UNIT 14 rates

WHY? Collision Theory

Multistep/elementary reactions and energy

Types of Catalysts

Collision theory: 1. Reactions happen with (more) collisions, 2. Reactants need enough energy when colliding, (> Ea) 3. and correct orientation

SO THINK, these will raise rate:

Increase Pressure

Increase concentration

Increase temp

Increase surface area/homogenity

Decrease Ea (activation energy)/add catalyst

Only when a reaction is boiled down into its elementary parts can its rate orders/behavior be ascertained (this is always true!)

Can sum steps to get overall equation, which shows reactants, products, intermediates, catalysts

RATE-DETERMINING STEP = SLOW STEP

KEY: For an elementary reaction (one step), the coefficients are the same as the rate orders!

  1. Homogeneous catalysts: same state as reactants, form intermediates?

CATALYST is present at beginning (and end; next beginning) while intermediate is created in the middle and used in the middle.

Decreased molecularity (pool ball ex.)

WHAT? Rates: Marriage of Math and Chemistry

Think about a given reaction... it will have constituents with their respective concentrations, and will take place over time. It will be a unique function like in math, but still can be translated into math. (x = time, y = concentration.) IT WILL:

  1. Be able to be observed, leading to AROC (and ideally instant ROC for rate law):
  1. Have respective constituent rates based on stoichiometry.

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  1. Be differentiable; derivative = rate law!
  1. Have changing concentrations; integral law..? (but kinda just y...)

Zero order:

1st Order:

2nd Order:

MISC:

Compare how concentrations of each reactant affect the overall rate... if no, 0, if proportional, 1, if squared, 2 (must be obtained experimentally, or you use experimental results)

  1. Have half-lives?

Zero Order:

k just expresses the rate of that given reaction; changes with temperature, or nature of the reaction. ALWAYS PUT UNITS OF K; solve for units w/dimensional analysis and rate law

Overall rate order is the sum of all reaction orders.

1st Order:

2nd Order:

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moles ∝ molarity ∝ pressure (gas) ∝ concentration

If FIRST STEP slow, those reactants and molecularity are the rate law.

If SECOND STEP slow, that step will be law but intermediate must be substituted for ITS reactants.

GRAPHS:

"Intermediate valleys" signify an intermediate

Peak is "activation complex", then it changes form, going down energy gradient

change from beginning to highest point is Ea, change from beginning to end is change in E

Higher peak/Ea means SLOWER, more energy-demanding reaction

  1. Surface catalysts: use adsorption to bring molecules together, allowing for opportunities to collide. (usually metal or metal oxide). Use unpaired electrons on metal.
  1. Acid-Base: donates/accepts a proton from reactant, easing reaction.
  1. Enzymes, form enzyme-substrate complex, biological molecules

Rates always positive; rate of disappearance must be change * -1 to make it positive.

LOOK FOR AXES; If it is linear, just concentration is 0, ln[A] is 1st order, 1/[A] is 2nd order

Smaller, simpler molecules (less chances for orientation to go wrong)

Lowered Ea means more stable transition state (potential energy means instability.)

If coefficients DONT match orders, must be multiple steps..

Best catalyst will actually allow reaction to proceed at lowest temp; if Ea is lower, required energy will be lower

Each step of catalyst mechanism must be lower than the original Ea to make it actually go faster