Chemistry 381 Advanced Module in Physical Chemistry Objectives: Learn how to do experiments Learn how to interpret experimental data Learn how to report experimental data

Coincidence mass spectrum K +(CH3NO2, CH3Br, air oriented + random O- random Mass2

CH3NO2 & CH3Br NO2- e- O2- ?? CH3NO2- Br- O-

Oriented + random random X X

S/N 28 65 NO2- e- 17 O2- No! CH3NO2- 18 Br- 48 4 O-

Errors Always present. At best we reduce to a tolerable level How to reduce? Improve methods Calibrate equipment Alternate methods Replicate runs Types of errors: Precision vs Accuracy Precision: how reproducible Accuracy: how close to right answer

Accuracy vs Precision True value Accuracy & Precision Accuracy No Precision Precision No Accuracy Neither True value

Determinate (Systematic)--Definite value. Accuracy Kinds of errors Determinate (Systematic)--Definite value. Accuracy Can be measured & corrected Personal Ignorance, carelessness Improper technique Arithmetic Prejudices--trying to get “right” answer Physical limits Instrumental Improper calibration Procedure These can be corrected for Nonideality Equilibrium not attained Side reactions Indeterminate (Random) can’t be controlled. Tend to average out.

Treatment of Systematic Errors A. Write down all possibilities for error Be imaginative. It’s easier to throw them out that to dream them up. B. Estimate the accuracy with which these are known. C. Using the chain rule of differentiation, evaluate the effect of each of these on the final result D. Retain those sources of error which are significant (and not stupid.)

Propagation of systematic errors Does not average out Measure X with error X How does it affect final result, R? a) Addition & subtraction R=(X1+ X1) + (X2+ X1) R=(X1+ X2) + (X1+ X2) RX1+X2

b) Multiplication & Division Now divide by R Fractional errors add & subtract

Propagation of random errors These have random sign How does it affect final result, R? Magnitude difficult mathematically, so square each term a) Addition & subtraction R=x-y If x≈y, R≈x2

b) Multiplication & Division Random errors

See Shoemaker, et al, Chapter 1 & 2 Reports See Shoemaker, et al, Chapter 1 & 2 Reports must be typed (except for data & equations) Introduction Legibility II. Experimental Clarity III Results Brevity Accuracy IV Discussion Proper English & spelling Meaning Accuracy (including error discussion