Chem 2 exam A Reaction Rates & Rate Laws (Sects. 12.1-12.3)

Define Rate of reaction

change in the amount of a reactant or product per unit time

Reaction rates are, by

convention, positive quantities.

Initial rate?

speed at t = 0 s

Instantaneous rate?

speed at any instant in time speedometer reading!

Average rate

speed over a defined period of time

Generalized relationship between rate expressions

aA —→ bB rate =

-(1/a)(deltaA/delta t) = (1/b)(delta B/ delta t)

Define Rate Law

mathematical expression describing the relationship between the rate of chemical reaction and the reactant concentrations

k = rate constant

(specific to each reaction at a specific temperature)

x and y = reaction orders

usually positive integers but don’t have to be; note that they are NOT the reactant coefficients from the balanced equation)

[A] and [B] =

reactant concentrations

What does a rate law equation look like

rate = k[A]x[B]y

Overall reaction order=

sum of individual reactant reaction orders

Rate = k[H2O2]

(rate law)

*Remember: The rate law is experimentally determined

(you can’t tell from the reaction coefficients!)

if we want to know the concentration of a given reactant after a certain amount of time?

Integrated rate laws!

Define Reaction Half-Life (t1/2)

time required for one-half of a given amount of reactant to be consumed or one-half of a given amount of reactant remains

What is an integrated rate law? A way to calculate reactant concentrations

after a certain amount of time

What is an integrated rate law? A way to calculate the time it takes for a

certain amount of reactant to disappear or when a certain amount will remain

What is an integrated rate law? A way to calculate

the rate constant

The rate law for the reaction A + B C + D is first order in [A] and second order in [B]. If [A] is halved and [B] is doubled, the rate of the reaction will

increase by a factor of 2.

For the reaction A + 3B —>2C, how does the rate of disappearance of B compare to the rate of production of C?

The rate of disappearance of B is 3/2 the rate of appearance of C.

For the reaction 2A + 3B —> 4C + 5D, the rate of the reaction in terms of delta C would be written as...

+1/4 deltaC/delta t

Consider the following rate law: rate = k[A]^1/2[B]. The order with respect to A is

1/2

Consider the following rate law = k[A]^1/2[B]. he order with respect to B is

1

Consider the following rate law: rate = k[A]^1/2[B]. the overall reaction order is

3/2

Equilibrium is

dynamic

Equilibrium = when the

rates of the forward and reverse reactions are equal (the concentrations of reactants and products remain constant over time)

reversible reaction =

can proceed in both the forward and reverse directions (note the special “double arrow”)

rate of vaporization =

rate of condensation

Law of mass action =

the rate of a chemical reaction is directly proportional to the product of the active masses (molar concentrations) of the reactants raised to their stoichiometric coefficients at a given temperature

Solids and liquids are excluded from such expressions only 𝑎𝐴 + 𝑏𝐵 ⇌ 𝑐𝐶 + 𝑑𝐷

𝑟𝑎𝑡𝑒𝑓𝑜𝑟 = 𝑘𝑓 𝐴 𝑎 𝐵 𝑏

𝑟𝑎𝑡𝑒𝑟𝑒𝑣 = 𝑘𝑟 𝐶 𝑐 𝐷 𝑑

only aqueous and gaseous species are included.

Reaction quotient (Q) =

expression describing the status of a reversible reaction; based on the concentrations of products and reactants raised to their stoichiometric coefficients

Equilibrium constant (K) = constant value of Q for

a system at equilibrium

what’s the point in calculating the reaction quotient (Q)? Q indicates

in which direction a reaction will proceed.

What does the equilibrium constant (K) tell us? K indicates

whether products or reactants are favored at equilibrium. (In other words: how far will the reaction proceed?)

Remember, at a given temperature and volume, pressure is directly proportional to the

# of moles of gas (according to PV = nRT).

Equilibrium constants are

dimensionless/unitless

Chatelier’s principle =

if an equilibrium system is stressed, the system will experience a shift in response to the stress that re establishes equilibrium

Examples of stressors and how they alter the equilibrium: Changes in concentrations of reactants and/or products example H2 + I2 ←> 2HI

What will happen if I increase the concentration of H2? Why

Rxn proceeds to the right (goes in the forward direction) to produce more products… until Qc = Kc (equilibrium reestablished) Qc < Kc

Examples of stressors and how they alter the equilibrium 2. Changes in pressures/volumes. Ex: H2+I2←>2HI What will happen if I increase the pressure (by decreasing the volume)? Why?

Concentrations (or partial pressures) of the gases increase. Rxn proceeds to the left (goes in the reverse direction) to produce more reactants… until Qc = Kc (equilibrium reestablished

General Rule: If the total number of moles of gas is different between reactants and products

changes in pressure will shift the equilibrium.

Examples of stressors and how they alter the equilibrium 3. Temperature

General Rule: Treat heat like a reactant or product!

the reaction is exothermic

(ΔH < 0)

the reaction is endothermic

(ΔH > 0))

catalysts effect on equilibria: Both the forward and reverse reactions

are accelerated

catalysts effect on equilibria: Lower Ea results in a

greater kf and kr (remember the Arrhenius equation?

catalysts effect on equilibria: ΔH, reactant and product

identities and concentrations remain unchanged

Catalysts do not affect the equilibrium position. They only change

the rate at which a system reaches equilibrium.)

Describe the ways an equilibrium can be stressed and how the system will respond to that stress (using Le Chatelier’s principle)

When in doubt, determine the effect of the stress on Q (makes it >, <, or = K?)

LeChatelier Principle photo