Presentation on theme: "Modified from P.W.W. Hunter, Chemistry Laboratory Manual (CHEM 162), Michigan State University."— Presentation transcript:
Modified from P.W.W. Hunter, Chemistry Laboratory Manual (CHEM 162), Michigan State University
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!)
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!
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!
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
A simpler example of a clock reaction Step 1: HSO 3 - + H 2 O SO 3 2- + H 3 O + Fast Step 2: H 2 O + HCHO + SO 3 2- CH 2 (OH)SO 3 - + 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
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
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
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
Trial[F] (mL)[BS-S] (mL) Reaction Time (s) Rate (1/s) 152.531.6 25531.2 351031.7 451531.9 55515.8 61057.9 71555.0 82053.5 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
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
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