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R's Chem 2 > Exam 1: Rates and Thermo > Flashcards

Exam 1: Rates and Thermo Flashcards

1
Q

What is the Rate Law

A

Rate=k[A]^x[B]^y
It expresses the relationship between the rate and concentrations
Directly proportional to the concentration and collisions per second.

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2
Q

What is the rate constant

A

K
relates the rate of a reaction with the concentration. Remains the same for that reaction.
efficiency of the reaction, higher k=more efficient
k=pze^-Ea/RT

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3
Q

What is the reaction order

A

the exponent on the reactants. Indicates how the concentration of that substance will effect the rate (zero order no effect, first order doubles, second order increases twofold)

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4
Q

How do you find the overall order?

A

The sum of the orders in a reaction.

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5
Q

Integrated Rate Law- what is this used for

A

Allows us to predict concentrations of reactants and products over TIME.
This equation will differ depending on order
Shows most accurate data of [ ] at any given time

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6
Q

Explain what is meant by half life

A

The amount of time it takes for half of the reactant to be consumed.
First order: ln2/k=t1/2

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7
Q

Arrhenius equation

A

The relationship between temperature and rate constant.
k=Ae^(-Ea/RT)
-RT(ln k- ln A)=Ea
needs to be manipulated into slope intercept form to be able to graph. once in slope intercept form, we can use that equation to find Ea.

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8
Q

Activation Energy

A

Ea- or activation barrier, the minimum amount of energy required to initiate a chemical reaction. If there is not suffient energy, the reaction cannot occur.

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9
Q

Collision Model

A

More collisions means faster reaction, means higher rate. Increasing temp and [ ] will increase collisions, increase rate. “Measure of success rate” by measuring collisions, temperature, rate constant, Ea, and frequency factor.

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10
Q

Orientation factor

A

P-steric factor, The collision must occur at the correct orientation to allow for a reaction

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11
Q

Collision frequency

A

Z- the frequency of collisions occuring

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12
Q

Frequency Factor

A

A- derived from collision frequency and steric factor.

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13
Q

Reaction Mechanism

A

Sequence of elementary steps by which the reaction will occur

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14
Q

Elementary Step

A

One step in a series that shows the progress of a reaction

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15
Q

Reaction intermediate

A

cannot be part of the proposed rate law. It is made then used up.

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16
Q

Rate determining step

A

Always the slow step. This must match the proposed rate law.

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17
Q

Catalyst

A

The addition of a catalyst will bring down the Ea by setting proper orientation to allow for more collisions, thereby increasing the rate. Collisions that do occur will be more efficient at producing products

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18
Q

Rate expression

A

An equation that shows the dissapearance of the reactants and the appearance of the products:
Rate=-A/nT = -B/nT=C/nT

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19
Q

What is reaction rate

A

The increase (or decrease) in molar concentration of the product (or reactant) per unit of time

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20
Q

Average Rate

A

Rate of change over the course of entire reaction

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21
Q

Instantaneous rate

A

Rate of change over a small period of time. Represented by finding the slope at any point on a curve. (change in y/change in x) Equal to the tangent to the curve. More useful than the average rate because we can decipher order based on the graph line

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22
Q

What are some ways we can determine rates?

A

Spectrophotometrically, Chromatographically, Pressure Changes, Mass.

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23
Q

Why is it more useful to graph rate and concentration than to graph concentration and time?

A

[ ]/t will give you either a straight line with a constant curve (zero order) a curve( 1 or 2 order).
Rate/[ ] will provide a linear graph which we can use to determine order.
(Remember rate= [ ]/t, meaning rate is our slope)

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24
Q

Draw an energy diagram and label the Ea, transition state, reactants, and products in exo and endo reactions

A
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25
Q

What are the 4 factors affecting Rates? (chemical kinetics)

A
  1. Concentration of reactants.
  2. Presence of a catalyst
  3. Temperature
  4. Surface area (increase sa = increase concentration of material that can react)
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26
Q

What happens when you increase the temperature of a reaction?

A

A higher temperature will increase the kinetic energy of the molecules, thereby allowing an increase of molecules to exceed the activation barrier, and the reaction rate will increase.
Increase in temp=increase collisions=faster rate

27
Q

Zero Order Reactions

A

Concentration does not dictate the rate. This is concentration independent.
Straight horizonal line if graphed rate/[ ]. Straight downward line if graphed [ ]/t

28
Q

First order reaction

A

Straight line if graphed rate/[ ], rate is directly dependent on the reactant concentration. If [ ] doubles, rate will double

29
Q

Second order reaction

A

graphed as a curved line. Second order means higher collisions

30
Q

What is the method of initial rates?

A

The mathematical calculations and experimentally observed comparisons to determine order of reactants.
Concentration and Rates relationship.
Determines the order of the reactants in order to write the rate law.

31
Q

The higher the reaction order the ________the reaction

A

More complex

32
Q

Integrated Rate law

A

Rate vs Time relationship. Allows us to predict concentrations over time.
Zero: [A]t= -kt+[A]o
1st: ln[A]t= -kt+[A]o
2nd: 1/[A]t= kt+1/[A]o

33
Q

Half life

A

First order equation: ln2/k=t1/2
or derived from the integrated rate law.

34
Q

Effects of temperature on rate

A

Increase temp=increased rate.
Think of energy diagrams here: increased temp=lower ea=faster reaction

35
Q

What are the three criteria for proposed rate in reaction mechanisms?

A
  1. Adding up elementary steps must equal the overall reaction
  2. Proposed rate law must equal the observed rate law. (observed can only be found experimentally)
  3. Must trap out the intermediate, intermediate is not found in the proposed rate law
36
Q

Which step in a reaction mechanism is the rate determining step?

A

Always the slow one. A reaction can only go as fast as its slowest reaction
Predicted rate law for the RDS will have to equal the observed rate law.

37
Q

What are the Laws of Thermodynamics?

A
  1. Energy cannot be created or destroyed, but can be transferred
  2. For a proces to be spontaneous, the entropy of the universe must be positive.
  3. Entropy of a perfect crystalline substance is zero at absolute 0 (0 Kelvin). As temp increases, motion increases.
38
Q

What is meant by a “spontaneous” reaction?

A

A reaction that occurs under specified set of conditions. (up to mother nature, without our intervention)

39
Q

Define Entropy

A

State function that measures the amount of disorder in a system, or how dispersed the energy is.

40
Q

Define enthalpy

A

The heat exchanged between the system and the surroundings

41
Q

Gibbs Free energy

A

The sign determines the reactions spontaneity. Relationship between entropy of the universe and the temperature.
-Gsys= spontaneous
DeltaGsys= -T(deltaSuniv)
DeltaGsys= deltaH - T(delta Ssys)

42
Q

2nd law of thermodynamics

A

Suniv= Ssys + Ssurr
Suniv must be + to be spontaneous.

43
Q

Finding S univ with temperature to determine spont

A

Suniv= Ssys + -Hsys/T

44
Q

What states increase entropy

A

Solid<Liquid< Gas

45
Q

How to predict increase in entropy

A
  1. determine state changes
  2. # of molecules increase in product
  3. molecular weight-larger mass=greater entropy
46
Q

What is meant by equilibrium

A

G=0
chemical potential or kinetic energy is 0. The most stable it can be. It is the bridge between spontaneous and non spont.
The energy forward matches the energy reverse

47
Q

If H is and S is, then G is
1: - +
2: + -
3: + +
4: - -

A

1: spontaneous
2: nonspontaneous
3: Spontaneous at high temps
4: Spontaneous at low temps

48
Q

How to find G

A

If at stp and values are given:
Grxn= G[products] - G[reactants]

If stp and H and S are given: Grxn= Hrxn - TSrxn

49
Q

What does stp mean

A

Standard Temperature and pressure, 25 C, 1 atm pressure (and 1 M where applicable)

50
Q

In Lab, how did we determine the rates for each sample?

A

By using the general rate expression
[ ] /t
taking the concentration of potassium permanganate and dividing by the average time in seconds.

51
Q

In Lab, why did we use the concentration of potassium permanganate for the rate, and no the oxylic acid?

A

Because the potassium permanganate was purple in color we could visual time when it had completely reacted. The oxylic acid is clear so we would not be able to see a change.

52
Q

In lab, how did we determine the final rate law equation?

A

solving for order of each substance then plugging in to solve for k

53
Q

What is the purpose of the Arrhenius equation?

A

To find Ea. In lab, we can already find k at different temps. This equation allows us to see the effect of T on Ea.
Arrhenius said we need energy to make or break bonds.

54
Q

Give examples of a spontaneous reaction

A

Ice melting at room temp

Sodium metal reacting violently with water

A ball rolling downhill

Rusting of iron at room temp

Water freezing at -10C

55
Q

Give examples of a nonspontaneous process

A

Water freezing at room temp

Sodium hydroxide reacting with hydrogen gas

ball rolling uphill

converting rust back to iron at room temp

Ice melting at -10 C

56
Q

What are the units for entropy?
Why do we care?

A

J/K mol
We care because gibbs and enthalpy are both in kj. Must remember to convert for accurate numbers. Remember Kelvin as well. (+273)

57
Q

What is the significance of the Gibbs sign?

A

Tells us the direction of a reaction and allows us to determine spontaneity.

58
Q

Equation to find the temperature at which reactions will become spontaneous

A

G(or 0)= H/S
Derived from H-T(S). Add T(S), divide by S, set T to 0 (since negative G becomes spontaneous)

59
Q

Instantaneous rate

A

a specific point on the curve, a more accurate slope is given with this than with the average rate.
This is the tangent line we will “eyeball”

60
Q

If you increase the activation barrier, what does that affect in terms of time and rate

A

Increasing the Ea will increase the time, decreasing the rate.
Decreasing the Ea will decrease the time, increasing the rate.
Example: turning granite into diamond. Takes a long long long time, therefore the Ea must be huge

61
Q

What is the goal in understanding rates? Why do we care?

A

So we can understand the behavior of a chemical reaction.

62
Q

Observed rate law

A

can only be determined by experimenting and testing method of initial rates in lab.

63
Q

Define Thermodynamics

A

The study of energy transfer (not always heat)

64
Q

Keq

A

equilibrium constant
Ratio between reactant and product

R's Chem 2 (5 decks)

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