Chem module 1

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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


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