General Chemistry II (Test II)

4 Factors that affect entropy: LINKED TO RIGIDITY: more rigidity (stronger bonds)? less entropy!

1. entropy increases when temperature increases 2. entropy increases when volume increases 3. Entropy increases when the number of independent particles increases 4. Entropy is linked to molecular mass and structure and increases with increasing molecular size

Half life of second order reaction:

1/k[A]0

Heterogeneous Catalyst

A catalyst in a different phase than the reactants • NO decomposes to N2 and O2. Most gas-phase reactions on heterogeneous catalysts exhibit zero order kinetics.

Reaction Mechanisms

A set of steps that describe how a reaction occurs at the molecular level; the mechanism must be consistent with the rate law for the reaction.

Cyclopropane, an anesthetic, decomposes to propene according to the following equation. The reaction is first order in cyclopropane with k = 9.2 s-1 at 10000C. Calculate the half life of cyclopropane at 10000C.

A. 0.075 s

Rate Law

An equation that defines the experimentally determined relation between the concentration and rate of the reaction

How much of a 3.0 g sample of cyclopropane remains after 0.50 seconds? (k = 9.2 s-1 )

B. 0.030 g

Suppose the activation energy of a certain reaction is 250 kJ/mol. If the rate constant at T1 = 300 K is k1, and the rate constant at T2 = 320 K is k2, then k2/k1 = ____. (The universal gas constant = 8.314 J/mol·K.)

B. 6.26

What will be the effect of increasing the temperature of reactants that are known to undergo an endothermic reaction?

Both the rate of reaction and the value of the equilibrium constant increase

The rate constant for the second order reaction below is 2.79 L/mol ·min at 48°C. If the initial concentration of NO2 is 1.05 M , what is the half-life? 2NO2 >> N2O4

C. 0.341 min

A catalyst increases the rate of a reaction by?

Changing the mechanism of the reaction provides an alternative route for the reaction with a lower activation energy

If a graph of lnK v 1/T has a positive slope, is the reaction exothermic or endothermic?

Exothermic

T or F Non-spontaneous reactions require a high activation energy.

False

T or F As a first order reaction proceeds at a constant temperature, the rate remains constant.

False, as reactant decreases, rate decreases

T or F A fast reaction must be thermodynamically spontaneous.

False, can be fast or slow

T or F Only exothermic processes are spontaneous.

False, some endothermic processes are spontaneous

T or F Spontaneous reactions require continuous addition of energy from an external source.

False, spontaneous processes proceed without outside intervention

Transition State

High energy state between reactants and products; short lived; activated complex; bond forming/ bond breaking

List liquid, gas, solid in order of increasing entropy.

Least: 1. Solid 2. Liquid 3. Gas Most:

THE RATE OF A REACTION IS DETERMINED EXPERIMENTALLY.

NEVEr from the balanced chemical equation!!

If ΔG is greater than 0, is the reaction spontaneous, non-spontaneous or at equilibrium?

Non-spontaneous in the forward direction and spontaneous in the reverse direction

Which of the following reactions has a positive entropy change? A. H2O(g) >> H2O(l) B. BF3(g) + NH3(g) >> F3BNH3(s) C. 2SO2(g) + O2(g) >> 2SO3(g) D. N2(g) + 3H2(g) >> 2NH3(g) E. 2NH4NO3(s) >> 2N2(g) + 4H2O(g) + O2(g)

Not A: negative entropy, increasing in order, less disorder Not C: negative entropy, less disorder, increasing in order Not D: same reason as C E is correct: decreasing in order (increasing in disorder), positive entropy NOT B: negative entropy, going from less order to more order (less disorder)

2NO2 → 2NO+O2 Rate?

Rate = k[NO2 ]^2

If ΔG is less than 0, is the reaction spontaneous, non-spontaneous or at equilibrium?

Spontaneous in the forward direction and non-spontaneous in the reverse direction

EFFECT OF CONCENTRATION ON REACTION RATE

The rate of the reaction slows down as the concentration decreases. The more reactants present, the more collisions and the faster the reaction takes place.

Rate Constant

The proportionality constant that relates the rate of a reaction to the concentration of reactants Units depend upon the order of the reaction. (k)

Overall Reaction Order

The sum of the exponents of the concentration terms in the rate law Order = m + n

T or F A spontaneous reaction is a fast or slow reaction.

True

T or F Spontaneous processes proceed without outside intervention.

True

T or F Non-spontaneous reactions proceed with outside intervention.

True, they cannot proceed without outside intervention

T or F Spontaneous reactions require a high activation energy.

True? LOOK IN BOOK!

Define Entropy

a measure of how dispersed the energy in a system is at a specific temperature

A positive change in entropy represents: a. an increase in dispersal of matter (molecular disorder) b. release of thermal energy c. a decrease in thermal energy d. a process that is always spontaneous e. a process that cannot occur spontaneously

a. an increase in dispersal of matter (molecular disorder)

Average rates

apply to a specific time interval different time interval = different rate

If ΔG is 0, is the reaction spontaneous, non-spontaneous or at equilibrium?

at equilibrium

Catalysts

change reaction rates by providing an alternative reaction pathway with a different activation energy.

Volatile organic compounds (VOCs)

combine with ozone and OH radicals to form noxious compounds

Consider the reacRon below at 25°C for which ΔS0 = 16.1 J/K. CH4(g) + N2(g) + 163.8 kJ >> HCN(g) + NH3(g) Which one of the following statements describes the reaction? A. Spontaneous at all temperatures B. Spontaneous at relaRvely low temperatures only C. Spontaneous at relaRvely high temperatures only D. Nonspontaneous at all temperatures E. Insufficient informaRon to esRmate temperature range of spontaneity

delta H will be (negative) delta S will be (positive, given in Q) C spontaneous at relatively high temperatures only!

If a graph of lnK v 1/T has a negative slope, is the reaction exothermic or endothermic?

endothermic

When ΔHrxn is greater than 0, is the reaction exothermic or endothermic?

endothermic

Entropy of the universe is equal to _______ + ________ > 0. (For Spontaneous) Non-Spontaneous?

entropy of the system plus entropy of the surrounds greater than 0 entropy of the system plus entropy of the surrounds less than 0

When particles disperse into a larger volume and/or gain freedom of motion then ________ increases and the process is __________.

entropy, spontaneous

When ΔHrxn is less than 0, is the reaction exothermic or endothermic?

exothermic

Gas to a Liquid is ____. Liquid to a Gas is ____.

exothermic endothermic

Gas to a Solid: Solid to a Liquid:

exothermic endothermic

Liquid to Solid: Solid to a Gas:

exothermic endothermic

Spontaneous processes often still need an energy boost...spontaneous does NOT equal ______ or _______ necessarily!

fast, exothermic

A spontaneous process produces an ________ of the universe.

increase in entropy

Reaction rate always ______ with increasing temperatures. The rate constant for a reaction ________ at lower temperatures.

increases decreases

When particles disperse into a larger volume and/or gain freedom of motion then ________ increases and the process is __________.

increases, spontaneous

Arrhenius Equation:

k = Ae^(-Ea/RT) or lnk = -Ea/RT + lnA

Define G, Gibbs Free Energy.

maximum useful energy obtainable in the form of work from a process at constant temperature and pressure

Reaction Rate

measure of how rapidly a reaction occurs OR the rate at which a product appears or a reactant disappears

Positive entropy = _______ Negative Entropy = _______

more disorder less disorder

Processes that increase the energy of a system tend to be ________.

non spontaneous

Processes that decrease entropy tend to be _______. Processes that increase entropy tend to be _____.

non-spontaneous spontaneous

A ________ process is a reaction that cannot happen on its own.

nonspontaneous

Reaction rate signs _____ for product appearing. ______ for reactants disappearing

positive negative

Instantaneous Rates

rate of reaction at a particular time after it began calculate slope of a line tangent to the curve

In an exothermic reaction, the _____ are at a higher energy level as compared to the _____.

reactants products

The ____________________ also known as the rate determining step is used to write the experimental rate law.

slow step

Photochemical smog

smoke + fog = smog

A ________ process proceeds without outside intervention.

spontaneous

Processes that decrease the energy of a system tend to be ________.

spontaneous

If Entropy of the universe > 0, then a process is _______. If entropy of the universe < 0, then a process is ________.

spontaneous nonspontaneous

Exothermic reactions tend to occur ______. Endothermic reactions tend to be ______. REMEMBER NOT ALL!!

spontaneously non-spontaneous

Entropy is a _________ (like enthalpy), which means that it depends only on the chemical and physical state of a system and NOT the ______ followed to reach that state.

state function, pathway

How do you find the change in entropy of a reaction?

sum of entropy of the products minus sum of entropy of the reactants

How do you find the chance in G of a reaction?

sum of moles X G of products minus sum of moles X G of reactants

Half life of zero order reaction:

t1/2 = [A]0 / 2k

Half life of first order reaction:

t1/2=ln2/k

The Third Law of Thermodynamics states, _______.

the entropy of a pure, perfect, crystalline solid at 0K is zero

Reaction rate coefficients

the stoichiometric coefficient becomes the reciprocal for rate Example: 2NO becomes + 1/2NO

The second law of thermodynamics states that the ___________. FIRST LAW?

total entropy of the universe increases in any spontaneous reaction First: total energy of an isolated system is constant; energy can be transformed from one form to another, but can be neither created nor destroyed.

T or F The rate constant for a reaction is independent of reactant concentrations.

true

If X versus t is linear...? If ln(X) versus t is linear...? If 1/(X) versus t is linear...?

zero order first order second order

Unit of k in a zero order reaction? first order? second order?

zero order: mol/L X s first order: 1/s second order: L/mol X s

Reaction Order`

• An experimentally determined number defining the dependence of the reaction rate on the concentration of a reactant

`Activation Energy (Ea)

• The minimum energy molecules need to react when they collide • Energy barrier that must be overcome • The greater the Ea, the slower the reaction. • Rates increase with increasing temperature.