Chem module 1
Consider the overall reaction AB + C →AC + B
1. reactant AB separates into A and B AB---A + B 2. A combines with C to form the product AC A + C --- AC each step has its own transitions state and can be described using a reaction energy diagram
Rate law
A mathematical equation that summarizes the dependence of reaction rate on concentration
Rate constant, k
A proportionality constant that relates rate and concentration of reaction species reaction and temperature dependent
Which statement explains why addition of H2O shifts the reaction toward the products side (right, as written). C(s) + H2O(g) ⇌ CO(g) + H2(g) Addition of H2O makes Q < K, and the rate of the reverse reaction must increase to get back to equilibrium. Addition of H2O makes Q > K, and the rate of the reverse reaction must increase to get back to equilibrium. Addition of H2O makes Q > K, and the rate of the forward reaction must increase to get back to equilibrium. Addition of H2O makes Q < K, and the rate of the forward reaction must increase to get back to equilibrium.
Addition of H2O makes Q < K, and the rate of the forward reaction must increase to get back to equilibrium.
The rate of a particular reaction, A B, decreases as the reaction proceeds. The half-→life of the reaction is dependent on the initial concentration of A. Which statement is likely to be true for this reaction? A) Tripling the concentration of A triples the rate of reaction B) A plot of 1/[A] versus time is linear. C) Each subsequent half-life is shorter than the previous one. D) The reaction could be zero order or second order. E) A plot of reaction rate versus time for the reaction will NOT be linear.
B) A plot of 1/[A] versus time is linear.
Consider the reaction below and the possible "stresses" that could be applied. Which of these stresses cause the value of K to decrease?PCl3(g) + Cl2(g) ⇌ PCl5(g) H = - 88 kJ/mol I.Increase the temperature II.Add PCl5 III.Remove Cl2 IV.Add PCl3 A) I, II, and II B) IV only C) I only D) I and IV E) II and III
C) I only
Which statement or statements explain why collision rate is greater than reaction rate for a given chemical reaction? I.Most collisions occur with an energy that is less than energy required to begin breaking bonds in reactants. II.Collisions don't occur that frequently because there are no attractions between molecules of a gas. III.Some collisions occur with orientations that are not conducive to product formation. A) I only B) II only C) Both II and III D) Both I and III E) All of I, II, and III
D) Both I and III
Which change(s) will shift this exothermic reaction to the right? Select all that apply. X(g) + 2 Y(g) ⇌ XY2(g) Adding a catalyst Adding argon gas while holding the container volume constant Decreasing the partial pressure of Y Decreasing the partial pressure of XY2 Heating the container
Decreasing the partial pressure of XY2
heterogeneous equilibria
Equilibria that include species in different phases equilibrium constant expressions do not include an solids or liquids
6CO2 (g) + 6H2O (l) -> C6H12O6 (s) + 6O2 (g) H= +2801.69 kJ/mol Which of the following would cause shift towards products I: increasing concentration of CO2 II: decreasing temperature of the sustem III: adding a catalyst IV: removing some of C6H12O6 from the system
I
The frequency factor, A, includes consideration for which of the following? Select all that apply. I. The fraction of collisions having molecular orientation that leads to the formation of desired products II. The fraction of collisions that occur with Ecollision > Ea III. How often collisions occur
I. The fraction of collisions having molecular orientation that leads to the formation of desired products III. How often collisions occur
Which answer option is a reason that a collision between molecules does NOT result in a chemical reaction? Select all that apply. I. The collision occurs with too much kinetic energy. II. The collision occurs with a molecular orientation that does not facilitation formation of the bonds in the desired products. III. The collision doesn't occur at the right pressure.
II. The collision occurs with a molecular orientation that does not facilitation formation of the bonds in the desired products.
LeChatelier's Principle
If a stress is applied to a system at equilibrium, the reaction will tend to shift in a direction to relieve the stress when a system is at equilibrium and conditions that affect equilibrium positions are changed, rates of forward and reverse reactions may be affected differently
The mechanism for the reaction of nitrogen dioxide with carbon monoxide to form nitric oxide and carbon dioxide is thought to be: NO2 + NO2 → NO3 + NO (???) NO3 + CO → NO2 + CO2 (???) It has not yet been determined which step is fast and which is slow. Which experimental design would permit you to determine the relative speeds of the steps?
Increase the concentration of CO Decrease the concentration of CO Decrease the concentration of NO Increase the concentration of NO
Which of these will cause the reaction to shift toward the products (to the right, as written)? Select all that apply. C3H8(g) + 5 O2(g) ⟷ 3 CO2(g) + 4 H2O(l) ΔHo = -2220 kJ Increase the partial pressure of O2 Decrease the partial pressure of CO2 Decreasing reaction volume Addition of Ar (an inert gas). Decrease the partial pressure of C3H8
Increase the partial pressure of O2 Decrease the partial pressure of CO2 Decreasing reaction volume
Which answer option best explains why increasing temperature tends to increase rate of reaction? Increasing temperature decreases the fraction of molecular collisions that occur with kinetic energy greater than the energy of the transition state. Increasing temperature increases the fraction of molecular collisions that occur with kinetic energy less than the energy of the transition state. Increasing temperature increases the fraction of molecular collisions that occur with kinetic energy greater than the energy of the transition state. Increasing temperature decreases the number of molecular collisions that occur with proper orientation for reaction. Increasing temperature increases the number of molecular collisions that occur with proper orientation for reaction.
Increasing temperature increases the fraction of molecular collisions that occur with kinetic energy greater than the energy of the transition state.
Expansion
Increasing volume = decreasing pressure
Half-life for first-order reaction
Integrated rate law: ln(At/A0) = -kt t1/2= ln2/k constant and independent of the initial concentration of the reactant inversely proportional to k the slower the reaction, the longer the half life
What happens when the slow step is not first
It still determines the overall rate of reaction, but often the condition results in the introduction of an intermediate to the rate law for the slow step intermediates can't appear in the rate law for the overall reaction intermediate formed is unstable and doesn't accumulate to any significant extent once formed, intermediate can be consumed as a reactant for the 2nd reaction or the first reaction can go in reverse
Units for a zero order reaction
M s^-1
Units for second order reaction
M^-1 s^-1
Consider the Arrhenius equation: Which statements are true? Select all that apply. Rate constant increases as temperature increases Rate constant increases as activation energy increases. Rate constant decreases as activation energy increases. Rate constant decreases as temperature increases
Rate constant increases as temperature increases Rate constant decreases as activation energy increases.
Compression
Reducing volume = increasing pressure
orders of reaction
Represented by m and n
Changes in temperature change the actual value of K
T
Only a certain percentage of the collisions will be in the correct orientation for bond-breaking/bond-making to occur
T
The reaction quotient has the exact same form as the equilibrium constant but the concentrations or pressures used for Q do NOT have to be equilibrium concentrations or pressures
T
all equilibrium constants are reaction quotients but not all reaction quotients are equilibrium constants
T
Equilibrium constant expressions do not include any solids or pure liquids.
T only aqueous and gaseous species have concentrations that change are are therefore included
Reaction quotient (Q)
The ratio of the product and reactant concentrations at any point during the chemical reaction
Equilibrium constant
The ratio, at equilibrium, of the concentrations of the products raised to their stoichiometric coefficients divided by the concentrations of the reactants raised to their stoichiometric coefficients
Chemical equilibrium
The stage in a chemical reaction when there is no further tendency for the composition of the reaction mixture to change
What effect will increasing the temperature have on the system at equilibrium? C3H8(g) + 5 O2(g) ⟷⟷ 3 CO2(g) + 4 H2O(l) ΔΔHo = -2220 kJ The value of K will decrease, and the reaction will shift toward the products. The value of K will decrease, and the reaction will shift toward the reactants. The value of K will increase, and the reaction will shift toward the reactants. The value of K will increase, and the reaction will shift toward the products.
The value of K will decrease, and the reaction will shift toward the reactants.
K can also be expressed in terms of pressure
a A(g) + b B(g) ⇌c C(g) + d D(g) Kp= [(Pc)^c (Pd)^d]/[(Pa)^a (Pb)^b)]
reaction mechanism
a collection of one or more molecular steps that account for the way reactants become products series of single steps by which reactants are converted to products
kinetic energy of molecules is proportional to the ____
absolute temperature
difference in energy between the reactants and the activated
activation energy
The presence of a catalyst does not affect Q or K.
catalyst lowers AE of both forward and reverse reactions by THE SAME AMOUNT equilibrium is established more quickly in the presence of a catalyst
Factors that affect reaction rates
concentration of reactants temperature nature of reactants presence of a catalyst particle size of solid reactants
The following K values were attained at 25 C. Hb (aq) + O2 (aq) --- Hb(O2) (aq) K= 1.5 x 10^-4 Hb(O2) (aq) + O2 (aq) --- Hb(O2) (aq) K= 3.5 x 10^-4 Hb(O2)2 (aq) + O2 (aq) --- Hb(O2)3 (aq) K= 5.9 x 10^-4 Hb(O2)3 (aq) + O2 (aq) --- Hb(O2)4 (aq) K= 1.5 x 10^-5 Each hemoglobin can bind to up to 4 O2 at a time. When a person travels to sea level to a higher altitude, which would immediately occur? a) reaction will shift right because Q<K b) reaction will shift right because no products have formed c) reaction will shift left because Q<K d) reaction will shift left because Q>K e) reaction continues in both direction because its already in equilibrium
d) reaction will shift left because Q>K
as activation energy increases, the percentage of molecules that have or exceed this energy ____
decreases
The rate of the generic reaction A →B can be expressed as the rate at which [A]_____ or as the rate at which [B] ______.
decreases increases
Collision theory
describes molecular motion that helps us explain rates of chemical reactions
Arrhenius equation
describes the relationship among molecular orientation, kinetic energy, and rate of reaction
There are two useful forms of the rate law
differential rate law integrated rate law
activation energy
energy threshold a reaction must overcome to begin
large K values
equilibrium favors products
small K values
equilibrium favors reactants
method of initial rates
experimental method used to determine order of reaction Start with a base set of concentrations of reactants and determine rate Change the concentration of only one reactant at a timeto isolate its impact on rate
If plot lnA vs time is linear, reaction is ______
first order
steric factor
fraction of collisions having proper orientation for the conversion of reactants to products the larger and more complex the molecular in a reaction are, the smaller the value of p
frequency factor(A)is related to
how often collisions occur (the rate of collisions can be increased by increasing the contraption and temperature) the probability that the collision will occur with the reactants in the appropriate orientation
Kp rules
if the number of moles of a gas is same on both side of an equation, K and Kp are numerically equal
The _____________ by the addition of an inert gas does NOT affect the position of equilibrium
increase in pressure
second order half life
inversely proportional to both initial concentration
zero order half life
inversely proportional to rate constant directly proportional to initial concentration
activation energy properties
magnitude of activation energy is inversely related to magnitude of constant as AE increases, rate constant decreases, reaction progresses more slowly
collision theory principles
molecules must collide to react collision only result in reactions if the occur with: a molecular orientation that facilitates foundation of a desired product sufficient kinetic energy to overawe the activation energy
intermediate K values
neither products or reactants are favored
2NOCl (g) ---- 2NO (g) + 2CL2 (g) The system is at equilibrium at a given temperature when PNOCl= .33 atm, PNO= .064 atm, PCl2 = .5 atm. An additional .2 atm of NOCl (g) is added to 1 L system at the same temperature. Which is true? a) rate of reverse is more than the rate of forward b) amount of NO (g) will decrease compared to initial amount because Q>K c) Once equilibrium is established, PNOCl= .53 atm d) reaction wouldn't shift 1 way or another. Reaction proceeds with rate of forward= rate of reverse e) none
none
increasing temperature for endothermic reactions shifts reaction to
products heat + reactants ---- products
Collision rate
proportional to reactant concentration
Catalysts
provide an alternative pathway for the reaction to occur, one with a lower activation energy, which speeds up the reaction follow a different set of elementary steps NOT consumed by reactions
integrated rate law
quantifies the relationship between concentration and time
zero order reactions
rate =k[A]0=k
second order reactions
rate =k[A]2 change in concentration is squared to determine rate if you double concentration, rate increases by 4
all chemical equilibria are dynamic
rate of forward reaction = rate of reverse reaction neither reaction stops at equilibrium. Both continue with no net change in concentration of products or reactants
First-order reactions
rate: k[A] rate and concentration are linearly related concentration results in exact same magnitude of change of rate if we double concentration of A, rate will double
increasing temperature for exothermic reactions shifts reaction to
reactants reactants ---- products + heat
Why does the increase in pressure by the addition of an inert gas does NOT affect the position of equilibrium.
reacting gases continue to occupy the same V so their individual mol concentrations and partial pressure don't change when a gas is ideal, equilibrium composition is unaffected despite the change in total pressure
Q=K
reaction is at equilibrium
Q<K
reaction must proceed to products to get back to equilibrium
Q>K
reaction must proceed to reactants to get back to equilibrium
Which is false? reaction rates increase as the temperature goes up reaction rates for reactions involving solids tend to decrease as the particle size increases as there is less surface area per unit mass reaction rates are never changed by pressure catalysts are not consumed during a chemical reaction
reaction rates for reactions involving solids tend to decrease as the particle size increases as there is less surface area per unit mass
equilibrium constant rules
regardless of initial concentration, at equilibrium, the rate of concentration raised to appropriate powers will always equal the same value K
Units for a first order reaction
s^-1
If plot 1/A vs time is linear, reaction is ______
second order
the rate of the overall reaction is limited by the rate of the _______
slowest elementary step
The value of K depends on the ________ and on the _______.
specific chemical equation temperature
overall order of reactions
sum of reaction orders for each reactant
Rates of chemical reactions depend on _________
temperature
average rate
the change in molar concentration of a reactant divided by the time interval
rate
the change in the concentration of a reactant or product per unit time
Which statement best describes activation energy? the difference in energy between the transition state and the products the difference in energy between the transition state and the reactants the difference in energy between reactants and products the energy of the transition state
the difference in energy between the transition state and the reactants
initial rate
the instantaneous rates as close as possible to the start of the reaction (time zero).
instantaneous rate
the rate at a single moment
Half-life
the time it takes for concentration to decrease to one-half its original value
transition state
when a reaction is occurring, a high energy unstable species exists in the reaction mixture
If plot a vs time is linear, reaction is ______
zero order