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Chem 142 > Unit 5 > Flashcards

Unit 5 Flashcards

Rates of Chemical Reactions and the Gaseous State (Ch 15 and 5)

1
Q

Reaction Rate
- definition
- unit

A
  • the change in concentration of reactants or products over time
  • aka, how fast a reaction occurs
  • k
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2
Q

Kinetics

A

the study of the rates of chemical reactions

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3
Q

What factors influence rate of reaction (k)?

A
  • Concentration (UU, DD)
  • Physical state of the reactants
  • Temperature (UU, DD)
  • Surface area of a reactant (UU, DD)
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4
Q

Rate of Reaction Equation

A
  • ( 1 / a )( Δ[A] / Δt ) = + (1 / c )( Δ[C] / Δt )

a, c = coefficients
A = reactant (-)!!!!
C = product (+)!!!

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5
Q

EQUATION ROLL CALL

A

every time you see this card, tell:
(1) Definition of equations
(2) Meanings of each variable

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6
Q

Average Reaction Rates vs Instantaneous Reaction Rates

A

Average: change in concentration divided by change in time
Instantaneous: Slope of the tangent line drawn at the time of interest. Generally what the term reaction rate refers to

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7
Q

Initial Rate

A

the instantaneous (deriv slope!) rate at time t=0

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8
Q

The Rate Law

Definition/purpose and equation

A

Gives the reaction rate, in terms of the concentration of species at any time.
Rate = k [A]^n [B]^m
k = rate constant
n & m = reaction orders

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9
Q

Zero Order Reactions
1. Equation
2. Behavior of [A]
2. Relationship of k - [A]
3. Units

A
  1. Rate = k[A]^0 = k
  2. [A] decreases linearly with time
  3. Rate is independent of [A]
  4. units of M x s-1
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10
Q

1st Order Reactions
1. Equation
2. Relationship of k - [A]
3. Units

A
  1. Rate = k[A]
  2. Rate is directly proportional to [A]
  3. Units of s-1
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11
Q

2nd Order Reactions
1. Equation
2. Relationship of k - [A]
3. Units

A
  1. Rate = k[A]^2
  2. Rate is proportional to [A]^2
  3. k has units of M-1s-1
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12
Q

How to find overall reaction order

if more than one reactant

A

Add together the orders of each constant.
Rate = k[H2]^1[NO]^2
Overall reaction = 1+2 = 3

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13
Q

How to find reaction orders

given table of experiment data

A

do the table thing
where only [x] to find is changing, not the other
Exp. 2 [X] / Exp. 1 [X], R(i) 2 / R(i) 1,
M^a = M/s
a = order

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14
Q

Purpose of integrated rate laws

A

They let you calculate the concentration of a species at any time after the start of the reaction.

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15
Q

Integrated Rate Law: Zero Order

  1. slope
  2. graph axes

1. equation

A

[A(t)] = -kt + [A(0)]
slope = -k
[A] vs time

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16
Q

Integrated Rate Law: 1st Order

  1. slope
  2. graph axes

1. equation

A

ln[A(t)] = -kt + ln [A(0)]
slope = -k
ln [A] vs time

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17
Q

Integrated Rate Law: 2nd Order

1. Equation
2. slope
3. graph axes

A

1/[A(t)] = kt + 1/[A(0)]
slope = +k
1/[A] vs time

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18
Q

Half-life

1. Definition
2. equation

A

the time required for the reactant concentration to reach half of its initial value

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19
Q

If not given but lowkey needing reaction order, how can you find it

if only given info like “X -> Y” and rate constant

A

look at the units of the rate constant!

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20
Q

Pressure

- Definition
- Units
- Equation

A
  • force per unit area
  • atm OR pascal (N per m^2)
  • P = F/A
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21
Q

Charles’s Law

A
  • volume of an ideal gas is directly proportional to its temperature measured in Kelvin
22
Q

Boyle’s Law

A
  • PV is constant
  • V is proportional to 1/P
  • PV = constant is called “ideal”
23
Q

Avagadro’s Law

A
  • The connection between V and n of gases
  • V is proportional to n
24
Q

Empirical Gas Law

A

V ∝ nT/P

25
Ideal Gas Law
PV = nRT | TEMP IN KELVIN!!
26
Dalton's Law of Partial Pressures
Total pressure = the sum of the individual gas pressures. Assumes gases are behaving ideally.
27
KMT
- Can predict macroscopic behavior of a gasous system with a molecular-level model - Answers the WHY of ideal gas behaviors.
28
KMT Postulates
- Gas particles are so small that their volume is negligible. - Gas particles are in constant, random motion - KE is directly proportional to the kelvin T. - Collisions are elastic (conserve energy) - Particles exert no force on one another
29
KMT Central Points
Pressure: arises from molecules banging into the container walls. Temperature: directly related to the KE. UU, DD
30
Average Velocity of Gas particles
* u(rms) * u(rms) = √(3k(B)T)/m OR * u(rms) = √(3RT)/M
31
"Maxwell-Boltzmann" curve
Representsthe fraction of gas molecules in a sample that are traveling at a given velocity. u(m): most probable speed u(avg): average speed u(rms): the speed of a molecule with the average molecular kinetic energy
32
Collision Theory
1. Atoms and/or molecules MUST collide for a reaction to occur. 2. Steric Effects: Collisions must occur in the correct orientation, or no reaction occurs 3. Atoms/molecules MUST possess a minimum amount of energy ot initiate the reaction. (Ea)
33
Activated State
The state particles reach when they collide effectively.
34
Transition State
- aka activated complex - when to molecules collide and reach the top of the potential energy curve - an unstable species that contains PARTIAL BONDS - cannot be isolated
35
The Arrhenius Equation
Relates the rate constant (k) to the activation energy (Ea) and the temperature (T) k = Ae ^ -(Ea/RT) A: pre-exponential factor e ^ -(Ea/RT): exponential factor
36
Pre-exponential factor (A) in the Arrhenius Equation
the number of times that reactants approach the activation barrier per unit time. - two parts: orientation factor & collision frequency
37
Exponential factor in the Arrhenius Equation
the fraction of approaches that are successful in surmounting the activation barrier and forming the products
38
Reaction Mechanisms
a series of individual chemical steps by which the overall chemical reaction occurs.
39
Elementary Step
* Each step in a reaction mechanism * Can't be broken down into simple steps. * Occur as they are written
40
Reaction intermediate
* a substance that forms in one elementart step and is consumed in another * Not a transition state! * On a graph, exist in a potential energy well between 2 different transition state peaks
41
How are elementary steps characterized?
* By their molecularity - the number of reactant particles involved in the step
42
How to find rate laws for elementary steps
* Can find orders through the balanced chemical equation!! * The coefficients in the balanced equation for each step are the order of the reaction for that molecule. * For each individual molecule! ex. 2A + B -> k[A]^2[B]
43
Rate Detemining Step
* The slowest elementary step * Determines the rate of the overall reaction cause its such a SLOWPOKE * So basically the orders for the overall reaction rate follow the orders of the slowest steps. * Are usually determined by who has the biggest Ea
44
What if the rate determining step involves an intermediate?
* cross that bitch out! * now replace it with a non-intermediate particle with an equivalent rate. set their k1[A]=k-1[B] equal to each other, then solve for the intermediate in terms of the other. Then replace the intermediate with the equation found, including the k's!
45
Equilibrium
When the rate forward (k1) = rate reverse (k-1).
46
3rd Order Reaction Units
M^-2 s^-1
47
Instantaneous rate of disappearance meaning
d[A]/dt
48
How to calculate the pressure exerted from the flask in mercury problems
1. Calculate the amount of m of mercury to balance out the pressure from the gas - (?) atm x 760 mm = (?) mm = (?) m mercury to balance out pressure - add whatever amount of mercury just calculated, and add to the original level of mercury.
49
How to calculate pressure from the atmosphere in mercury problem
1. Calculate the pressure from the gas inside the flask first 2. 1 atm = 760mm = 0.76 m mercury. So, (?) atm x 760mm = (?) m mercury to push DOWN (subtract from) the level calculated in step one.
50
Ideal bond angles: Linear
180

Chem 142 (3 decks)

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