2 Chemical Kinetics Flashcards
What is chemical kinetics?
A science regarding to how a chemical reaction happens
Why is chemical kinetics important
Equilibrium does not always happen
Combustion is more often an undergoing process than an initial and final states of an equilibrium process
What is detailed chemistry?
A rigorous description of the chemical kinetics
What is reduced chemistry?
A simplified description of the chemical kinetics
What does the law of mass action state?
It states that the rate of a chemical reation is proportional to the product of the activities or concentration of the REACTANTS
When is the law of mass action applicable?
To true elementary reactions.
How do the backward and forward rate of reaction change at equilibrium?
They don’t. They’re equal
How can the progress of a backward reaction be slowed?
- endothermic
- high energy barrier
What does the SSA state?
The consumption and regeneration of intermediates occurs at equal rates such that the concentrations can be considered to remain constant.
When does the PEA apply?
To a particular reaction rath er than species
Which are the limits of SSA and PEA
They apply to vigorously burning conditions which are characterized by large temperatures and radical concentrations
Not addapted for pre-ignition and pre-extinction conditions
What’s the molecularity of a reaction?
The number of atoms or molecules that interact with each other at molecular level, leading to the completion of the reaction
What happens with higher molecularity?
The less frequnt the event
Why is a third body needed in a threebody collision?
To carry away the excess energy.
For large, complex molecules the excess energy can be absorbed by the various vibrational modes of the combined molecules and a third body may not be needed.
What does the order of reaction indicate?
The influence of the concentration of the ith reactant on the reactoin rate
How does the frequency factor A change with temperature?
It is dependent over an extanded temperature range
What does the sensitivity of a chemical reaction to temperature depend on?
To the activation energy.
The larger the (forward activation energy), the more sensitive is the reaction to temperature.
What does the activation energy present?
The minimum energy the colliding molecules must posses for the reaction to be possible.
When is the maximum reaction rate attained?
At maximum temperature
Is the backward reaction for an exothermic process slower or faster?
Slower because of the larger activation energy.
Sine the backward reaction is endothermic, the backward activation energy has to be at least as large as the heat absorbed in the reaction.
When is the backward reaction of an exothermic reaction not slow?
At high temperatures.
What does the Arrhenius number measure?
The largeness of the activation energy.
How does the reaction progress for small Ar?
The reaction rate increases progressively From T=0 to the maximum temperature
How does the reaction progress for large Ar?
The reaction rate is suppressed until T
approaches the maximum temperature ,at which t increases rapidly
The reactant is consumed rapidly within a thin layer
The reaction is confined within vers short times or within very thin space intervals
What does the collision theory of reaction rates do?
It equates
the reaction rate with
the rate of molecular collision having a collision energy exceeding the activation energy.
Thus if we assume that the collision energy necessary to effect moleuclar change is derived from the relative translational energy between the colliding molecules,
then the reaction rate can be determined by summing over all possible collisions satisfying the requirement of minimum collision energy.
What does the transition state theory of reaction rates do?
It examines the state of the activated complex and its influence on the reaction rate.
Which assumptions does the transition state theory make?
- Partial equilibrium exists between the reactants and the activated complex
- The comple is a steady-state species
What’s a unimolecular reaction?
It describes a chemical process in which a reactant undergoes an isomerization or decomposition process.
Which order does a unimolecular reaction have?
It is really second order in nature, exhibiting a pseudo first-order behavior only under certain conditions.
Since the reactant of a unimolecular reaction must acquire sufficient energy before the reaction can take place, it requires intermolecular energy transfer through collision with another molecule.
What influences the specific rate constant of a unimolecular reaction?
Both temperature and pressure.
Which decreasing pressure, reaction undergoes transition from first order toward second order.
What does the Lindemann theory state?
It states that since stable molecules cannot spontaneously break up into products, the reactant R must attain sufficient energy to undergo reaction.
What does the process of forming an energized molecule involve?
It infolves collision between the reactant R and another molecule M.
The energized molecule R* may
- undergo a unimolecular reaction to form products
- undergo de-energization by collision with other molecules
What’s a characteristic of straight-chain reactions?
They produce one radical per radical consumed
We don’t have a net production of chain carriers.
Which thermicity do the initiation reactions of halogens have?
They are all endothermic.
Therefore, halogen-hydrogen reactions are not spontaneous.
What’s the main characteristic of branched chain reactions?
The reactions produce ore than one (typically two) radicals per radical consumed)
We have a net production of chain carriers
By what is explosion favored?
- fast chain-branching reactions
- slow chain-termination reactions
Why does the gas density decrease with p -> 0 for general explosive mixtures?
The chain cycle becomes less efficient because it requires the collision between two molecules.
The wall termination however depends only on the concentration of the chain carrier and therefore becomes more efficient.
The net effect is the tendency to inhibit explosion
Why does the gas density decrease with p -> infinity for general explosive mixtures?
The increase in density favors the three-body gas termination action, hence inhibiting explosion
Which termicity do chain-branching reactions have?
They are endothermic
How do chain-branching reactions react to temperature?
Since endothermic, they are more sensitive to temperature variation and have a large activation energy
With increasing temperature, the gas becomes more explosive
Is the activation energy of chain-branching reactions large or small?
Large
Are gas and wall termination reactions temperature dependent?
No, they are not temperature sensitive, with very small activation energies (nearly zero)
Which thermicity do gas termination reactions have?
They are highly exothermic because of the release of energy from the activated radicals
What is chain ignition
Type of ignition in which the temperature of the reactans has reached a point at which the rate of radical production through branching exceeds the rate of radical destruction through gas or wall termination
What is thermal ignition?
Type of ignition in which a critical mass of the reactive mixture is heated up to the point that the chemical heat generation exceeds the rate of the various modes of heat loss
Which modes of heat loss do we have?
- conductive
- convective
- radiative
Measures of inhibiting highly explosive branched-chain reactions
Flame inhibitors
- lower temperature
- reduce radical pool by addition of halogen
