Chemistry Notes: Chapter 14
What are the two different phases that the reactants and products are in in a heterogeneous catalyst?
1. Haber synthesis of ammonia 2. Catalytic converters
What two requirements must a proposed mechanism satisfy?
1. The sum of the elementary reactions must be the overall balanced equation for the reaction. 2. The rate-determining step must have the same rate law as that determined from the experimental data.
How can the overall progress of a chemical reaction be represented?
1. at the molecular level by a series of simple elementary steps or elementary reactions 2. The sequence of elementary steps that leads to product formation is the reaction mechanism 3. The overall reaction is always the sum of the elementary steps of the reaction mechanism.
What does the Arrhenius equation do?
1. if we know the rate constant at two different temperatures, we can calculate the activation energy. 2. If we know the activation energy, we can determine the value of the rate constant at any other temperature.
What are the three conditions that must be met for a reaction to occur according to collision theory
1. the reacting molecules must collide with one another. 2. The reacting molecules must collide with sufficient energy to initiate the process of breaking bonds. 3. The molecules must collide in an orientation that can lead to rearrangement of the atoms and the formation of products.
What is a second order reaction?
A second-order reaction is a reaction whose rate depends on concentration of one reactant raised to the second power or the on the product of the concentrations of two different reactants (each raised to the first power)
Part 2 of Collision Theory
A greater frequency of collisions usually leads to a higher reaction rate
Part 3 of Collision Theory
According to the collision theory of chemical kinetics, the reaction rate is directly proportional to the number of molecular collisions per second
Concentration of Reactants
At a higher concentration, reactant molecules collide more often, giving rise to a greater number of effective collisions thereby increasing reaction rate.
Temperature
At higher temperature, reactant molecules are moving faster, causing more frequent collisions with greater energy and rate increases
Addition of a catalyst
Catalysts speed up reactions by changing the mechanism of the reaction
Part 1 of Collision Theory
Chemical reactions generally occur as a result of collisions between reacting molecules
Part 4 of Collision Theory
Collisions that result in a chemical reaction are called effective collisions
What factors affect reactions rates?
Concentration of reactants, Temperature, Surface area of a solid reactant, Addition of a catalyst
Where is an intermediated formed and consumed
Intermediates are formed in an early elementary step and consumed in a later elementary step
For the general reactions aA + bB ---> cC + dD, what is the rate equal to?
Rate = k[A]^x[B]^y
What is the slowest step in the reaction mechanism?
The rate determining step
In general, how are the rate of chemical reactions and temperature related
The rate of a chemical reaction increases with increasing temperature
Surface area of a solid reactant
With greater surface area, reactant molecules can collide more frequently, thereby increasing reaction rate
What is a way of visualizing a chemical reaction
a reaction profile, also known as a reaction energy diagram
What is a first order reaction?
a reaction whose rate depends on the reactant concentration raised to the first power
How are reactants and catalyst dispersed in a homogeneous catalyst?
a single phase, usually liquid.
What is a catalyst?
a substance that increases the rate of a chemical reaction without itself being consumed
What does a catalyst provide?
an alternative reaction pathway that has a lower activation energy
What is the rate law?
an equation that relates the rate of reaction to the concentrations of reactants
What is the equation for delta h in an endothermic reaction?
delta h = ea - ea prime(delta h is positive)
What is the equation for delta h in an exothermic reaction?
delta h = ea prime - ea (delta h is negative)
What does an Arrhenius equation express?
dependence of the rate constant on temperature
Unimolecular reaction
elementary step with 1 molecules
Bimolecular reaction
elementary step with 2 molecules
Termolecular reaction
elementary step with 3 molecules
What is it called if the products are higher in energy than the reactants the reaction?
endothermic, this kind of reaction is favorable.
What is it called if the products are lower in energy than their reactions?
exothermic, this kind of reaction is favorable.
How must the exponents x and y be determined?
experimentally
What are the most common reaction types?
first and second order reactions
How are the exponents in a rate law determined?
from a table of experimental data
What does [ ] imply?
mol/L or M
Are the exponents in a rate law related to the stoichiometric coefficients in the balanced chemical equation?
no
What is rate directly proportional to
number of collisions / second
What are intermediates?
species that appear in a reaction mechanism but not in the overall balanced equation
What does stoichiometry require?
that the rate of disappearance of reactants must be equal to the rate of appearance of products
What is the instantaneous rate t=0 called
the initial rate
What is activation energy?
the minimum amount of energy required to initiate a chemical reaction.
What is the molecularity of an elementary reaction?
the number of reactant molecules in an elementary step
In a reaction mechanism what step has the highest activation energy?
the rate determining step (slow step)
What does instantaneous rate give?
the rate for a specific instant in time
Why are we so interested in the elementary steps of a reaction?
the rate law can be written directly from the equation for that step
What does the rate-determining step give?
the rate law for the overall process in a reaction mechanism consisting of more than one elementary step
What is average reaction rate?
the rate of a chemical reaction refers to the change in concentration of a substance per unit in time
What is Chemical kinetics the study of?
the rate of chemical reactions and the mechanisms by which they occur
What is the instantaneous rate equal to?
the slope of a tangent to the curve at any particular time
What is the half-life of a reaction?
the time required for the concentration of a reactant to decrease to half of its initial concentration.