HYS CH5.1,5.2, CH6Chemical Kinetics Flashcards
what determines whether a reaction will be spontaneous or non spontaneous?
change in Gibbs free energy (delta G)
if a reaction is spontaneous it doesnt mean it runs fast –> thats reaction rate
many biological spontaneous reactions happen slowly without enzyme or catalysts
How do you break up a reaction?
Overall: A2 + 2B –> 2 AB
A2B is an intermediate
what is the Rate determining step?
the slowest step which determines rate of the overall reaction
what is collision theory of chemical kinetics?
rate of a reaction is proportional to the number of collisions per second between reacting molecules
not all collisions result in reactions
Effective collisions (creating product) need molecules to collide with correct orientation and with sufficient energy to break bonds and form news ones
what is activation energy Ea or energy barrier?
the minimum energy of collision necessary for a reaction to take place. Only some colliding particles have enough energy
What is the rate of the reaction in terms of number of collisions and number of effective collisions
rate = Z x f
Z = total number of collisions per second
f = fraction of collisions that are effective
What is the frequency factor (attempt frequency)
measure of how often molecules in a certain reaction collide with units s-1
as frequency factor increases, rate constant increases
how can you increase frequency factor?
- increase number of molecules
opportunities for collision increases
what is a reaction coordinate?
a reaction coordinate is an abstract one-dimensional coordinate chosen to represent progress along a reaction pathway.
What is a transition state?
transition state (activated complex) It is defined as the state corresponding to the highest potential energy along the reaction coordinate.
- It is often marked with the double dagger (‡) symbol.
- energy required to reach transition state is called activation energy
- theoretical concepts
- once formed can dissociate into products or revert to reactants without additional energy input
What is free energy change of the reaction? delta G
difference between the free energy of the products and the free energy of the reactants
neg free energy change (exergonic reaction- gives off)
pos free energy change (endergonic reaction-absorbs)
What is difference between free energy of forward reaction and reverse reaction?
The difference in free energy between the transition state and the reactants is the activation energy of the forward reaction;
the difference in free energy between the transition state and the products is the activation energy of the reverse reaction.
contrast delta G vs Ea forward vs Ea reverse
can activation energy be lowered without effecting delta G?
yes, kinetics and thermodynamics are not the same
this is how catalysts work
is it true that raising temp by 10 Celcius will result in double reaction rate?
generally true for biological systems
but if temp too high, a catalyst may denature and reaction rate plummets
enzymatic reaction rates are optimal between 35-40 celcius (body temp) but denaturation starts occuring at 40 celcius
what kind of medium is preferred in reactions?
some reactions are more likely in aqeous, or non aqeous
physical state has an effect
polar solvents are preferred because molecular dipoles tend to polarize the bonds of the reactants and lengthen or weaken them, which permits the reaction to occur faster
how do catalysts work
lower activation energy of reaction (for forward and reverse) not changing free energy
- change forward and reverse reaction rate by same factor
interact with reactants by absorption or through formation of intermediates and stabilize them to reduce Ea necessary for reaction to proceed
return to original state upon formation of products
- increase frequency of collisions between reactants
- change reactant orientation for higher percentage of effective collisions
- donate electron density to reactants
- reduce intramolecular bonding within reactants
homogenous vs heterogenous catalysts
In homogeneous catalysis, the catalyst is in the same phase (solid, liquid, gas) as the reactants. In heterogeneous catalysis, the catalyst is in a distinct phase.
do catalysts change the forward and reverse reaction rate by same factor?
True
do catalysts have impact on equilibrium position or measurement of K eq?
can catalysts transform non spontaneous into spontaneous reactions?
Nope, no impact on equilibrium
Nope, they can only make spontaneous reactions move more quickly tpwards equilibrium
What is gibbs free energy for spontaneous and non spontaneous
A negative value for ΔG indicates a spontaneous process; a positive ΔG indicates a nonspontaneous process; and a ΔG of zero indicates that the system is at equilibrium
spontaneous vs non spontaneous vs endergonic vs exergonic
Exergonic reactions are also called spontaneous reactions, because they can occur without the addition of energy. Reactions with a positive ∆G (∆G > 0), on the other hand, require an input of energy and are called endergonic reactions.
is endothermic/exothermic same as endergonic/exergonic
Endergonic and exergonic refer to free energy changes (delta G). Endothermic and exothermic refer to the changes in internal energy of molecules, measured as heat given off or taken up, delta H.
(enthalpy)
In summary, endothermic and exothermic refer specifically to heat transfer, while endergonic and exergonic describe the overall energy changes in a reaction.
what is the unit for rate of reaction?
what is the equation of reaction rate?
for almost all forward irreversible reactions –> the rate is proportional to the concentration of reactants with each raised to a experimental exponent. How do i write this?
k is the prop constant or the reaction rate coefficient or rate constant.
x and y are the orders of the reaction
overall order addes sum of exponents. Each individual order is the order of reaction with respect to a particular reactant
law of mass action vs rate law
Rate law is an experimental law i.e. it is based on observations. Rate laws provide a mathematical description of how changes in the amount of a substance affect the rate of a chemical reaction. USES ONLY REACTANTS
law of mass action is a theoretical law based on the balanced chemical reaction. its the proposition that the rate of the chemical reaction is directly proportional to the product of the activities or concentrations of the reactants. USES REACTANTS AND PRODUCTs
Find the rate law: identify trials where one variable is constant, other changes and look at the reaction rate.
Use reaction rate and concentration to solve for order of reactant
combine for overall equation
How do you find the value of k once you have the rate law?
what is a zero order reaction?
A zero-order reaction is one in which the rate of formation of product C (aA + bB → cC + dD ) is independent of changes in concentrations of any of the reactants, A and B. These reactions have a constant reaction rate equal to the rate constant (rate coefficient), k. The rate law for a zero-order reaction is:
can you change a zero order reaction with increased temperature?
Remember that the rate constant itself is dependent on temperature; thus, it is possible to change the rate for a zero-order reaction by changing the temperature. The only other way to change the rate of a zero-order reaction is by the addition of a catalyst, which lowers the activation energy, thereby increasing the value of k.
Temperature and the addition of a catalyst are the only factors that can change the rate of a zero-order reaction.
What does this show?
Plotting a zero-order reaction on a concentration vs. time curve results in a linear graph, as shown in Figure 5.8. This line shows that the rate of formation of product is independent of the concentration of reactant. The slope of such a line is the opposite of the rate constant, k.
what is a first order reaction?
A first-order reaction has a rate that is directly proportional to only one reactant, such that doubling the concentration of that reactant results in a doubling of the rate of formation of the product. The rate law for a first-order reaction is
example of first order reaction
What does this show?
Plotting a first-order reaction on a concentration vs. time curve results in a nonlinear graph, as shown in Figure 5.9. This curve shows that the rate of formation of product is dependent on the concentration of reactant. Plotting ln [A] vs. time reveals a straight line; the slope of such a line is the opposite of the rate constant, k.
On the left, note that the rate of reaction is dependent on reactant concentration; on the right, note that the rate constant is the opposite of the slope of a graph of ln [A] vs. time.
what is second order reaction?
A second-order reaction has a rate that is proportional to either the concentrations of two reactants or to the square of the concentration of a single reactant. The following rate laws all reflect second-order reactions:
what does this show?
On the left, note that the rate of reaction is dependent on reactant concentration; on the right, note that the rate constant is equal to the slope of a graph of 1/[A]
vs. time.
how does mixed order describe reaction rate over time?
A represents a single reactant and C, a catalyst.
he result of the large value for [A] at the beginning of the reaction is that k3[A] ≫ k2, and the reaction will appear to be first-order with respect to A. (Note: The symbol “≫” means “much-greater-than”, implying, in this context, that the contribution of k2 to the denominator “k2 + [A]k3” is negligible.) At the end of the reaction, when [A] is low, k2 ≫k3[A], making the reaction appear second-order with respect to A. While the MCAT will not ask you to derive a rate expression for a mixed-order reaction, you are responsible for being able to recognize how the rate order changes as the reactant concentration changes.
fill in table
solve for rate law
Start by writing the generic rate law for the reaction: rate = k[A]x[B]y[C]z.
In a complex rate law problem, always check for the possibility of a reagent that has no impact on the rate law. Looking at Trials 1 and 2, the concentration of B is doubled with no change in the rate. Thus, reagent B has no impact on the rate law, and its exponent is zero. The rate law can be updated to rate = k[A]x[B]0[C]z.
we can still figure out the rate of A even when B is also changing since we know B is second order so we that the reaction rate is *4 (1.4 *4 = 5.6)
doubling A causes the reaction rate to double from 5.6 to 11.2 –> this means it must be first order for A
reaction rate is written with the net reaction ignoring intermediates and using reactant side
net equation: Cl2 + CO –> Cl2CO
rate law depends only on chloride and monoxide. D is only choice
exergonic//endergonic or spont and non spont
according to the collision kinetics and the arrhenius equation, which of the following will NOT necessarily increase reaction rate?
catalyst can lower energy of transition states but
NOT free energy of reactant or intermediates or products
