{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}

# Lecture34A - Mechanisms to Rate Laws(29.4-6 Today we will...

This preview shows pages 1–4. Sign up to view the full content.

Lecture 34 1 Mechanisms to Rate Laws (29.4-6) Today we will cover 3 important concepts in deriving rate equations from elementary reaction steps: the steady state approximation; pre- equilibrium; and the Lindemann-Hinshelwood Mechanism for unimolecular dissociation. Recall from last lecture our two step mechanism: [ ] [ ] [ ] [ ] 1 2 0 0 0 at 0 0 k k A I P t A A I P = = = = We solved for [A], [I], and [P] under general conditions and limiting cases where k 1 <<k 2 and k 2 <<k 1 . Fig 29.1 ( 29 [ ] [ ] [ ] 1 2 1 2 a) 10 This case had A disappearing at a faster rate than P forming. The reason: a substantial buildup of I . k k k k = ( 29 [ ] 2 1 2 1 b) 10 This case has I low and nearly constant for most times. k k k k =

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
Lecture 34 2 [ ] The latter case (b) suggests 0 for most times, t. d I dt 2245 [ ] [ ] [ ] 1 2 From our elementary reaction steps: d I k A k I dt = - [ ] [ ] [ ] [ ] 1 2 Assuming I remains constant and low, the steady state approximation gives 0 ss ss d I k A k I dt = = - [ ] [ ] [ ] 1 1 1 0 2 2 So I k t ss k k A A e k k - = = [ ] [ ] Note that I is time-dependent. So what is ? ss ss d I dt [ ] [ ] 1 2 1 0 2 which we expect to be 0. k t ss d I k A e dt k - = - 2245 [ ] 2 1 0 2 2 1 Indeed this will be true when is very small. Since , this is a good approximation. k A k k k
Lecture 34 3 Fig 29.2 The steady state approximations are the solid lines for k 2 =10k 1 . In the last lecture I mentioned that the simple reaction: ( 29 ( 29 3 2 2 3 O g O g A [ ] [ ] [ ] [ ] 2 3 3 2 3 1 had a rather nasty rate law equation: 2 ' '' d O k O dt k O k O - = + The proposed mechanism is ( 29 ( 29 ( 29 ( 29 ( 29 ( 29 ( 29 ( 29 1 1 2 3 2 3 2 2 k k k O g M g O g O g M g O g O g O g - + + + + Note: M activates the decomposition of O 3 by collision, but is not a reactant.

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

### Page1 / 11

Lecture34A - Mechanisms to Rate Laws(29.4-6 Today we will...

This preview shows document pages 1 - 4. Sign up to view the full document.

View Full Document
Ask a homework question - tutors are online