Chem quiz chapters 14/15 in 8th edition

Rate laws are determined by

observing how initial reaction rate depends on reactant concentration

True or false: Molecular orientation during intermolecular collisions has no effect on whether a reaction will occur.

False. Needs proper geometric orientation and sufficient energy

True or false: Reducing the activation energy for a rate-limiting step changes the value of the equilibrium constant, Kc.

False, it has no effect on Kc but it does make the reaction faster (thermodynamics)

A reaction intermediate is ___________ then ______________ in a later step

Produced then consumed

A catalyst is ____________ then ______________ in a later step

Consumed then regenerated

Doubling the pressure in a reaction means that it will shift…..

To the side with fewer moles of gas (coefficients)

When solving for the halflife (t_1/2) in a first order reaction, we can use what equation

t_1/2 = 0.693/k (given)

Increasing temperature increases reaction rates because….

A greater percentage of molecules undergo sufficiently energetic intermolecular collisions

We determine rate order when given a rate law by…

Looking at exponents - 2 is 2nd order.
For overall order, add up all exponents

The method of initial rates is used determine rate laws experimentally because….

The reverse reaction is negligible at the start since no products have formed yet

T or F: One can always determine the rate law by examining the overall (net) equation for a multi-step chemical reaction

False - Rate law depends on individual steps

If a reaction is zero order, the amount of product generated/reactant consumed is….

Directly proportional to time

Units of the rate constant k vary depending on the….

Order of the reaction
They can be used to figure out the overall order of a reaction

Catalysts ___________________________ of a reaction

Increase the speed

Explain what two factors influence whether an intermolecular collision between A and B will result in a reaction.

Need enough energy to break bonds (activation energy)
Correct orientation so they properly touch and can form bonds

A catalyst __________ the forward and reverse reaction rates _________

Increases - equally

Reaction rates unit

Moles per liter per second - M/s or mol/L*s
Molarity (M) = mol/L
M/s

Using initial rates to measure reaction rate makes sure….

The reactant concentrations are accurate and the reverse reaction hasn’t started/interferred

A zero order reaction has a reaction rate…

that is independent of reactant concentration

T or F: It is ok to have the concentrations of intermediates appear in a net (overall) rate law equation

False - they are supposed to be generated then consumed

T or F: Reaction steps that occur after the rate limiting step do not factor into the rate law we write for a series of steps in a multistep reaction mechanism.

TRUE - reaction can’t go any faster than the rate-limiting step

Which order reaction has a reactant half-life that is constant throughout the course of the reaction?

First order - the half life only depends on the rate constant

Slope of Arrhenius equation gives?

-E_a/R
R is the gas constant (8.314 J/mol*K)

Y intercept of Arrhenius equation gives?

ln (pZ)
p is the steric factor - the fraction of the collisions where the molecules are lined up right

Z is the collision frequency - how often molecules collide with each other

Units of k for zero order

M s^-1

Units of k for first order

s^-1

Units of k for second order

M^-1 s^-1

Integrated rate law for zero order

[A] = [A]₀ - kt
[A]₀ means initial concentration
t is time in seconds

Integrated rate law for first order

ln[A] = ln[A]₀ - kt
[A]₀ means initial concentration
t is time in seconds

Integrated rate law for second order

1/[A] = 1/[A]₀ + kt

[A]₀ means initial concentration
t is time in seconds

Lowering activiation energy causes rate to?

Increase

T or F: Catalysts will shift the position of an equilibrium

False - helps it reach equilibrium faster but does not change it

Characteristics of fast reactions

Low activation energy, high temp, frequent collisions (Z), collisions are well oriented (p), catalyst is present

Reversible reactions

Products can turn back into the reactants - reaction stays at equilibrium between completion

Irreversible reactions

Reaction essentially goes to completion - no reactants remain, K is very large
K is products over reactants [C][D]/[A][B]

Molecularity is the number of reactant molecules that come together in a….

Single elementary step
Unimolecular = 1
Bimolecular = 2
Termolecular = 3

Rate constants for reaction steps after the rate determining step ____ appear in the overall rate law.

Do not

Coefficients only become exponents in a rate law for….

Elementary steps, not the net/overall equation

Catalysts _____ the activation barrier

Lower

Do catalysts change the equilibrium ratio?

No

Reaction rate

the rate of product production or reactant consumption

The steric factor (p) for an enzyme is __________ than that of a simple diatomic molecule.

Significantly smaller

The collision frequency between two reacting species is ________________ to their concentrations

Directly proportional

T or F: For a reaction step involving two chemical species, A+B, increasing one or both concentrations will increase the rate of that step

True - product (multiplication) gets larger so rate goes up

We use the method of initial rates to determine the concentration dependence of a reaction rate because…

Initially, there is no equilibrium and the reverse rate is zero, and products accumulating slows the rate

The transition state ? be lower than the energy of the products

Cannot

Pure solids and pure liquids are __________ when calculating equilbrium constant

Left out

[C][D]/[A][B]

Le Chȃtelier’s principle states that

a reaction in equilibrium will response to external stresses to minimize the applied stress

At equilibrium, the forward rate of reaction is…..

Equal to the reverse rate of the reaction

Chemical equilibrium occurs when

the forward rate equals the reverse rate

Quotient constant and equilibrium constant rules

• Q < K: shifts forward (right)

• Q > K: shifts reverse (left)

• Q = K: already at equilibrium

Variable S means what?

Entropy - disorder, amount of particles moving around
ΔS > 0 means more disorder

Variable H means what?

Enthalpy - heat energy of a reaction
ΔH > 0 = endothermic - reaction absorbs heat
ΔH < 0 = exothermic - reaction releases heat

K_c (equilbrium constant) uses?

Concentrations at equilibrium

Q_c (quotient constant) uses?

Concentrations right now

Quotient constant and equilibrium constant rules

• Q < K: shifts forward (right)

• Q > K: shifts reverse (left)

• Q = K: already at equilibrium

What is the reaction quotient?

the same calculation as for Kc, but possibly for non-equilibrium conditions

Why do we omit the concentration of water from the equilibrium expressions we write for reactions taking place in aqueous media that involve water as a reaction participant?

The concentration of water remains constant throughout the reaction - its a pure liquid
Pure solids and liquids are not included

When calcuating equilbrium/quotient constant, coefficents become…

Exponents outside of the bracket
2 CO₂ becomes [CO₂]²

If a mixture is known to be at equilibrium, the concentrations of the molecules in the mixture….

Don’t change - they stay constant

At chemical equilibrium, both the forward and reverse reactions ____?____ occur on a microscopic scale

Continue to

Le Châtelier’s principle is a (crude / sophisticated) model

Crude model - makes qualitiative predictions (shifts)

When can a reaction be said to not be at equilibrium?

• When Q_c doesn’t equal K_c - concentrations now aren’t equal to concentrations at equilibrium

• If concentrations are changing - should be constant at equilibrium

What does it mean when an equilibrium constant has a value near one?

at equilibrium, appreciable amounts of both reactant and product should be present

PV=nrt transformation for K_p equilibrium expression with chemical species A

P_A = [A]RT

n/v becomes [A] because n is moles and V is liters, moles/liters = Molarity (concentration

A reaction that is both _______ and _________ will attain equilibrium quickly

Fast and reversible

When Q_c is higher than K_c…..

There are more products than reactants - not equilibrium
Needs to shift left

When Q_c is lower than K_c…..

There are more reactants than products - not equlibrium
Needs to shift right