Gen Chem Exam 2 Chapters 15-16

what is the sign on the reactants in a chemical reaction?

negative

what is the sign on the products in a chemical reaction?

positive

rate

a measure of how fast something occurs

instantaneous rate

the rate at a particular point in time

how do you determine the rate of a reaction?

you use experimental data; you can’t tell what the rate will be from the balanced equation

rate law expression

k[A]^n

k[A]^n: what is k?

the rate law constant (temperature dependent)

k[A]ⁿ: what is n?

the “order” with respect to A; n determines how much the rate depends on the concentration of A

zero order: what is n and the rate law expression?

n=0; rate=k[A]⁰=k

first order: what is n and the rate law expression?

n=1; rate=k[A]¹=k[A]

second order: what is n and the rate law expression?

n=2; rate=k[A]²

zero, first, or second order: the rate is independent of the concentration of A; the rate doesn’t change if you change the concentration of A

zero order

zero, first, or second order: the rate is directly proportional to [A]; if you double [A], the rate also doubles

first order

zero, first, or second order: the rate is proportional to [A]²; if you double [A], the rate will quadruple

second order

what are the units of k in a first order reaction?

/time

what are the units of k in a zero order reaction?

M/s

what are the units of k in a second order reaction?

/M*s

what is the order?

the relationship between the rate and concentration

rate=k[A]^m[B]ⁿ: what is m?

the order with respect to A

rate=k[A]^m[B]ⁿ: what is n?

the order with respect to B

how is overall order calculated?

m + n = overall order

what relationship does the integrated rate law show?

the relationship between the concentration of reactants and time

first order integrated rate law

ln[A]_t=-kt+ln[A]₀

second order integrated rate law

1/[A]_t=kt+1/[A]₀

zero order integrated rate law

[A]_t=-kt+[A]₀

half-life

the time required for the concentration of a reactant to fall to ½ its initial concentration

what does half-life depend on in a first order reaction?

the value of k

first order reaction half-life

t_1/2=0.693/k

second order reaction half-life

t_1/2=1/k[A]₀; as [A]₀ decreases, the half-life gets longer

zero order reaction half-life

t_1/2=[A]₀/2k; as [A]₀ decreases, the half-life gets shorter

k=Ae^-Ea/RT: what is k?

the rate constant

k=Ae^-Ea/RT: what is A?

the frequency factor

k=Ae^-Ea/RT: what is e?

the exponential factor

k=Ae^-Ea/RT: what is Ea?

the activation energy

k=Ae^-Ea/RT: what is R?

8.314 J/mol * K

k=Ae^-Ea/RT: what is T?

the temperature (K)

what is the peak called in an activation energy graph?

the transition state or activated complex

the _____ the Ea, the _____ the reaction rate

larger, slower

activation energy

the energy required to have reactants form products

frequency factor

the number of times the reactants approach the activation barrier per unit time

exponential factor

the fraction of molecules that have enough energy to overcome the activation barrier and form products; depends on the Ea and temperature

what will happen to the rate when you cool the reaction?

it will slow down

the warmer the reaction, the _____ the k value

larger

reaction mechanism

a set of individual steps by which an overall chemical reaction occurs

elementary step

each individual step of a reaction mechanism

reaction intermediates

formed in one step and consumed in another; won’t appear in the overall reaction

unimolecular

rate=k[A]

bimolecular

rate=k[A]² or rate=k[A][B]

termolecular

rate=k[A]³ or rate=k[A]²[B] or rate=k[A][B]² or rate=k[A][B][C]

rate-determining step (rds)

the slowest elementary step in the reaction mechanism; determines the rate of the reaction

what two conditions must be met to prove if a proposed mechanism is valid?

1. the elementary steps must add up to give the overall reaction

2. the rate law must be consistent with experimental data

what are the three ways to speed up a reaction?

increasing the temperature, the [reactants], and adding a catalyst

catalyst

a substance that increases the reaction rate but is not consumed in the reaction

homogeneous catalyst

in the same phase as the reactants

heterogeneous catalyst

in a different phase than the reactants

equilibrium

the rate of the forward reaction is equal to the rate of the reverse reaction

equilibrium constant (K)

quantifies the concentrations of reactants and products at equilibrium

what is different between the kinetics k and the equilibrium K?

the equilibrium K can be determined using the balanced equation

what does a large K value indicate?

there are more products than reactants present at equilibrium

what does a small K value indicate?

there are more reactants than products present at equilibrium

what does a K value of 1 indicate?

the amount of reactants and products is equal at equilibrium

what would the equilibrium expression for the opposite reaction be?

1/K_n

what would the equilibrium expression be if you multiplied the coefficients in the equation by some factor m?

K_n^m

what would the equilibrium expression be if you add two chemical equations together?

K_l=(K_m)(K_n)

K_p

the equilibrium expression for the partial pressures of gases

K_c

the equilibrium expression for the concentrations of gases and aqueous solutions

K_p=K_c(RT)^Δn; what is Δn?

the moles of gas product - the moles of gas reactants

what do equilibrium expressions not include?

solids, liquids, and other pure substances

what is K dependent on?

temperature

three important concepts for K:

1. the value of K remains constant unless you change the temperature

2. the value of K doesn’t depend on initial concentrations

3. the equilibrium concentrations of products and reactants depend on the initial concentration

what does the acronym ICE stand for?

initial, change, and equilibrium

K=

[products]/[reactants]

Q=

[products]/[reactants]

what is the difference between Q and K?

K is at equilibrium while Q is not

Q=K

equilibrium

Q>K

there are more products present than there would be at equilibrium, so the reaction will shift towards the reactants

Q<K

there are more reactants present than there would be at equilibrium, so the reaction will shift towards the products

what can Q be used to predict?

the direction a reaction will go to reach equilibrium