Gen chem 5

Question 1

whats another name for the slow step of a rxn, and what does it prevent

Answer 1

its called the rate det’ing step and stops the overall rxn from going any faster than that slow step

Question 2

rate of a rxn equation

Answer 2

rate = z x f z is total number of collisions per second f is fraction of effective collisions

Question 3

what theory does the Arrhenius equation explain? what is the eqn and variables?

what is the relationship between A and K(what happens to k as the frequency factor increases?

what happens to the exponent as T goes up to infinity and all other variables are constant? what happens to k and why does what happens to it make sense

Answer 3

collision theory

as A goes up, k goes up(direct relationship)

as T goes up to infinity, with all other values constant, the value of the exponent is less than 1; as T goes up, k goes up because as you increase the rate of a rxn the T goes up too!

Question 4

what does a low Ea and high T do the negative exponent of the Arrhenius equation, and what does this do to k?

Answer 4

it makes the negative exponent become smaller(less negative) and k goes up

Question 5

what forms when molecules collide with energy greater than or equal to the activation energy? also what happens to the old and new bonds?

Answer 5

a transition state forms And old bonds are weakened, and new bonds form

Question 6

what is another name for transition state

what kind of rxn is indicated by a pos and neg Grxn

how do you find the: free energy change of the reaction, the activation energy of the forward reaction, and of the reverse reaction?

Answer 6

another name is activated complex

neg is exergonic(energy is given off)

pos is endergonic(energy s absorbed)

free energy change of the reaction is the difference between the free energy of the products and the free energy of the reactants

Ea of the forward reaction can be found by difference in energy between the transition state and the reactants; Ea of the reverse reaction can be found by the difference in energy between the transition state and the products

Question 7

what do transition states have more of than the products and reactants?

when the free energy of the products is less than the free energy of the reactants, what kind of rxn is it (exo or endo)

Answer 7

they have more energy than the prods and reacs

when G of prods is less than the G of the reactants the rxn is exergonic(energy is released and the G of the reaction is negative)

Question 8

when the conc of the reactants is Great, what happens to the number of effective collisions, and what change does this cause in A(frequency factor) and k?

Answer 8

this number of effective collisions will go up too

A will go up and k will go up too(k wont go up for zero order reactions)

Question 9

what do all reactions depend on

Answer 9

they all depend on T(even nuclear reactions)

Question 10

why do reactions love polar solvents(hint: what does it to to the bonds of the reactants, which causes what two things that cause the reaction to go faster)?

what is homogeneous and heterogeneous catalysts?

can a catalyst turn a nonspon rxn into a spon?

Answer 10

they love polar solvents because their molec dipole polarizes the bonds of the reactants, lengthening and weakening them, to let the rxn go faster

homo catalysts are in the same phase (solid, liq, gas) as the reactants; hetero catalysts are in a diff phase from the reactants

no, catalysts AINT miracle workers!!!

Question 11

what sign does the rate of reactants part of rxn have and what is the formula for rate expression for a reactant in this reaction?

(2A + B –> C)

what units is rate expressed in?

on mcat, how to start figuring out the correct rate law?

Answer 11

it is negative becuz the reactants are being consumed; -A/t

it is expressed in units of moles per liter per second (mol/L . s)

gotta look for experimental data

Question 12

what is the general formula for the rate law?

on the mcat, concerning rate, what can we say about the exponents x and y and the stoichiometric coefficients? Give two instances where the reverse of this is the case!!

Answer 12

rate= k[A]^x[B]^y

on the mcat, the x and y exponents and the stoichiometric coefficients will ALMOST NEVER be the same.

they will be the same when 1: the rxn mech is a single step and the balanced overall reaction is reflective of the entire chemical process AND 2: the complete rxn mech is given and the rate det’ing step is indicated!

Question 13

STATE THE RELATIONSHIP BETWEEN THE STOICHIOMETRIC COEFFICIENTS ON THE REACTANTS SIDE OF THE RATE DET’ING STEP AND THE ORDERS OF THE REACTION

Answer 13

THEY ARE EQUAL!!!

Question 14

when dealing with rate law and the rate det’ing step includes an intermediate as a reactant, what should you do to avoid the trap??

whats another trap to avoid concerning the equilibrium constant expression and the rate law(and how to tell the difference between the two?)

Answer 14

gotta figure out the intermediate’s conc by the law of mass action OR the equilibrium constant expression for the step that produced the intermediate

confusing the one for the other(becuz they look similar) >> you can tell the difference between the two becuz the equilibrium expression includes the concs of BOTH the reactants and the products; the expression for chemical kinetics- the rate law expression - only includes the reactants

Question 15

what two things does the rate constant depend on

for a reversible reaction, what does Keq equal?

usually, when is the reaction rate measured ?

Answer 15

temp and activation energy for that rxn

for a reversible rxn, Keq equals the ratio of the rate constant for the forward rxn, k, divided by the rate constant for the reverse reaction, k-1

reaction rate is measured @ or near the beginning of the reaction to minimize the effects of the reverse reaction

Question 16

how to determine rate order on the MCAT from a chart of initial concs of reactants and initial rates of product formation

how to figure out k

Answer 16

1st look for a pair of trials that has the conc of one reactant changed while the others stayed the same

if you have this then if there is a change in the product formation from one trial to the other, its because of that one reactant conc that changed

do the same steps for the other reactant, using another pair of data(make sure only one of the reactant concs has changed from one to the other, while the others stay the same)

once we got the orders, then we write the complete rate law

figure out k by plugging in actual values from any one of the trials

Question 17

consider this example, you have two reactants, A and B, that make product C. Lets say the conc of A is constant, but B’s conc doubles. Now lets say that the conc of product C quadruples. what will the exponent be on the order of B and why?

Answer 17

it will be 2; set your rate law generic equation up first
rate=k[A]^x[B]^y

so doubling B caused the quadrupling of the rate(since A’s conc is constant); so to determine the reaction, y with respect to B, i gotta find the exponent that the number 2 must be raised to to equal 4, that’s why we get 2^2 = 4 y=2

Question 19

consider our generic reaction

aA + bB –> cC + dD

what does zero order mean? describe the reaction rate for a zero order reaction

whats the rate law for a zero order reaction?

Answer 19

the rate of formation of products doesn’t depend on ay of the concs of the reactants

these reactions have a constant reaction rate equal to the rate constant k

rate= k[A]^0[B]^0= k

Question 20

what does the rate constant depend on

how do you change the rate for a zero order reaction(2 ways)

Answer 20

it depends on temperature

you can change the rate for a zero order reaction by changing the temp

you can also change it by adding a catalyst which lowers the activation energy and INCREASES K

Question 21

what kind of graph does a zero order reaction have

on this graph, how does the rate of reaction relate to the slope

Answer 21

linear graph

rate of reaction, k, is opposite to the slope

Question 22

what is a first order reaction( in terms of rate)

Answer 22

the rate is directly related to only one reactant and doubling the conc of that reactant doubles the rate of formation of the product

rate= k[A]^1 or k[B]^1

Question 23

in the example for a first order reaction(radioactive decay), what can we say about the amount of A and the decrease of the amount of a radioactive isotope A

Answer 23

its proportional to the amount of A

Question 24

the rate of decrease of the amount of radioactive isotope A is proportional to the amount of A

Question 25

what is suggested about a first order rate law with a single reactant ?

Answer 25

it suggests that the reaction begins when the molecule goes thru a chemical change all by itself, without chemical interaction and without a physical interaction from another molecule

Question 26

For a first order reaction, what does a conc vs time graph look like(and what does it show)? natural log vs time graph look like(and what is the correlation between the slope and k?

Answer 26

the conc vs time graph looks like a curve results in a nonlinear graph, which shows that the rate of formation of product is dependent on the conc of reactant

plotting natural log vs time shows a linear graph; and the slope of this line is opposite of the rate constant, k

Question 27

describe what two things the rate depends on

what 3 ways can the rate law for a second order reaction be rep’d?

what does a second order rate law often suggest about two reactant molecules, especially under what special condition?

Answer 27

depends on the concs of two reactants or the square of the conc of a single reactant

rate= k[A]^1[B]^1 or rate= k[A]^2 or rate= k[B]^2

it often suggests a physical collision especially if the rate law is first order with respect to both reactants

Question 28

what kind of graph do you get when you plot a conc vs time for a second order reaction with respect to a single reactant? what happens when you plot 1/conc of A vs time?

Answer 28

when you plot a conc vs time for a second order reaction with respect to a single reactant, you get a nonlinear (curve) graph

when you plot 1/conc of A vs time, you get a linear curve and this curve is equal to the rate constant k

Question 29

what two things does mixed order refer to

what does broken order refer to

what does a mixed order rate law look like(what does A and C represent?

Answer 29

it refers to non-integer orders (fractions) and in other cases to reactions with rate orders that change over the course over the reaction

broken order refers to a fraction

rate= k1[C][A]^2/k2 + k3[A]; A reps a single reactant and C reps a catalyst

Question 30

for a mixed order reaction, what happens at the beginning of the reaction and why does the reaction appear first order?

for a mixed order reaction, what happens at the end of the reaction and why does the reaction appear second order?

Answer 30

at the beginning of the reaction, [A] has a large value because k3[A] >> k2, and the reaction appears first order with respect to A

[A] is low and k2 >> k3[A], and the reaction looks second order with respect to A.