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Level 141

Equilibrium & Rate Law


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concentration, pressure, and temperature
3 factors that can affect a system at equilibrium
Le Chatelier's Principle
application of stress (change in temp, pressure, or concentration) on a system at equilibrium causes it to minimize the stress and attain new equilibrium position
gases
intermolecular forces are less in what state?
solids or liquids
pressure changes do not affect the equilibrium of what
lowest number of total moles
an increase in pressure favors what side of the reaction
highest number of total moles
a decrease in pressure favors what side of the reaction
shift to product
heat is absorbed shifts to what
shift to reactant
heat is released shifts to what
Factors that affect the reaction rate
concentration or pressure for gases, temperature, surface area of reactants, pressure of catalyst, nature of the reactants
Concentration or pressure of gases explanation
the greater the concentration of molecules, the greater number of collisions
increase of temperature
increases rate and energy of collisions with each 10 degrees Celsius increase, double reaction rate
surface area
an increase in this increases the rate of a heterogeneous reaction
catalyst
a substance that increase reaction rate w/o being permanently used up or changed. it lowers the activation energy needed for the reaction so more molecules have the necessary energy for a reaction
homogeneous catalyst
a catalyst that is in the same phase as all the reactants and products
heterogeneous catalyst
a catalyst that is in a different phase
Nature of the reactants
ionic reacts faster than covalent
Potential Energy
energy due to position or composition
reaction coordinates
Energy Diagram's X axis
Activated complex
a transitional structure that results from and effective collision that persists while old bonds are breaking and new bonds are forming.
catalyzed pathway
Energy Diagram's smaller hill
activation energy
Energy Diagram's hill bisector line
[ ]
concentration in moles per liter
K
the rate constant for the reaction
Rate
Molecules must collide with an effective orientation for them to react, so structural complexity decreases...?
mechanism
if two or more steps are involved in creating products, the succession of those steps
the slowest step
rate determining step
the overall reaction
The rate law for the rate-determining step is the rate law for...?
intermediate
is a species that is a product in an early step of the mechanism and a reactant in a later step of the mechanism.
Mechanism Example
A+B--> C (slow)
1st order
rate is 2x
2nd order
rate is 4x
3rd order
rate is 8x
0 order
rate is same
Define: Reaction Rate
the change in concentration of a reactant or product per unit of time
Write out the rate law
Rate = ?A / ?T
disappearance
Reaction Rate Rule: can measure _ of reactants
equal, tangent
Reaction Rate Rule: are _ to the slope _ to that point
changes, dependent
Reaction Rate Rule: _ as the reaction proceeds, if the rate is _ upon concentration
Define: differential rate laws
express relationships between concentration of reactants and rate of the reaction
rate law
an equation that relates reaction rate and concentration at a certain temperature
Define: integrated rate law
express relationship between concentration of reactants and time
Rate= k[A]^m * [B]^n
Write out the integrated law
Zero: rate does not change
How to determine order from rate value changes
time vs. concentration
Integrated rate law formula: Zero order
time vs. ln(concentration)
Integrated rate law formula: first order
time vs. 1/concentration
Integrated rate law: second order
Slope= k
When using integrated rate law, how do you determine the constant, k?
second order graphs
Slope is negative for all order graphs except for _
Half-life formula: zero order
t1/2 = [A]0 /2k
t1/2= 0.693/k
Half-life formula: first order
t1/2= 1/ k*[A]0
Half-life formula: second order
Define: reaction mechanism
series of elementary steps by which a chemical reaction occurs
elementary steps
The sum of the _ must give the overall balanced equation for the reaction
experimentally; rate law
The mechanism must agree with the _ determined _
rate determining step
In a multi-step reaction, the slowest step is _
agree; rate determing
The experimental rate law must _ with the _ step
How to identify rate determining step (steps)
Identify which step agrees with the experimental rate law
Define: intermediate
species that do not appear in the final, balanced equation; but appear in the elementary steps
What is the collision model key idea?
Molecules must collide to react. However, only a small fraction of collisions produces a reaction.
sufficient; reaction
Collisions must have _ energy to produce the _
equal, exceed
Collisions must _ or _ the activation energy
oriented, produce
Colliding particles must be correctly _ to one another in order to _ a reaction
increase
as the size of halogens increase, the melting and boiling points
increases
Increasing temp _______ the vapor pressure. (increases/decreases)
usually increases
Increasing concentration _ reaction rate
lower; alternate pathways
The presence of catalysts _ activation energy by providing _
Define: catalyst
substance that speeds up a reaction without being consumed
Define: enzyme
large molecule (can be a protein) that catalyzes biological reactions
Define: homogeneous catalyst
present in same phases as reacting molecules
Define: heterogeneous catalyst
present in different phase than reacting molecules
see notes
Study: lowering of activation energy by a catalyst
uncatalyzed
Effective collisions that are _ will have a lower Ea as the reaction progresses.
catalyzed
Effective collisions that are _ will have a higher Ea as the reaction progresses
What determines an exothermic reaction?
Reactants' energy potential is higher than product energy potential
Define: activation energy
energy needed so reactant bonds break and reform to make product
What determines an endothermic reaction?
Reactant energy potential is lower than product energy potential
endothermic, forward
In an _ reaction, more energy is needed for the _ reaction to take place.
Define: activated complex
highest energy arrangement of molecules as they change from reactants to products
Old, breaking, new bonds
_ bonds of the reactants are in the process of _ and _ are forming
minimum, activation complex, successful reaction
Activation energy (Ea) is the _ energy required for the _ to form and for a _ to occur
smaller, faster
The _ the activation energy, the _ the reaction will occur (regardless of being exothermic or endothermic)
large, more
If there is a _ activation energy needed, _ energy and time is used for successful collisions to take place
greater; proceed
At higher temperatures there is a _ proportion of molecules that would have enough activation energy for the reaction to _. This is why high temperatures increase rate.
Arrhenius; temperautre
The _ equation shows the effect of _ on the rate constant, k.
What does the Arrhenius equation indicate?
That k depends exponentially on temperature
k= Ae^(-Ea/RT)
Write out the Arrhenius equation
increases, larger, increases
For Arrhenius equation: as T _, the negative exponent becomes smaller, so that the value of k becomes _, which means that the rate _
graphically
ln(k) and 1/T is linear: With R known, find Ea _ from a series of k values at different temperatures
Write out Arrhenius equation for ln(k)
ln(k2/k1)= Ea/R * (1/T2 - 1/T1)
Write out Arrhenius equation for 1/T
Ea= -R[ln(k2/k1)] * (1/T2 - 1/T1)
ln(k)= -Ea/R * 1/T + ln(A)
If the Arrhenius equation is rearranged, what will it convert to?
linear; -Ea/R; ln(A)
In a graph of ln(k) against of 1/T will be _ with a slope/gradient of _ and an intercept on the y-axis of _.
endothermic
A chemical reaction that absorbs heat
exothermic
A chemical reaction that releases energy
?H changes from positive to negative.
How does ?H change between endo- and exo-thermic rxn's?
absorb
Endothermic reactions _ heat
release
Exothermic reactions _ heat
Higher
Electron affinity tends to become from left to right across a row of the periodic table
lower
electrons that are closer to the nucleus have a _______ energy
Decreases
Atomic radius _______ across a period
true
(ex mg)
Slowest
In a reaction with several steps, which step limits the rate of the rxn?
2 (second order)
What is the overall order of a rxn with a rate constant having units L*mol?¹(s?¹) ?
Compare coefficients in the chemical reaction equation
How do you determine rate of decomposition to rate of formation?
particle size
Which property, if decreased, will cause an increase in rxn rate involving a solid?
Zero
A plot of reactant concentration versus time gives a straight line. What is the order of the rxn for this reactant?
increasing concentration of reactants
Which kind of change does not increase value of the rate constant for a rxn?
What represents the instantaneous rate of the rxn?
Slope of the tangent line at any point (t, [A])
What four factors affect rxn rates?
concentrations of reactants, temperature, physical state of reactants, catalyst
Define: rate law
mathematical relationship that shows how rate of reaction depends on concentrations of reactants
first
For a _ order rxn, a plot of ln[A] vs. time will yield a straight line with slope of -k
second
base unit of time in the metric system
The half-life of a reaction is the time _______.
required for the initial concentration of a reactant to fall to half its value
first; second
The half life of a simple _ order rxn does not depend on initial concentration of reactant, whereas in a _ order rxn the half life depends on initial concentration
Define: elementary step
each single event or step in a mechanism
Define: rxn mechanism
step-by-step process by which a chem rxn occurs
Define: unimolecular elementary rxn
Rxn involving one reactant molecule
Define: bimolecular elementary rxn
Rxn involving collision 2 reactant molecules
Define: termolecular rxn
Simultaneous collision reactant molecules (rare but not imposibru)
How to determine rates of appearance and disapperance
Reaction rates relate directly to stoichiometry. Create ratios between compounds shown in the chemical rxn equation. Compare with what the question is asking (multiply by moles) and convert to units needed.
How to determine the slowest step
Rate law includes only those reactant molecules which react during and before the rate determining step. Cannot be determined from overall balanced equation.
Rate of rxn and concentration of reactant
What changes with time for a first order rxn?
concentration of products
For irreversible chemical rxn, rate will NOT be affected by change in _
mol/ L*s
Zero order unit
1/s
First order unit
L/ mol*s
Second order unit
Third order unit
L²/ mol² * s
0th order rate law
rate = k[A]° = k
rate = k[A]¹
1st order rate law
rate = k[A]²
2nd order rate law
0th order integrated rate law
[A] at final time = -kt + [A] at initial time
1st order integrated rate law
ln[A] at final time = -kt +ln[A] at initial time
2nd order integrated rate law
1/[A] at final time = kt + 1/[A] at initial time
[A] vs. time
0th order linear plot
ln[A] vs. time
1st order linear plot
1/[A] vs time
2nd order linear plot
0th order y-intercept
[A] at initial time
1st order y-intercept
ln[A] at initial time
2nd order y-intercept
1/[A] at initial time
0th order half-life
[A] at initial time/2k
0.693/k
1st order half-life
2nd order half-life
1/k[A] at initial time