.
The molecularity is often reflected in the exponents of the reactant concentrations in the rate law.
What is a rate law for a reaction dependent on?
- Concentrations of reactants in the rate-determining step.
- The rate constant (k).
The rate law is independent of the concentration of products.
