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Modified from P.W.W. Hunter, Chemistry Laboratory Manual (CHEM 162), Michigan State University.

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Presentation on theme: "Modified from P.W.W. Hunter, Chemistry Laboratory Manual (CHEM 162), Michigan State University."— Presentation transcript:

1 Modified from P.W.W. Hunter, Chemistry Laboratory Manual (CHEM 162), Michigan State University

2 Thermodynamics – if a reaction occurs Kinetics – how fast the reaction occurs and route the reaction takes Examples : how fast unwanted chemical substances break down (environmental) how long before chemicals like drugs metabolize in the body(biological) how fast will this fuel ignite to run my car (energy!)

3 Tells us how the reaction proceeds Chemical reactions occur through a series of steps (intermediate reactions) Some steps are fast, some slow Slowest step in mechanism determines rate Cant go any faster than slowest step!

4 A visible change occurs when a certain point in the reaction is reached (typically the end point) Time is inversely proportional to rate The longer a reaction takes, the slower the rate must be Something we can measure in the lab!

5 This is a complicated clock reaction called the Briggs-Rauscher Reaction Goes through 10 to 15 cycles Chemical oscillator is due to changes in iodine and iodide ion concentrations in solution (several chemical equations) B.Z. Shakhashiri, Chemical Demonstrations: A handbook for Teachers of Chemistry, V2, 1983, p248

6 A simpler example of a clock reaction Step 1: HSO H 2 O SO H 3 O + Fast Step 2: H 2 O + HCHO + SO 3 2- CH 2 (OH)SO OH - Slow Produce OH - (basic!) Can use pH indicator phenolphthalein Colorless to pink at certain pH Modified from P.W.W. Hunter, Chemistry Laboratory Manual (CHEM 162), Michigan State University B.Z. Shakhashiri, Chemical Demonstrations: A handbook for Teachers of Chemistry, V4, 1983, p70

7 Rates depend on initial concentrations of reactants A change in either one will affect reaction rate Rate = k [formaldehyde] a [bisulfate-sulfite] b The order of each reactant (a & b) add to give the overall reaction order

8 For the general reaction X Z Say a reaction is first order in X Then as the concentration of X doubles, the rate also doubles Say a reaction is second order in X Then as the concentration of X doubles, the rate quadruples Shows concentration dependence on rate

9 Rate = k [formaldehyde] a [bisulfate-sulfite] b Hard to understand what to change and what to hold constant… so lets rearrange by taking log of both sides log(rate) = log(k) + a log[formaldehyde] + b log[bisulfate-sulfite] If the concentration of F is held constant, then (a log[F] ) is constant and we can plot log(rate) vs (b log[BS]) to get order with respect to BS

10 Trial[F] (mL)[BS-S] (mL) Reaction Time (s) Rate (1/s) The concentrations of each reactant will be varied, while holding the other reactant constant. The time for each reaction to go to completion (in seconds), via a color change of phenolphthalein, will be recorded in the table. T. Cassen, J. Chem. Ed. 53(3), 1976, pg 197

11 Trial[BS-S] (mL)log[BS-S] Reaction Rate (1/s) log(rate) Trial[F] (mL)log[F] Reaction Rate (1/s) log(rate)

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14 ReactantSlope of lineOrder F11 BS-S00 Overall reaction order: 1 Order of overall reaction is one. Reaction depends directly on concentration of formaldehyde, but does not depend on bisulfite B.Z. Shakhashiri, Chemical Demonstrations: A handbook for Teachers of Chemistry, V4, 1983, p70

15 Dependent vs independent variables Calculation of slope Using log function to simplify expression Significant figures/digits Fluctuation in measurement/results Can perform experiment if desired and use discovery style


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